Yudai Tsuji

Structural variations of N-glycans critically regulate glycoprotein functions and are involved in various human diseases. N-Acetylglucosaminyltransferase-III (GnT-III or MGAT3) is highly expressed in the brain and kidney and is an N-glycan branching enzyme that biosynthesizes the unique N-glycan branch designated as bisecting GlcNAc. Its roles in Alzheimer's disease and cancer have been revealed, but the functions of bisecting GlcNAc in the kidney are poorly understood. Here, we show that kidneys in the GnT-III-knockout (KO) mouse exhibit impaired body fluid balance and present interstitial edema. To understand the molecular mechanisms further, we biochemically purified the glycoproteins modified by GnT-III in the mouse kidney and identified these proteins using proteomics. We found that the proteins involved in the pathway for angiotensin II (Ang II) metabolism are modified by GnT-III, and that the subcellular localization of angiotensin-converting enzyme was altered in GnT-III-KO cells. Furthermore, the pathology in models of Ang II-related disease was slightly more severe in GnT-III-KO than in wild-type mice. Our data indicate a protective role for bisecting GlcNAc in the mouse kidney, highlighting a newly described link between specific N-glycan structures and renal functions.

BACKGROUND: IgA nephropathy (IgAN) is the most common type of primary glomerulonephritis. Elevation in the blood levels of aberrantly glycosylated IgA1 is a crucial initial step in IgAN pathogenesis. Here, we aimed to determine the longitudinal changes in the serum levels of IgA1 O- and N-glycoforms in patients with IgAN receiving different treatments. METHODS: We enrolled Japanese patients diagnosed with primary IgAN: 10 patients who underwent tonsillectomy and corticosteroid therapy (T-CST), 7 who received corticosteroid therapy (CST), 8 who received conservative therapy (CO), and 5 with other renal diseases who received corticosteroid therapy (ORD) as disease controls. IgA was purified from patient sera collected at diagnosis and post-treatment. After sample preparation, O-glycoforms of the hinge region (HR) and N-glycoforms of the fragment crystallizable region were analyzed using high-resolution mass spectrometry (MS). RESULTS: The MS analysis of O-glycoforms of IgA1 showed that the relative abundance of IgA1 with 3GalNAc3Gal, which we previously identified as a characteristic IgA1 O-glycoform in IgAN, decreased post-treatment only in the T-CST group (P = 0.0195). Regarding N-glycoforms, the relative abundance of fucosylated N-glycan at asparagine (Asn)340 increased in the IgAN group compared with that in the ORD group (P = 0.0189) and decreased post-treatment only in the T-CST group (P = 0.0195). CONCLUSION: The MS analysis of O- and N-glycoforms of IgA1 revealed substantial changes in their abundance in the T-CST group but not in the CST, CO, and ORD groups. Our study provides new insights into how specific treatments alter the IgA1 glycoform abundance.