Role of Sphingolipid Signaling in Glomerular Diseases: Focus on DKD and FSGS

Sphingolipids are well-recognized as major players in the pathogenesis of many human diseases, including chronic kidney disease. The kidney is a very sensitive organ to alterations in sphingolipid metabolism. The critical issues to be addressed in this review relate to the role of sphingolipids and enzymes involved in sphingolipid metabolism in the pathogenesis of glomerular diseases with a special focus on podocytes, a key cellular component of the glomerular filtration barrier. Among several sphingolipids, we will highlight the role of ceramide, sphingosine, sphingosine-1-phosphate and ceramide-1-phosphate. Additionally, we will summarize the current knowledge with regard to the use of sphingolipids as therapeutic agents for the treatment of podocyte injury in kidney disease.

Modulation of radiation-induced damage of human glomerular endothelial cells by SMPDL3B

The intracellular molecular pathways involved in radiation-induced nephropathy are still poorly understood. Glomerular endothelial cells are key components of the structure and function of the glomerular filtration barrier but little is known about the mechanisms implicated in their injury and repair. The current study establishes the response of immortalized human glomerular endothelial cells (GEnC) to ionizing radiation (IR). We investigated the role of sphingolipids and the lipid-modifying enzyme sphingomyelin phosphodiesterase acid-like 3b (SMPDL3b) in radiation-induced GEnC damage. After delivering a single dose of radiation, long and very-long-chain ceramide species, and the expression levels of SMPDL3b were elevated. In contrast, levels of ceramide-1-phosphate (C1P) dropped in a time-dependent manner although mRNA and protein levels of ceramide kinase (CERK) remained stable. Treatment with C1P or knocking down SMPDL3b partially restored cell survival and conferred radioprotection. We also report a novel role for the NADPH oxidase enzymes (NOXs), namely NOX1, and NOX-derived reactive oxygen species (ROS) in radiation-induced GEnC damage. Subjecting cultured endothelial cells to radiation was associated with increased NOX activity and superoxide anion generation. Silencing NOX1 using NOX1-specific siRNA mitigated radiation-induced oxidative stress and cellular injury. In addition, we report a novel connection between NOX and SMPDL3b. Treatment with the NOX inhibitor, GKT, decreased radiation-induced cellular injury and restored SMPDL3b basal levels of expression. Our findings indicate the importance of SMPDL3b as a potential therapeutic target in radiation-induced kidney damage.

Regulation of the amount of ceramide-1-phosphate synthesized in differentiated human podocytes

Sphingolipids have important functions as structural components of cells but they also function as signaling molecules regulating different cellular processes such as apoptosis, cell proliferation, cell migration, cell division and inflammation. Hence, a tight regulation of the sphingolipid homeostasis is essential to maintain proper cellular functions. Mammalian ORMDL proteins are orthologues of the yeast ORM1/2 proteins, which regulate ceramide synthesis in yeast. ORMDL proteins inhibit serine palmitoyltransferase (SPT), the enzyme regulating a rate-limiting step of the sphingolipid pathway to control the levels of ceramides and other sphingolipids. Sphingomyelinase phosphodiesterase like 3b (SMPDL3b) is a glycosylphosphatidylinositol (GPI) anchored protein in the plasma membrane (PM) and determines membrane fluidity in macrophages. We previously showed that differential expression of SMPDL3b alters the availability of Ceramide-1-phosphate (C1P) in human podocytes, which are terminally differentiated cells of the kidney filtration barrier. This observation lead us to investigate if SMPDL3b controls C1P availability in human podocytes by interfering with ceramide kinase (CERK) expression and function. We found that SMPDL3b interacts with CERK and can bind to C1P in vitro. Furthermore, CERK expression is reduced when SMPDL3b expression is silenced. These observations led us to propose that one of the mechanisms by which SMPDL3b influences the amount of C1P available in the podocytes is by interfering with the function of CERK thereby maintaining a balance in the levels of the C1P in podocytes.

Podocyte pathology and nephropathy - sphingolipids in glomerular diseases

Sphingolipids are components of the lipid rafts in plasma membranes, which are important for proper function of podocytes, a key element of the glomerular filtration barrier. Research revealed an essential role of sphingolipids and sphingolipid metabolites in glomerular disorders of genetic and non-genetic origin. The discovery that glucocerebrosides accumulate in Gaucher disease in glomerular cells and are associated with clinical proteinuria initiated intensive research into the function of other sphingolipids in glomerular disorders. The accumulation of sphingolipids in other genetic diseases including Tay-Sachs, Sandhoff, Fabry, hereditary inclusion body myopathy 2, Niemann-Pick, and nephrotic syndrome of the Finnish type and its implications with respect to glomerular pathology will be discussed. Similarly, sphingolipid accumulation occurs in glomerular diseases of non-genetic origin including diabetic kidney disease (DKD), HIV-associated nephropathy, focal segmental glomerulosclerosis (FSGS), and lupus nephritis. Sphingomyelin metabolites, such as ceramide, sphingosine, and sphingosine-1-phosphate have also gained tremendous interest. We recently described that sphingomyelin phosphodiesterase acid-like 3b (SMPDL3b) is expressed in podocytes where it modulates acid sphingomyelinase activity and acts as a master modulator of danger signaling. Decreased SMPDL3b expression in post-reperfusion kidney biopsies from transplant recipients with idiopathic FSGS correlates with the recurrence of proteinuria in patients and in experimental models of xenotransplantation. Increased SMPDL3b expression is associated with DKD. The consequences of differential SMPDL3b expression in podocytes in these diseases with respect to their pathogenesis will be discussed. Finally, the role of sphingolipids in the formation of lipid rafts in podocytes and their contribution to the maintenance of a functional slit diaphragm in the glomerulus will be discussed.