Gangliosides in Podocyte Biology and Disease

Interplay of lipid metabolism and inflammation in podocyte injury

Podocytes are critical for maintaining permselectivity of the glomerular filtration barrier, and podocyte injury is a major cause of proteinuria in various primary and secondary glomerulopathies. Lipid dysmetabolism and inflammatory activation are the distinctive hallmarks of podocyte injury. Lipid accumulation and lipotoxicity trigger cytoskeletal rearrangement, insulin resistance, mitochondrial oxidative stress, and inflammation. Subsequently, inflammation promotes the progression of glomerulosclerosis and renal fibrosis via multiple pathways. These data suggest that lipid dysmetabolism positively or negatively regulates inflammation during podocyte injury. In this review, we summarize recent advances in the understanding of lipid metabolism and inflammation, and highlight the potential association between lipid metabolism and podocyte inflammation.

Progression of albuminuria and podocyte injury in focal segmental glomerulosclerosis inhibited by enhanced glycosphingolipid GM3 via valproic acid

Focal segmental glomerulosclerosis, characterized by decreased numbers of podocytes in glomeruli, is a common cause of refractory nephrotic syndrome. Recently, we showed that enhanced glycosphingolipid GM3 expression after administration of valproic acid, an upregulator of ST3GAL5/St3gal5, was effective in preventing albuminuria and podocyte injury. We also revealed the molecular mechanism for this preventive effect, which involves GM3 directly binding nephrin that then act together in glycolipid-enriched membrane (GEM) fractions under normal conditions and in non-GEM fractions under nephrin injury conditions. Kidney disease is frequently referred to as a "silent killer" because it is often difficult to detect subjective symptoms. Thus, primary treatment for these diseases is initiated after the onset of disease progression. Consequently, the efficacy of enhanced levels of GM3 induced by valproic acid needs to be evaluated after the onset of the disease with severe albuminuria such as focal segmental glomerulosclerosis. Here, we report the therapeutic effect of enhanced GM3 expression induced via administration of valproic acid on albuminuria and podocyte injury after the onset focal segmental glomerulosclerosis in anti-nephrin antibody treated mice. Our findings suggest elevated levels of GM3 following treatment with valproic acid has therapeutic utility for kidney disease associated with severe albuminuria and podocyte injury.

Glycosphingolipid GM3 prevents albuminuria and podocytopathy induced by anti-nephrin antibody

Podocytopathy, which is characterized by injury to podocytes, frequently causes proteinuria or nephrotic syndrome. There is currently a paucity of effective therapeutic drugs to treat proteinuric kidney disease. Recent research suggests the possibility that glycosphingolipid GM3 maintains podocyte function by acting on various molecules including nephrin, but its mechanism of action remains unknown. Here, various analyses were performed to examine the potential relationship between GM3 and nephrin, and the function of GM3 in podocytes using podocytopathy mice, GM3 synthase gene knockout mice, and nephrin injury cells. Reduced amounts of GM3 and nephrin were observed in podocytopathy mice. Intriguingly, this reduction of GM3 and nephrin, as well as albuminuria, were inhibited by administration of valproic acid. However, when the same experiment was performed using GM3 synthase gene knockout mice, valproic acid administration did not inhibit albuminuria. Equivalent results were obtained in model cells. These findings indicate that GM3 acts with nephrin in a collaborative manner in the cell membrane. Taken together, elevated levels of GM3 stabilize nephrin, which is a key molecule of the slit diaphragm, by enhancing the environment of the cell membrane and preventing albuminuria. This study provides novel insight into new drug discovery, which may offer a new therapy for kidney disease with albuminuria.

Decreased GM3 correlates with proteinuria in minimal change nephrotic syndrome and focal segmental glomerulosclerosis

BACKGROUND

Glycolipids on cell membrane rafts play various roles by interacting with glycoproteins. Recently, it was reported that the glycolipid GM3 is expressed in podocytes and may play a role in podocyte protection. In this report, we describe the correlation between changes in GM3 expression in glomeruli and proteinuria in minimal change nephrotic syndrome (MCNS) and focal segmental glomerulosclerosis (FSGS) patients.

METHODS

We performed a case-control study of the correlation between nephrin/GM3 expression levels and proteinuria in MCNS and FSGS patients who underwent renal biopsy at our institution between 2009 and 2014. Normal renal tissue sites were used from patients who had undergone nephrectomy at our institution and gave informed consent.

RESULTS

Both MCNS and FSGS had decreased GM3 and Nephrin expression compared with the normal (normal vs. MCNS, FSGS; all p < 0.01). Furthermore, in both MCNS and FSGS, GM3 expression was negatively correlated with proteinuria (MCNS: r = - 0.61, p < 0.01, FSGS: r = - 0.56, p < 0.05). However, nephrin expression had a trend to correlate with proteinuria in FSGS (MCNS: r = 0.19, p = 0.58, FSGS: r = - 0.48, p = 0.06). Furthermore, in a simple linear regression analysis, GM3 expression also correlated with proteinuric change after 12 months of treatment (MCNS: r = 0.40, p = 0.38, FSGS: r = 0. 68, p < 0.05).

CONCLUSION

We showed for the first time that decreased GM3 expression correlates with proteinuria in MCNS and FSGS patients. Further studies are needed on the podocyte-protective effects of GM3.

A Lack of GD3 Synthase Leads to Impaired Renal Expression of Connexins and Pannexin1 in St8sia1 Knockout Mice

The aim of this study was to determine the effects of altered ganglioside composition on the expression of Cx37, Cx40, Cx43, Cx45, and Panx1 in different kidney regions of St8sia1 gene knockout mice (St8sia1 KO) lacking the GD3 synthase enzyme. Experiments were performed in twelve male 6-month-old mice: four wild-type (C57BL/6-type, WT) and eight St8sia1 KO mice. After euthanasia, kidney tissue was harvested, embedded in paraffin wax, and processed for immunohistochemistry. The expression of connexins and Panx1 was determined in different regions of the kidney: cortex (CTX.), outer stripe of outer medulla (O.S.), inner stripe of outer medulla (IN.S.), and inner medulla (IN.MED.). We determined significantly lower expression of Cx37, Cx40, Cx45, and Panx1 in different parts of the kidneys of St8sia1 KO mice compared with WT. The most consistent decrease was found in the O.S. where all markers (Cx 37, 40, 45 and Panx1) were disrupted in St8si1 KO mice. In the CTX. region, we observed decrease in the expression of Cx37, Cx45, and Panx1, while reduced expression of Cx37 and Panx1 was more specific to IN.S. The results of the present study suggest that deficiency of GD3 synthase in St8sia1 KO mice leads to disruption of renal Cx expression, which is probably related to alteration of ganglioside composition.

The Updates of Podocyte Lipid Metabolism in Proteinuric Kidney Disease

BACKGROUND

Podocytes, functionally specialized and terminally differentiated glomerular visceral epithelial cells, are critical for maintaining the structure and function of the glomerular filtration barrier. Podocyte injury is considered as the most important early event contributing to proteinuric kidney diseases such as obesity-related renal disease, diabetic kidney disease, focal segmental glomerulosclerosis, membranous nephropathy, and minimal change disease. Although considerable advances have been made in the understanding of mechanisms that trigger podocyte injury, cell-specific and effective treatments are not clinically available.

SUMMARY

Emerging evidence has indicated that the disorder of podocyte lipid metabolism is closely associated with various proteinuric kidney diseases. Excessive lipid accumulation in podocytes leads to cellular dysfunction which is defined as lipotoxicity, a phenomenon characterized by mitochondrial oxidative stress, actin cytoskeleton remodeling, insulin resistance, and inflammatory response that can eventually result in podocyte hypertrophy, detachment, and death. In this review, we summarize recent advances in the understanding of lipids in podocyte biological function and the regulatory mechanisms leading to podocyte lipid accumulation in proteinuric kidney disease.

KEY MESSAGES

Targeting podocyte lipid metabolism may represent a novel therapeutic strategy for patients with proteinuric kidney disease.

Sialoglyco-Conjugate Abnormalities, IL-6 Trans-Signaling and Anti-Ganglioside Immune Response-Potential Interferences in Lupus Nephritis Pathogenesis

We have investigated glycoconjugates sialization profile, endogen synthesis rate of antiganglioside antibodies (AGA), IL-6 signaling pathways correlated with activity disease in systemic lupus erythematous (SLE) and lupus nephritis (LN). Material and methods. A case-control study was developed and included 109 patients with SLE with or without renal impairment, 32 patients with IgA nephropathy and 60 healthy volunteers, clinically and paraclinically monitored. The following parameters were evaluated in volunteers serum: total sialic acid (TSA), orosomucoids, lipid bound sialic acid (LSA), interleukin-6 (IL-6), soluble factors IL-6R, gp130, anti –GM1, -GM2, -GM3, -GD1a, -GD1b, -GT1b, -GQ1b antigangliosides antibodies of IgG and IgM type. Results. Experimental data analysis showed: increase in synthesis rhythm of sialoglyco-conjugated in SLE (TSA increased in SLE and LN compared to control), accelerated catabolism of LSA in LN (LSA/TSA ratio was higher in SLE and LN than in control group), overexpression of IL-6 mediated trans-signaling (sIL-6R/sgp 130 ratio was subunit in SLE and IgA nephropathy and superunit in LN), large AGA profile synthesis of IgM isotype (over 45.1% in SLE and over 20.7% in LN). Conclusions. Hypersialization, accelerated glycosphingolipids degradation, IL-6 trans-signaling amplify and AGA pattern could represent essential mechanisms in LN pathogenesis.