Sphingomyelin Phosphodiesterase Acid-like 3b Research Articles

The Lipid-Modifying Enzyme SMPDL3B Negatively Regulates Innate Immunity.

SummaryLipid metabolism and receptor-mediated signaling are highly intertwined processes that cooperate to fulfill cellular functions and safeguard cellular homeostasis. Activation of Toll-like receptors (TLRs) leads to a complex cellular response, orchestrating a diverse range of inflammatory events that need to be tightly controlled. Here, we identified the GPI-anchored Sphingomyelin Phosphodiesterase, Acid-Like 3B (SMPDL3B) in a mass spectrometry screening campaign for membrane proteins co-purifying with TLRs. Deficiency of Smpdl3b in macrophages enhanced responsiveness to TLR stimulation and profoundly changed the cellular lipid composition and membrane fluidity. Increased cellular responses could be reverted by re-introducing affected ceramides, functionally linking membrane lipid composition and innate immune signaling. Finally, Smpdl3b-deficient mice displayed an intensified inflammatory response in TLR-dependent peritonitis models, establishing its negative regulatory role in vivo. Taken together, our results identify the membrane-modulating enzyme SMPDL3B as a negative regulator of TLR signaling that functions at the interface of membrane biology and innate immunity.

The glomerular permeability factors in idiopathic nephrotic syndrome.

It is currently postulated that steroid-sensitive idiopathic nephrotic syndrome (SSNS) and steroid-resistant idiopathic nephrotic syndrome (SRNS), which are not related to the mutation of a gene coding for podocyte structures or for glomerular basement membrane proteins, result from a circulating factor affecting podocyte shape and function. T lymphocytes have for a long time been suspected to be involved in the pathophysiology of these diseases. The successful treatment of steroid-dependant nephrotic syndrome with rituximab suggests a potential role for B lymphocytes. Clinical and experimental data indicate roles for cytokines IL-13, TNFα, circulating cardiotrophin-like cytokine factor 1 (member of the IL-6 family), circulating hemopexin, radical oxygen species, and the soluble urokinase-type plasminogen activator receptor (suPAR) in the development of nephrotic syndrome. Podocyte metabolism modifications—leading to the overexpression of the podocyte B7-1antigen (CD 80), hypoactivity of the podocyte enzyme sphingomyelin phosphodiesterase acid-like 3 b (SMPDL3b), and to the podocyte production of a hyposialylated form of the angiopoietin-like 4 (Angptl4)—are mechanisms possibly involved in the changes in the podocyte cytoskeleton leading to SSNS and or SRNS. Different multifactorial pathophysiological mechanisms can be advocated for SSNS and SRNS. The present paper reviews the experimental and clinical data upon which the different hypotheses are based and reports their possible clinical applications.

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.

Rituximab Treatment Prevents the Early Development of Proteinuria following Pig-to-Baboon Xeno-Kidney Transplantation

We previously reported life-supporting α1,3-galactosyltransferase knockout (GalTKO) thymokidney xenograft survival of >2 months in baboons. However, despite otherwise normal renal function, recipients developed proteinuria with morphologic changes (podocyte effacement), a condition that presents a major obstacle to long-term studies in this model. A recent clinical study showed that rituximab therapy after allogeneic transplant prevented proteinuria possibly associated with loss of sphingomyelin phosphodiesterase acid-like 3b (SMPDL-3b). Here, we demonstrate that rituximab prevents the disruption of pig podocytes in an SMPDL-3b-dependent manner in vitro and the early development of proteinuria after xenogeneic kidney transplantation in baboons. Immunofluorescence showed SMPDL-3b expression in pig glomerular epithelium; immunoprecipitation demonstrated rituximab binding to SMPDL-3b in glomeruli. Culture of isolated pig podocytes with naive baboon sera, which has preformed antipig natural antibodies, reduced SMPDL-3b expression, disrupted podocyte morphology, and decreased podocyte proliferation, whereas pretreatment with rituximab prevented these effects. Six baboons received rituximab before transplantation to deplete B cells and again in the peri-transplant period; 18 baboons treated only before transplantation served as historical controls. The onset of post-transplant proteinuria was significantly delayed in a B cell-independent manner in the animals that received peri-transplant rituximab treatment. Although further optimization of this protocol is required, these data provide intriguing clues to the mechanisms of post-transplant proteinuria in xenogeneic kidney transplantation and a potential strategy for its prevention.

Prevention of Early Development of Proteinuria by Rituximab in a Pig to Baboon Thymokidney Model Likely Associated with Modulating Podocyte Function in An SMPDL-3b-Dependent Manner

Background: We have previously reported life-supporting GalT-KO thymokidney (TK) xenograft survival greater than 80 days in baboons. One of major obstacles to long-term success in this model is the persistent development of proteinuria. Despite otherwise normal renal function, EM findings showed podocyte effacement. Recent clinical data suggested that Rituximab might prevent recurrent FSGS by modulating podocyte function in a sphingomyelin phosphodiesterase acid-like 3b (SMPDL-3b)-dependent manner. In this study, we determined whether Rituximab prevents proteinuria in this xenogeneic TK model. Methods: 33 baboon recipients of GalT-KO TK received both T and B cell depletion prior to TK transplantation (Tx), followed by anti-CD40L based maintenance therapy. All animals (n=33) received Rituximab to deplete B cells 2 weeks prior to TKTx. In order to test the effect of Rituximab therapy on preventing proteinuria, 29 of 33 did not receive additional dose of Rituximab (Group1), and 4 baboons received Rituximab (10mg/kg) in peri-TKTx period (day 0, POD2 or 5) (Group2). We assessed the development of proteinuria following TKTx as well as immunohistological analysis to determine the mechanism by which Rituximab affects the onset of proteinuria. Results: CD20+ cells in all recipients were nearly completely depleted (< 20/ul) in the peri-Tx period. On average, all baboons without Rituximab therapy in the peri-TKTx period (Group1) developed 2+ (100mg/dl) proteinuria on POD2. In contrast, 4 baboons with Rituximab (Group2) exhibited a markedly delayed first onset of 2+ proteinuria at PODs 9, >10, 11 and 22. One animal given Rituximab on POD5 reversed its proteiunuria from 2+ to trace until POD22. Immunofluorescence data showed (1) Rituximab bound to porcine glomerular epithelium, (2) SMPDL-3b was expressed on porcine glomerular epithelium confirmed by both immunohistologic staining and western-blotting. (3) Anti-SMPDL-3b Abs blocked binding of Rituximab and (4) SMPDL-3b expression decreased after severe proteinuria developed. Conclusion: These data suggest that Rituximab therapy in peri-TKTx period can markedly delay the development of proteinuria. This effect is not due to T or B cell depletion because both T/B cells were depleted similarly in both groups. Histologic examination suggested that this effect is associated with modulating podocyte function in an SMPDL-3b-dependent manner. To our knowledge, this is the first evidence of preventing early development of proteinuria following xenogeneic GalTKO KTx in nonhuman primates.

Circulating Anti-PLA2R Autoantibodies to Monitor Immunological Activity in Membranous Nephropathy

Experimental and human data converge to indicate that deposition along the glomerular basement membrane of immunoglobulins produced by autoreactive B cells may initiate a sequence of events resulting in complement activation,[1][1] impaired sieving function of the glomerular capillary wall and the