Progress in molecular and genetic studies of IgA nephropathy

Association between geriatric nutritional risk index and pathological phenotypes of IgA nephropathy

Background

IgA nephropathy (IgAN) is an immune disease related to oxidative stress and inflammation. It is the most common type of glomerulonephritis in the world and is the cause of chronic kidney disease and end-stage renal disease (ESRD). The Geriatric Nutritional Risk Index (GNRI) is a practical and uncomplicated method to assess the risk of morbidity and mortality, but its ability to assess IgAN is still unclear. Here, we evaluated the association between the GNRI and clinical and histologic findings of IgAN.

Methods

In a cross-sectional study, we included 348 biopsy-verified IgAN patients. The Oxford classification was used to analyze the pathological characteristics of the included patients. Based on previous studies, the participants were divided into two groups using a cutoff value of 92. Differences in clinicopathological indices between the two groups were compared. The correlation between the GNRI and the indicators was evaluated by using a bivariate correlation analysis. A binary logistic regression analysis was conducted to determine the factors associated with the crescent lesions in IgAN.

Results

In this study, 138 out of 348 patients (39.7%) had low GNRI scores (GNRI < 92). Patients in the low GNRI group tended to have a significantly lower body mass index; lower hemoglobin, serum albumin, serum IgG, and serum C3 levels; and higher 24-h proteinuria. The proportions of females, Oxford M1 and Oxford C1/2 were higher in the low GNRI group. The GNRI was positively correlated with body mass index (r = 0.57, P < 0.001), hemoglobin (r = 0.35, P < 0.001), serum albumin (r = 0.83, P < 0.001), serum IgG (r = 0.32, P < 0.001), and serum C3 (r = 0.26, P < 0.001) and negatively correlated with 24-h proteinuria (r = -0.36, P < 0.001) and the proportion of crescents (r = -0.24, P < 0.001). The GNRI scores and serum IgG levels were considered independent factors influencing the crescent lesions in IgAN.

Conclusions

The GNRI can reflect the severity of clinical and histologic phenotypes in IgAN patients. Lower GNRI and serum IgG levels may suggest an increased risk of crescent lesions and are potential markers for disease monitoring in IgAN.

Mucosal Immune System Dysregulation in the Pathogenesis of IgA Nephropathy

The mucosal immune system, via a dynamic immune network, serves as the first line of defense against exogenous antigens. Mucosal immune system dysregulation is closely associated with the pathogenesis of immunoglobulin A nephropathy (IgAN), as illustrated by IgAN having the clinical feature of gross hematuria, often concurrent with mucosal infections. Notably, previous studies have demonstrated the efficacy of tonsillectomy and found that a targeted-release formulation of budesonide reduced proteinuria in patients with IgAN. However, it remains unclear how exogenous antigens interact with the mucosal immune system to induce or exacerbate IgAN. Thus, in this review, we focus on the dysregulation of mucosal immune response in the pathogenesis of IgAN.

Pathogenesis of IgA Nephropathy: Current Understanding and Implications for Development of Disease-Specific Treatment

IgA nephropathy, initially described in 1968 as a kidney disease with glomerular “intercapillary deposits of IgA-IgG”, has no disease-specific treatment and is a common cause of kidney failure. Clinical observations and laboratory analyses suggest that IgA nephropathy is an autoimmune disease wherein the kidneys are damaged as innocent bystanders due to deposition of IgA1-IgG immune complexes from the circulation. A multi-hit hypothesis for the pathogenesis of IgA nephropathy describes four sequential steps in disease development. Specifically, patients with IgA nephropathy have elevated circulating levels of IgA1 with some O-glycans deficient in galactose (galactose-deficient IgA1) and these IgA1 glycoforms are recognized as autoantigens by unique IgG autoantibodies, resulting in formation of circulating immune complexes, some of which deposit in glomeruli and activate mesangial cells to induce kidney injury. This proposed mechanism is supported by observations that (i) glomerular immunodeposits in patients with IgA nephropathy are enriched for galactose-deficient IgA1 glycoforms and the corresponding IgG autoantibodies; (ii) circulatory levels of galactose-deficient IgA1 and IgG autoantibodies predict disease progression; and (iii) pathogenic potential of galactose-deficient IgA1 and IgG autoantibodies was demonstrated in vivo. Thus, a better understanding of the structure–function of these immunoglobulins as autoantibodies and autoantigens will enable development of disease-specific treatments.

Aberrantly Glycosylated IgA1 in IgA Nephropathy: What We Know and What We Don't Know

IgA nephropathy (IgAN), the most common primary glomerular disease worldwide, is characterized by glomerular deposition of IgA1-containing immune complexes. The IgA1 hinge region (HR) has up to six clustered O-glycans consisting of Ser/Thr-linked N-acetylgalactosamine usually with β1,3-linked galactose and variable sialylation. Circulating levels of IgA1 with abnormally O-glycosylated HR, termed galactose-deficient IgA1 (Gd-IgA1), are increased in patients with IgAN. Current evidence suggests that IgAN is induced by multiple sequential pathogenic steps, and production of aberrantly glycosylated IgA1 is considered the initial step. Thus, the mechanisms of biosynthesis of aberrantly glycosylated IgA1 and the involvement of aberrant glycoforms of IgA1 in disease development have been studied. Furthermore, Gd-IgA1 represents an attractive biomarker for IgAN, and its clinical significance is still being evaluated. To elucidate the pathogenesis of IgAN, it is important to deconvolute the biosynthetic origins of Gd-IgA1 and characterize the pathogenic IgA1 HR O-glycoform(s), including the glycan structures and their sites of attachment. These efforts will likely lead to development of new biomarkers. Here, we review the IgA1 HR O-glycosylation in general and the role of aberrantly glycosylated IgA1 in the pathogenesis of IgAN in particular.

Quantitative proteomics analysis of lysine 2-hydroxyisobutyrylation in IgA nephropathy

Background

Protein posttranslational modification is an indispensable regulatory element that can fine-tune protein functions and regulate diverse cellular processes. Lysine 2-hydroxyisobutyrylation (Khib) is a protein posttranslational modification that was recently identified and is thought to play a role in a wide variety of active cellular functions.

Methods

In this report, for the first time, we comparatively studied the 2-hydroxyisobutyrylation proteome in peripheral blood mononuclear cells from a biopsy-proven immunoglobulin A nephropathy (IgAN) group and a normal control group based on liquid chromatography-tandem mass spectrometry.

Results

Altogether, 7405 proteins were identified and added to a Khib library. Of these proteins, we identified 111 with upregulated expression and 83 with downregulated expression. Furthermore, we identified 428 Khib modification sites on 290 Khib-modified proteins, including 171 sites with increased modification on 122 Khib-modified proteins and 257 specific sites with reduced modification on 168 Khib-modified proteins.

Conclusions

Importantly, the abundance of lipocalin 2 was increased in the differentially expressed proteins, and a KEGG-based functional enrichment analysis showed that Khib proteins clustered in the IL-17 signaling pathway and phagosome category, which may have important associations with IgAN. Our data enlighten our understanding of Khib in IgAN and indicate that Khib may have important regulatory roles in IgAN.

Mucin-Type O-GalNAc Glycosylation in Health and Disease

Mucin-type GalNAc O-glycosylation is one of the most abundant and unique post-translational modifications. The combination of proteome-wide mapping of GalNAc O-glycosylation sites and genetic studies with knockout animals and genome-wide analyses in humans have been instrumental in our understanding of GalNAc O-glycosylation. Combined, such studies have revealed well-defined functions of O-glycans at single sites in proteins, including the regulation of pro-protein processing and proteolytic cleavage, as well as modulation of receptor functions and ligand binding. In addition to isolated O-glycans, multiple clustered O-glycans have an important function in mammalian biology by providing structural support and stability of mucins essential for protecting our inner epithelial surfaces, especially in the airways and gastrointestinal tract. Here the many O-glycans also provide binding sites for both endogenous and pathogen-derived carbohydrate-binding proteins regulating critical developmental programs and helping maintain epithelial homeostasis with commensal organisms. Finally, O-glycan changes have been identified in several diseases, most notably in cancer and inflammation, where the disease-specific changes can be used for glycan-targeted therapies. This chapter will review the biosynthesis, the biology, and the translational perspectives of GalNAc O-glycans.

Quantitative assessment of successive carbohydrate additions to the clustered O-glycosylation sites of IgA1 by glycosyltransferases

Mucin-type O-glycosylation occurs on many proteins that transit the Golgi apparatus. These glycans impact structure and function of many proteins and have important roles in cellular biosynthetic processes, signaling and differentiation. Although recent technological advances have enhanced our ability to profile glycosylation of glycoproteins, limitations in the understanding of the biosynthesis of these glycan structures remain. Some of these limitations stem from the difficulty to track the biosynthetic process of mucin-type O-glycosylation, especially when glycans occur in dense clusters in repeat regions of proteins, such as the mucins or immunoglobulin A1 (IgA1). Here, we describe a series of nano-liquid chromatography (LC)-mass spectrometry (MS) analyses that demonstrate the range of glycosyltransferase enzymatic activities involved in the biosynthesis of clustered O-glycans on IgA1. By utilizing nano-LC-MS relative quantitation of in vitro reaction products, our results provide unique insights into the biosynthesis of clustered IgA1 O-glycans. We have developed a workflow to determine glycoform-specific apparent rates of a human UDP-N-acetylgalactosamine:polypeptide N-acetylgalactosaminyltrasnfersase (GalNAc-T EC 2.4.1.41) and demonstrated how pre-existing glycans affect subsequent activity of glycosyltransferases, such as core 1 galactosyltransferase and α2,3- and α2,6-specific sialyltransferases, in successive additions in the biosynthesis of clustered O-glycans. In the context of IgA1, these results have potential to provide insight into the molecular mechanisms implicated in the pathogenesis of IgA nephropathy, an autoimmune renal disease involving aberrant IgA1 O-glycosylation. In a broader sense, these methods and workflows are applicable to the studies of the concerted and competing functions of other glycosyltransferases that initiate and extend mucin-type core 1 clustered O-glycosylation.

IgA and FcαRI: Pathological Roles and Therapeutic Opportunities

Immunoglobulin A (IgA) is the most abundant antibody class present at mucosal surfaces. The production of IgA exceeds the production of all other antibodies combined, supporting its prominent role in host-pathogen defense. IgA closely interacts with the intestinal microbiota to enhance its diversity, and IgA has a passive protective role via immune exclusion. Additionally, inhibitory ITAMi signaling via the IgA Fc receptor (FcαRI; CD89) by monomeric IgA may play a role in maintaining homeostatic conditions. By contrast, IgA immune complexes (e.g., opsonized pathogens) potently activate immune cells via cross-linking FcαRI, thereby inducing pro-inflammatory responses resulting in elimination of pathogens. The importance of IgA in removal of pathogens is emphasized by the fact that several pathogens developed mechanisms to break down IgA or evade FcαRI-mediated activation of immune cells. Augmented or aberrant presence of IgA immune complexes can result in excessive neutrophil activation, potentially leading to severe tissue damage in multiple inflammatory, or autoimmune diseases. Influencing IgA or FcαRI-mediated functions therefore provides several therapeutic possibilities. On the one hand (passive) IgA vaccination strategies can be developed for protection against infections. Furthermore, IgA monoclonal antibodies that are directed against tumor antigens may be effective as cancer treatment. On the other hand, induction of ITAMi signaling via FcαRI may reduce allergy or inflammation, whereas blocking FcαRI with monoclonal antibodies, or peptides may resolve IgA-induced tissue damage. In this review both (patho)physiological roles as well as therapeutic possibilities of the IgA-FcαRI axis are addressed.

How to treat patients with chronic kidney disease: With special focus on IgA nephropathy

Chronic kidney disease has become a worldwide problem. Among chronic kidney disease patients, IgA nephropathy is common in the world. Serum levels of galactose deficient (Gd)-IgA1 and Gd-IgA1-specific antibodies are elevated in most IgA nephropathy patients. Glomerular Gd-IgA1 deposition has been observed by immunofluorescence. There are many reports that the anti-proteinuric effect is significantly greater in groups who receive tonsillectomy with steroid pulse therapy in IgA nephropathy patients. Furthermore, patients with tonsillectomy with steroid pulse therapy have shown a strong down-regulation of delta serum IgA/C3 per year and have conserved their renal function. New treatments, that is, Atacicept and glucocorticoid budesonide, have been developed for this disease.

Assay for galactose-deficient IgA1 enables mechanistic studies with primary cells from IgA nephropathy patients

AIMS

IgA nephropathy, the most common primary glomerulonephritis worldwide, is characterized by glomerular deposition of galactose-deficient IgA1 and elevated serum levels of this IgA1 glycoform. Current ELISA methods lack sensitivity to assess galactose deficiency using small amounts of IgA1, which limits studies in primary cells due to modest IgA1 production in isolated peripheral-blood lymphocytes.

METHODS

Lectin from Helix pomatia was conjugated to biotin or acridinium ester and used in ELISA to detect galactose deficiency of IgA1 using small amounts of IgA1.

RESULTS

Lectin conjugated to acridinium had an approximately a log-fold increased sensitivity compared with biotin-labeled lectin.

CONCLUSIONS

The new method of using lectin from Helix pomatia conjugated to acridinium increased assay sensitivity, allowing future mechanistic studies with cultured primary cells.

Comparative proteomic analysis of urine and laser microdissected glomeruli in IgA nephropathy

The purpose of the present study was to identify the differential proteins that synchronously change in urine and glomeruli and could be used to monitor glomerular lesions of IgA nephropathy (IgAN). The proteomes of urine and glomeruli from four IgAN patients who were graded III/IV according to the grading system of Lee et al. were compared to the urine proteomes of four healthy volunteers and the glomeruli proteomes of adjacent normal tissue from four patients with renal tumors, respectively. Western blot, enzyme-linked immunosorbent assay and immunofluorescence assay were applied to verify the results of the proteomic analysis. In the proteomic analysis of urine from IgAN patients and healthy volunteers, 714 proteins were identified, with 246 proteins identified as differential proteins. In the proteomic analysis of glomeruli from renal biopsy tissue of IgAN patients and from adjacent normal tissue of patients with renal tumors, 161 proteins were identified altogether, and 20 proteins of these were recognized as differential proteins. After comparatively analyzing the differential proteins identified in the urine and glomeruli, five synchronously changed differential proteins were found: complement C9, Ig kappa chain C region and three cytoskeleton proteins. In summary, our findings indicate that certain immunological indices in urine appear to be associated with glomerular lesions of IgAN.

Icariin ameliorates IgA nephropathy by inhibition of nuclear factor kappa b/Nlrp3 pathway

Immunoglobulin A nephropathy (IgAN) is the most frequent form of glomerulonephritis, which is characterized by glomerular proliferation and renal inflammation. Icariin is a flavonoid from the Chinese herb Epimedium, and its anti‐inflammatory effect has been reported. This study aimed to investigate the effects of icariin on the renal damage in IgAN rats and the mechanisms behind these effects. IgAN model was established in Sprague–Dawley rats by oral and intravenous immunization with bovine gamma‐globulin for 12 weeks, and rats were treated with icariin from 12 to 18 weeks. At the end of experimental period, kidneys, urine, and blood samples were collected for further analysis. Our results showed that icariin ameliorated the increase in the levels of proteinuria, serum creatinine, and urea nitrogen without severe side effects. IgAN rats exhibited significantly increased IgA deposition, mesangial matrix expansion, and glomerular fibrosis, while icariin treatment markedly attenuated these alterations. Moreover, treatment with icariin also dramatically blocked nuclear factor kappa b (NF‐κB) nuclear translocation and Nlrp3 inflammasome activation in IgAN rats, leading to reduced downstream proinflammatory cytokines production. Mechanistically, we found that icariin treatment inhibited IKKβ and IκBα phosphorylation and IκBα degradation in IgAN rats. Our data demonstrate that icariin ameliorates renal damage in IgAN rats via inhibition of NF‐κB‐mediated Nlrp3 inflammasome activation. These findings provide insight into an application of icariin for the treatment of IgAN disease, and represent a novel mechanism behind these effects.

In silico prediction of specific pathways that regulate mesangial cell proliferation in IgA nephropathy

IgA nephropathy is one of the most common forms of primary glomerulonephritis worldwide leading to end-stage renal disease. Proliferation of mesangial cells, i.e., the multifunctional cells located in the intracapillary region of glomeruli, after IgA- dominant immune deposition is the major histologic feature in IgA nephropathy. In spite of several studies on molecular basis of proliferation in these cells, specific pathways responsible for regulation of proliferation are still to be discovered. In this study, we predicted a specific signaling pathway started from transferrin receptor (TFRC), a specific IgA1 receptor on mesangial cells, toward a set of proliferation-related proteins. The final constructed subnetwork was presented after filtration and evaluation. The results suggest that estrogen receptor (ESR1) as a hub protein in the significant subnetwork has an important role in the mesangial cell proliferation and is a potential target for IgA nephropathy therapy. In conclusion, this study suggests a novel hypothesis for the mechanism of pathogenesis in IgA nephropathy and is a reasonable start point for the future experimental studies on mesangial proliferation process in this disease.

Diagnosis and treatment of patients with IgA nephropathy in Japan

Chronic kidney disease (CKD) is a worldwide public health problem that affects millions of people from all racial and ethnic groups. Although CKD is not one specific disease, it is a comprehensive syndrome that includes IgA nephropathy. As reported by the Japanese Society of Nephrology, 13.0 million people have CKD. In Japan, major causes of end-stage kidney disease are type 2 diabetic nephropathy, chronic glomerulonephritis, especially IgA nephropathy, hypertensive nephrosclerosis, and polycystic kidney disease. IgA nephropathy is characterized by polymeric IgA1 with aberrant galactosylation (galactose-deficient IgA1) increased in the blood and deposited in the glomerular mesangial areas, as well as partially in the capillary walls. The tonsils are important as one of the responsible regions in this disease. The clarification of the mechanism of galactose-deficient IgA1 production will pave the way for the development of novel therapies. The results of future research are eagerly awaited. At present, the most important therapeutic goals in patients with IgA nephropathy are the control of hypertension, the decrease of urinary protein excretion, and the inhibition of progression to end-stage kidney disease. Several investigators have reported that renin-angiotensin-aldosterone system inhibitors reduce levels of urinary protein excretion and preserve renal function in patients with IgA nephropathy. In Japan, tonsillectomy and steroid pulse therapy are more effective for patients with IgA nephropathy.

IgA nephropathy enigma

IgA nephropathy (IgAN) is the leading cause of primary glomerulonephritis in the world. The disease is characterized by the presence of IgA-containing immune complexes in the circulation and in mesangial deposits with ensuing glomerular injury. Although in humans there are two IgA subclasses, only IgA1 molecules are involved. The exclusivity of participation of IgA1 in IgAN prompted extensive structural and immunological studies of the unique hinge region (HR) of IgA1, which is absent in otherwise highly homologous IgA2. HR of IgA1 with altered O-glycans serves as an antigen recognized by autoantibodies specific for aberrant HR glycans leading to the generation of nephritogenic immune complexes. However, there are several unresolved questions concerning the phylogenetic origin of human IgA1 HR, the structural basis of its antigenicity, the origin of antibodies specific for HR with altered glycan moieties, the regulatory defects in IgA1 glycosylation pathways, and the potential approaches applicable to the disease-specific interventions in the formation of nephritogenic immune complexes. This review focuses on the gaps in our knowledge of molecular and cellular events that are involved in the immunopathogenesis of IgAN.

[IgA nephropathy]

IgA nephropathy is the most common form of primary glomerulonephritis worldwide and an important cause of chronic kidney disease and end-stage kidney failure. Its pathophysiology remains in part unsolved but it is recognized as an immune complex disease. Recent years have brought progress in the field through the discovery of several genetic susceptibility loci and the formulation of the multi-hit pathogenesis model. Presentation, clinical course and histology can be extremely variable, making any histological classification still difficult. Indeed, most therapeutic studies until now include patients based only on the severity of clinical criteria but the new classification of Oxford should change that. Only the management of patients with nephropathy with minimal change glomerular lesions and nephrotic syndrome, or extra-capillary glomerulonephritis and rapidly progressive renal failure, is consensual: Corticosteroids alone for the first and associated with immunosuppressive drugs for the latter. The recent Kidney Disease Improving Global Outcomes (KDIGO) consensus treatment guideline is still controversial, especially in light of the last clinical studies. Corticosteroid therapy can be discussed in patients with proteinuria greater than 1 g/day without renal failure. All IgA nephropathy patients should benefit from the global management of chronic glomerular disease, including a renin-angiotensin system blocker in the presence of hypertension or proteinuria.

The Origin and Activities of IgA1-Containing Immune Complexes in IgA Nephropathy

IgA nephropathy (IgAN) is the most common primary glomerulonephritis, frequently leading to end-stage renal disease, as there is no disease-specific therapy. IgAN is diagnosed from pathological assessment of a renal biopsy specimen based on predominant or codominant IgA-containing immunodeposits, usually with complement C3 co-deposits and with variable presence of IgG and/or IgM. The IgA in these renal deposits is galactose-deficient IgA1, with less than a full complement of galactose residues on the O-glycans in the hinge region of the heavy chains. Research from the past decade led to the definition of IgAN as an autoimmune disease with a multi-hit pathogenetic process with contributing genetic and environmental components. In this process, circulating galactose-deficient IgA1 (autoantigen) is bound by antiglycan IgG or IgA (autoantibodies) to form immune complexes. Some of these circulating complexes deposit in glomeruli, and thereby activate mesangial cells and induce renal injury through cellular proliferation and overproduction of extracellular matrix components and cytokines/chemokines. Glycosylation pathways associated with production of the autoantigen and the unique characteristics of the corresponding autoantibodies in patients with IgAN have been uncovered. Complement likely plays a significant role in the formation and the nephritogenic activities of these complexes. Complement activation is mediated through the alternative and lectin pathways and probably occurs systemically on IgA1-containing circulating immune complexes as well as locally in glomeruli. Incidence of IgAN varies greatly by geographical location; the disease is rare in central Africa but accounts for up to 40% of native-kidney biopsies in eastern Asia. Some of this variation may be explained by genetically determined influences on the pathogenesis of the disease. Genome-wide association studies to date have identified several loci associated with IgAN. Some of these loci are associated with the increased prevalence of IgAN, whereas others, such as deletion of complement factor H-related genes 1 and 3, are protective against the disease. Understanding the molecular mechanisms and genetic and biochemical factors involved in formation and activities of pathogenic IgA1-containing immune complexes will enable the development of future disease-specific therapies as well as identification of non-invasive disease-specific biomarkers.

New Insights into the Pathogenesis of IgA Nephropathy

BACKGROUND

IgA nephropathy, a frequent cause of end-stage renal disease, is an autoimmune disease wherein immune complexes consisting of IgA1 with galactose-deficient O-glycans (autoantigen) and anti-glycan autoantibodies deposit in glomeruli and induce renal injury. Multiple genetic loci associated with disease risk have been identified. The prevalence of risk alleles varies geographically, highest in eastern Asia and northern Europe, fewer in other parts of Europe and North America, and the least in Africa. IgA nephropathy is diagnosed from pathological assessment of a renal biopsy specimen. Currently, therapy is not disease-targeted but rather is focused on maintaining control of blood pressure and proteinuria, ideally with suppression of angiotensin II. Possible additional approaches differ between countries. Disease-specific therapy as well as new tools for diagnosis, prognosis, and assessment of responses to therapy are needed.

SUMMARY

Glycosylation pathways associated with aberrant O-glycosylation of IgA1 and, thus, production of autoantigen, have been identified. Furthermore, unique characteristics of the autoantibodies in IgA nephropathy have been uncovered. Many of these biochemical features are shared by patients with IgA nephropathy and Henoch-Schönlein purpura nephritis, suggesting that the two diseases may represent opposite ends of a spectrum of a disease process. Understanding the molecular mechanisms involved in formation of pathogenic IgA1-containing immune complexes will enable development of disease-specific therapies as well as diagnostic and prognostic biomarkers.

KEY MESSAGES

IgA nephropathy is an autoimmune disease caused by glomerular deposition of nephritogenic circulating immune complexes consisting of galactose-deficient IgA1 (autoantigen) bound by anti-glycan autoantibodies. A better understanding of the multi-step process of pathogenesis of IgA nephropathy and the genetic and environmental contributing factors will lead to development of biomarkers to identify patients with progressive disease who would benefit from a future disease-specific therapy.

Nephrokeli, a Chinese herbal formula, may improve IgA nephropathy through regulation of the sphingosine-1-phosphate pathway

Nephrokeli (NPKL) is a Chinese herbal formula that has been used to treat patients with IgA nephropathy (IgAN) for improvement of proteinuria and kidney injury. However, the mechanism remains unclear. Sphingosine-1-phosphate (S1P) and its receptors S1PR2 and S1PR3 are known to play an important role in kidney disease. Here, we tested whether NPKL is able to regulate the S1P pathway in the kidney of IgAN rats. Four groups of rats were included in the study: Control, IgAN, IgAN treated with losartan, and IgAN treated with NPKL. The IgAN model was generated by injection of bovine serum albumin and staphylococcus enterotoxin B. We found that IgAN rats had increased staining for proliferating cell nuclear antigen (PCNA) in the mesangial area and increased mRNA and protein levels of S1PR2 and S1PR3 in the kidney compared to control rats. Connective tissue growth factor (CTGF), a downstream growth factor in the S1P pathway, was also elevated in the kidney of IgAN rats. Treatment with either NPKL or losartan was able to reduce PCNA staining and the expression of both S1PR2 and S1PR3 in the kidney of IgAN rats. However, NPKL (but not losartan treatment) reduced the expression of CTGF in the kidney of IgAN rats. In addition, we treated rat mesangial cells with sera collected from either NPKL-treated rats or control rats and found that NPKL-serum was able to reduce S1P-induced mesangial cell proliferation and the expression of S1PR2/S1PR3 and CTGF. NPKL also attenuates expression of fibrosis, inflammation, and oxidative stress markers in the kidney of IgAN rats. Our studies provide the mechanism by which NPKL attenuates kidney injury in IgAN rats.

N-acetylgalactosaminide α2,6-sialyltransferase II is a candidate enzyme for sialylation of galactose-deficient IgA1, the key autoantigen in IgA nephropathy

BACKGROUND

Galactose-deficient O-glycans in the hinge region (HR) of immunoglobulin A1 (IgA1) play a key role in the pathogenesis of IgA nephropathy (IgAN). O-Glycans of circulatory IgA1 consist of N-acetylgalactosamine (GalNAc) with a β1,3-linked galactose; both sugars may be sialylated. In patients with IgAN, α2,6-sialylated GalNAc is a frequent form of the galactose-deficient O-glycans. Prior analyses of IgA1-producing cells had indicated that α2,6-sialyltransferase II (ST6GalNAc-II) is likely responsible for sialylation of GalNAc of galactose-deficient IgA1, but direct evidence is missing.

METHODS

We produced a secreted variant of recombinant human ST6GalNAc-II and an IgA1 fragment comprised of Cα1-HR-Cα2. This IgA1 fragment and a synthetic HR peptide with enzymatically attached GalNAc residues served as acceptors. ST6GalNAc-II activity was assessed in vitro and the attachment of sialic acid to these acceptors was detected by lectin blot and mass spectrometry.

RESULTS

ST6GalNAc-II was active with both acceptors. High-resolution mass spectrometry analysis revealed that up to three sialic acid residues were added to the GalNAc residues of the HR glycopeptide.

CONCLUSIONS

Our data provide direct evidence that ST6GalNAc-II can sialylate GalNAc of galactose-deficient IgA1. As serum levels of galactose-deficient IgA1 with sialylated glycoforms are increased in IgAN patients, our data explain the corresponding part of the biosynthetic pathway.

Pathogenesis of immunoglobulin A nephropathy

PURPOSE OF REVIEW

In this article, we review recent findings on the pathogenesis and genetics of immunoglobulin A (IgA) nephropathy.

RECENT FINDINGS

During the past 2 years, the understanding of the pathogenesis of IgA nephropathy has evolved as a result of progress in technology and new tools that have been developed. Since 1968, when IgA nephropathy was described as an IgA-IgG immune-complex disease, the knowledge base expanded to allow definition of IgA nephropathy as an autoimmune disease with a multihit pathogenetic process. Specifically, galactose-deficient immunoglobulin A1 (IgA1) is recognized by unique autoantibodies, resulting in the formation of pathogenic immune complexes that ultimately deposit in the glomerular mesangium and induce renal injury. New approaches using high-resolution mass spectrometry have provided unique insight at the molecular level into IgA1 O-glycosylation. Cutting-edge genome-wide association studies revealed multiple disease-associated risk loci and have mapped their geographic and racial distribution.

SUMMARY

Recent studies of molecular and genetic defects operating in IgA nephropathy can define new biomarkers specific for the disease that can be developed into clinical assays to aid in the diagnosis, assessment of prognosis, and monitoring of disease progression. Moreover, disease-specific targets are being discovered that may lead to development of new approaches for treatment.

Cellular signaling and production of galactose-deficient IgA1 in IgA nephropathy, an autoimmune disease

Immunoglobulin A (IgA) nephropathy (IgAN), the leading cause of primary glomerulonephritis, is characterized by IgA1-containing immunodeposits in the glomeruli. IgAN is a chronic disease, with up to 40% of patients progressing to end-stage renal disease, with no disease-specific treatment. Multiple studies of the origin of the glomerular immunodeposits have linked elevated circulating levels of aberrantly glycosylated IgA1 (galactose-deficient in some O-glycans; Gd-IgA1) with formation of nephritogenic Gd-IgA1-containing immune complexes. Gd-IgA1 is recognized as an autoantigen in susceptible individuals by anti-glycan autoantibodies, resulting in immune complexes that may ultimately deposit in the kidney and induce glomerular injury. Genetic studies have revealed that an elevated level of Gd-IgA1 in the circulation of IgAN patients is a hereditable trait. Moreover, recent genome-wide association studies have identified several immunity-related loci that associated with IgAN. Production of Gd-IgA1 by IgA1-secreting cells of IgAN patients has been attributed to abnormal expression and activity of several key glycosyltransferases. Substantial evidence is emerging that abnormal signaling in IgA1-producing cells is related to the production of Gd-IgA1. As Gd-IgA1 is the key autoantigen in IgAN, understanding the genetic, biochemical, and environmental aspects of the abnormal signaling in IgA1-producing cells will provide insight into possible targets for future disease-specific therapy.

Serum under-O-glycosylated IgA1 level is not correlated with glomerular IgA deposition based upon heterogeneity in the composition of immune complexes in IgA nephropathy

BACKGROUND

Although serum under-O-glycosylated IgA1 in IgA nephropathy (IgAN) patients may deposit more preferentially in glomeruli than heavily-O-glycosylated IgA1, the relationship between the glomerular IgA deposition level and the O-glycan profiles of serum IgA1 remains obscure.

METHODS

Serum total under-O-glycosylated IgA1 levels were quantified in 32 IgAN patients by an enzyme-linked immunosorbent assay (ELISA) with Helix aspersa (HAA) lectin. Serum under-O-glycosylated polymeric IgA1 (pIgA1) was selectively measured by an original method using mouse Fcα/μ receptor (mFcα/μR) transfectant and flow cytometry (pIgA1 trap). The percentage area of IgA deposition in the whole glomeruli (Area-IgA) was quantified by image analysis on the immunofluorescence of biopsy specimens. Correlations were assessed between the Area-IgA and data from HAA-ELISA or pIgA1 trap. The relationships between clinical parameters and data from HAA-ELISA or pIgA1 trap were analyzed by data mining approach.

RESULTS

While the under-O-glycosylated IgA1 levels in IgAN patients were significantly higher than those in healthy controls when measured (p<0.05), there was no significant difference in under-O-glycosylated pIgA1. There was neither a correlation observed between the data from HAA-ELISA and pIgA1 trap (r2=0.09) in the IgAN patients (r2=0.005) nor was there a linear correlation between Area-IgA and data from HAA-ELISA or the pIgA1 trap (r2=0.005, 0.03, respectively). Contour plots of clinical parameters versus data from HAA-ELISA and the pIgA1 trap revealed that patients with a high score in each clinical parameter concentrated in specific areas, showing that patients with specific O-glycan profiles of IgA1 have similar clinical parameters. A decision tree analysis suggested that dominant immune complexes in glomeruli were consisted of: 1) IgA1-IgG and complements, 2) pIgA1 and complements, and 3) monomeric IgA1-IgA or aggregated monomeric IgA1.

CONCLUSIONS

Serum under-O-glycosylated IgA1 levels are not correlated with glomerular IgA deposition based upon heterogeneity in the composition of glomerular immune complexes in IgAN patients.

Enzymatic sialylation of IgA1 O-glycans: implications for studies of IgA nephropathy

Patients with IgA nephropathy (IgAN) have elevated circulating levels of IgA1 with some O-glycans consisting of galactose (Gal)-deficient N-acetylgalactosamine (GalNAc) with or without N-acetylneuraminic acid (NeuAc). We have analyzed O-glycosylation heterogeneity of naturally asialo-IgA1 (Ale) myeloma protein that mimics Gal-deficient IgA1 (Gd-IgA1) of patients with IgAN, except that IgA1 O-glycans of IgAN patients are frequently sialylated. Specifically, serum IgA1 of healthy controls has more α2,3-sialylated O-glycans (NeuAc attached to Gal) than α2,6-sialylated O-glycans (NeuAc attached to GalNAc). As IgA1-producing cells from IgAN patients have an increased activity of α2,6-sialyltransferase (ST6GalNAc), we hypothesize that such activity may promote premature sialylation of GalNAc and, thus, production of Gd-IgA1, as sialylation of GalNAc prevents subsequent Gal attachment. Distribution of NeuAc in IgA1 O-glycans may play an important role in the pathogenesis of IgAN. To better understand biological functions of NeuAc in IgA1, we established protocols for enzymatic sialylation leading to α2,3- or α2,6-sialylation of IgA1 O-glycans. Sialylation of Gal-deficient asialo-IgA1 (Ale) myeloma protein by an ST6GalNAc enzyme generated sialylated IgA1 that mimics the Gal-deficient IgA1 glycoforms in patients with IgAN, characterized by α2,6-sialylated Gal-deficient GalNAc. In contrast, sialylation of the same myeloma protein by an α2,3-sialyltransferase yielded IgA1 typical for healthy controls, characterized by α2,3-sialylated Gal. The GalNAc-specific lectin from Helix aspersa (HAA) is used to measure levels of Gd-IgA1. We assessed HAA binding to IgA1 sialylated at Gal or GalNAc. As expected, α2,6-sialylation of IgA1 markedly decreased reactivity with HAA. Notably, α2,3-sialylation also decreased reactivity with HAA. Neuraminidase treatment recovered the original HAA reactivity in both instances. These results suggest that binding of a GalNAc-specific lectin is modulated by sialylation of GalNAc as well as Gal in the clustered IgA1 O-glycans. Thus, enzymatic sialylation offers a useful model to test the role of NeuAc in reactivities of the clustered O-glycans with lectins.

A panel of serum biomarkers differentiates IgA nephropathy from other renal diseases

BACKGROUND AND OBJECTIVES

There is increasing evidence that galactose-deficient IgA1 (Gd-IgA1) and Gd-IgA1-containing immune complexes are important for the pathogenesis of IgA nephropathy (IgAN). In the present study, we assessed a novel noninvasive multi-biomarker approach in the diagnostic test for IgAN.

MATERIALS AND METHODS

We compared serum levels of IgA, IgG, Gd-IgA1, Gd-IgA1-specific IgG and Gd-IgA1-specific IgA in 135 IgAN patients, 79 patients with non-IgAN chronic kidney disease (CKD) controls and 106 healthy controls. Serum was collected at the time of kidney biopsy from all IgAN and CKD patients.

RESULTS

Each serum marker was significantly elevated in IgAN patients compared to CKD (P<0.001) and healthy controls (P<0.001). While 41% of IgAN patients had elevated serum Gd-IgA1 levels, 91% of these patients exhibited Gd-IgA1-specific IgG levels above the 90th percentile for healthy controls (sensitivity 89%, specificity 92%). Although up to 25% of CKD controls, particularly those with immune-mediated glomerular diseases including lupus nephritis, also had elevated serum levels of Gd-IgA1-specific IgG, most IgAN patients had elevated levels of Gd-IgA1-specific antibody of both isotypes. Serum levels of Gd-IgA1-specific IgG were associated with renal histological grading. Furthermore, there was a trend toward higher serum levels of Gd-IgA1-specific IgG in IgAN patients with at least moderate proteinuria (≥1.0 g/g), compared to patients with less proteinuria.

CONCLUSIONS

Serum levels of Gd-IgA1-specific antibodies are elevated in most IgAN patients, and their assessment, together with serum levels of Gd-IgA1, improves the specificity of the assays. Our observations suggest that a panel of serum biomarkers may be helpful in differentiating IgAN from other glomerular diseases.

Cytokines alter IgA1 O-glycosylation by dysregulating C1GalT1 and ST6GalNAc-II enzymes

IgA nephropathy (IgAN), the most common primary glomerulonephritis, is characterized by renal immunodeposits containing IgA1 with galactose-deficient O-glycans (Gd-IgA1). These immunodeposits originate from circulating immune complexes consisting of anti-glycan antibodies bound to Gd-IgA1. As clinical disease onset and activity of IgAN often coincide with mucosal infections and dysregulation of cytokines, we hypothesized that cytokines may affect IgA1 O-glycosylation. We used IgA1-secreting cells derived from the circulation of IgAN patients and healthy controls and assessed whether IgA1 O-glycosylation is altered by cytokines. Of the eight cytokines tested, only IL-6 and, to a lesser degree, IL-4 significantly increased galactose deficiency of IgA1; changes in IgA1 O-glycosylation were robust for the cells from IgAN patients. These cytokines reduced galactosylation of the O-glycan substrate directly via decreased expression of the galactosyltransferase C1GalT1 and, indirectly, via increased expression of the sialyltransferase ST6GalNAc-II, which prevents galactosylation by C1GalT1. These findings were confirmed by siRNA knockdown of the corresponding genes and by in vitro enzyme reactions. In summary, IL-6 and IL-4 accentuated galactose deficiency of IgA1 via coordinated modulation of key glycosyltransferases. These data provide a mechanism explaining increased immune-complex formation and disease exacerbation during mucosal infections in IgAN patients.

Tn and sialyl-Tn antigens, aberrant O-glycomics as human disease markers

In many different human disorders, the cellular glycome is altered. An interesting but poorly understood alteration occurs in the mucin-type O-glycome, in which there is aberrant expression of the truncated O-glycans Tn (GalNAcα1-Ser/Thr) and its sialylated version sialyl-Tn (STn) (Neu5Acα2,6GalNAcα1-Ser/Thr). Both Tn and STn are tumor-associated carbohydrate antigens and tumor biomarkers, since they are not expressed normally and appear early in tumorigenesis. Moreover, their expression is strongly associated with poor prognosis and tumor metastasis. The Tn and STn antigens are also expressed in other human diseases and disorders, such as Tn syndrome and IgA nephropathy. The major pathological mechanism for expression of the Tn and STn antigens is compromised T-synthase activity, resulting from alteration of the X-linked gene that encodes for Cosmc, a molecular chaperone specifically required for the correct folding of T-synthase to form active enzyme. This review will summarize our current understanding of the Tn and STn antigens in terms of their biochemistry and role in pathology.

CHO microRNA engineering is growing up: recent successes and future challenges

microRNAs with their ability to regulate complex pathways that control cellular behavior and phenotype have been proposed as potential targets for cell engineering in the context of optimization of biopharmaceutical production cell lines, specifically of Chinese Hamster Ovary cells. However, until recently, research was limited by a lack of genomic sequence information on this industrially important cell line. With the publication of the genomic sequence and other relevant data sets for CHO cells since 2011, the doors have been opened for an improved understanding of CHO cell physiology and for the development of the necessary tools for novel engineering strategies. In the present review we discuss both knowledge on the regulatory mechanisms of microRNAs obtained from other biological models and proof of concepts already performed on CHO cells, thus providing an outlook of potential applications of microRNA engineering in production cell lines.

Elucidating heterogeneity of IgA1 hinge-region O-glycosylation by use of MALDI-TOF/TOF mass spectrometry: role of cysteine alkylation during sample processing

Determining disease-associated changes in protein glycosylation provides a better understanding of pathogenesis. This work focuses on human immunoglobulin A1 (IgA1), where aberrant O-glycosylation plays a key role in the pathogenesis of IgA nephropathy (IgAN). Normal IgA1 hinge region carries 3 to 6 O-glycans consisting of N-acetylgalactosamine (GalNAc) and galactose (Gal); both sugars may be sialylated. In IgAN patients, some O-glycans on a fraction of IgA1 molecules are Gal-deficient. Here we describe a sample preparation protocol with optimized cysteine alkylation of a Gal-deficient polymeric IgA1 myeloma protein prior to in-gel digestion and analysis of the digest by MALDI-TOF/TOF mass spectrometry (MS). Following a novel strategy, IgA1 hinge-region O-glycopeptides were fractionated by reversed-phase liquid chromatography using a microgradient device and identified by MALDI-TOF/TOF tandem MS (MS/MS). The acquired MS/MS spectra were interpreted manually and by means of our own software. This allowed assigning up to six O-glycosylation sites and demonstration, for the first time, of the distribution of isomeric O-glycoforms having the same molecular mass, but a different glycosylation pattern. The most abundant Gal-deficient O-glycoforms were GalNAc4Gal3 and GalNAc5Gal4 with one Gal-deficient site and GalNAc5Gal3 and GalNAc4Gal2 with two Gal-deficient sites. The most frequent Gal-deficient sites were at Ser230 and/or Thr236.

BIOLOGICAL SIGNIFICANCE

In this work, we studied the O-glycosylation in the hinge region of human immunoglobulin A1 (IgA1). Aberrant glycosylation of the protein plays a key role in the pathogenesis of IgA nephropathy. Thus identification of the O-glycan composition of IgA1 is important for a deeper understanding of the disease mechanism, biomarker discovery and validation, and implementation and monitoring of disease-specific therapies. We developed a new procedure for elucidating the heterogeneity of IgA1 O-glycosylation. After running a polyacrylamide gel electrophoresis under denaturing conditions, the heavy chain of IgA1 was subjected to in-gel digestion by trypsin. O-glycopeptides were separated from the digest on capillary columns using a microgradient chromatographic device (replacing commonly used liquid chromatographs) and subjected to MALDI-TOF/TOF mass spectrometry (MS) and tandem mass spectrometry (MS/MS) involving post-source decay fragmentation. We show that the complete modification of cysteines by iodoacetamide prior to electrophoresis is critical for successful MS/MS analyses on the way to deciphering the microheterogeneity of O-glycosylation in IgA1. Similarly, the removal of the excess of the reagent is equally important. The acquired MS/MS allowed assigning up to six O-glycosylation sites and identification of isomeric O-glycoforms. We show that our simplified approach is efficient and has a high potential to provide a method for the rapid assessment of IgA1 heterogeneity that is a less expensive and yet corroborating alternative to LC-(high-resolution)-MS protocols. The novelty and biological significance reside in the demonstration, for the first time, of the distribution of the most abundant isoforms of HR O-glycopeptides of IgA1. As another new feature, we introduce a software solution for the interpretation of MS/MS data of O-glycopeptide isoforms, which provides the possibility of fast and easier data processing. This article is part of a Special Issue entitled: Posttranslational Protein modifications in biology and Medicine.

IgA nephropathy: molecular mechanisms of the disease

Studies of molecular and cellular interactions involved in the pathogenesis of IgA nephropathy have revealed the autoimmune nature of this most common primary glomerulonephritis. In patients with this disease, altered glycan structures in the unique hinge region of the heavy chains of IgA1 molecules lead to the exposure of antigenic determinants, which are recognized by naturally occurring antiglycan antibodies of the IgG and/or IgA1 isotype. As a result, nephritogenic immune complexes form in the circulation and deposit in the glomerular mesangium. Deposited immune complexes induce proliferation of resident mesangial cells, increased production of extracellular matrix proteins and cytokines, and ultimately loss of glomerular function. Structural elucidation of the nature of these immune complexes and their biological activity should provide a rational basis for an effective, immunologically mediated inhibition of the formation of nephritogenic immune complexes that could be used as a disease-specific therapeutic approach.

Determination of severity of murine IgA nephropathy by glomerular complement activation by aberrantly glycosylated IgA and immune complexes

The pathogenic roles of glomerular deposition of components of the complement cascade in IgA nephropathy (IgAN) are not completely clarified. To investigate the pathologic role of complement pathways in IgAN, two IgAN-prone mouse models were examined. Grouped ddY (gddY) mice showed significant high proteinuria, severe glomerular lesions, and extracellular matrix expansion compared with high serum IgA (HIGA) mice but with similar intensity of glomerular IgA deposition. Glomerular activation of the classical, lectin, and alternative pathways was demonstrated by significantly stronger staining for complement (C)3, C5b-9, C1q, C4, mannose-binding lectin (MBL)-A/C, MBL-associated serine protease-2, and factor B and properdin in gddY mice than in HIGA mice. Similarly, the serum levels of IgA-IgG2a/IgM and IgA-MBL-A/C immune complexes and polymeric IgA were significantly higher in gddY mice than in HIGA mice. Moreover, the serum levels of aberrantly glycosylated IgA characterized by the binding of Sambucus nigra bark lectin and Ricinus communis agglutinin I were significantly higher in gddY mice than in HIGA mice. This aberrancy in glycosylation was confirmed by monosaccharide compositional analysis of purified IgA using gas-liquid chromatography. This study is the first to demonstrate that aberrantly glycosylated IgA may influence the formation of macromolecular IgA including IgA-IgG immune complexes and subsequent complement activation, leading to full progression of IgAN.

Clinical significance of IgG deposition in the glomerular mesangial area in patients with IgA nephropathy

Background

Immunoglobulin (Ig) A nephropathy (IgAN) is characterized by mesangial deposits of IgA1 and C3, often with co-deposits of IgG. We attempted to clarify the clinical significance of mesangial IgG deposition in patients with IgAN.

Methods

We retrospectively reviewed 57 patients who were diagnosed with IgAN on the basis of pathological examination of renal biopsy specimens obtained between October 2006 and December 2010. Subjects were divided into two groups: IgA+IgG deposition (IgA-IgG) group (n = 29) and IgA deposition alone (IgA) group (n = 28). The study outcome was complete remission (CR), defined as negative proteinuria by dipstick urinalysis and urinary erythrocytes of less than 1–4/high-power field.

Results

Proteinuria was greater in the IgA-IgG group than the IgA group (1.1 ± 0.8 vs. 0.7 ± 0.6 g/day, Mann–Whitney U test, P = 0.042). Capillary wall IgA deposits were noted more frequently in the IgA-IgG group than the IgA group (59 vs. 11 %, Fisher’s exact test, P = 0.014). During the median follow-up period of 33.3 months (range 6–55 months) in the 57 patients, we observed CR in 24 cases (42.1 %). After the start of treatment, urinary abnormalities disappeared earlier in the IgA group than in the IgA-IgG group (log rank test, P = 0.012). Cox’s regression model showed that IgG deposition reduced the hazard ratio for CR (hazard ratio 0.35; 95 % confidence interval 0.14–0.82, P = 0.014). Therefore, IgG deposition is a risk factor for persistent urinary abnormalities.

Conclusion

Mesangial IgG deposition is associated with more severe clinical features in patients with IgAN.

Glycosylation of IgA1 and pathogenesis of IgA nephropathy

IgA nephropathy, described in 1968 as IgA-IgG immune-complex disease, is an autoimmune disease. Galactose-deficient IgA1 is recognized by unique autoantibodies, resulting in the formation of pathogenic immune complexes that ultimately induce glomerular injury. Thus, formation of the galactose-deficient IgA1-containing immune complexes is a critical factor in the pathogenesis of IgA nephropathy. Studies of molecular defects of IgA1 can define new biomarkers specific for IgA nephropathy that can be developed into clinical assays to aid in the diagnosis, assessment of prognosis, and monitoring of disease progression.

Abnormal miR-148b expression promotes aberrant glycosylation of IgA1 in IgA nephropathy

Aberrant O-glycosylation in the hinge region of IgA1 characterizes IgA nephropathy. The mechanisms underlying this abnormal glycosylation are not well understood, but reduced expression of the enzyme core 1, β1,3-galactosyltransferase 1 (C1GALT1) may contribute. In this study, high-throughput microRNA (miRNA) profiling identified 37 miRNAs differentially expressed in PBMCs of patients with IgA nephropathy compared with healthy persons. Among them, we observed upregulation of miR-148b, which potentially targets C1GALT1. Patients with IgA nephropathy exhibited lower C1GALT1 expression, which negatively correlated with miR-148b expression. Transfection of PBMCs from healthy persons with a miR-148b mimic reduced endogenous C1GALT1 mRNA levels threefold. Conversely, loss of miR-148b function in PBMCs of patients with IgA nephropathy increased C1GALT1 mRNA and protein levels to those observed in healthy persons. Moreover, we found that upregulation of miR-148b directly correlated with levels of galactose-deficient IgA1. In vitro, we used an IgA1-producing cell line to confirm that miR-148b modulates IgA1 O-glycosylation and the levels of secreted galactose-deficient IgA1. Taken together, these data suggest a role for miRNAs in the pathogenesis of IgA nephropathy. Abnormal expression of miR-148b may explain the aberrant glycosylation of IgA1, providing a potential pharmacologic target for IgA nephropathy.

Galectin-8 in IgA nephritis: decreased binding of IgA by galectin-8 affinity chromatography and associated increased binding in non-IgA serum glycoproteins

Background

Immunoglobulin A nephritis (IgAN) is the most common primary glomerulonephritis worldwide. It is caused by accumulation of IgA1-containing immune complexes in the kidney resulting in renal failure, which is thought to be due to altered glycosylation of IgA with a decrease of 2–3-sialylated galactosides (NeuAcα2-3Gal).

Purpose

The purpose of this study was to analyze whether altered glycosylation of IgA would lead to an altered binding to galectin-8, an endogenous lectin with strong affinity for 2–3-sialylated galactosides. Galectins are a family of β-galactoside-binding proteins; by binding various glycoproteins, they play important roles in the regulation of cellular functions in inflammation and immunity. Hence, an altered binding of IgA to galectin-8 could lead to pathologic immune functions, such as glomerulonephritis.

Methods

Affinity chromatography of serum glycoproteins on the human sialogalactoside-binding lectin galectin-8N permitted quantitation of bound and unbound fractions, including IgA.

Results

Analysis of ∼100 IgA nephritis sera showed that the galectin-8N unbound fraction of IgA increased compared to ∼100 controls, consistent with the known loss of galactosylation. A subgroup of ∼15% of the IgAN patients had a ratio of galectin-8 bound/unbound IgA <0.09, not found for any of the controls. Unexpectedly, the galectin-8N-binding fraction of serum glycoproteins other than IgA increased in the sera of IgAN patients but not in controls, suggesting a previously unrecognized change in this disease.

Conclusion

This is the first study that relates a galectin, an endogenous lectin family, to IgA nephritis and thus should stimulate new avenues of research into the pathophysiology of the disease.

Electronic supplementary material

The online version of this article (doi:10.1007/s10875-011-9618-3) contains supplementary material, which is available to authorized users.

IgA1 immune complexes from pediatric patients with IgA nephropathy activate cultured human mesangial cells

BACKGROUND

Circulating immune complexes (CIC) containing galactose (Gal)-deficient IgA1 from adults with IgA nephropathy (IgAN) induce proliferation of cultured mesangial cells, but activities of CIC from pediatric patients with the disease have not been studied.

METHODS

CIC of different sizes were isolated from sera of pediatric and adult IgAN patients and their effects on cultured human mesangial cells (MC) were assessed by measuring cellular proliferation, expression of IL-6 and IL-8 and laminin and phosphotyrosine signaling.

RESULTS

Large CIC from pediatric IgAN patients (>800 kDa) containing Gal-deficient IgA1 stimulated cellular proliferation, whereas in some patients, smaller CIC were inhibitory. Addition of stimulatory and inhibitory CIC to MC differentially altered phosphorylation patterns of three major tyrosine-phosphorylated proteins of molecular mass 37, 60 and 115 kDa. The stimulatory CIC transiently increased tyrosine-phosphorylation of the 37-kDa protein and decreased phosphorylation of the other two proteins, whereas the inhibitory CIC increased phosphorylation of all three proteins. Furthermore, we investigated the influence of IgA1-containing CIC from sera of children with IgAN with clinically active disease (i.e., abnormal urinalysis and/or serum creatinine concentration) or inactive disease (i.e., normal urinalysis and serum creatinine concentration) on the expression of IL-6 and IL-8 genes by mesangial cells. Real-time reverse transcription-polymerase chain reaction results showed that the CIC from a patient with active disease stimulated MC to express the two cytokine genes at higher levels than did the CIC from a patient with inactive disease. Moreover, stimulatory CIC increased production of the extracellular matrix protein laminin.

CONCLUSION

These data indicate that sera of pediatric IgAN patients contain biologically active CIC with Gal-deficient IgA1.

The Tn antigen-structural simplicity and biological complexity

Glycoproteins in animal cells contain a variety of glycan structures that are added co- and/or posttranslationally to proteins. Of over 20 different types of sugar-amino acid linkages known, the two major types are N-glycans (Asn-linked) and O-glycans (Ser/Thr-linked). An abnormal mucin-type O-glycan whose expression is associated with cancer and several human disorders is the Tn antigen. It has a relatively simple structure composed of N-acetyl-D-galactosamine with a glycosidic α linkage to serine/threonine residues in glycoproteins (GalNAcα1-O-Ser/Thr), and was one of the first glycoconjugates to be chemically synthesized. The Tn antigen is normally modified by a specific galactosyltransferase (T-synthase) in the Golgi apparatus of cells. Expression of active T-synthase is uniquely dependent on the molecular chaperone Cosmc, which is encoded by a gene on the X chromosome. Expression of the Tn antigen can arise as a consequence of mutations in the genes for T-synthase or Cosmc, or genes affecting other steps of O-glycosylation pathways. Because of the association of the Tn antigen with disease, there is much interest in the development of Tn-based vaccines and other therapeutic approaches based on Tn expression.

Association of -27T>C and its haplotype at the putative promoter for IgA-specific receptor gene with IgA nephropathy among the Chinese Han population

BACKGROUND

One-third to half of IgA nephropathy (IgAN) patients have raised serum IgA levels. Decreased clearance of IgA/IgA complex has been observed in IgAN patients. FCAR codes for IgA-specific receptor and plays an important role in IgA metabolism. Previous small sample-sized studies reported controversial findings in its association with IgAN.

METHODS

We re-sequenced the FCAR in 107 IgAN patients and 112 controls. Association of -27T/C and their haplotypes were performed in 606 patients versus 606 controls, its two independent subsets: 293 single patients with family members and 313 cases versus 606 controls. Functional impact of -27T>C and their haplotypes were analyzed by bioinformatics, allelic differential expression and luciferase activity assays. Cell surface FCAR density between -27T/C heterozygous patients and -27T/T homozygous controls was assessed by flow cytometry.

RESULTS

-27T>C, on the consensus TATA box of transcription factor-binding motif in the putative promoter of the gene was the only variation identified in all coding, splice-site and known protein-binding sequence in re-sequencing. -27C and its haplotype were associated with IgAN (P = 0.0034/0.0013, 0.0099/0.0054, 0.0129/0.0076 and 0.00039/0.00014 in 606 cases versus 606 controls, family-based study, 313 cases versus 606 controls and meta-analysis, respectively). Bioinformatics predicted 2 bp binding changes by -27C. Allelic differential expression and luciferase activity assays showed a reduced expression/activity by the associated haplotype/allele (P < 0.001). -27T/C heterozygous patients had a lower receptor density on cell surface compared to -27T/T homozygous controls (P < 0.001).

CONCLUSIONS

Our results provide evidence for genetic variation at the putative promoter region of FCAR conferring susceptibility to IgAN, suggesting -27C and its haplotype may be causative for the susceptibility among the Chinese Han population.

Galactosylation of serum IgA1 O-glycans in celiac disease

In celiac disease, gluten ingestion provokes small-bowel mucosal injury and production of IgA autoantibodies against transglutaminase 2 (TG2). It has been suggested that in celiac patients IgA could mediate the transepithelial passage of gluten peptides in a mechanism involving the transferrin receptor. As IgA1 with galactose-deficient O-linked glycans has elevated affinity for the transferrin receptor, we assessed whether total serum IgA1 and IgA1 anti-TG2 autoantibodies in celiac patients are aberrantly glycosylated. We report that males with celiac disease have higher total serum levels of galactose-deficient IgA1 than non-celiac males. Furthermore, O-glycans of the disease-specific TG2 IgA1 autoantibodies in celiac patients exhibited elevated galactose deficiency. A gluten-free diet had no effect on the total serum levels of galactose-deficient IgA1, whereas the amount of galactose-deficient anti-TG2 IgA1 decreased. Thus, the undergalactosylated IgA1 molecules are not pathognomonic for celiac disease, but galactose deficiency in IgA1 could be an aggravating factor.

Clustered O-glycans of IgA1: defining macro- and microheterogeneity by use of electron capture/transfer dissociation

IgA nephropathy (IgAN) is the most common primary glomerulonephritis in the world. Aberrantly glycosylated IgA1, with galactose (Gal)-deficient hinge region (HR) O-glycans, plays a pivotal role in the pathogenesis of the disease. It is not known whether the glycosylation defect occurs randomly or preferentially at specific sites. We have described the utility of activated ion-electron capture dissociation (AI-ECD) mass spectrometric analysis of IgA1 O-glycosylation. However, locating and characterizing the entire range of O-glycan attachment sites are analytically challenging due to the clustered serine and threonine residues in the HR of IgA1 heavy chain. To address this problem, we analyzed all glycoforms of the HR glycopeptides of a Gal-deficient IgA1 myeloma protein, mimicking the aberrant IgA1 in patients with IgAN, by use of a combination of IgA-specific proteases + trypsin and AI-ECD Fourier transform ion cyclotron resonance (FT-ICR) tandem mass spectrometry (MS/MS). The IgA-specific proteases provided a variety of IgA1 HR fragments that allowed unambiguous localization of all O-glycosylation sites in the six most abundant glycoforms, including the sites deficient in Gal. Additionally, this protocol was adapted for on-line liquid chromatography (LC)-AI-ECD MS/MS and LC-electron transfer dissociation MS/MS analysis. Our results thus represent a new clinically relevant approach that requires ECD/electron transfer dissociation-type fragmentation to define the molecular events leading to pathogenesis of a chronic kidney disease. Furthermore, this work offers generally applicable principles for the analysis of clustered sites of O-glycosylation.

Recognition of galactose-deficient O-glycans in the hinge region of IgA1 by N-acetylgalactosamine-specific snail lectins: a comparative binding study

Aberrancies in IgA1 glycosylation have been linked to the pathogenesis of IgA nephropathy (IgAN), a kidney disease characterized by deposits of IgA1-containing immune complexes in the glomerular mesangium. IgA1 from IgAN patients is characterized by the presence of galactose (Gal)-deficient O-glycans in the hinge region that can act as epitopes for anti-glycan IgG or IgA1 antibodies. The resulting circulating immune complexes are trapped in the glomerular mesangium of the kidney where they trigger localized inflammatory responses by activating mesangial cells. Certain lectins recognize the terminal N-acetylgalactosamine (GalNAc)-containing O-glycans on Gal-deficient IgA1 and can be potentially used as diagnostic tools. To improve our understanding of GalNAc recognition by these lectins, we have conducted binding studies to assess the interaction of Helix aspersa agglutinin (HAA) and Helix pomatia agglutinin (HPA) with Gal-deficient IgA1. Surface plasmon resonance spectroscopy revealed that both HAA and HPA bind to a Gal-deficient synthetic hinge region glycopeptide (HR-GalNAc) as well as various aberrantly glycosylated IgA1 myeloma proteins. Despite having six binding sites, both HAA and HPA bind IgA1 in a functionally bivalent manner, with the apparent affinity for IgA1 related to the number of exposed GalNAc groups in the IgA1 hinge. Finally, HAA and HPA were shown to discriminate very effectively between the IgA1 secreted by cell lines derived from peripheral blood cells of patients with IgAN and that from cells of healthy controls. These studies provide insight into lectin recognition of the Gal-deficient IgA1 hinge region and lay the groundwork for the development of reliable diagnostic tools for IgAN.

[Glomerulonephritis with IgA mesangial deposits]

IgA nephropathy is the primitive glomerulonephritis the most frequently encountered worldwide. In about one case out of three, it is responsible for the progression from progressive renal failure to end-stage renal failure. The pathophysiological mechanisms of this disease which is mediated by immune complexes remain unclear. The presentation, clinical progression and optical microscope aspect of the renal biopsy may widely vary, making any histological classification very difficult. Most therapeutic studies include the patients only on clinical criteria of severity. The only consensual management is that of patients with a nephropathy and mild glomerular lesions and a nephritic syndrome, or with an extracapillar glomerulonephritis and a rapidly progressive renal failure; corticoids are indicated in former cases while corticoids must be combined with immunosuppressive agents in the latter ones. Corticotherapy may be considered in patients with a proteinuria higher than 1g/day without renal failure. In any patient with primitive IgA nephropathy, the overall management used for chronic glomerulopathy must be initiated, including, in case of arterial hypertension or proteinuria, the renin-angiotensin system blockade.

Role of IgA and IgA fc receptors in inflammation

INTRODUCTION

Signals delivered by serum monomeric IgA (mIgA) are essential in controlling the immune system by preventing the development of autoimmunity and inflammation. However, IgA can also, when aggregated, be deleterious to the host, inducing inflammatory diseases. This Janus-like nature of IgA is mainly due to their heterogeneity in molecular forms and their interaction with IgA receptors.

DISCUSSION

While serum mIgA are mainly involved in FcalphaRI-mediated inhibition of immune responses, macromolecular serum IgA or circulating IgA immune complexes are often deleterious to the host by inducing sustained activation through IgA receptors including FcalphaRI and transferrin receptor.

CONCLUSION

FcalphaRI-mediated inhibitory function is able to suppress several inflammatory diseases in mice including asthma and glomerulonephritis. Intravenous mIgA (mIgAIV) and anti-FcalphaR monovalent antibodies represent thus promising tools for immunotherapy.

Concanavalin A and polyvinyl butyral use as a potential dengue electrochemical biosensor

Immobilization of concanavalin A on gold electrode by means of gold nanoparticles and polyvinyl butyral was carried out and investigated by cyclic voltammetry and electrochemical impedance spectroscopy. The system was tested with sera from patients infected by dengue fever (DF) and dengue hemorrhagic fever (DHF). Electrochemical impedance spectroscopy (in the frequency range from 100mHz to 100KHz), and cyclic voltammetry (from -0.2 to 0.7V vs. Ag/AgCl), was performed in phosphate buffer solution containing 10mM K(3)[Fe(CN)(6)]/K(4)[Fe(CN)(6)] (1:1) mixture as a redox probe. As biomolecules accumulated on the electrode surface the voltammetric response changed from a clear diffusional to an irreversible behavior. Impedance spectroscopy showed a clear increase of the electron-transfer resistance when the sensor is exposed to contaminated sera (DF or DHF) as compared to exposure to uncontaminated serum (NDF). The results were analyzed through an equivalent circuit and values of charge transfer resistance and capacitance were obtained. Variations in charge transfer resistance were used to distinguish the sensor response for the different sera investigated (DF, DHF and NDF). Alternatively, a three-dimensional graph gave the best response for differentiation of all three blood sera. The distinctive patterns of impedimetric responses observed were ascribed to different glycoprotein patterns in the sera investigated. Therefore, the lectin immobilization on electrode surface with gold nanoparticles and polyvinyl butyral, combined with the three-dimensional impedance analysis introduced herein are valuable tools in the development of a biosensor for immunological response to diseases.

Glycoprofiling of the Human Salivary Proteome

Glycosylation is important for a number of biological processes and is perhaps the most abundant and complicated of the known post-translational modifications found on proteins. This work combines two-dimensional polyacrylamide gel electrophoresis (2-DE) and lectin blotting to map the salivary glycome, and mass spectrometry to identity the proteins that are associated with the glycome map. A panel of 15 lectins that recognize six sugar-specific categories was used to visualize the type and extent of glycosylation in saliva from two healthy male individuals. Lectin blots were compared to 2-D gels stained either with Sypro Ruby (protein stain) or Pro-Q Emerald 488 (glycoprotein stain). Each lectin shows a distinct pattern, even those belonging to the same sugar-specific category. In addition, the glycosylation profiles generated from the lectin blots show that most of the salivary proteins are glycosylated and that the pattern is more widespread than is demonstrated by the glycoprotein stained gel. Finally, the co-reactivity between two lectins was measured to determine the glycan structures that are most and least often associated with one another along with the population variation of the lectin reactivity for 66 individuals.

Paraneoplastic glomerular diseases and malignancies

Paraneoplastic glomerulopathies are rare manifestations of neoplastic disease to be distinguished from iatrogenic renal damage. Solid tumors are preferentially associated with membranous nephropathy, whereas Hodgkin's lymphomas are associated with minimal change disease. The most common neoplasia associated with paraneoplastic glomerular disease are carcinomas of the lung and of the gastrointestinal tract. Nephrotic syndrome is the most frequent presentation of paraneoplastic glomerulopathy and the most critical glomerular disease regarding prognosis and patient care. Renal biopsy is recommended in patients with glomerular proteinuria or nephrotic syndrome and cancer, depending on life expectancy and therapeutic options. The primary treatment must be directed at the cancer in all cases. Symptomatic treatment of the nephrotic syndrome with diuretics and ACE inhibitors is justified. Prevention of nephrotic syndrome complications, i.e. thromboses and infections, should also be addressed and systematic regular renal follow-up is warranted. All treatments should be regularly reviewed to avoid toxicity, associated renal function loss or low albumin levels for patients receiving albumin-binding drugs. Epidemiologic studies have low evidence-based value. There is no widely accepted experimental model of the association of glomerulopathy and cancer. Thus, epidemiologic and mechanistic studies are needed to determine the true prevalence of paraneoplastic glomerulopathies and investigate new pathophysiologic approaches.

Specificity in cancer immunotherapy

From the earliest days in the field of tumor immunology three questions have been asked: do cancer cells express tumor-specific antigens, does the immune system recognize these antigens and if so, what is their biochemical nature? We now know that truly tumor-specific antigens exist, that they are caused by somatic mutations, and that these antigens can induce both humoral and cell-mediated immune responses. Because tumor-specific antigens are exclusively expressed by the cancer cell and are often crucial for tumorigenicity, they are ideal targets for anti-cancer immunotherapy. Nevertheless, the antigens that are targeted today by anti-tumor immunotherapy are not tumor-specific antigens, but antigens that are normal molecules also expressed by normal tissues (so-called "tumor-associated" antigens). If tumor-specific antigens exist and are ideal targets for immunotherapy, why are they not being targeted? In this review, we summarize current knowledge of tumor-specific antigens: their identification, immunological relevance and clinical use. We discuss novel tumor-specific epitopes and propose new approaches that could improve the success of cancer immunotherapy, especially for the treatment of solid tumors.