Background/Purpose: Lupus is a chronic autoimmune disease characterized by autoantibody production and aberrant activation and proliferation of lymphocytes. Subsequent deposition of immune complexes in target tissues leads to an inflammatory reaction and tissue damage. Glomerulonephritis (GN) is the most severe complication of lupus, lupus nephritis (LN) affecting up to two-thirds of lupus patients and is associated with high morbidity and mortality. LN is characterized by podocyte dysfunction, proteinuria and a decrease in renal function. Glycosphingolipids (GSLs) are a heterogeneous class of lipids in the sphingolipid family that play a role in the regulation of cellular processes. Highly abundant in the kidney, GSLs play a role in a variety of kidney diseases including GN, nephropathy, and polycystic kidney disease. GSLs are present in most cells and are thought to play roles in signal transduction, cell-cell adhesion and immune responses. GSLs are enriched in the kidney. Loss of sialic acid residues from the surface of podocytes is linked to proteinuria in GN. Neuraminidases (NEUs) remove sialic acids from gangliosides to generate lactosylceramide (LacCer) and glucosylceramide, (GlcCer) ganglioside precursors.

Methods: Kidney homogenates, serum and urine samples were prepared from 11, 14 and 18 week-old MRL/lpr lupus mice and human lupus patients and controls. GSL levels were measured by Supercritical Fluid Chromatography coupled with tandem mass spectrometry. NEU protein and activity were measured by western immunoblot and/or enzyme activity assays. Gene expression was analyzed by real-time RTPCR on RNA isolated from kidney cortex. Matrix-assisted laser desorption/ionization-imaging mass spectrometry (MALDI-IMS) and immunohistochemistry for LacCer was performed on frozen kidney sections. 

Results: GlcCer and LacCer levels are significantly elevated 3-5-fold, NEU activity and Neu1 expression levels are significantly elevated 2-fold and 12-fold, respectively, in the kidney of lupus mice with nephritis compared to lupus mice without nephritis and/or normal, healthy controls. Levels of ceramide and other enzymes in the GSL metabolic pathway are unchanged. Urine LacCer levels appear to be significantly elevated prior to significant increases in proteinuria. Using MALDI-IMS, we also observed a striking decrease in the levels of gangliosides GM1 and GM3 in the LN mice compared to controls. Of translational significance, human LN patients compared to controls have a 1) significant 25-fold increase in urine LacCer levels, 2) significant increase in urine Neu1 levels, and 3) observed increased LacCer levels in the mesangial regions in renal biopsy sections. No significant differences were observed in serum LacCer levels in LN patients compared to controls. 

Conclusion: Our results demonstrate that elevated LacCer is likely due to an upregulation of the GSL catabolic pathway in the kidney of lupus mice and patients with nephritis. Furthermore, the elevated LacCer levels in the urine of patients may be largely due to kidney rather than systemic contributions and our mouse studies suggest it may be an earlier marker than proteinuria of nephritis.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/dysfunction-of-glycosphingolipid-metabolism-in-lupus-nephritis/