Distribution of acid sphingomyelinase in human various body fluids

Relationships between ocular surface sphingomyelinases, Meibum and Tear Sphingolipids, and clinical parameters of meibomian gland dysfunction

PURPOSE

Sphingolipids (SPL) are a class of lipid molecules that play important functional and structural roles in our body and are a component of meibum. Sphingomyelinases (SMases) are key enzymes in sphingolipid metabolism that hydrolyze sphingomyelin (SM) and generate ceramide (Cer). The purpose of this study was to examine relationships between ocular surface SMases, SPL composition, and parameters of Meibomian gland dysfunction (MGD).

METHODS

Individuals were grouped by meibum quality (n = 25 with poor-quality, MGD, and n = 25 with good-quality, control). Meibum and tears were analyzed with LC-MS to quantify SPL classes: Cer, Hexosyl-Ceramide (Hex-Cer), SM, Sphingosine (Sph), and sphingosine 1-phosphate (S1P). SMase activity in tears were quantified using a commercially available 'SMase assay'. Statistical analysis included multiple linear regression analyses to assess the impact of SMase activity on lipid composition, as well as ocular surface symptoms and signs of MGD.

RESULTS

Demographic characteristics were similar between the two groups. nSMase and aSMase levels were lower in the poor vs good quality group. aSMase activity in tears negatively correlated with SM in meibum and tears and positively with Sph in meibum and S1P in tears. Lower SMase activity were associated with signs of MGD, most notably Meibomian gland dropout.

CONCLUSION

This study suggests that individuals with MGD have reduced enzymatic activity of SMases in tears. Specifically, individuals with poor vs good meibum quality were noted to have alterations in SMase activity and SPL composition of meibum and tears which may reflect deviations from normal lipid metabolism in individuals with MGD.

Study on the protective effect and mechanism of Liriodendrin on radiation enteritis in mice

Patients receiving pelvic or abdominal radiotherapy may experience acute and/or chronic side effects due to gastrointestinal changes. However, effective medicine for treating radiation enteritis has not been found yet. Sargentodoxa cuneata is a famous Chinese medicine used to treat intestinal inflammation, and our research team has found the main biologically active compound through its extraction, which is Liriodendrin. In this study, we found that Liriodendrin can reduce the expression of Cer, Cer1P and S1P in the sphingolipid pathway, thereby reducing the histological damage to the intestinal tract of mice and inhibiting the apoptosis of intestinal tissue cells. In addition, Liriodendrin can reduce the levels of pro-inflammatory cytokines (IL-6 and TNF-α), and it is suggested through flow cytometry that the proportion of neutrophils in the intestinal tissue can decrease due to the existence of Liriodendrin. At the same time, the western blot evaluation revealed that Liriodendrin significantly inhibited the activation of Bcl-2/Bax/Caspase-3 and NF-κB signaling pathways. The results show that Liriodendrin can inhibit intestinal inflammation and intestinal cell apoptosis through the sphingolipid pathway. Therefore, the aforementioned results demonstrated that Liriodendrin may be a promising drug for the treatment of radiation enteritis.

A sublethal ATP11A mutation associated with neurological deterioration causes aberrant phosphatidylcholine flipping in plasma membranes

ATP11A translocates phosphatidylserine (PtdSer), but not phosphatidylcholine (PtdCho), from the outer to the inner leaflet of plasma membranes, thereby maintaining the asymmetric distribution of PtdSer. Here, we detected a de novo heterozygous point mutation of ATP11A in a patient with developmental delays and neurological deterioration. Mice carrying the corresponding mutation died perinatally of neurological disorders. This mutation caused an amino acid substitution (Q84E) in the first transmembrane segment of ATP11A, and mutant ATP11A flipped PtdCho. Molecular dynamics simulations revealed that the mutation allowed PtdCho binding at the substrate entry site. Aberrant PtdCho flipping markedly decreased the concentration of PtdCho in the outer leaflet of plasma membranes, whereas sphingomyelin (SM) concentrations in the outer leaflet increased. This change in the distribution of phospholipids altered cell characteristics, including cell growth, cholesterol homeostasis, and sensitivity to sphingomyelinase. Matrix-assisted laser desorption ionization-imaging mass spectrometry (MALDI-IMS) showed a marked increase of SM levels in the brains of Q84E-knockin mouse embryos. These results provide insights into the physiological importance of the substrate specificity of plasma membrane flippases for the proper distribution of PtdCho and SM.

Digestion and Absorption of Milk Phospholipids in Newborns and Adults

Milk polar lipids provide choline, ethanolamine, and polyunsaturated fatty acids, which are needed for the growth and plasticity of the tissues in a suckling child. They may also inhibit cholesterol absorption by interacting with cholesterol during micelle formation. They may also have beneficial luminal, mucosal, and metabolic effects in both the neonate and the adult. The milk fat globule membrane contains large proportions of sphingomyelin (SM), phosphatidylcholine (PC), and phosphatidylethanolamine (PE), and some phosphatidylserine (PS), phosphatidylinositol (PI), and glycosphingolipids. Large-scale technical procedures are available for the enrichment of milk fat globule membrane (MFGM) in milk replacement formulations and food additives. Pancreatic phospholipase A2 (PLA2) and mucosal phospholipase B digest glycero-phospholipids in the adult. In the neonate, where these enzymes may be poorly expressed, pancreatic lipase-related protein 2 probably has a more important role. Mucosal alkaline SM-ase and ceramidase catalyze the digestion of SM in both the neonate and the adult. In the mucosa, the sphingosine is converted into sphingosine-1-phosphate, which is both an intermediate in the conversion to palmitic acid and a signaling molecule. This reaction sequence also generates ethanolamine. Here, we summarize the pathways by which digestion and absorption may be linked to the biological effects of milk polar lipids. In addition to the inhibition of cholesterol absorption and the generation of lipid signals in the gut, the utilization of absorbed choline and ethanolamine for mucosal and hepatic phospholipid synthesis and the acylation of absorbed lyso-PC with polyunsaturated fatty acids to chylomicron and mucosal phospholipids are important.

Lipid-Iron Nanoparticle with a Cell Stress Release Mechanism Combined with a Local Alternating Magnetic Field Enables Site-Activated Drug Release

Most available cancer chemotherapies are based on systemically administered small organic molecules, and only a tiny fraction of the drug reaches the disease site. The approach causes significant side effects and limits the outcome of the therapy. Targeted drug delivery provides an alternative to improve the situation. However, due to the poor release characteristics of the delivery systems, limitations remain. This report presents a new approach to address the challenges using two fundamentally different mechanisms to trigger the release from the liposomal carrier. We use an endogenous disease marker, an enzyme, combined with an externally applied magnetic field, to open the delivery system at the correct time only in the disease site. This site-activated release system is a novel two-switch nanomachine that can be regulated by a cell stress-induced enzyme at the cellular level and be remotely controlled using an applied magnetic field. We tested the concept using sphingomyelin-containing liposomes encapsulated with indocyanine green, fluorescent marker, or the anticancer drug cisplatin. We engineered the liposomes by adding paramagnetic beads to act as a receiver of outside magnetic energy. The developed multifunctional liposomes were characterized in vitro in leakage studies and cell internalization studies. The release system was further studied in vivo in imaging and therapy trials using a squamous cell carcinoma tumor in the mouse as a disease model. In vitro studies showed an increased release of loaded material when stress-related enzyme and magnetic field was applied to the carrier liposomes. The theranostic liposomes were found in tumors, and the improved therapeutic effect was shown in the survival studies.

Secretion of Acid Sphingomyelinase and Ceramide by Endothelial Cells Contributes to Radiation-Induced Intestinal Toxicity

Ceramide-induced endothelial cell apoptosis boosts intestinal stem cell radiosensitivity. However, the molecular connection between these two cellular compartments has not been clearly elucidated. Here we report that ceramide and its related enzyme acid sphingomyelinase (ASM) are secreted by irradiated endothelial cells and act as bystander factors to enhance the radiotoxicity of intestinal epithelium. Ceramide and the two isoforms of ASM were acutely secreted in the blood serum of wild-type mice after 15 Gy radiation dose, inducing a gastrointestinal syndrome. Interestingly, serum ceramide was not enhanced in irradiated ASMKO mice, which are unable to develop intestinal failure injury. Because ASM/ceramide were secreted by primary endothelial cells, their contribution was studied in intestinal epithelium dysfunction using coculture of primary endothelial cells and intestinal T84 cells. Adding exogenous ASM or ceramide enhanced epithelial cell growth arrest and death. Conversely, blocking their secretion by endothelial cells using genetic, pharmacologic, or immunologic approaches abolished intestinal T84 cell radiosensitivity. Use of enteroid models revealed ASM and ceramide-mediated deleterious mode-of-action: when ceramide reduced the number of intestinal crypt-forming enteroids without affecting their structure, ASM induced a significant decrease of enteroid growth without affecting their number. Identification of specific and different roles for ceramide and ASM secreted by irradiated endothelial cells opens new perspectives in the understanding of intestinal epithelial dysfunction after radiation and defines a new class of potential therapeutic radiomitigators. SIGNIFICANCE: This study identifies secreted ASM and ceramide as paracrine factors enhancing intestinal epithelial dysfunction, revealing a previously unknown class of mediators of radiosensitivity.

The role of sphingolipids in endothelial barrier function

Sphingolipids are a ubiquitous family of essential lipids with an increasingly understood role as biologically active mediators in numerous physiologic and pathologic processes. Two particular sphingolipid species, sphingosine-1-phosphate and ceramide, and their metabolites interact both directly and indirectly with endothelial cells to regulate vascular permeability. Sphingosine-1-phosphate generally augments endothelial integrity while ceramide tends to promote vascular leak, and a tight balance between the two is necessary to maintain normal physiologic function. The mechanisms by which sphingolipids regulate endothelial barrier function are complex and occur through multiple different pathways, and disruptions or imbalances in these pathways have been implicated in a number of specific disease processes. With improved understanding of sphingolipid biology, endothelial function, and the interactions between the two, several targets for therapeutic intervention have emerged and there is immense potential for further advancement in this field.

Regulation of sphingomyelin metabolism

Sphingolipids (SFs) represent a large class of lipids playing diverse functions in a vast number of physiological and pathological processes. Sphingomyelin (SM) is the most abundant SF in the cell, with ubiquitous distribution within mammalian tissues, and particularly high levels in the Central Nervous System (CNS). SM is an essential element of plasma membrane (PM) and its levels are crucial for the cell function. SM content in a cell is strictly regulated by the enzymes of SM metabolic pathways, which activities create a balance between SM synthesis and degradation. The de novo synthesis via SM synthases (SMSs) in the last step of the multi-stage process is the most important pathway of SM formation in a cell. The SM hydrolysis by sphingomyelinases (SMases) increases the concentration of ceramide (Cer), a bioactive molecule, which is involved in cellular proliferation, growth and apoptosis. By controlling the levels of SM and Cer, SMSs and SMases maintain cellular homeostasis. Enzymes of SM cycle exhibit unique properties and diverse tissue distribution. Disturbances in their activities were observed in many CNS pathologies. This review characterizes the physiological roles of SM and enzymes controlling SM levels as well as their involvement in selected pathologies of the Central Nervous System, such as ischemia/hypoxia, Alzheimer disease (AD), Parkinson disease (PD), depression, schizophrenia and Niemann Pick disease (NPD).

Topical application of phosphatidyl-inositol-3,5-bisphosphate for acute lung injury in neonatal swine

Hypoxemic respiratory failure of the neonatal organism involves increased acid sphingomyelinase (aSMase) activity and production of ceramide, a second messenger of a pro-inflammatory pathway that promotes increased vascular permeability, surfactant alterations and alveolar epithelial apoptosis. We comparatively assessed the benefits of topical aSMase inhibition by either imipramine (Imi) or phosphatidylinositol-3,5-bisphosphate (PIP2) when administered into the airways together with surfactant (S) for fortification. In this translational study, a triple-hit acute lung injury model was used that entails repeated airway lavage, injurious ventilation and tracheal lipopolysaccharide instillation in newborn piglets subject to mechanical ventilation for 72 hrs. After randomization, we administered an air bolus (control), S, S+Imi, or S+PIP2. Only in the latter two groups we observed significantly improved oxygenation and ventilation, dynamic compliance and pulmonary oedema. S+Imi caused systemic aSMase suppression and ceramide reduction, whereas the S+PIP2 effect remained compartmentalized in the airways because of the molecule's bulky structure. The surfactant surface tensions improved by S+Imi and S+PIP2 interventions, but only to a minor extent by S alone. S+PIP2 inhibited the migration of monocyte-derived macrophages and granulocytes into airways by the reduction of CD14/CD18 expression on cell membranes and the expression of epidermal growth factors (amphiregulin and TGF-β1) and interleukin-6 as pro-fibrotic factors. Finally we observed reduced alveolar epithelial apoptosis, which was most apparent in S+PIP2 lungs. Exogenous surfactant “fortified” by PIP2, a naturally occurring surfactant component, improves lung function by topical suppression of aSMase, providing a potential treatment concept for neonates with hypoxemic respiratory failure.

Characterization of acid sphingomyelinase activity in human cerebrospinal fluid

Background

As a key enzyme in sphingolipid metabolism, acid sphingomyelinase (ASM) is involved in the regulation of cell fate and signaling via hydrolysis of sphingomyelin to form ceramide. While increased activity of the lysosomal form has been associated with various pathological conditions, there are few studies on secretory ASM limited only to cell models, plasma or serum.

Methods

An optimized assay based on a fluorescent substrate was applied to measure the ASM activity in cerebrospinal fluid (CSF) collected from mice and from 42 patients who were classified as controls based on normal routine CSF values.

Results

We have detected ASM activity in human CSF, established a sensitive quantitative assay and characterized the enzyme’s properties. The enzyme resembles plasmatic ASM including protein stability and Zn²⁺-dependence but the assays differ considerably in the optimal detergent concentration. Significantly increased activities in the CSF of ASM transgenic mice and undetectable levels in ASM knock-out mice prove that the measured ASM activity originates from the ASM-encoding gene SMPD1. CSF localized ASM activities were comparable to corresponding serum ASM levels at their respective optimal reaction conditions, but no correlation was observed. The large variance in ASM activity was independent of sex, age or analyzed routine CSF parameters.

Conclusions

Human and mouse CSF contain detectable levels of secretory ASM, which are unrelated to serum ASM activities. Further investigations in humans and in animal models will help to elucidate the role of this enzyme in human disease and to assess its value as a potential biomarker for disease type, severity, progress or therapeutic success.

The role of sphingolipids in respiratory disease

Sphingolipids form a broad class of lipids with diverse functions ranging from membrane constituents to intracellular second messengers and extracellular mediators. They can be rapidly generated or converted into each other and they play pivotal roles in various cellular processes, many of which are broadly associated with inflammation and apoptosis. Among the numerous sphingolipids, ceramide and sphingosine-1-phosphate (S1P) have received the greatest attention. Ceramide is a hydrophobic molecule that is increased in the lungs of patients with cystic fibrosis and chronic obstructive pulmonary disease (COPD). Ceramide is the eponym for ceramide-rich membrane platforms. that need to form as a prerequisite to the uptake of several microorganisms including Pseudomonas aeruginosa, and as a prerequisite to many signaling processes including apoptosis and increased vascular permeability. Accordingly, abnormal amounts of enzymes involved in the synthesis of ceramide, such as neutral or acid sphingomyelinase, are found in emphysematic smokers and in patients with severe sepsis, and are considered as novel pharmacological targets. S1P acts as an extracellular mediator that opposes several actions of ceramide and acts by binding to G-protein coupled S1P receptors (S1P(1)-S1P(5)). Of particular interest are S1P(1) receptors that enhance vascular barrier functions and are antiapoptotic. Therefore, S1P(1)-receptor ligands are suggested as novel drugs for COPD and acute lung injury. S1P is a potent chemotaxin for many leukocytes, it organizes lymphocyte trafficking and is involved in several key symptoms of asthma such as airway hyperresponsiveness and pulmonary eosinophil sequestration. S1P is formed by sphingosine kinases that have been identified as possible drug targets for the treatment of asthma. Based on these findings, several new drugs have recently been developed to specifically target sphingomyelinases, sphingosine kinases and S1P receptors for the treatment of COPD, cystic fibrosis, asthma and acute lung injury.

Sphingolipids in the lungs

Sphingolipids such as sphingosine-1-phosphate (S1P), ceramide, or sphingomyelin are essential constituents of plasma membranes and regulate many (patho)physiological cellular responses inducing apoptosis and cell survival, vascular permeability, mast cell activation, and airway smooth muscle functions. The complexity of sphingolipid biology is generated by a great variety of compounds, diverse receptors, and often antagonistic functions of different sphingolipids. For instance, apoptosis is promoted by ceramide and prevented by S1P, and pulmonary vascular permeability is increased by S1P2/3 receptors and by ceramide, whereas S1P1 receptors stabilize barrier integrity. Several enzymes of the sphingolipid metabolism respond to external stimuli such as sphingomyelinase isoenzymes that are activated by many stress stimuli and the sphingosine kinase isoenzymes that are activated by allergens. The past years have provided increasing evidence that these processes contribute to pulmonary disorders including asthma, chronic obstructive pulmonary disease, acute lung injury, and cystic fibrosis. Sphingolipid metabolism offers several novel therapeutic targets for the treatment of lung diseases such as emphysema, asthma, cystic fibrosis, respiratory tract infection, sepsis, and acute lung injury.