Cloning and characterization of the mammalian brain-specific, Mg2+-dependent neutral sphingomyelinase

Ceramides are fuel gauges on the drive to cardiometabolic disease

Ceramides are signals of fatty acid excess that accumulate when a cell's energetic needs have been met and its nutrient storage has reached capacity. As these sphingolipids accrue, they alter the metabolism and survival of cells throughout the body including in the heart, liver, blood vessels, skeletal muscle, brain, and kidney. These ceramide actions elicit the tissue dysfunction that underlies cardiometabolic diseases such as diabetes, coronary artery disease, metabolic-associated steatohepatitis, and heart failure. Here, we review the biosynthesis and degradation pathways that maintain ceramide levels in normal physiology and discuss how the loss of ceramide homeostasis drives cardiometabolic pathologies. We highlight signaling nodes that sense small changes in ceramides and in turn reprogram cellular metabolism and stimulate apoptosis. Finally, we evaluate the emerging therapeutic utility of these unique lipids as biomarkers that forecast disease risk and as targets of ceramide-lowering interventions that ameliorate disease.

Practical Three-Component Regioselective Synthesis of Drug-Like 3-Aryl(or heteroaryl)-5,6-dihydrobenzo[h]cinnolines as Potential Non-Covalent Multi-Targeting Inhibitors To Combat Neurodegenerative Diseases

Neurodegenerative diseases (NDs) are one of the prominent health challenges facing contemporary society, and many efforts have been made to overcome and (or) control it. In this research paper, we described a practical one-pot two-step three-component reaction between 3,4-dihydronaphthalen-1(2H)-one (1), aryl(or heteroaryl)glyoxal monohydrates (2a-h), and hydrazine monohydrate (NH2NH2•H2O) for the regioselective preparation of some 3-aryl(or heteroaryl)-5,6-dihydrobenzo[h]cinnoline derivatives (3a-h). After synthesis and characterization of the mentioned cinnolines (3a-h), the in silico multi-targeting inhibitory properties of these heterocyclic scaffolds have been investigated upon various Homo sapiens-type enzymes, including hMAO-A, hMAO-B, hAChE, hBChE, hBACE-1, hBACE-2, hNQO-1, hNQO-2, hnNOS, hiNOS, hPARP-1, hPARP-2, hLRRK-2(G2019S), hGSK-3β, hp38α MAPK, hJNK-3, hOGA, hNMDA receptor, hnSMase-2, hIDO-1, hCOMT, hLIMK-1, hLIMK-2, hRIPK-1, hUCH-L1, hPARK-7, and hDHODH, which have confirmed their functions and roles in the neurodegenerative diseases (NDs), based on molecular docking studies, and the obtained results were compared with a wide range of approved drugs and well-known (with IC50, EC50, etc.) compounds. In addition, in silico ADMET prediction analysis was performed to examine the prospective drug properties of the synthesized heterocyclic compounds (3a-h). The obtained results from the molecular docking studies and ADMET-related data demonstrated that these series of 3-aryl(or heteroaryl)-5,6-dihydrobenzo[h]cinnolines (3a-h), especially hit ones, can really be turned into the potent core of new drugs for the treatment of neurodegenerative diseases (NDs), and/or due to the having some reactionable locations, they are able to have further organic reactions (such as cross-coupling reactions), and expansion of these compounds (for example, with using other types of aryl(or heteroaryl)glyoxal monohydrates) makes a new avenue for designing novel and efficient drugs for this purpose.

The Role of Neutral Sphingomyelinase-2 (NSM2) in the Control of Neutral Lipid Storage in T Cells

The accumulation of lipid droplets (LDs) and ceramides (Cer) is linked to non-alcoholic fatty liver disease (NAFLD), regularly co-existing with type 2 diabetes and decreased immune function. Chronic inflammation and increased disease severity in viral infections are the hallmarks of the obesity-related immunopathology. The upregulation of neutral sphingomyelinase-2 (NSM2) has shown to be associated with the pathology of obesity in tissues. Nevertheless, the role of sphingolipids and specifically of NSM2 in the regulation of immune cell response to a fatty acid (FA) rich environment is poorly studied. Here, we identified the presence of the LD marker protein perilipin 3 (PLIN3) in the intracellular nano-environment of NSM2 using the ascorbate peroxidase APEX2-catalyzed proximity-dependent biotin labeling method. In line with this, super-resolution structured illumination microscopy (SIM) shows NSM2 and PLIN3 co-localization in LD organelles in the presence of increased extracellular concentrations of oleic acid (OA). Furthermore, the association of enzymatically active NSM2 with isolated LDs correlates with increased Cer levels in these lipid storage organelles. NSM2 enzymatic activity is not required for NSM2 association with LDs, but negatively affects the LD numbers and cellular accumulation of long-chain unsaturated triacylglycerol (TAG) species. Concurrently, NSM2 expression promotes mitochondrial respiration and fatty acid oxidation (FAO) in response to increased OA levels, thereby shifting cells to a high energetic state. Importantly, endogenous NSM2 activity is crucial for primary human CD4+ T cell survival and proliferation in a FA rich environment. To conclude, our study shows a novel NSM2 intracellular localization to LDs and the role of enzymatically active NSM2 in metabolic response to enhanced FA concentrations in T cells.

SMPD3 expression is spatially regulated in the developing embryo by SOXE factors

During epithelial-to-mesenchymal transition (EMT), significant rearrangements occur in plasma membrane protein and lipid content that are important for membrane function and acquisition of cell motility. To gain insight into how neural crest cells regulate their lipid content at the transcriptional level during EMT, here we identify critical enhancer sequences that regulate the expression of SMPD3, a gene responsible for sphingomyelin hydrolysis to produce ceramide and necessary for neural crest EMT. We uncovered three enhancer regions within the first intron of the SMPD3 locus that drive reporter expression in distinct spatial and temporal domains, together collectively recapitulating the expression domains of endogenous SMPD3 within the ectodermal lineages. We further dissected one enhancer that is specifically active in the migrating neural crest. By mutating putative transcriptional input sites or knocking down upstream regulators, we find that the SOXE-family transcription factors SOX9 and SOX10 regulate the expression of SMPD3 in migrating neural crest cells. Further, ChIP-seq and nascent transcription analysis reveal that SOX10 directly regulates expression of an SMPD3 enhancer specific to migratory neural crest cells. Together these results shed light on how core components of developmental gene regulatory networks interact with metabolic effector genes to control changes in membrane lipid content.

Two novel cases of biallelic SMPD4 variants with brain structural abnormalities

Sphingomyelin phosphodiesterase 4 (SMPD4) encodes a member of the Mg2+-dependent, neutral sphingomyelinase family that catalyzes the hydrolysis of the phosphodiester bond of sphingomyelin to form phosphorylcholine and ceramide. Recent studies have revealed that biallelic loss-of-function variants of SMPD4 cause syndromic neurodevelopmental disorders characterized by microcephaly, congenital arthrogryposis, and structural brain anomalies. In this study, three novel loss-of-function SMPD4 variants were identified using exome sequencing (ES) in two independent patients with developmental delays, microcephaly, seizures, and brain structural abnormalities. Patient 1 had a homozygous c.740_741del, p.(Val247Glufs*21) variant and showed profound intellectual disability, hepatomegaly, a simplified gyral pattern, and a thin corpus callosum without congenital dysmorphic features. Patient 2 had a compound heterozygous nonsense c.2124_2125del, p.(Phe709*) variant and splice site c.1188+2dup variant. RNA analysis revealed that the c.1188+2dup variant caused exon 13 skipping, leading to a frameshift (p.Ala406Ser*6). In vitro transcription analysis using minigene system suggested that mRNA transcribed from mutant allele may be degraded by nonsense-mediated mRNA decay system. He exhibited diverse manifestations, including growth defects, muscle hypotonia, respiratory distress, arthrogryposis, insulin-dependent diabetes mellitus, sensorineural hearing loss, facial dysmorphism, and various brain abnormalities, including cerebral atrophy, hypomyelination, and cerebellar hypoplasia. Here, we review previous literatures and discuss the phenotypic diversity of SMPD4-related disorders.

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.

Extracellular vesicles modulate key signalling pathways in refractory wound healing

Chronic wounds are wounds that cannot heal properly due to various factors, such as underlying diseases, infection or reinjury, and improper healing of skin wounds and ulcers can cause a serious economic burden. Numerous studies have shown that extracellular vesicles (EVs) derived from stem/progenitor cells promote wound healing, reduce scar formation and have significant advantages over traditional treatment methods. EVs are membranous particles that carry various bioactive molecules from their cellular origins, such as cytokines, nucleic acids, enzymes, lipids and proteins. EVs can mediate cell-to-cell communication and modulate various physiological processes, such as cell differentiation, angiogenesis, immune response and tissue remodelling. In this review, we summarize the recent advances in EV-based wound healing, focusing on the signalling pathways that are regulated by EVs and their cargos. We discuss how EVs derived from different types of stem/progenitor cells can promote wound healing and reduce scar formation by modulating the Wnt/β-catenin, phosphoinositide 3-kinase/protein kinase B/mammalian target of rapamycin, vascular endothelial growth factor, transforming growth factor β and JAK-STAT pathways. Moreover, we also highlight the challenges and opportunities for engineering or modifying EVs to enhance their efficacy and specificity for wound healing.

Neutral sphingomyelinase 2 inhibitors based on the pyrazolo[1,5-a]pyrimidin-3-amine scaffold

Neutral sphingomyelinase 2 (nSMase2) has gained increasing attention as a therapeutic target to regulate ceramide production in various disease conditions. Phenyl (R)-(1-(3-(3,4-dimethoxyphenyl)-2,6-dimethylimidazo[1,2-b]pyridazin-8-yl)-pyrrolidin-3-yl)carbamate (PDDC) is a submicromolar nSMase2 inhibitor and has been widely used to study the pharmacological effects of nSMase2 inhibition. Through screening of compounds containing a bicyclic 5-6 fused ring, larotrectinib containing a pyrazolo[1,5-a]pyrimidine ring was identified as a low micromolar inhibitor of nSMase2. This prompted us to investigate the pyrazolo[1,5-a]pyrimidin-3-amine ring as a novel scaffold to replace the imidazo[1,2-b]pyridazine-8-amine ring of PDDC. A series of molecules containing a pyrazolo[1,5-a]pyrimidin-3-amine ring were synthesized and tested for their ability to inhibit human nSMase2. Several compounds exhibited nSMase2 inhibitory potency superior to that of PDDC. Among these, N,N-dimethyl-5-morpholinopyrazolo[1,5-a]pyrimidin-3-amine (11j) was found to be metabolically stable in liver microsomes and orally available with a favorable brain-to-plasma ratio, demonstrating the potential of pyrazolo[1,5-a]pyrimidine ring as an effective scaffold for nSMase2 inhibition.

Regulated translocation of neutral sphingomyelinase-2 to the plasma membrane drives insulin resistance in steatotic hepatocytes

Obesity-associated diabetes is linked to the accumulation of ceramide in various organs, including the liver. The exact mechanisms by which ceramide contributes to diabetic pathology are unclear, but one proposed scenario is that ceramide accumulation may inhibit insulin signaling pathways. It is unknown however whether the excess ceramide is generated proximal to the insulin receptor, that is, at the plasma membrane (PM), where it could affect the insulin signaling pathway directly, or the onset of insulin resistance is due to ceramide-induced mitochondrial dysfunction and/or lipotoxicity. Using hepatic cell lines and primary cultures, gain- and loss- of function approach, and state-of-the art lipid imaging, this study shows that PM-associated neutral sphingomyelinase 2 (nSMase2) regulates ceramide homeostasis in fat-loaded hepatocytes and drives the onset of insulin resistance. Our results provide evidence of a regulated translocation of nSMase2 to the PM which leads to local generation of ceramide and insulin resistance in cells treated with palmitic acid (PAL), a type of fat commonly found in diabetogenic diets. Oleic acid, which also causes accumulation of lipid droplets, does not induce nSMase2 translocation and insulin resistance. Experiments using the acyl-biotin exchange method to quantify protein palmitoylation show that cellular PAL abundance regulates the rate of nSMase2 palmitoylation. Furthermore, while inhibition of nSMase2 with GW4869 prevents PAL-induced insulin resistance, the overexpression of wild type nSMase2 but not palmitoylation-defective mutant protein potentiates the suppressive effect of PAL on insulin signaling. Overall, this study identifies nSMase2 as a novel component of the mechanism of insulin resistance onset in fat-loaded hepatocytes, that is, cell-autonomous and driven by PAL.

Tumor Cells Transmit Drug Resistance via Cisplatin-Induced Extracellular Vesicles

Cisplatin is a first-line clinical agent used for treating solid tumors. Cisplatin damages the DNA of tumor cells and induces the production of high levels of reactive oxygen species to achieve tumor killing. Tumor cells have evolved several ways to tolerate this damage. Extracellular vesicles (EVs) are an important mode of information transfer in tumor cells. EVs can be substantially activated under cisplatin treatment and mediate different responses of tumor cells under cisplatin treatment depending on their different cargoes. However, the mechanism of action of tumor-cell-derived EVs under cisplatin treatment and their potential cargoes are still unclear. This review considers recent advances in cisplatin-induced release of EVs from tumor cells, with the expectation of providing a new understanding of the mechanisms of cisplatin treatment and drug resistance, as well as strategies for the combined use of cisplatin and other drugs.

Hepatic deletion of serine palmitoyl transferase 2 impairs ceramide/sphingomyelin balance, bile acids homeostasis and leads to liver damage in mice

Ceramides (Cer) have been shown as lipotoxic inducers, which disturb numerous cell-signaling pathways, leading to metabolic disorders such as type 2 diabetes. In this study, we aimed to determine the role of de novo hepatic ceramide synthesis in energy and liver homeostasis in mice. We generated mice lacking serine palmitoyltransferase 2 (Sptlc2), the rate limiting enzyme of ceramide de novo synthesis, in liver under albumin promoter. Liver function, glucose homeostasis, bile acid (BA) metabolism and hepatic sphingolipids content were assessed using metabolic tests and LC-MS. Despite lower expression of hepatic Sptlc2, we observed an increased concentration of hepatic Cer, associated with a 10-fold increase in neutral sphingomyelinase 2 (nSMase2) expression, and a decreased sphingomyelin content in the liver. Sptlc2ΔLiv mice were protected against obesity induced by high fat diet and displayed a defect in lipid absorption. In addition, an important increase in tauro-muricholic acid was associated with a downregulation of the nuclear BA receptor FXR target genes. Sptlc2 deficiency also enhanced glucose tolerance and attenuated hepatic glucose production, while the latter effect was dampened in presence of nSMase2 inhibitor. Finally, Sptlc2 disruption promoted apoptosis, inflammation and progressive development of hepatic fibrosis, worsening with age. Our data suggest a compensatory mechanism to regulate hepatic ceramides content from sphingomyelin hydrolysis, with deleterious impact on liver homeostasis. In addition, our results show the involvement of hepatic sphingolipid modulation in BA metabolism and hepatic glucose production in an insulin-independent manner, which highlight the still under-researched role of ceramides in many metabolic functions.

Inhibition of neutral sphingomyelinase 2 impairs HIV-1 envelope formation and substantially delays or eliminates viral rebound

Although HIV-1 Gag is known to drive viral assembly and budding, the precise mechanisms by which the lipid composition of the plasma membrane is remodeled during assembly are incompletely understood. Here, we provide evidence that the sphingomyelin hydrolase neutral sphingomyelinase 2 (nSMase2) interacts with HIV-1 Gag and through the hydrolysis of sphingomyelin creates ceramide that is necessary for proper formation of the viral envelope and viral maturation. Inhibition or depletion of nSMase2 resulted in the production of noninfectious HIV-1 virions with incomplete Gag lattices lacking condensed conical cores. Inhibition of nSMase2 in HIV-1-infected humanized mouse models with a potent and selective inhibitor of nSMase2 termed PDDC [phenyl(R)-(1-(3-(3,4-dimethoxyphenyl)-2, 6-dimethylimidazo[1,2-b]pyridazin-8-yl) pyrrolidin-3-yl)-carbamate] produced a linear reduction in levels of HIV-1 in plasma. If undetectable plasma levels of HIV-1 were achieved with PDDC treatment, viral rebound did not occur for up to 4 wk when PDDC was discontinued. In vivo and tissue culture results suggest that PDDC selectively kills cells with actively replicating HIV-1. Collectively, this work demonstrates that nSMase2 is a critical regulator of HIV-1 replication and suggests that nSMase2 could be an important therapeutic target with the potential to kill HIV-1-infected cells.

Neuronal deletion of nSMase2 reduces the production of Aβ and directly protects neurons

Extracellular vesicles (EVs) have been proposed to regulate the deposition of Aβ. Multiple publications have shown that APP, amyloid processing enzymes and Aβ peptides are associated with EVs. However, very little Aβ is associated with EVs compared with the total amount Aβ present in human plasma, CSF, or supernatants from cultured neurons. The involvement of EVs has largely been inferred by pharmacological inhibition or whole body deletion of the sphingomyelin hydrolase neutral sphingomyelinase-2 (nSMase2) that is a key regulator for the biogenesis of at-least one population of EVs. Here we used a Cre-Lox system to selectively delete nSMase2 from pyramidal neurons in APP/PS1 mice (APP/PS1-SMPD3-Nex1) and found a ∼ 70% reduction in Aβ deposition at 6 months of age and ∼ 35% reduction at 12 months of age in both cortex and hippocampus. Brain ceramides were increased in APP/PS1 compared with Wt mice, but were similar to Wt in APP/PS1-SMPD3-Nex1 mice suggesting that elevated brain ceramides in this model involves neuronally expressed nSMase2. Reduced levels of PSD95 and deficits of long-term potentiation in APP/PS1 mice were normalized in APP/PS1-SMPD3-Nex1 mice. In contrast, elevated levels of IL-1β, IL-8 and TNFα in APP/PS1 mice were not normalized in APP/PS1-SMPD3-Nex1 mice compared with APP/PS1 mice. Mechanistic studies showed that the size of liquid ordered membrane microdomains was increased in APP/PS1 mice, as were the amounts of APP and BACE1 localized to these microdomains. Pharmacological inhibition of nSMase2 activity with PDDC reduced the size of the liquid ordered membrane microdomains, reduced the localization of APP with BACE1 and reduced the production of Aβ_1-40 and Aβ_1-42. Although inhibition of nSMase2 reduced the release and increased the size of EVs, very little Aβ was associated with EVs in all conditions tested. We also found that nSMase2 directly protected neurons from the toxic effects of oligomerized Aβ and preserved neural network connectivity despite considerable Aβ deposition. These data demonstrate that nSMase2 plays a role in the production of Aβ by stabilizing the interaction of APP with BACE1 in liquid ordered membrane microdomains, and directly protects neurons from the toxic effects of Aβ. The effects of inhibiting nSMase2 on EV biogenesis may be independent from effects on Aβ production and neuronal protection.

Characterization of a Neutral Sphingomyelinase Activity in Human Serum and Plasma

Alterations of sphingolipids and their metabolizing enzymes play a role in various diseases. However, peripheral biomarkers for such changes are limited. Particularly, in the increasingly reported involvement of neutral sphingomyelinase (NSM) with four described isoforms in tissues or cells, a peripheral marker is lacking. We here describe the detection of an NSM activity in human serum and plasma samples which hydrolyses fluorescently labeled sphingomyelin to ceramide in a time- and volume-dependent manner. Reaction rates were linear up to 10 days, and serum volumes above 2 vol-% were inhibitory. Biochemical properties were different from acid sphingomyelinase (ASM) with respect to detergent specificity (sodium deoxycholate), pH profile (pH 7-9), and cation dependence: Serum NSM activity was inhibited by EDTA ≥ 1 µM and restored in EDTA-anticoagulated plasma with the addition of ≥ 100 µM Co²⁺. It was independent of Mg²⁺, the typical cofactor of cellular NSM species, and even inhibited by [Mg²⁺] ≥ 20 mM. Serum NSM activity was not correlated with ASM activity and was independent of sex and age in 24 healthy adults. Since human peripheral NSM activity is very low and activities in rodents are even lower or undetectable, future research should aim to increase the reaction rate and determine the source of this enzymatic activity. The established activity could serve as a future biomarker or therapeutic target in diseases affected by sphingolipid derangements.

Cytotoxic and Apoptotic Impacts of Ceranib-2 on RAW 264.7 Macrophage Cells

BACKGROUND

Many ceramidase inhibitors have been developed and identified as potential treatment agents for various types of tumors in the last several decades. In recent years, their therapeutic potential against tumors has gained great attention. Inhibition of ceramidase is r eportedly related to apoptosis and cytotoxicity in macrophages, which are closely related to tumor development and progression. However, whether and how ceranib-2, a novel ceramidase inhibitor, can exert its cytotoxic and apoptotic effects on RAW 264.7, a macrophage cell line established from a tumor in a male mouse induced with the Abelson murine leukemia virus, remains unknown.

OBJECTIVE

In this study, we aimed to investigate whether and how ceranib-2 can exert cytotoxic, antiproliferative, and apoptotic effects on the RAW264.7 macrophages.

METHODS

We performed the MTT assay, Annexin V staining assay, and confocal microscopy to detect the cytotoxicity, apoptosis, and morphological changes, respectively, in the RAW264.7 cells.

RESULTS

The viability of RAW264.7 cells treated with ceranib-2 was decreased as the doses of ceranib-2 increased at 24 h and 48 h due to apoptosis resulting from ceranib-2-reduced integrity of the mitochondrial membrane. Moreover, morphological changes were observed in these ceranib-2 exposed cells, further indicating the role of ceranib-2 in inducing apoptosis in these cells.

CONCLUSION

Ceranib-2 is cytotoxic to RAW 264.7 macrophages and can induce apoptosis in these cells.

Discovery of Orally Bioavailable and Brain-Penetrable Prodrugs of the Potent nSMase2 Inhibitor DPTIP

Extracellular vesicles (EVs) can carry pathological cargo and play an active role in disease progression. Neutral sphingomyelinase-2 (nSMase2) is a critical regulator of EV biogenesis, and its inhibition has shown protective effects in multiple disease states. 2,6-Dimethoxy-4-(5-phenyl-4-thiophen-2-yl-1H-imidazol-2-yl)phenol (DPTIP) is one of the most potent (IC₅₀ = 30 nM) inhibitors of nSMase2 discovered to date. However, DPTIP exhibits poor oral pharmacokinetics (PK), limiting its clinical development. To overcome DPTIP's PK limitations, we synthesized a series of prodrugs by masking its phenolic hydroxyl group. When administered orally, the best prodrug (P18) with a 2',6'-diethyl-1,4'-bipiperidinyl promoiety exhibited >fourfold higher plasma (AUC_0-t = 1047 pmol·h/mL) and brain exposures (AUC_0-t = 247 pmol·h/g) versus DPTIP and a significant enhancement of DPTIP half-life (2 h vs ∼0.5 h). In a mouse model of acute brain injury, DPTIP released from P18 significantly inhibited IL-1β-induced EV release into plasma and attenuated nSMase2 activity. These studies report the discovery of a DPTIP prodrug with potential for clinical translation.

Targeting the Sphingolipid Rheostat in Gliomas

Gliomas are highly aggressive cancer types that are in urgent need of novel drugs and targeted therapies. Treatment protocols have not improved in over a decade, and glioma patient survival remains among the worst of all cancer types. As a result, cancer metabolism research has served as an innovative approach to identifying novel glioma targets and improving our understanding of brain tumors. Recent research has uncovered a unique metabolic vulnerability in the sphingolipid pathways of gliomas that possess the IDH1 mutation. Sphingolipids are a family of lipid signaling molecules that play a variety of second messenger functions in cellular regulation. The two primary metabolites, sphingosine-1-phosphate (S1P) and ceramide, maintain a rheostat balance and play opposing roles in cell survival and proliferation. Altering the rheostat such that the pro-apoptotic signaling of the ceramides outweighs the pro-survival S1P signaling in glioma cells diminishes the hallmarks of cancer and enhances tumor cell death. Throughout this review, we discuss the sphingolipid pathway and identify the enzymes that can be most effectively targeted to alter the sphingolipid rheostat and enhance apoptosis in gliomas. We discuss each pathway’s steps based on their site of occurrence in the organelles and postulate novel targets that can effectively exploit this vulnerability.

Vascular Smooth Muscle Cell Neutral Sphingomyelinase 2 in the Release of Exosomes and Vascular Calcification

Vascular calcification (VC) is the pathological precipitation of calcium salts in the walls of blood vessels. It is a risk factor for cardiovascular events and their associated mortality. VC can be observed in a variety of cardiovascular diseases and is most prominent in diseases that are associated with dysregulated mineral homeostasis such as in chronic kidney disease. Local factors and mechanisms underlying VC are still incompletely understood, but it is appreciated that VC is a multifactorial process in which vascular smooth muscle cells (VSMCs) play an important role. VSMCs participate in VC by releasing extracellular vesicles (EVs), the extent, composition, and propensity to calcify of which depend on VSMC phenotype and microenvironment. Currently, no targeted therapy is available to treat VC. In-depth knowledge of molecular players of EV release and the understanding of their mechanisms constitute a vital foundation for the design of pharmacological treatments to combat VC effectively. This review highlights our current knowledge of VSMCs in VC and focuses on the biogenesis of exosomes and the role of the neutral Sphingomyelinase 2 (nSMase2).

Therapeutically harnessing extracellular vesicles

The field of extracellular vesicle (EV) research has developed rapidly over the last decade from the study of fundamental biology to a subject of significant clinical relevance. The potential of harnessing EVs in the diagnosis and treatment of diseases - including cancer and neurological and cardiovascular disorders - is now being recognized. Accordingly, the applications of EVs as therapeutic targets, biomarkers, novel drug delivery agents and standalone therapeutics are being actively explored. This Review provides a brief overview of the characteristics and physiological functions of the various classes of EV, focusing on their association with disease and emerging strategies for their therapeutic exploitation.

Proximity Ligation Mapping of Microcephaly Associated SMPD4 Shows Association with Components of the Nuclear Pore Membrane

SMPD4 is a neutral sphingomyelinase implicated in a specific type of congenital microcephaly. Although not intensively studied, SMPD4 deficiency has also been found to cause cell division defects. This suggests a role for SMPD4 in cell-cycle and differentiation. In order to explore this role, we used proximity ligation to identify the partners of SMPD4 in vivo in HEK293T cells. We found that these partners localize near the endoplasmic reticulum (ER) and the nuclear membrane. Using mass spectrometry, we could identify these partners and discovered that SMPD4 is closely associated with several nucleoporins, including NUP35, a nucleoporin directly involved in pore membrane curvature and pore insertion. This suggests that SMPD4 may play a role in this process.

Neutral Sphingomyelinase 2 controls exosomes secretion via counteracting V-ATPase-mediated endosome acidification

During endosome maturation, neutral sphingomyelinase 2 (nSMase2, encoded by SMPD3) is involved in budding of intraluminal vesicles (ILVs) into late endosomes or multivesicular bodies (MVBs). Fusion of these with the plasma membrane results in secretion of exosomes or small extracellular vesicles (sEVs). Here, we report that nSMase2 activity controls sEV secretion through modulation of vacuolar H⁺-ATPase (V-ATPase) activity. Specifically, we show that nSMase2 inhibition induces V-ATPase complex assembly that drives MVB lumen acidification and consequently reduces sEV secretion. Conversely, we further demonstrate that stimulating nSMase2 activity with the inflammatory cytokine TNFα (also known as TNF) decreases acidification and increases sEV secretion. Thus, we find that nSMase2 activity affects MVB membrane lipid composition to counteract V-ATPase-mediated endosome acidification, thereby shifting MVB fate towards sEV secretion.

This article has an associated First Person interview with the first author of the paper.

Summary: Changing neutral sphingomyelinase 2 activity regulates small extracellular vesicle secretion through modulation of V-ATPase activity.

Ceramide Transfer Protein (CERT): An Overlooked Molecular Player in Cancer

Sphingolipids are a class of essential lipids implicated in constructing cellular membranes and regulating nearly all cellular functions. Sphingolipid metabolic network is centered with the ceramide-sphingomyelin axis. Ceramide is well-recognized as a pro-apoptotic signal; while sphingomyelin, as the most abundant type of sphingolipids, is required for cell growth. Therefore, the balance between these two sphingolipids can be critical for cancer cell survival and functioning. Ceramide transfer protein (CERT) dictates the ratio of ceramide to sphingomyelin within the cell. It is the only lipid transfer protein that specifically delivers ceramide from the endoplasmic reticulum to the Golgi apparatus, where ceramide serves as the substrate for sphingomyelin synthesis. In the past two decades, an increasing body of evidence has suggested a critical role of CERT in cancer, but much more intensive efforts are required to draw a definite conclusion. Herein, we review all research findings of CERT, focusing on its molecular structure, cellular functions and implications in cancer. This comprehensive review of CERT will help to better understand the molecular mechanism of cancer and inspire to identify novel druggable targets.

A Comprehensive Review on the Interplay between Neisseria spp. and Host Sphingolipid Metabolites

Sphingolipids represent a class of structural related lipids involved in membrane biology and various cellular processes including cell growth, apoptosis, inflammation and migration. Over the past decade, sphingolipids have become the focus of intensive studies regarding their involvement in infectious diseases. Pathogens can manipulate the sphingolipid metabolism resulting in cell membrane reorganization and receptor recruitment to facilitate their entry. They may recruit specific host sphingolipid metabolites to establish a favorable niche for intracellular survival and proliferation. In contrast, some sphingolipid metabolites can also act as a first line defense against bacteria based on their antimicrobial activity. In this review, we will focus on the strategies employed by pathogenic Neisseria spp. to modulate the sphingolipid metabolism and hijack the sphingolipid balance in the host to promote cellular colonization, invasion and intracellular survival. Novel techniques and innovative approaches will be highlighted that allow imaging of sphingolipid derivatives in the host cell as well as in the pathogen.

[The role of sphingolipids in pathogenesis of amyotrophic lateral sclerosis]

Amyotrophic lateral sclerosis (ALS) is an incurable neurodegenerative disease characterized by selective degeneration of motor neurons of the spinal cord and motor cortex and brain stem. The key features of the course of this disease are excitotoxicity, oxidative stress, mitochondrial dysfunction, neuro-inflammatory and immune reactions. Recently, the mechanisms of programmed cell death (apoptosis), which may be responsible for the degeneration of motor neurons in this disease, have been intensively studied. In this regard, sphingolipids, which are the most important sources of secondary messengers that transmit cell proliferation, differentiation and apoptosis signals, and are involved in the development of neuroinflammatory and immune responses, are of particular interest in the context of ALS pathogenesis. The review provides information from domestic and foreign authors on the involvement of various sphingolipids (sphingomyelins, ceramides, sphingosine, sphinganin, sphingosine-1-phosphate, galactosylceramides, glucosylceramides, gangliosides) in the development of pro-inflammatory reactions and apoptosis of motor neurons in ALS. The authors discuss the prospects of using new drugs that control the metabolism of sphingolipids for the treatment of ALS.

Role of ceramide/sphingomyelin (SM) balance regulated through "SM cycle" in cancer

Sphingomyelin synthase (SMS), which comprises of two isozymes, SMS1 and SMS2, is the only enzyme that generates sphingomyelin (SM) by transferring phosphocholine of phosphatidylcholine to ceramide in mammals. Conversely, ceramide is generated from SM hydrolysis via sphingomyelinases (SMases), ceramide de novo synthesis, and the salvage pathway. The biosynthetic pathway for SM and ceramide content by SMS and SMase, respectively, is called "SM cycle." SM forms a SM-rich microdomain on the cell membrane to regulate signal transduction, such as proliferation/survival, migration, and inflammation. On the other hand, ceramide acts as a lipid mediator by forming a ceramide-rich platform on the membrane, and ceramide exhibits physiological actions such as cell death, cell cycle arrest, and autophagy induction. Therefore, the regulation of ceramide/SM balance by SMS and SMase is responsible for diverse cell functions not only in physiological cells but also in cancer cells. This review outlines the implications of ceramide/SM balance through "SM cycle" in cancer progression and prevention. In addition, the possible involvement of "SM cycle" is introduced in anti-cancer tumor immunity, which has become a hot topic to innovate a more effective and safer way to conquer cancer in recent years.

Neutral sphingomyelinase-2 and cardiometabolic diseases

Sphingolipids, in particular ceramides, play vital role in pathophysiological processes linked to metabolic syndrome, with implications in the development of insulin resistance, pancreatic ß-cell dysfunction, type 2 diabetes, atherosclerosis, inflammation, nonalcoholic steatohepatitis, and cancer. Ceramides are produced by the hydrolysis of sphingomyelin, catalyzed by different sphingomyelinases, including neutral sphingomyelinase 2 (nSMase2), whose dysregulation appears to underlie many of the inflammation-related pathologies. In this review, we discuss the current knowledge on the biochemistry of nSMase2 and ceramide production and its regulation by inflammatory cytokines, with particular reference to cardiometabolic diseases. nSMase2 contribution to pathogenic processes appears to involve cyclical feed-forward interaction with proinflammatory cytokines, such as TNF-α and IL-1ß, which activate nSMase2 and the production of ceramides, that in turn triggers the synthesis and release of inflammatory cytokines. We elaborate these pathogenic interactions at the molecular level and discuss the potential therapeutic benefits of inhibiting nSMase2 against inflammation-driven cardiometabolic diseases.

TMEM135 is a Novel Regulator of Mitochondrial Dynamics and Physiology with Implications for Human Health Conditions

Transmembrane proteins (TMEMs) are integral proteins that span biological membranes. TMEMs function as cellular membrane gates by modifying their conformation to control the influx and efflux of signals and molecules. TMEMs also reside in and interact with the membranes of various intracellular organelles. Despite much knowledge about the biological importance of TMEMs, their role in metabolic regulation is poorly understood. This review highlights the role of a single TMEM, transmembrane protein 135 (TMEM135). TMEM135 is thought to regulate the balance between mitochondrial fusion and fission and plays a role in regulating lipid droplet formation/tethering, fatty acid metabolism, and peroxisomal function. This review highlights our current understanding of the various roles of TMEM135 in cellular processes, organelle function, calcium dynamics, and metabolism.

Ceramide Metabolism and Parkinson's Disease-Therapeutic Targets

Ceramide is a bioactive sphingolipid involved in numerous cellular processes. In addition to being the precursor of complex sphingolipids, ceramides can act as second messengers, especially when they are generated at the plasma membrane of cells. Its metabolic dysfunction may lead to or be a consequence of an underlying disease. Recent reports on transcriptomics and electrospray ionization mass spectrometry analysis have demonstrated the variation of specific levels of sphingolipids and enzymes involved in their metabolism in different neurodegenerative diseases. In the present review, we highlight the most relevant discoveries related to ceramide and neurodegeneration, with a special focus on Parkinson's disease.

Characterization of neutral sphingomyelinase activity and isoform expression in rodent skeletal muscle mitochondria

Skeletal muscle is composed of fiber types that differ in mitochondrial content, antioxidant capacity, and susceptibility to apoptosis. Ceramides have been linked to oxidative stress-mediated apoptotic intracellular signalling and the enzyme neutral sphingomyelinase (nSMase) is, in part, responsible for generating these ceramides through the hydrolysis of sphingomyelin. Despite the role of ceramides in mediating apoptosis, there is a gap in the literature regarding nSMase in skeletal muscle mitochondria. This study aimed to characterize total nSMase activity and individual isoform expression in isolated subsarcolemmal (SS) mitochondria from soleus, diaphragm, plantaris, and extensor digitorum longus (EDL). Total nSMase activity did not differ between muscle types. nSMase2 content was detectable in all muscles and higher in EDL, soleus, and plantaris compared to diaphragm whereas nSMase3 was undetectable in all muscles. Finally, total nSMase activity positively correlated to nSMase2 protein content in soleus but not the other muscles. These findings suggest that nSMase associated with SS mitochondria may play a role in intracellular signalling processes involving ceramides in skeletal muscle and nSMase2 may be the key isoform, specifically in slow twitch muscle like soleus. Further studies are needed to fully elucidate the specific contribution of nSMase, along with the role of the various isoforms and mitochondrial subpopulation in generating mitochondrial ceramides in skeletal muscle, and its potential effects on mediating apoptosis.

Nonspecific phospholipase C4 hydrolyzes phosphosphingolipids and sustains plant root growth during phosphate deficiency

Phosphate is a vital macronutrient for plant growth, and its availability in soil is critical for agricultural sustainability and productivity. A substantial amount of cellular phosphate is used to synthesize phospholipids for cell membranes. Here, we identify a key enzyme, nonspecific phospholipase C4 (NPC4) that is involved in phosphosphingolipid hydrolysis and remodeling in Arabidopsis during phosphate starvation. The level of glycosylinositolphosphorylceramide (GIPC), the most abundant sphingolipid in Arabidopsis thaliana, decreased upon phosphate starvation. NPC4 was highly induced by phosphate deficiency, and NPC4 knockouts in Arabidopsis decreased the loss of GIPC and impeded root growth during phosphate starvation. Enzymatic analysis showed that NPC4 hydrolyzed GIPC and displayed a higher activity toward GIPC as a substrate than toward the common glycerophospholipid phosphatidylcholine. NPC4 was associated with the plasma membrane lipid rafts in which GIPC is highly enriched. These results indicate that NPC4 uses GIPC as a substrate in planta and the NPC4-mediated sphingolipid remodeling plays a positive role in root growth in Arabidopsis response to phosphate deficiency.

Physiological functions and therapeutic applications of neutral sphingomyelinase and acid sphingomyelinase

Sphingomyelin (SM) can be converted into ceramide (Cer) by neutral sphingomyelinase (NSM) and acid sphingomyelinase (ASM). Cer is a second messenger of lipids and can regulate cell growth and apoptosis. Increasing evidence shows that NSM and ASM play key roles in many processes, such as apoptosis, immune function and inflammation. Therefore, NSM and ASM have broad prospects in clinical treatments, especially in cancer, cardiovascular diseases (such as atherosclerosis), nervous system diseases (such as Alzheimer's disease), respiratory diseases (such as chronic obstructive pulmonary disease) and the phenotype of dwarfisms in adolescents, playing a complex regulatory role. This review focuses on the physiological functions of NSM and ASM and summarizes their roles in certain diseases and their potential applications in therapy.

Pharmacological inhibition of nSMase2 reduces brain exosome release and α-synuclein pathology in a Parkinson's disease model

Aim

We have previously reported that cambinol (DDL-112), a known inhibitor of neutral sphingomyelinase-2 (nSMase2), suppressed extracellular vesicle (EV)/exosome production in vitro in a cell model and reduced tau seed propagation. The enzyme nSMase2 is involved in the production of exosomes carrying proteopathic seeds and could contribute to cell-to-cell transmission of pathological protein aggregates implicated in neurodegenerative diseases such as Parkinson’s disease (PD). Here, we performed in vivo studies to determine if DDL-112 can reduce brain EV/exosome production and proteopathic alpha synuclein (αSyn) spread in a PD mouse model.

Methods

The acute effects of single-dose treatment with DDL-112 on interleukin-1β-induced extracellular vesicle (EV) release in brain tissue of Thy1-αSyn PD model mice and chronic effects of 5 week DDL-112 treatment on behavioral/motor function and proteinase K-resistant αSyn aggregates in the PD model were determined.

Results/discussion

In the acute study, pre-treatment with DDL-112 reduced EV/exosome biogenesis and in the chronic study, treatment with DDL-112 was associated with a reduction in αSyn aggregates in the substantia nigra and improvement in motor function. Inhibition of nSMase2 thus offers a new approach to therapeutic development for neurodegenerative diseases with the potential to reduce the spread of disease-specific proteopathic proteins.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13041-021-00776-9.

Neutral Sphingomyelinase 2 Heightens Anti-Melanoma Immune Responses and Anti-PD-1 Therapy Efficacy

Dysregulation of lipid metabolism affects the behavior of cancer cells, but how this happens is not completely understood. Neutral sphingomyelinase 2 (nSMase2), encoded by SMPD3, catalyzes the breakdown of sphingomyelin to produce the anti-oncometabolite ceramide. We found that this enzyme was often downregulated in human metastatic melanoma, likely contributing to immune escape. Overexpression of nSMase2 in mouse melanoma reduced tumor growth in syngeneic wild-type but not CD8-deficient mice. In wild-type mice, nSMase2-overexpressing tumors showed accumulation of both ceramide and CD8⁺ tumor-infiltrating lymphocytes, and this was associated with increased level of transcripts encoding IFNγ and CXCL9. Overexpressing the catalytically inactive nSMase2 failed to alter tumor growth, indicating that the deleterious effect nSMase2 has on melanoma growth depends on its enzymatic activity. In vitro, small extracellular vesicles from melanoma cells overexpressing wild-type nSMase2 augmented the expression of IL12, CXCL9, and CCL19 by bone marrow-derived dendritic cells, suggesting that melanoma nSMase2 triggers T helper 1 (Th1) polarization in the earliest stages of the immune response. Most importantly, overexpression of wild-type nSMase2 increased anti-PD-1 efficacy in murine models of melanoma and breast cancer, and this was associated with an enhanced Th1 response. Therefore, increasing SMPD3 expression in melanoma may serve as an original therapeutic strategy to potentiate Th1 polarization and CD8⁺ T-cell-dependent immune responses and overcome resistance to anti-PD-1.

Small Molecule Inhibitors Targeting Biosynthesis of Ceramide, the Central Hub of the Sphingolipid Network

Ceramides are composed of a sphingosine and a single fatty acid connected by an amide linkage. As one of the major classes of biologically active lipids, ceramides and their upstream and downstream metabolites have been implicated in several pathological conditions including cancer, neurodegeneration, diabetes, microbial pathogenesis, obesity, and inflammation. Consequently, tremendous efforts have been devoted to deciphering the dynamics of metabolic pathways involved in ceramide biosynthesis. Given that several distinct enzymes can produce ceramide, different enzyme targets have been pursued depending on the underlying disease mechanism. The main objective of this review is to provide a comprehensive overview of small molecule inhibitors reported to date for each of these ceramide-producing enzymes from a medicinal chemistry perspective.

Sowing the Seeds of Discovery: Tau-Propagation Models of Alzheimer's Disease

The propagation of pathological proteins throughout the brain is the primary physiological hallmark of the progression of Alzheimer's Disease (AD). A growing body of evidence indicates that hyperphosphorylated Tau proteins are spread transcellularly between neurons in a prionlike fashion, inducing misfolding and aggregation into neurofibrillary tangles which accumulate along specific connectivity pathways. Earlier transgenic rodent AD models did not capture this disease-relevant spread, and therefore, seeded Tau-propagation models have been developed. Here, mutant human Tau (as isolated protein or packaged into an adeno-associated virus (AAV) viral vector) is stereotaxically injected into select brain regions and its histopathological propagation to downstream neurons quantified. These models offer a faster and more direct mechanism to evaluate genetic components and therapeutic approaches which attenuate Tau spreading in vivo. Recently, these Tau-seeding models have revealed several new targets for AD drug discovery, including nSMase2, SIRT1, p300/CBP, LRP1, and TYROBP, as well as the potential therapeutics based on melatonin and chondroitinase ABC. Importantly, these Tau-propagation rodent models more closely phenocopy the progression of AD in humans and are therefore likely to improve preclinical studies and derisk future moves into clinical trials.

Sphingomyelinases and Liver Diseases

Sphingolipids (SLs) are critical components of membrane bilayers that play a crucial role in their physico-chemical properties. Ceramide is the prototype and most studied SL due to its role as a second messenger in the regulation of multiple signaling pathways and cellular processes. Ceramide is a heterogeneous lipid entity determined by the length of the fatty acyl chain linked to its carbon backbone sphingosine, which can be generated either by de novo synthesis from serine and palmitoyl-CoA in the endoplasmic reticulum or via sphingomyelin (SM) hydrolysis by sphingomyelinases (SMases). Unlike de novo synthesis, SMase-induced SM hydrolysis represents a rapid and transient mechanism of ceramide generation in specific intracellular sites that accounts for the diverse biological effects of ceramide. Several SMases have been described at the molecular level, which exhibit different pH requirements for activity: neutral, acid or alkaline. Among the SMases, the neutral (NSMase) and acid (ASMase) are the best characterized for their contribution to signaling pathways and role in diverse pathologies, including liver diseases. As part of a Special Issue (Phospholipases: From Structure to Biological Function), the present invited review summarizes the physiological functions of NSMase and ASMase and their role in chronic and metabolic liver diseases, of which the most relevant is nonalcoholic steatohepatitis and its progression to hepatocellular carcinoma, due to the association with the obesity and type 2 diabetes epidemic. A better understanding of the regulation and role of SMases in liver pathology may offer the opportunity for novel treatments of liver diseases.

Neutral Sphingomyelinase-2 (NSM 2) Controls T Cell Metabolic Homeostasis and Reprogramming During Activation

Neutral sphingomyelinase-2 (NSM2) is a member of a superfamily of enzymes responsible for conversion of sphingomyelin into phosphocholine and ceramide at the cytosolic leaflet of the plasma membrane. Upon specific ablation of NSM2, T cells proved to be hyper-responsive to CD3/CD28 co-stimulation, indicating that the enzyme acts to dampen early overshooting activation of these cells. It remained unclear whether hyper-reactivity of NSM2-deficient T cells is supported by a deregulated metabolic activity in these cells. Here, we demonstrate that ablation of NSM2 activity affects metabolism of the quiescent CD4⁺ T cells which accumulate ATP in mitochondria and increase basal glycolytic activity. This supports enhanced production of total ATP and metabolic switch early after TCR/CD28 stimulation. Most interestingly, increased metabolic activity in resting NSM2-deficient T cells does not support sustained response upon stimulation. While elevated under steady-state conditions in NSM2-deficient CD4⁺ T cells, the mTORC1 pathway regulating mitochondria size, oxidative phosphorylation, and ATP production is impaired after 24 h of stimulation. Taken together, the absence of NSM2 promotes a hyperactive metabolic state in unstimulated CD4⁺ T cells yet fails to support sustained T cell responses upon antigenic stimulation.

Transbilayer Movement of Sphingomyelin Precedes Catastrophic Breakage of Enterobacteria-Containing Vacuoles

Pathogenic bacteria enter the cytosol of host cells through uptake into bacteria-containing vacuoles (BCVs) and subsequent rupture of the vacuolar membrane [1]. Bacterial invaders are sensed either directly, through cytosolic pattern-recognition receptors specific for bacterial ligands, or indirectly, through danger receptors that bind host molecules displayed in an abnormal context, for example, glycans on damaged BCVs [2, 3, 4]. In contrast to damage caused by Listeria monocytogenes, a Gram-positive bacterium, BCV rupture by Gram-negative pathogens such as Shigella flexneri or Salmonella Typhimurium remains incompletely understood [5, 6]. The latter may cause membrane damage directly, when inserting their Type Three Secretion needles into host membranes, or indirectly through translocated bacterial effector proteins [7, 8, 9]. Here, we report that sphingomyelin, an abundant lipid of the luminal leaflet of BCV membranes, and normally absent from the cytosol, becomes exposed to the cytosol as an early predictive marker of BCV rupture by Gram-negative bacteria. To monitor subcellular sphingomyelin distribution, we generated a live sphingomyelin reporter from Lysenin, a sphingomyelin-specific toxin from the earthworm Eisenia fetida [10, 11]. Using super resolution live imaging and correlative light and electron microscopy (CLEM), we discovered that BCV rupture proceeds through two distinct successive stages: first, sphingomyelin is gradually translocated into the cytosolic leaflet of the BCV, invariably followed by cytosolic exposure of glycans, which recruit galectin-8, indicating bacterial entry into the cytosol. Exposure of sphingomyelin on BCVs may therefore act as an early danger signal alerting the cell to imminent bacterial invasion.

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Lysenin serves as a reporter of sphingomyelin exposure in the mammalian cytosol

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Chemical-, toxin-, or pathogen-induced membrane damage exposes sphingomyelin

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Sphingomyelin exposure precedes catastrophic breakage of bacteria-containing vacuoles

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Cytosolic sphingomyelin is indicative of membrane stress and imminent pathogen entry

Extracellular Vesicles as Delivery Vehicles of Specific Cellular Cargo

Extracellular vesicles (EVs) mediate cell-to-cell communication via the transfer of biomolecules locally and systemically between organs. It has been elucidated that the specific EV cargo load is fundamental for cellular response upon EV delivery. Therefore, revealing the specific molecular machinery that functionally regulates the precise EV cargo intracellularly is of importance in understanding the role of EVs in physiology and pathophysiology and conveying therapeutic use. The purpose of this review is to summarize recent findings on the general rules, as well as specific modulator motifs governing EV cargo loading. Finally, we address available information on potential therapeutic strategies to alter cargo loading.

Interaction of Synaptosomal-Associated Protein 25 with Neutral Sphingomyelinase 2: Functional Impact on the Sphingomyelin Pathway

Neurotransmitter release is mediated by ceramide, which is generated by sphingomyelin hydrolysis. In the present study, we examined whether synaptosomal-associated protein 25 (SNAP-25) is involved in ceramide production and exocytosis. Neutral sphingomyelinase 2 (nSMase2) was partially purified from bovine brain and we found that SNAP-25 was enriched in the nSMase2-containing fractions. In rat synaptosomes and PC12 cells, the immunoprecipitation pellet of anti-SNAP-25 antibody showed higher nSMase activity than the immunoprecipitation pellet of anti-nSMase2 antibody. In PC12 cells, SNAP-25 was colocalized with nSMase2. Transfection of SNAP-25 small interfering RNA (siRNA) significantly inhibited nSMase2 translocation to the plasma membrane. A23187-induced ceramide production was concomitantly reduced in SNAP-25 siRNA-transfected PC12 cells compared with that in scrambled siRNA-transfected cells. Moreover, transfection of SNAP-25 siRNA inhibited dopamine release, whereas addition of C₆-ceramide to the siRNA-treated cells moderately reversed this inhibition. Additionally, nSMase2 inhibition reduced dopamine release. Collectively, our results indicate that SNAP-25 interacts with nSMase2 during ceramide production, which mediates exocytosis and neurotransmitter release.

Phospholipases in neuronal function: A role in learning and memory?

Despite the human brain being made of nearly 60% fat, the vast majority of studies on the mechanisms of neuronal communication which underpin cognition, memory and learning, primarily focus on proteins and/or (epi)genetic mechanisms. Phospholipids are the main component of all cellular membranes and function as substrates for numerous phospholipid-modifying enzymes, including phospholipases, which release free fatty acids (FFAs) and other lipid metabolites that can alter the intrinsic properties of the membranes, recruit and activate critical proteins, and act as lipid signalling molecules. Here, we will review brain specific phospholipases, their roles in membrane remodelling, neuronal function, learning and memory, as well as their disease implications. In particular, we will highlight key roles of unsaturated FFAs, particularly arachidonic acid, in neurotransmitter release, neuroinflammation and memory. In light of recent findings, we will also discuss the emerging role of phospholipase A₁ and the creation of saturated FFAs in the brain.

Role of Neutral Sphingomyelinase-2 (NSM 2) in the Control of T Cell Plasma Membrane Lipid Composition and Cholesterol Homeostasis

The activity of neutral sphingomyelinase-2 (NSM2) to catalyze the conversion of sphingomyelin (SM) to ceramide and phosphocholine at the cytosolic leaflet of plasma membrane (PM) is important in T cell receptor (TCR) signaling. We recently identified PKCζ as a major NSM2 downstream effector which regulates microtubular polarization. It remained, however, unclear to what extent NSM2 activity affected overall composition of PM lipids and downstream effector lipids in antigen stimulated T cells. Here, we provide a detailed lipidomics analyses on PM fractions isolated from TCR stimulated wild type and NSM2 deficient (ΔNSM) Jurkat T cells. This revealed that in addition to that of sphingolipids, NSM2 depletion also affected concentrations of many other lipids. In particular, NSM2 ablation resulted in increase of lyso-phosphatidylcholine (LPC) and lyso-phosphatidylethanolamine (LPE) which both govern PM biophysical properties. Crucially, TCR dependent upregulation of the important T cell signaling lipid diacylglycerol (DAG), which is fundamental for activation of conventional and novel PKCs, was abolished in ΔNSM cells. Moreover, NSM2 activity was found to play an important role in PM cholesterol transport to the endoplasmic reticulum (ER) and production of cholesteryl esters (CE) there. Most importantly, CE accumulation was essential to sustain human T cell proliferation. Accordingly, inhibition of CE generating enzymes, the cholesterol acetyltransferases ACAT1/SOAT1 and ACAT2/SOAT2, impaired TCR driven expansion of both CD4⁺ and CD8⁺ T cells. In summary, our study reveals an important role of NSM2 in regulating T cell functions by its multiple effects on PM lipids and cholesterol homeostasis.

Dietary inulin decreases circulating ceramides by suppressing neutral sphingomyelinase expression and activity in mice

Elevated circulating levels of ceramides (Cers) are associated with increased risk of cardiometabolic diseases, and Cers may play a causative role in metabolic dysfunction that precedes cardiac events, such as mortality as a result of coronary artery disease. Although the mechanisms involved are likely complex, these associations suggest that lowering circulating Cer levels could be protective against cardiovascular diseases. Conversely, dietary fibers, such as inulin, have been reported to promote cardiovascular and metabolic health. However, the mechanisms involved in these protective processes also are not well understood. We studied the effects of inulin on lipid metabolism with a model of atherosclerosis in LDL receptor-deficient mice using lipidomics and transcriptomics. Plasma and tissues were collected at 10 days and/or 12 weeks after feeding mice an atherogenic diet supplemented with inulin or cellulose (control). Compared with controls, inulin-fed mice displayed a decreased C16:0/C24:0 plasma Cer ratio and lower levels of circulating Cers associated with VLDL and LDL. Liver transcriptomic analysis revealed that Smpd3, a gene that encodes neutral SMase (NSMase), was downregulated by 2-fold in inulin-fed mice. Hepatic NSMase activity was 3-fold lower in inulin-fed mice than in controls. Furthermore, liver redox status and compositions of phosphatidylserine and FFA species, the major factors that determine NSMase activity, were also modified by inulin. Taken together, these results showed that, in mice, inulin can decrease plasma Cer levels through reductions in NSMase expression and activity, suggesting a mechanism by which fiber could reduce cardiometabolic disease risk.

Translational Approaches Targeting Ceramide Generation From Sphingomyelin in T Cells to Modulate Immunity in Humans

In T cells, as in all other cells of the body, sphingolipids form important structural components of membranes. Due to metabolic modifications, sphingolipids additionally play an active part in the signaling of cell surface receptors of T cells like the T cell receptor or the co-stimulatory molecule CD28. Moreover, the sphingolipid composition of their membranes crucially affects the integrity and function of subcellular compartments such as the lysosome. Previously, studying sphingolipid metabolism has been severely hampered by the limited number of analytical methods/model systems available. Besides well-established high resolution mass spectrometry new tools are now available like novel minimally modified sphingolipid subspecies for click chemistry as well as recently generated mouse mutants with deficiencies/overexpression of sphingolipid-modifying enzymes. Making use of these tools we and others discovered that the sphingolipid sphingomyelin is metabolized to ceramide to different degrees in distinct T cell subpopulations of mice and humans. This knowledge has already been translated into novel immunomodulatory approaches in mice and will in the future hopefully also be applicable to humans. In this paper we are, thus, summarizing the most recent findings on the impact of sphingolipid metabolism on T cell activation, differentiation, and effector functions. Moreover, we are discussing the therapeutic concepts arising from these insights and drugs or drug candidates which are already in clinical use or could be developed for clinical use in patients with diseases as distant as major depression and chronic viral infection.

Inhibition of nSMase2 Reduces the Transfer of Oligomeric α-Synuclein Irrespective of Hypoxia

Recently, extracellular vesicles (EVs), such as exosomes, have been proposed to play an influential role in the cell-to-cell spread of neurodegenerative diseases, including the intercellular transmission of α-synuclein (α-syn). However, the regulation of EV biogenesis and its relation to Parkinson’s disease (PD) is only partially understood. The generation of EVs through the ESCRT-independent pathway depends on the hydrolysis of sphingomyelin by neutral sphingomyelinase 2 (nSMase2) to produce ceramide, which causes the membrane of endosomal multivesicular bodies to bud inward. nSMase2 is sensitive to oxidative stress, a common process in PD brains; however, little is known about the role of sphingomyelin metabolism in the pathogenesis of PD. This is the first study to show that inhibiting nSMase2 decreases the transfer of oligomeric aggregates of α-syn between neuron-like cells. Furthermore, it reduced the accumulation and aggregation of high-molecular-weight α-syn. Hypoxia, as a model of oxidative stress, reduced the levels of nSMase2, but not its enzymatic activity, and significantly altered the lipid composition of cells without affecting EV abundance or the transfer of α-syn. These data show that altering sphingolipids can mitigate the spread of α-syn, even under hypoxic conditions, potentially suppressing PD progression.

Invited review: Sphingolipid biology in the dairy cow: The emerging role of ceramide

The physiological control of lactation through coordinated adaptations is of fundamental importance for mammalian neonatal life. The putative actions of reduced insulin sensitivity and responsiveness and enhanced adipose tissue lipolysis spare glucose for the mammary synthesis of milk. However, severe insulin antagonism and body fat mobilization may jeopardize hepatic health and lactation in dairy cattle. Interestingly, lipolysis- and dietary-derived fatty acids may impair insulin sensitivity in cows. The mechanisms are undefined yet have major implications for the development of postpartum fatty liver disease. In nonruminants, the sphingolipid ceramide is a potent mediator of saturated fat-induced insulin resistance that defines in part the mechanisms of type 2 diabetes mellitus and nonalcoholic fatty liver disease. In ruminants including the lactating dairy cow, the functions of ceramide had remained virtually undescribed. Through a series of hypothesis-centered studies, ceramide has emerged as a potential antagonist of insulin-stimulated glucose utilization by adipose and skeletal muscle tissues in dairy cattle. Importantly, bovine data suggest that the ability of ceramide to inhibit insulin action likely depends on the lipolysis-dependent hepatic synthesis and secretion of ceramide during early lactation. Although these mechanisms appear to fade as lactation advances beyond peak milk production, early evidence suggests that palmitic acid feeding is a means to augment ceramide supply. Herein, we review a body of work that focuses on sphingolipid biology and the role of ceramide in the dairy cow within the framework of hepatic and fatty acid metabolism, insulin function, and lactation. The potential involvement of ceramide within the endocrine control of lactation is also considered.

Neutral Sphingomyelinase 2 (SMPD3) Deficiency in Mice Causes Chondrodysplasia with Unimpaired Skeletal Mineralization

SMPD3 deficiency in the neutral sphingomyelinase (Smpd3^-/-) mouse results in a novel form of juvenile dwarfism, suggesting smpd3 is a polygenetic determinant of body height. SMPD3 controls homeostasis of the sphingomyelin cycle in the Golgi compartment, essential for membrane remodeling, initiating multiform vesicle formation and transport in the Golgi secretory pathway. Using the unbiased Smpd3^-/- genetic model, this study shows that the perturbed Golgi secretory pathway of chondrocytes of the epiphyseal growth zone leads to dysproteostasis, skeletal growth inhibition, malformation, and chondrodysplasia, but showed unimpaired mineralization in primary and secondary enchondral ossification centers. This has been elaborated by biochemical analyses and immunohistochemistry of long bones of Smpd3^-/- mice. A more precise definition of the microarchitecture and three-dimensional structure of the bone was shown by peripheral quantitative computed tomography, high-resolution microcomputed tomography, and less precisely by dual-energy X-ray absorptiometry for osteodensitometry. Ablation of the Smpd3 locus as part of a 980-kb deletion on chromosome 8 in the fro/fro mutant, generated by chemical mutagenesis, is held responsible for skeletal hypomineralization, osteoporosis, and multiple fractures of long bones, which are hallmarks of human osteogenesis imperfecta. The phenotype of the genetically unbiased Smpd3^-/- mouse, described here, precludes the proposed role of Smpd3 as a candidate gene of human osteogenesis imperfecta, but suggests SMPD3 deficiency as the pathogenetic basis of a novel form of chondrodysplasia.

The function of sphingomyelinases in mycobacterial infections

Tuberculosis (TB), caused by Mycobacterium tuberculosis, is one of the deadliest and most important infectious diseases worldwide. The sphingomyelinase/ceramide system, which has been shown several times to be a crucial factor in the internalization, processing and killing of diverse pathogens, also modulates the pro-inflammatory response and the state of mycobacteria in macrophages. Both acid and neutral sphingomyelinases are important in this activity. However, studies of the role of sphingomyelinases in TB are still at an early stage.

A blend containing docosahexaenoic acid, arachidonic acid, vitamin B12, vitamin B9, iron and sphingomyelin promotes myelination in an in vitro model

During the development of the central nervous system, oligodendrocytes (OLs) are responsible for myelination, the formation of the myelin sheath around axons. This process enhances neuronal connectivity and supports the maturation of emerging cognitive functions. In humans, recent evidence suggests that early life nutrition may affect myelination. In the present study, we investigated the impact of a blend containing docosahexaenoic acid, arachidonic acid, vitamin B12, vitamin B9, iron and sphingomyelin, or each of these nutrients individually, on oligodendrocyte precursor cells (OPCs) proliferation and maturation into OLs as well as their myelinating properties. By using an in vitro model, developed to study each step of myelination, we found that the nutrient blend increased the number of OPCs and promoted their differentiation and maturation into OLs, as measured by quantifying A2B5 positive cells, myelin-associated glycoprotein (MAG) positive cells and area, myelin binding protein (MBP) positive cells and area, respectively. Moreover, measuring myelination by quantifying the overlapping signal between neurofilament and either MAG or MBP revealed a positive effect of the blend on OLs myelinating properties. In contrast, treatment with each individual nutrient resulted in differential effects on the various readouts. This work suggests that dietary intake of these nutrients during early life, might be beneficial for myelination.

Lipids in Exosome Biology

Extracellular vesicles (EVs), and exosomes in particular, were initially considered as "garbage bags" for secretion of undesired cellular components. This view has changed considerably over the last two decades, and exosomes have now emerged as important organelles controlling cell-to-cell signaling. They are present in biological fluids and have important roles in the communication between cells in physiological and pathological processes. They are envisioned for clinical use as carriers of biomarkers, therapeutic targets, and vehicles for drug delivery. Important efforts are being made to characterize the contents of these vesicles and to understand the mechanisms that govern their biogenesis and modes of action. This chapter aims to recapitulate the place given to lipids in our understanding of exosome biology. Besides their structural role and their function as carriers, certain lipids and lipid-modifying enzymes seem to exert privileged functions in this mode of cellular communication. By extension, the use of selective "lipid inhibitors" might turn out to be interesting modulators of exosomal-based cell signaling.