Defective immunoglobulin A (IgA) glycosylation and IgA deposits in patients with IgA nephropathy

Deciphering roles of protein post-translational modifications in IgA nephropathy progression and potential therapy

Immunoglobulin A nephropathy (IgAN), one type of glomerulonephritis, displays the accumulation of glycosylated IgA in the mesangium. Studies have demonstrated that both genetics and epigenetics play a pivotal role in the occurrence and progression of IgAN. Post-translational modification (PTM) has been revealed to critically participate in IgAN development and progression because PTM dysregulation results in impaired degradation of proteins that regulate IgAN pathogenesis. A growing number of studies identify that PTMs, including sialylation, o-glycosylation, galactosylation, phosphorylation, ubiquitination and deubiquitination, modulate the initiation and progression of IgAN. Hence, in this review, we discuss the functions and mechanisms of PTMs in regulation of IgAN. Moreover, we outline numerous compounds that govern PTMs and attenuate IgAN progression. Targeting PTMs might be a useful strategy to ameliorate IgAN.

Use of liquid chromatography-tandem mass spectrometry to perform urinary proteomic analysis of children with IgA nephropathy and Henoch-Schönlein purpura nephritis

The emerging technology of urinary proteomics has become an efficient biological approach for identifying biomarkers and characterizing pathogenesis in renal involvement. In this study, we attempted to elucidate the relationship between IgAN and HSPN in children, employing LC-MS/MS to perform urinary proteomic analyses using the DIA method. Early-morning spot urine was collected from patients with biopsy-proven IgAN (n = 19) and HSPN (n = 19) prior to treatment and renal biopsy in the Department of Pediatrics, Jinling Hospital, Nanjing, China, and did healthy volunteers (n = 14), from June 2018 to December 2019. Two hundred seventy-six urinary proteins and 125 urinary proteins were determined to be differentially expressed in children with IgAN (n = 4) and HSPN (n = 4), respectively, compared to the urinary proteins of healthy children (n = 4) (p < 0.05). GO analysis demonstrated that the differentially expressed proteins of the two groups, which were located in the extracellular matrix and cell membrane, were primarily involved in biological processes, including metabolic processes, immune system processes, cellular adhesion, cell proliferation, signaling, and biological regulation. KEGG analysis revealed that the differentially expressed proteins of the two groups were associated with cell adhesion molecules, ECM-receptor interactions, the PI3K-Akt signaling pathway, the complement and coagulation cascades, regulation of actin cytoskeleton, cholesterol metabolism, and platelet activation. The target proteins (alpha-1B-glycoprotein (A1BG) and afamin (AFM)), which participated in the complement and coagulation cascades and the regulation of complement activation, were further investigated in the independent validation cohort by ELISA. These proteins were significantly increased in children with IgAN (n = 15) and HSPN (n = 15) compared with the proteins observed in healthy controls (n = 10, P < 0.05). The validated results were consistent with the mass spectrometry results. SIGNIFICANCE: IgAN and HSPN both result from the glomerular deposition of abnormally glycosylated IgA1 with mesangial proliferative changes, and both diseases are common glomerulopathies in the pediatric population that are believed to be correlated. Interestingly, our data, by combining urinary proteomic analyses, showed that several uniform enrichment pathways played an important role in the progression of IgAN and HSPN, suggesting that we might reduce the renal involvement of the two diseases in children through these pathways. The same urinary proteins along these pathways were observed to be differentially expressed in children with IgAN and HSPN, implying that these proteins may be potential biomarkers to identify the two diseases. Future studies examining larger cohorts are warranted to confirm the validity of our findings.

Serum Levels of Joining Chain-Containing IgA1 Are Not Elevated in Patients with IgA Nephropathy

Background

It has been suggested that mesangial IgA deposits are dimeric or polymeric in IgA nephropathy (IgAN). However, evidence concerning the molecular form of serum IgA in IgAN is controversial. And there is no direct evidence that the serum levels of joining chain- (J chain-) containing IgA (J-IgA) are elevated in IgAN. In this study, we aimed to measure serum J-IgA and glomerular J chain deposition with anti-J chain monoclonal antibody in IgAN.

Methods

BALB/c mice were immunized with human J chain-GST recombinant peptide to obtain anti-J chain monoclonal antibody. The levels of serum total IgA and J-IgA were measured by sandwich enzyme-linked immunosorbent assay in 115 patients with IgAN and 117 healthy volunteers. J chain deposition in kidney specimens was analyzed by immunohistochemistry staining.

Results

Serum levels of total IgA1 were elevated in IgAN patients compared to healthy subjects. However, serum levels of IgA, J-IgA, and J chain-containing IgA1 (J-IgA1), the J-IgA to total IgA ratio, and the J-IgA1 to total IgA1 ratio were not significantly different between IgAN patients and healthy subjects. Western blot analysis and gel filtration analysis using purified IgA1 also showed that the proportion of J chain-containing polymeric IgA1 was lower in IgAN patients compared to healthy subjects. No correlation was found between serum J-IgA or J-IgA1 and clinical features in IgAN. Immunohistochemistry analysis showed that glomerular J chain was positive in 12 IgAN patients (57.1%). The values of the J-IgA to IgA ratio and J-IgA1 to IgA ratio were significantly higher in IgAN patients with glomerular J chain deposition than those without. However, the serum levels of J-IgA and J-IgA1 and the J-IgA1 to IgA1 ratio were not significantly higher in two subgroups.

Conclusions

Although serum levels of total IgA1 were elevated in IgAN, the serum levels of J-IgA1 were not elevated. And serum J-IgA, serum J-IgA1, and J chain deposition were not correlated with disease severity in IgAN.

Recent Advances in Clinical Glycoproteomics of Immunoglobulins (Igs)

Antibody glycosylation analysis has seen methodological progress resulting in new findings with regard to antibody glycan structure and function in recent years. For example, antigen-specific IgG glycosylation analysis is now applicable for clinical samples because of the increased sensitivity of measurements, and this has led to new insights in the relationship between IgG glycosylation and various diseases. Furthermore, many new methods have been developed for the purification and analysis of IgG Fc glycopeptides, notably multiple reaction monitoring for high-throughput quantitative glycosylation analysis. In addition, new protocols for IgG Fab glycosylation analysis were established revealing autoimmune disease-associated changes. Functional analysis has shown that glycosylation of IgA and IgE is involved in transport across the intestinal epithelium and receptor binding, respectively.