Increased Sialylation of Complement Factor H Promotes Alternative Pathway Activation and Disease Progression in Diabetic Kidney Disease

Background: Increasing evidence points to an essential role for activation of the complement alternative pathway (AP) in the pathogenesis of diabetic kidney disease (DKD). However, the precise mechanism of AP activation in DKD remains poorly understood. Complement factor H (CFH), the key regulator of AP, exerts decay acceleration and cofactor activities to tightly control AP activation. Notably, CFH is a glycoprotein with 9 N-glycosites, where N-glycation probably affects its structure and function. Methods: CFH was purified from a total of 21 plasma pools comprising 71 DKD patients and 54 controls, with each pool containing 6 samples, except for one DKD pool, which contained 5 samples. 18 patients with type 2 diabetes but without renal involvement and 18 patients with renal amyloidosis were included as disease controls, while 18 healthy individuals without diabetes or kidney disease served as normal controls. The site-specific N-glycation profile of CFH, including glycosite occupancies, glycan compositions, structural types, and branching patterns, was analyzed using high-performance liquid chromatography with tandem mass spectrometry (HPLC-MS/MS) and Byonic software. The decay acceleration and cofactor activities of CFH were evaluated using specially designed ELISA and SDS-PAGE. To confirm the functional relevance of CFH sialylation, we compared the activity of desialylated CFH, treated with neuraminidase (which removes terminal α-linked sialic acids), with that of native CFH. Results: CFH from DKD patients exhibited higher glycosite occupancy, greater N-glycan diversity, and increased sialylation compared to controls. Both the decay acceleration activity and cofactor activity of CFH from DKD patients were reduced relative to controls. After desialylation, CFH from DKD patients showed regulatory abilities similar to those of the controls, indicating partial functional recovery from the native form. In contrast, CFH from controls showed no significant functional change before and after desialylation. Conclusion: Increased sialylation of CFH reduced its regulatory capacities, leading to complement AP activation, in DKD patients. Funding: Government Support – Non-U.S.