Crystal Structure of the Acid Sphingomyelinase-like Phosphodiesterase SMPDL3B Provides Insights into Determinants of Substrate Specificity

Contribution of sphingomyelin phosphodiesterase acid-like 3B to the proliferation, migration, and invasion of ovarian cancer cells

Background

Cancer has the highest mortality rate among gynecological cancers and poses a serious threat to women's lives. However, the treatment options for ovarian cancer are still limited, and exploring effective targeted biomarkers is particularly important for predicting and treating ovarian cancer. Therefore, it is necessary to explore the molecular mechanisms of the occurrence and development of ovarian cancer.

Methods

This investigation encompassed the analysis of gene expression profiles, measurement of transcription levels of potential target genes in peripheral blood samples from ovarian cancer patients and characterization of the ovarian cancer-related secretory protein sphingomyelin phosphodiesterase acid-like 3B (SMPDL3B). Through bioinformatics analysis, potential target genes were identified, and their association with overall survival (OS) and progression-free survival (PFS) in ovarian cancer patients was assessed utilizing relevant databases. Subsequently, differences in target gene expression in ovarian cancer tissue samples were validated through protein blotting and quantitative real-time PCR (qRT-qPCR). Cell proliferation assays using the cell count kit-8 (CCK-8) method, as well as transwell chamber assay and pre coated matrix gel chamber assay were employed to elucidate the role of SMPDL3B in ovarian cancer cell migration and invasion.

Results

This study revealed a substantial upregulation of SMPDL3B in the serum of ovarian cancer patients, correlating with an unfavorable prognosis. High SMPDL3B expression was linked not only to increased proliferation of ovarian cancer cells, but also enhanced migration and invasion. Remarkably, the knockdown the human alkaline ceramidase 2 (ACER2) gene in cancer cells with heightened SMPDL3B expression significantly inhibited cell proliferation, migration, and invasion induced by SMPDL3B activation (P<0.05), highlighting the functional interplay between ACER2 and SMPDL3B in ovarian cancer.

Conclusions

In summary, this study proposes SMPDL3B as a prognostic marker for ovarian cancer, with implications for potential therapeutic intervention targeting the ACER2-SMPDL3B axis.

Molecular basis for the catalytic mechanism of human neutral sphingomyelinases 1 (hSMPD2)

Enzymatic breakdown of sphingomyelin by sphingomyelinase (SMase) is the main source of the membrane lipids, ceramides, which are involved in many cellular physiological processes. However, the full-length structure of human neutral SMase has not been resolved; therefore, its catalytic mechanism remains unknown. Here, we resolve the structure of human full-length neutral SMase, sphingomyelinase 1 (SMPD2), which reveals that C-terminal transmembrane helices contribute to dimeric architecture of hSMPD2 and that D111 - K116 loop domain is essential for substrate hydrolysis. Coupled with molecular docking, we clarify the binding pose of sphingomyelin, and site-directed mutagenesis further confirms key residues responsible for sphingomyelin binding. Hybrid quantum mechanics/molecular mechanics (QM/MM) molecular dynamic (MD) simulations are utilized to elaborate the catalysis of hSMPD2 with the reported in vitro substrates, sphingomyelin and lyso-platelet activating fator (lyso-PAF). Our study provides mechanistic details that enhance our knowledge of lipid metabolism and may lead to an improved understanding of ceramide in disease and in cancer treatment.

Proteomic, Metabolomic, and Lipidomic Analyses of Lung Tissue Exposed to Mustard Gas

Sulfur mustard (HD) poses a serious threat due to its relatively simple production process. Exposure to HD in the short-term causes an inflammatory response, while long-term exposure results in DNA and RNA damage. Respiratory tract tissue models were exposed to relatively low concentrations of HD and collected at 3 and 24 h post exposure. Histology, cytokine ELISAs, and mass spectrometric-based analyses were performed. Histology and ELISA data confirmed previously seen lung damage and inflammatory markers from HD exposure. The multi-omic mass spectrometry data showed variation in proteins and metabolites associated with increased inflammation, as well as DNA and RNA damage. HD exposure causes DNA and RNA damage that results in variation of proteins and metabolites that are associated with transcription, translation and cellular energy.

Sphingomyelin Phodiesterase Acid-Like 3A Promotes Hepatocellular Carcinoma Growth Through the Enhancer of Rudimentary Homolog

Background

Hepatocellular carcinoma (HCC) is one of the most common malignant tumors worldwide, with unclear pathogenesis. Sphingomyelin phodiesterase acid-like 3A (SMPDL3A) affects cell differentiation and participates in immune regulation. However, its molecular biological function in HCC has not yet been elucidated.

Methods

Data from 180 HCC patients were analyzed the relationship between the expression of SMPDL3A in liver cancer tissues and the prognosis of liver cancer patients. Crispr-Cas9 dual vector lentivirus was used to knock out SMPDL3A in HCC cell lines. The effects of SMPDL3A on cell viability were determined by CCK8 assay, clone formation experiment, cell cycle assay, cell scratch, TUNEL experiment and flow cytometry. Xenograft tumor assays in BALB/c nude mice confirmed that SMPDL3A promoted tumor growth and in vivo. Preliminary exploration of SMPDL3A interacting protein by mass spectrometry analysis and co-immunoprecipitation.

Results

This study showed that the expression of SMPDL3A in HCC tissue differed from that in tumor-adjacent tissues. Moreover, the overall survival rate and tumor-free survival rate of patients with high-SMPDL3A expression were significantly lower than those with low-SMPDL3A expression. SMPDL3A expression was closely related to the level of protein induced by PIVKA-II, liver cirrhosis, tumor diameter, microvascular invasion, and Barcelona clinic liver cancer staging. Thus, SMPDL3A is an independent risk factor that affects the tumor-free survival rate and overall survival rate of HCC patients. In vitro study using Crispr-Cas9 genome editing technology revealed the knockout effect of SMPDL3A on cell proliferation, apoptosis, and migration. Cell counting kit-8 assay and clone formation experiment showed that sgSMPDL3A inhibited tumor cell proliferation and migration. Flow cytometry and TUNEL assay showed that sgSMPDL3A promoted apoptosis in tumors. Moreover, sgSMPDL3A inhibited tumor growth during subcutaneous tumor formation in nude mice. Immunohistochemistry of Ki67 and PNCA also indicated that sgSMPDL3A inhibited subcutaneous tumor proliferation in tumor-bearing nude mice. Further experiments showed that SMPDL3A interacts with the enhancer of rudimentary homolog (ERH).

Conclusions

High-SMPDL3A expression was related to poor prognosis of patients with HCC. Knockout of SMPDL3A inhibited the proliferation and migration and accelerated the migration of HCC cells. SMPDL3A interacted with ERH to affect the tumorigenesis and progression of HCC.

SMPDL3B Predicts Poor Prognosis and Contributes to Development of Acute Myeloid Leukemia

Background: Acute myeloid leukemia (AML), characterized by the low cure rate and high relapse, urgently needs novel diagnostic or prognostic biomarkers and potential therapeutic targets. Sphingomyelin Phosphodiesterase Acid Like 3B (SMPDL3B) is a negative regulator of Toll-like receptor signaling that plays important roles in the interface of membrane biology and innate immunity. However, the potential role of SMPDL3B in human cancer, especially in AML, is still unknown.

Methods: The expression of SMPDL3B in AML samples was investigated through data collected from Gene Expression Omnibus (GEO). Association between SMPDL3B expression and clinicopathologic characteristics was analyzed with the chi-square test. Survival curves were calculated by the Kaplan–Meier method. Cox univariate and multivariate analyses were used to detect risk factors for overall survival. The biological functions of SMPDL3B in human AML were investigated both in vitro and in vivo.

Results: Expression of SMPDL3B mRNA was significantly upregulated in human AML samples and closely correlated to cytogenetics risk and karyotypes. Elevated expression of SMPDL3B was associated with poor overall survival and emerged as an independent predictor for poor overall survival in human AML. Blocked SMPDL3B expression inhibited AML cells growth both in vitro and in vivo via promoting cell apoptosis.

Conclusion: Taken together, our results demonstrate that SMPDL3B could be used as an efficient prognostic biomarker and represent a potential therapeutic target for human AML.

Podocyte Sphingolipid Signaling in Nephrotic Syndrome

Podocytes play a vital role in the pathogenesis of nephrotic syndrome (NS), which is clinically characterized by heavy proteinuria, hypoalbuminemia, hyperlipidemia, and peripheral edema. The pathogenesis of NS has evolved through several hypotheses ranging from immune dysregulation theory and increased glomerular permeability theory to the current concept of podocytopathy. Podocytopathy is characterized by dysfunction or depletion of podocytes, which may be caused by unknown permeability factor, genetic disorders, drugs, infections, systemic disorders, and hyperfiltration. Over the last two decades, numerous studies have been done to explore the molecular mechanisms of podocyte injuries or NS and to develop the novel therapeutic strategies targeting podocytopathy for treatment of NS. Recent studies have shown that normal sphingolipid metabolism is essential for structural and functional integrity of podocytes. As a basic component of the plasma membrane, sphingolipids not only support the assembly of signaling molecules and interaction of receptors and effectors, but also mediate various cellular activities, such as apoptosis, proliferation, stress responses, necrosis, inflammation, autophagy, senescence, and differentiation. This review briefly summarizes current evidence demonstrating the regulation of sphingolipid metabolism in podocytes and the canonical or noncanonical roles of podocyte sphingolipid signaling in the pathogenesis of NS and associated therapeutic strategies.

Intravital imaging of interactions between iNKT and kupffer cells to clear free lipids during steatohepatitis

Rationale: Invariant natural killer T (iNKT) cells and Kupffer cells represent major hepatic populations of innate immune cells. However, their roles in steatohepatitis remain poorly understood. To elucidate their functions in steatohepatitis development, real-time, in vivo analysis is necessary to understand the pathophysiological events in the dynamic interactions between them during diet-induced steatohepatitis.

Methods: We used a steatohepatitis animal model induced by a methionine-choline-deficient (MCD) diet. Multi-photon confocal live imaging and conventional experimental techniques were employed to investigate the hepatic pathological microenvironment of iNKT and Kupffer cells, interactions between them, and the biological effects of these interactions in steatohepatitis.

Results: We found that iNKT cells were recruited and aggregated into small clusters and interacted dynamically with Kupffer cells in the early stage of steatohepatitis. Most significantly, the iNKT cells in the cluster cleared free lipids released by necrotic hepatocytes and presented a non-classical activation state with high IFN-γ expression. Furthermore, the Kupffer cells in the cell cluster were polarized to type M1. The transcriptome sequencing of iNKT cells showed upregulation of genes related to phagocytosis and lipid processing. Adoptive transfer of iNKT cells to Jα18^-/- mice showed that iNKT and Kupffer cell clusters were essential for balancing the liver and peripheral lipid levels and inhibiting liver fibrosis development.

Conclusions: Our study identified an essential role for dynamic interactions between iNKT cells and Kupffer cells in promoting lipid phagocytosis and clearance by iNKT cells during early liver steatohepatitis. Therefore, modulating iNKT cells is a potential therapeutic strategy for early steatohepatitis.

Macrophage-derived thrombospondin 1 promotes obesity-associated non-alcoholic fatty liver disease

Background & Aims

Thrombospondin 1 (TSP1) is a multifunctional matricellular protein. We previously showed that TSP1 has an important role in obesity-associated metabolic complications, including inflammation, insulin resistance, cardiovascular, and renal disease. However, its contribution to obesity-associated non-alcoholic fatty liver disease/non-alcoholic steatohepatitis (NAFLD or NASH) remains largely unknown; thus, we aimed to determine its role.

Methods

High-fat diet or AMLN (amylin liver NASH) diet-induced obese and insulin-resistant NAFLD/NASH mouse models were utilised, in addition to tissue-specific Tsp1-knockout mice, to determine the contribution of different cellular sources of obesity-induced TSP1 to NAFLD/NASH development.

Results

Liver TSP1 levels were increased in experimental obese and insulin-resistant NAFLD/NASH mouse models as well as in obese patients with NASH. Moreover, TSP1 deletion in adipocytes did not protect mice from diet-induced NAFLD/NASH. However, myeloid/macrophage-specific TSP1 deletion protected mice against obesity-associated liver injury, accompanied by reduced liver inflammation and fibrosis. Importantly, this protection was independent of the levels of obesity and hepatic steatosis. Mechanistically, through an autocrine effect, macrophage-derived TSP1 suppressed Smpdl3b expression in liver, which amplified liver proinflammatory signalling (Toll-like receptor 4 signal pathway) and promoted NAFLD progression.

Conclusions

Macrophage-derived TSP1 is a significant contributor to obesity-associated NAFLD/NASH development and progression and could serve as a therapeutic target for this disease.

Lay summary

Obesity-associated non-alcoholic fatty liver disease is a most common chronic liver disease in the Western world and can progress to liver cirrhosis and cancer. No treatment is currently available for this disease. The present study reveals an important factor (macrophage-derived TSP1) that drives macrophage activation and non-alcoholic fatty liver disease development and progression and that could serve as a therapeutic target for non-alcoholic fatty liver disease/steatohepatitis.

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.

Phosphodiesterase SMPDL3B Gene Expression as Independent Outcome Prediction Marker in Localized Prostate Cancer

Current outcome prediction markers for localized prostate cancer (PCa) are insufficient. The impact of the lipid-modifying Sphingomyelin Phosphodiesterase Acid Like 3B (SMPDL3B) in PCa is unknown. Two cohorts of patients with PCa who underwent radical prostatectomy (n = 40, n = 56) and benign prostate hyperplasia (BPH) controls (n = 8, n = 11) were profiled for SMPDL3B expression with qRT-PCR. Publicly available PCa cohorts (Memorial Sloane Kettering Cancer Centre (MSKCC; n = 131, n = 29 controls) and The Cancer Genome Atlas (TCGA; n = 497, n = 53 controls)) served for validation. SMPDL3B's impact on proliferation and migration was analyzed in PC3 cells by siRNA knockdown. In both cohorts, a Gleason score and T stage independent significant overexpression of SMPDL3B was seen in PCa compared to BPH (p < 0.001 each). A lower expression of SMPDL3B was associated with a shorter overall survival (OS) (p = 0.005) in long term follow-up. A SMPDL3B overexpression in PCa tissue was confirmed in the validation cohorts (p < 0.001 each). In the TCGA patients with low SMPDL3B expression, biochemical recurrence-free survival (p = 0.011) and progression-free interval (p < 0.001) were shorter. Knockdown of SMPDL3B impaired PC3 cell migration but not proliferation (p = 0.0081). In summary, SMPLD3B is highly overexpressed in PCa tissue, is inversely associated with localized PCa prognosis, and impairs PCa cell migration.

Human alkaline ceramidase 2 promotes the growth, invasion, and migration of hepatocellular carcinoma cells via sphingomyelin phosphodiesterase acid-like 3B

Hepatocellular carcinoma (HCC) is the most common type of liver cancer. It has a poor prognosis because it is often diagnosed at the advanced stage when treatments are limited. In addition, HCC pathogenesis is not fully understood, and this has affected early diagnosis and treatment of this disease. Human alkaline ceramidase 2 (ACER2), a key enzyme that regulates hydrolysis of cellular ceramides, affects cancer cell survival, however its role in HCC has not been well characterized. Our results showed that ACER2 is overexpressed in HCC tissues and cell lines. In addition, high ACER2 protein expression was associated with tumor growth; ACER2 knockdown resulted in decreased cell growth and migration. Sphingomyelin phosphodiesterase acid‐like 3B (SMPDL3B) promoted HCC cell growth, invasion, and migration; SMPDL3B knockdown had a significant inhibitory effect on HCC tumor growth in vivo. Moreover, ACER2 positively regulated the protein level of SMPDL3B. Of note, ACER2/SMPDL3B promoted ceramide hydrolysis and S1P production. This axis induced HCC survival and could be blocked by inhibition of S1P formation. In conclusion, ACER2 promoted HCC cell survival and migration, possibly via SMPDL3B. Thus, inhibition of ACER2/SMPDL3B may be a novel therapeutic target for HCC treatment.

In vitro and bioinformatics mechanistic-based approach for cadmium carcinogenicity understanding

Cadmium is a toxic metal able to enter the cells through channels and transport pathways dedicated to essential ions, leading, among others, to the dysregulation of divalent ions homeostasis. Despite its recognized human carcinogenicity, the mechanisms are still under investigation. A powerful tool for mechanistic studies of carcinogenesis is the Cell Transformation Assay (CTA). We have isolated and characterized by whole genome microarray and bioinformatics analysis of differentially expressed genes (DEGs) cadmium-transformed cells from different foci (F1, F2, and F3) at the end of CTA (6 weeks). The systematic analysis of up- and down-regulated transcripts and the comparison of DEGs in transformed cells evidence different functional targets and the complex picture of cadmium-induced transformation. Only 34 in common DEGs are found in cells from all foci, and among these, only 4 genes are jointly up-regulated (Ccl2, Ccl5, IL6 and Spp1), all responsible for cytokines/chemokines coding. Most in common DEGs are down-regulated, suggesting that the switching-off of specific functions plays a major role in this process. In addition, the comparison of dysregulated pathways immediately after cadmium treatment with those in transformed cells provides a valuable means to the comprehension of the overall process.

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Cell transformation Assay and toxicogenomics are integrated to study cadmium carcinogenesis mechanisms

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Inflammatory response is the only common feature in Cd-transformed cells from all different foci

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Switching-off of specific functions plays a major role in Cd-induced carcinogenesis

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Comparison of triggering signals and deregulated pathways in transformed cells provides hints on cadmium mechanisms

Exploring the Therapeutic Landscape of Sphingomyelinases

Sphingosine, ceramide, sphingosine-1-phosphate, and other related sphingolipids have emerged as important bioactive molecules involved in a variety of key cellular processes such as cell growth, differentiation, apoptosis, exosome release, and inter- and intracellular cell communication, making the pathways of sphingolipid metabolism a key domain in maintaining cell homeostasis (Hannun and Obeid, Trends Biochem Sci 20:73-77, 1995; Hannun and Obeid, Nat Rev Mol Cell Biol 9:139-150, 2008; Kosaka et al., J Biol Chem 288:10849-10859, 2013). Various studies have determined that these pathways play a central role in regulating intracellular production of ceramide and the other bioactive sphingolipids and hence are an important component of signaling in various diseases such as cancer, diabetes, and neurodegenerative and cardiovascular diseases (Chaube et al., Biochim Biophys Acta 1821:313-323, 2012; Clarke et al., Adv Enzyme Regul 51:51-58, 2011b; Horres and Hannun, Neurochem Res 37:1137-1149, 2012). In this chapter, we discuss one of the major enzyme classes in producing ceramide, sphingomyelinases (SMases), from a biochemical and structural perspective with an emphasis on their applicability as therapeutic targets.

iTRAQ-Based Quantitative Proteomic Analysis of Digestive Juice across the First 48 Hours of the Fifth Instar in Silkworm Larvae

The silkworm is an oligophagous insect for which mulberry leaves are the sole diet. The nutrients needed for vital activities of the egg, pupal, and adult stages, and the proteins formed in the cocoon, are all derived from the larval stages. The silkworm feeds and grows quickly during the larval stages. In particular, the amount of leaf ingested and digested quickly increases from the ecdysis to the gluttonous stage in the fifth instar period. In this study, we used the iTRAQ proteomic technique to identify and analyze silkworm larval digestive juice proteins during this period. A total of 227 proteins were successfully identified. These were primarily serine protease activity, esterase activity, binding, and serine protease inhibitors, which were mainly involved in the digestion and overcoming the detrimental effects of mulberry leaves. Moreover, 30 genes of the identified proteins were expressed specifically in the midgut. Temporal proteomic analysis of digestive juice revealed developmental dynamic features related to molecular mechanisms of the principal functions of digesting, resisting pathogens, and overruling the inhibitory effects of mulberry leaves protease inhibitors (PIs) with a dynamic strategy, although overruling the inhibitory effects has not yet been confirmed by previous study. These findings will help address the potential functions of digestive juice in silkworm larvae.

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.

Sphingomyelin Breakdown in T Cells: Role of Membrane Compartmentalization in T Cell Signaling and Interference by a Pathogen

Sphingolipids are major components of cellular membranes, and at steady-state level, their metabolic fluxes are tightly controlled. On challenge by external signals, they undergo rapid turnover, which substantially affects the biophysical properties of membrane lipid and protein compartments and, consequently, signaling and morphodynamics. In T cells, external cues translate into formation of membrane microdomains where proximal signaling platforms essential for metabolic reprograming and cytoskeletal reorganization are organized. This review will focus on sphingomyelinases, which mediate sphingomyelin breakdown and ensuing ceramide release that have been implicated in T-cell viability and function. Acting at the sphingomyelin pool at the extrafacial or cytosolic leaflet of cellular membranes, acid and neutral sphingomyelinases organize ceramide-enriched membrane microdomains that regulate T-cell homeostatic activity and, upon stimulation, compartmentalize receptors, membrane proximal signaling complexes, and cytoskeletal dynamics as essential for initiating T-cell motility and interaction with endothelia and antigen-presenting cells. Prominent examples to be discussed in this review include death receptor family members, integrins, CD3, and CD28 and their associated signalosomes. Progress made with regard to experimental tools has greatly aided our understanding of the role of bioactive sphingolipids in T-cell biology at a molecular level and of targets explored by a model pathogen (measles virus) to specifically interfere with their physiological activity.

SMPDL3b modulates insulin receptor signaling in diabetic kidney disease

Sphingomyelin phosphodiesterase acid-like 3b (SMPDL3b) is a lipid raft enzyme that regulates plasma membrane (PM) fluidity. Here we report that SMPDL3b excess, as observed in podocytes in diabetic kidney disease (DKD), impairs insulin receptor isoform B-dependent pro-survival insulin signaling by interfering with insulin receptor isoforms binding to caveolin-1 in the PM. SMPDL3b excess affects the production of active sphingolipids resulting in decreased ceramide-1-phosphate (C1P) content as observed in human podocytes in vitro and in kidney cortexes of diabetic db/db mice in vivo. Podocyte-specific Smpdl3b deficiency in db/db mice is sufficient to restore kidney cortex C1P content and to protect from DKD. Exogenous administration of C1P restores IR signaling in vitro and prevents established DKD progression in vivo. Taken together, we identify SMPDL3b as a modulator of insulin signaling and demonstrate that supplementation with exogenous C1P may represent a lipid therapeutic strategy to treat diabetic complications such as DKD.

Sphingolipid signaling in renal fibrosis

Over the last decade, various sphingolipid subspecies have gained increasing attention as important signaling molecules that regulate a multitude of physiological and pathophysiological processes including inflammation and tissue remodeling. These mediators include ceramide, sphingosine 1-phosphate (S1P), the cerebroside glucosylceramide, lactosylceramide, and the gangliosides GM3 and Gb3. These lipids have been shown to accumulate in various chronic kidney diseases that typically end in renal fibrosis and ultimately renal failure. This review will summarize the effects and contributions of those enzymes that regulate the generation and interconversion of these lipids, notably the acid sphingomyelinase, the acid sphingomyelinase-like protein SMPDL3B, the sphingosine kinases, the S1P lyase, the glucosylceramide synthase, the GM3 synthase, and the α-galactosidase A, to renal fibrotic diseases. Strategies of manipulating these enzymes for therapeutic purposes and the impact of existing drugs on renal pathologies will be discussed.

N-glycosylation of human sphingomyelin phosphodiesterase acid-like 3A (SMPDL3A) is essential for stability, secretion and activity

Sphingomyelin phosphodiesterase acid-like 3A (SMPDL3A) is a recently identified phosphodiesterase, which is a secreted N-linked glycoprotein. SMPDL3A is highly homologous to acid sphingomyelinase (aSMase), but unlike aSMase cannot cleave sphingomyelin. Rather, SMPDL3A hydrolyzes nucleotide tri- and diphosphates and their derivatives. While recent structural studies have shed light on these unexpected substrate preferences, many other aspects of SMPDL3A biology, which may give insight into its function in vivo, remain obscure. Here, we investigate the roles of N-glycosylation in the expression, secretion and activity of human SMPDL3A, using inhibitors of N-glycosylation and site-directed mutagenesis, with either THP-1 macrophages or CHO cells expressing human SMPDL3A. Tunicamycin (TM) treatment resulted in expression of non-glycosylated SMPDL3A that was not secreted, and was largely degraded by the proteasome. Proteasomal inhibition restored levels of SMPDL3A in TM-treated cells, although this non-glycosylated protein lacked phosphodiesterase activity. Enzymatic deglycosylation of purified recombinant SMPDL3A also resulted in significant loss of phosphodiesterase activity. Site-directed mutagenesis of individual N-glycosylation sites in SMPDL3A identified glycosylation of Asn69 and Asn222 as affecting maturation of its N-glycans and secretion. Glycosylation of Asn356 in SMPDL3A, an N-linked site conserved throughout the aSMase-like family, was critical for protection against proteasomal degradation and preservation of enzymatic activity. We provide the first experimental evidence for a predicted 22 residue N-terminal signal peptide in SMPDL3A, which is essential for facilitating glycosylation and is removed from the mature protein secreted from CHO cells. In conclusion, site-specific N-glycosylation is essential for the intracellular stability, secretion and activity of human SMPDL3A.