Analysis of lipid-composition changes in plasma membrane microdomains

A polarity-responsive lysosomes-nucleus translocation probe for the dual-emissive visualization of cell apoptosis

Visualization of cell apoptosis is a critical task playing central roles in the fundamental studies in biology, pathology, and biomedicine. Dual-emissive fluorescent probes are desired molecular tools for study on apoptosis, which however were rarely reported. Herein, utilizing the polarity differences between lysosomes and nucleus, a translocation type of fluorescent probe (NA-S) was developed for the dual-color visualization of cell apoptosis. NA-S was designed to be polarity sensitive, bearing alkalescence group, and with DNA affinity. In living cells, NA-S targeted the lysosomes to give blue fluorescence, which translocated into the nucleus during cell apoptosis to give green emission. Thereby, the cell apoptosis could be visualized with NA-S in dual-emissive manner. With the unique probe, the cell apoptosis induced by oxidative stress, UV irradiation, rotenone, colchicine, and paclitaxel have been successfully visualized.

Lipids in Equine Airway Inflammation: An Overview of Current Knowledge

Mild-moderate and severe equine asthma (MEA and SEA) are prevalent inflammatory airway conditions affecting horses of numerous breeds and disciplines. Despite extensive research, detailed disease pathophysiology and the differences between MEA and SEA are still not completely understood. Bronchoalveolar lavage fluid cytology, broadly used in clinical practice and in equine asthma research, has limited means to represent the inflammatory status in the lower airways. Lipidomics is a field of science that can be utilized in investigating cellular mechanisms and cell-to-cell interactions. Studies in lipidomics have a broad variety of foci, of which fatty acid and lipid mediator profile analyses and global lipidomics have been implemented in veterinary medicine. As many crucial proinflammatory and proresolving mediators are lipids, lipidomic studies offer an interesting yet largely unexplored means to investigate inflammatory reactions in equine airways. The aim of this review article is to collect and summarize the findings of recent lipidomic studies on equine airway inflammation.

Lipid organization and turnover in the plasma membrane of human differentiating neural progenitor cells revealed by time-of-flight secondary ion mass spectrometry imaging

Membrane lipids have been known to influence multiple signalling and cellular processes. Dysregulation of lipids at the neuronal membrane is connected to a significant alteration of the brain function and morphology, leading to brain diseases and neurodegeneration. Understanding the lipid composition and turnover of neuronal membrane will provide a significant insight into the molecular events underlying the regulatory effects of these biomolecules in a neuronal system. In this study, we aimed to characterize the composition and turnover of the plasma membrane lipids in human neural progenitor cells (NPCs) at an early differentiation stage into midbrain neurons using ToF-SIMS imaging. Lipid composition of the native plasma membrane was explored, followed by an examination of the lipid turnover using different isotopically labelled lipid precursors, including 13C-choline, 13C-lauric acid, 15N-linoleic, and 13C-stearic. Our results showed that differentiating NPCs contain a high abundance of ceramides, glycerophosphoserines, neutral glycosphingolipids, diradylglycerols, and glycerophosphocholines at the plasma membrane. In addition, different precursors were found to incorporate into different membrane lipids which are specific for the short- or long-carbon chains, and the unsaturation or saturation stage of the precursors. The lipid structure of neuronal membrane reflects the differentiation status of NPCs, and it can be altered significantly using a particular lipid precursor. Our study illustrates a potential of ToF-SIMS imaging to study native plasma membrane lipids and elucidate complex cellular processes by providing molecular -rich information at a single cell level.

Human sphingomyelin synthase 1 generates diacylglycerol in the presence and absence of ceramide via multiple enzymatic activities

Sphingomyelin (SM) synthase 1 (SMS1), which is involved in lipodystrophy, deafness, and thrombasthenia, generates diacylglycerol (DG) and SM using phosphatidylcholine (PC) and ceramide as substrates. Here, we found that SMS1 possesses DG-generating activities via hydrolysis of PC and phosphatidylethanolamine (PE) in the absence of ceramide and ceramide phosphoethanolamine synthase (CPES) activity. In the presence of the same concentration (4.7 mol%) of PC and ceramide, the amounts of DG produced by SMS and PC-phospholipase C (PLC) activities of SMS1 were approximately 65% and 35% of total DG production, respectively. PC-PLC activity showed substrate selectivity for saturated and/or monounsaturated fatty acid-containing PC species. A PC-PLC/SMS inhibitor, D609, inhibited only SMS activity. Mn2+ inhibited only PC-PLC activity. Intriguingly, DG attenuated SMS/CPES activities. Our study indicates that SMS1 is a unique enzyme with PC-PLC/PE-PLC/SMS/CPES activities.

Contrasting the phospholipid profiles of two neoplastic cell lines reveal a high PC:PE ratio for SH-SY5Y cells relative to A431 cells

Lipids have been implicated in Parkinson's Disease (PD). We therefore studied the lipid profile of the neuroblastoma SH-SY5Y cell line, which is used extensively in PD research and compared it to that of the A431 epithelial cancer cell line. We have isolated whole cell extracts (WC) and plasma membrane (PM) fractions of both cell lines. The isolates were analyzed with ³¹P NMR. We observed a significant higher abundance of phosphatidylcholine (PC) for SH-SY5Y cells for both WC (55 ± 4.1%) and PM (63.3 ± 3.1%) compared to WC (40.5 ± 2.2%) and PM (43.4 ± 1.3%) of A431. Moreover, a higher abundance of phosphatidylethanolamine was detected for the WC of A431 compared to the SH-SY5Y. Using LC-MS/MS, we also determined the relative abundance of fatty acid (FA) moieties for each phospholipid class, finding that SH-SY5Y had high polyunsaturated FA levels, including arachidonic acid compared to A431 cells. When comparing our results to reported compositions of brain and neural tissues, we note the much higher PC levels, as well as very low levels of docosahexaenoic acid. However, relative levels of arachidonic acid and other polyunsaturated fatty acids were elevated, in line with what is desirable for a neural model system.

Molecular Engineering of Near-Infrared Fluorescent Probes for Cell Membrane Imaging

Cell membrane (CM) is a phospholipid bilayer that maintains integrity of a whole cell and relates to many physiological and pathological processes. Developing CM imaging tools is a feasible method for visualizing membrane-related events. In recent decades, small-molecular fluorescent probes in the near-infrared (NIR) region have been pursued extensively for CM staining to investigate its functions and related events. In this review, we summarize development of such probes from the aspect of design principles, CM-targeting mechanisms and biological applications. Moreover, at the end of this review, the challenges and future research directions in designing NIR CM-targeting probes are discussed. This review indicates that more efforts are required to design activatable NIR CM-targeting probes, easily prepared and biocompatible probes with long retention time regarding CM, super-resolution imaging probes for monitoring CM nanoscale organization and multifunctional probes with imaging and phototherapy effects.

Biomimetic lipid-fluorescein probe for cellular bioimaging

Fluorescence probe is one of the most powerful tools for cellular imaging. Here, three phospholipid-mimicking fluorescent probes (FP1-FP3) comprising fluorescein and two lipophilic groups of saturated and/or unsaturated C18 fatty acids were synthesized, and their optical properties were investigated. Like in biological phospholipids, the fluorescein group acts as a hydrophilic polar headgroup and the lipid groups act as hydrophobic non-polar tail groups. Laser confocal microscope images illustrated that FP3, which contains both saturated and unsaturated lipid tails, showed great uptake into the canine adipose-derived mesenchymal stem cells.

Analysis of Lipids, Fatty Acid, and Cholesterol in Membrane Microdomains

The original concept that lipid and protein components are randomly distributed in cellular membranes has been challenged by evidence of compartmentalization of such components into discrete membrane microdomains (known as lipid rafts). The lipid microdomain hypothesis has generated significant controversy and rigorous inquiry to test the idea that such domains concentrate machinery to mediate cellular processes such as signaling, synaptic plasticity, and endocytosis. As such, a large number of studies have used biochemical, cell biological, and biophysical methodologies to define the composition of membrane microdomains in experimental contexts. Although biochemical preparation strategies are not without limitations (as discussed herein), the isolation of detergent-resistant and detergent-free membrane domains can provide important information about the segregation of lipids and proteins in membranes. In this chapter, we describe methodologies to isolate membranes from cell or tissue sources with biophysical/biochemical properties of membrane microdomains and also provide methods for subsequent classical or mass spectrometry-based lipid analytical approaches.

Regulation of signal transduction by gangliosides in lipid rafts: focus on GM3-IR and GM1-TrkA interactions

The interactions between gangliosides and proteins belonging to the same or different lipid domains and their influence on physiological and pathological states have been analysed in detail. A well-known factor impacting on lipid-protein interactions and their biological outcomes is the dynamic composition of plasma membrane. This review focuses on GM1 and GM3 gangliosides because they are an integral part of protein-receptor complexes and dysregulation of their concentration shows a direct correlation with the onset of pathological conditions. We first discuss the interaction between GM3 and insulin receptor in relation to insulin responses, with an increase in GM3 correlating with the onset of metabolic dysfunction. Next, we describe the case of the GM1-TrkA interaction, relevant to nerve-cell differentiation and homeostasis as deficiency in plasma-membrane GM1 is known to promote neurodegeneration. These two examples highlight the fact that interactions between gangliosides and receptor proteins within the plasma membrane are crucial in controlling cell signalling and pathophysiological cellular states.

Ceramide Metabolism Regulated by Sphingomyelin Synthase 2 Is Associated with Acquisition of Chemoresistance via Exosomes in Human Leukemia Cells

Ceramide levels controlled by the sphingomyelin (SM) cycle have essential roles in cancer cell fate through the regulation of cell proliferation, death, metastasis, and drug resistance. Recent studies suggest that exosomes confer cancer malignancy. However, the relationship between ceramide metabolism and exosome-mediated cancer malignancy is unclear. In this study, we elucidated the role of ceramide metabolism via the SM cycle in exosomes and drug resistance in human leukemia HL-60 and adriamycin-resistant HL-60/ADR cells. HL-60/ADR cells showed significantly increased exosome production and release compared with parental chemosensitive HL-60 cells. In HL-60/ADR cells, increased SM synthase (SMS) activity reduced ceramide levels, although released exosomes exhibited a high ceramide ratio in both HL-60- and HL-60/ADR-derived exosomes. Overexpression of SMS2 but not SMS1 suppressed intracellular ceramide levels and accelerated exosome production and release in HL-60 cells. Notably, HL-60/ADR exosomes conferred cell proliferation and doxorubicin resistance properties to HL-60 cells. Finally, microRNA analysis in HL-60 and HL-60/ADR cells and exosomes showed that miR-484 elevation in HL-60/ADR cells and exosomes was associated with exosome-mediated cell proliferation. This suggests that intracellular ceramide metabolism by SMS2 regulates exosome production and release, leading to acquisition of drug resistance and enhanced cell proliferation in leukemia cells.

A set of gene knockouts as a resource for global lipidomic changes

Enzyme specificity in lipid metabolic pathways often remains unresolved at the lipid species level, which is needed to link lipidomic molecular phenotypes with their protein counterparts to construct functional pathway maps. We created lipidomic profiles of 23 gene knockouts in a proof-of-concept study based on a CRISPR/Cas9 knockout screen in mammalian cells. This results in a lipidomic resource across 24 lipid classes. We highlight lipid species phenotypes of multiple knockout cell lines compared to a control, created by targeting the human safe-harbor locus AAVS1 using up to 1228 lipid species and subspecies, charting lipid metabolism at the molecular level. Lipid species changes are found in all knockout cell lines, however, some are most apparent on the lipid class level (e.g., SGMS1 and CEPT1), while others are most apparent on the fatty acid level (e.g., DECR2 and ACOT7). We find lipidomic phenotypes to be reproducible across different clones of the same knockout and we observed similar phenotypes when two enzymes that catalyze subsequent steps of the long-chain fatty acid elongation cycle were targeted.

A novel mechanism of thrombocytopenia by PS exposure through TMEM16F in sphingomyelin synthase 1 deficiency

Membrane SM reduction by SMS1deficiency enhances PS exposure and thrombocytopenia.

Depression of membrane SM potentiates Ca²⁺ influx and PS externalization through TMEM16F.

Sphingomyelin synthase 1 (SMS1) contributes to the generation of membrane sphingomyelin (SM) and affects SM-mediated physiological functions. Here, we describe the hematologic phenotypes, such as reduced circulating platelets and dysfunctional hemostasis, in SMS1-deficient (SMS1-KO) mice. SMS1-KO mice display pathologic manifestations related to idiopathic thrombocytopenia (ITP), including relatively high amounts of peripheral blood reticulated platelets, enhanced megakaryopoiesis in the bone marrow and spleen, and splenomegaly. Deficiency of SMS1, but not SMS2, prevented SM production and enhanced phosphatidylserine (PS) externalization on the plasma membranes of platelets and megakaryocytes. Consequently, SMS1-KO platelets were excessively cleared by macrophages in the spleen. Multimer formation in the plasma membrane of TMEM16F, a known calcium (Ca²⁺)-activated nonselective ion channel and Ca²⁺-dependent PS scramblase, was enhanced; the result was PS externalization to outer leaflets through increased Ca²⁺ influx in immortalized mouse embryonic fibroblasts established from SMS1-KO mice (SMS1-KO tMEFs), as seen with SMS1-KO platelets. Thus, SMS1 deficiency changed the TMEM16F distribution on the membrane microdomain, regulating Ca²⁺ influx-dependent PS exposure. SMS1-KO tMEFs in which TMEM16F was knocked out by using the CRISPR/Cas9 system lacked both the Ca²⁺ influx and excess PS exposure seen in SMS1-KO tMEFs. Therefore, SM depletion on platelet membrane microdomains due to SMS1 deficiency enhanced PS externalization via a Ca²⁺ influx through TMEM16F activation, leading to elevated platelet clearance and causing hemostasis dysfunction through thrombocytopenia. Our current findings show that the SM-rich microdomain generated by SMS1 is a potent regulator of thrombocytopenia through TMEM16F, suggesting that its dysfunction may be a novel additional mechanism of ITP.

Tumor-Specific Delivery of 5-Fluorouracil-Incorporated Epidermal Growth Factor Receptor-Targeted Aptamers as an Efficient Treatment in Pancreatic Ductal Adenocarcinoma Models

BACKGROUNDS & AIMS

Fluoropyrimidine c (5-fluorouracil [5FU]) increasingly represents the chemotherapeutic backbone for neoadjuvant, adjuvant, and palliative treatment of pancreatic ductal adenocarcinoma (PDAC). Even in combination with other agents, 5FU efficacy remains transient and limited. One explanation for the inadequate response is insufficient and nonspecific delivery of 5FU to the tumor.

METHODS

We designed, generated, and characterized 5FU-incorporated systematic evolution of ligands by exponential enrichment (SELEX)-selected epidermal growth factor receptor (EGFR)-targeted aptamers for tumor-specific delivery of 5FU to PDAC cells and tested their therapeutic efficacy in vitro and in vivo.

RESULTS

5FU-EGFR aptamers reduced proliferation in a concentration-dependent manner in mouse and human pancreatic cancer cell lines. Time-lapsed live imaging showed EGFR-specific uptake of aptamers via clathrin-dependent endocytosis. The 5FU-aptamer treatment was equally effective in 5FU-sensitive and 5FU-refractory PDAC cell lines. Biweekly treatment with 5FU-EGFR aptamers reduced tumor burden in a syngeneic orthotopic transplantation model of PDAC, in an autochthonously growing genetically engineered PDAC model (LSL-KrasG12D/+;LSL-Trp53flox/+;Ptf1a-Cre [KPC]), in an orthotopic cell line-derived xenograft model using human PDAC cells in athymic mice (CDX; Crl:NU-Foxn1nu), and in patient-derived organoids. Tumor growth was significantly attenuated during 5FU-EGFR aptamer treatment in the course of follow-up.

CONCLUSIONS

Tumor-specific targeted delivery of 5FU using EGFR aptamers as the carrier achieved high target specificity; overcame 5FU resistance; and proved to be effective in a syngeneic orthotopic transplantation model, in KPC mice, in a CDX model, and in patient-derived organoids and, therefore, represents a promising backbone for pancreatic cancer chemotherapy in patients. Furthermore, our approach has the potential to target virtually any cancer entity sensitive to 5FU treatment by incorporating 5FU into cancer cell-targeting aptamers as the delivery platform.

How Does Liquid-Liquid Phase Separation in Model Membranes Reflect Cell Membrane Heterogeneity?

Although liquid-liquid phase separation of cytoplasmic or nuclear components in cells has been a major focus in cell biology, it is only recently that the principle of phase separation has been a long-standing concept and extensively studied in biomembranes. Membrane phase separation has been reconstituted in simplified model systems, and its detailed physicochemical principles, including essential phase diagrams, have been extensively explored. These model membrane systems have proven very useful to study the heterogeneity in cellular membranes, however, concerns have been raised about how reliably they can represent native membranes. In this review, we will discuss how phase-separated membrane systems can mimic cellular membranes and where they fail to reflect the native cell membrane heterogeneity. We also include a few humble suggestions on which phase-separated systems should be used for certain applications, and which interpretations should be avoided to prevent unreliable conclusions.

Composition of receptor tyrosine kinase-mediated lipid micro-domains controlled by adaptor protein interaction

Receptor tyrosine kinases (RTKs) are highly regulated, single pass transmembrane proteins, fundamental to cellular function and survival. Aberrancies in regulation lead to corruption of signal transduction and a range of pathological outcomes. Although control mechanisms associated with the receptors and their ligands are well understood, little is known with respect to the impact of lipid/lipid and lipid/protein interactions in the proximal plasma membrane environment. Given that the transmembrane regions of RTKs change in response to extracellular ligand binding, the lipid interactions have important consequences in influencing signal transduction. Fibroblast growth factor receptor 2 (FGFR2) is a highly regulated RTK, including under basal conditions. Binding of the adaptor protein, growth factor receptor-bound protein 2 (GRB2) to FGFR2 prevents full activation and recruitment of downstream signalling effector proteins in the absence of extracellular stimulation. Here we demonstrate that the FGFR2-GRB2 complex is sustained in a defined lipid environment. Dissociation of GRB2 from this complex due to ligand binding, or reduced GRB2 expression, facilitates the dispersion of FGFR2 into detergent-resistant membrane (DRM) micro-domains. This modification of the plasma membrane proximal to FGFR2 provides a further regulatory checkpoint which controls receptor degradation, recycling and recruitment of intracellular signalling proteins.

E-cig vapor condensate alters proteome and lipid profiles of membrane rafts: impact on inflammatory responses in A549 cells

Electronic cigarettes (e-cigs) are battery-operated heating devices that aerosolize e-liquid, typically containing nicotine and several other chemicals, which is then inhaled by a user. Over the past decade, e-cigs have gained immense popularity among both smokers and non-smokers. One reason for this is that they are advertised as a safe alternative to conventional cigarettes. However, the recent reports of e-cig use associated lung injury have ignited a considerable debate about the relative harm and benefits of e-cigs. The number of reports about e-cig-induced inflammation and pulmonary health is increasing as researchers seek to better understand the effects of vaping on human health. In line with this, we investigated the molecular events responsible for the e-cig vapor condensate (ECVC)-mediated inflammation in human lung adenocarcinoma type II epithelial cells (A549). In an attempt to limit the variables caused by longer ingredient lists of flavored e-cigs, tobacco-flavored ECVC (TF-ECVC±nicotine) was employed for this study. Interestingly, we observed significant upregulation of cytokines and chemokines (IL-6, IL-8, and MCP-1) in A549 cells following a 48 h TF-ECVC challenge. Furthermore, there was a significant increase in the expression of pattern recognition receptors TLR-4 and NOD-1, lipid raft-associated protein caveolin-1, and transcription factor NF-кB in TF-ECVC with and/or without nicotine-challenged lung epithelial cells. Our results further demonstrate the harboring of TLR-4 and NOD-1 in the caveolae of TF-ECVC-challenged A549 cells. Proteomic and lipidomic analyses of lipid raft fractions from control and challenged cells revealed a distinct protein and lipid profile in TF-ECVC (w/wo nicotine)-exposed A549 cells. Interestingly, the inflammatory effects of TF-ECVC (w/wo nicotine) were inhibited following the caveolin-1 knockdown, thus demonstrating a critical role of caveolae raft-mediated signaling in eliciting inflammatory responses upon TF-ECVC challenge. Graphical Abstract Graphical Abstract.

Ceramide/Sphingomyelin Rheostat Regulated by Sphingomyelin Synthases and Chronic Diseases in Murine Models

Ceramide and sphingomyelin (SM) are major components of the double membrane-bound sphingolipids. Ceramide is an essential bioactive lipid involved in numerous cell processes including apoptosis, necrosis, and autophagy-dependent cell death. Inversely, SM regulates opposite cellular processes such as proliferation and migration by changing receptor-mediated signal transduction in the lipid microdomain. SM is generated through a transfer of phosphocholine from phosphatidylcholine to ceramide by SM synthases (SMSs). Research during the past several decades has revealed that the ceramide/SM balance in cellular membranes regulated by SMSs is important to decide the cell fate, survival, and proliferation. In addition, recent experimental studies utilizing SMS knockout mice and murine disease models provide evidence that SMS-regulated ceramide/SM balance is involved in human diseases. Here, we review the basic structural and functional characteristics of SMSs and focus on their cellular functions through the regulation of ceramide/SM balance in membrane microdomains. In addition, we present the pathological or physiological implications of SMSs by analyzing their role in SMS-knockout mice and human disease models. This review finally presents evidence indicating that the regulation of ceramide/SM balance through SMS could be a therapeutic target for human disorders.

Membrane Rearrangements in the Maturation of Circulating Human Reticulocytes

Red blood cells (RBCs) begin their circulatory life as reticulocytes (Retics) after their egress from the bone marrow where, as R1 Retics, they undergo significant rearrangements in their membrane and intracellular components, via autophagic, proteolytic, and vesicle-based mechanisms. Circulating, R2 Retics must complete this maturational process, which involves additional loss of significant amounts of membrane and selected membrane proteins. Little is known about the mechanism(s) at the basis of this terminal differentiation in the circulation, which culminates with the production of a stable biconcave discocyte. The membrane of R1 Retics undergoes a selective remodeling through the release of exosomes that are enriched in transferrin receptor and membrane raft proteins and lipids, but are devoid of Band 3, glycophorin A, and membrane skeletal proteins. We wondered whether a similar selective remodeling occurred also in the maturation of R2 Retics. Peripheral blood R2 Retics, isolated by an immunomagnetic method, were compared with mature circulating RBCs from the same donor and their membrane protein and lipid content was analyzed. Results show that both Band 3 and spectrin decrease from R2 Retics to RBCs on a "per cell" basis. Looking at membrane proteins that are considered as markers of membrane rafts, flotillin-2 appears to decrease in a disproportionate manner with respect to Band 3. Stomatin also decreases but in a more proportionate manner with respect to Band 3, hinting at a heterogeneous nature of membrane rafts. High resolution lipidomics analysis, on the contrary, revealed that those lipids that are typically representative of the membrane raft phase, sphingomyelin and cholesterol, are enriched in mature RBCs with respct to Retics, relative to total cell lipids, strongly arguing in favor of the selective retention of at least certain subclasses of membrane rafts in RBCs as they mature from Retics. Our hypothesis that rafts serve as additional anchoring sites for the lipid bilayer to the underlying membrane-skeleton is corroborated by the present results. It is becoming ever more clear that a proper lipid composition of the reticulocyte is necessary for the production of a normal mature RBC.

Knockdown of sphingomyelin synthase 2 inhibits osteoclastogenesis by decreasing RANKL expression in mouse primary osteoblasts

Sphingomyelin is a major lipid of the plasma membrane and is enriched in microdomains of the plasma membrane that are critical for signal transduction. However, the function of sphingomyelin in the cell membrane of osteoblasts has not been clarified. Therefore, we examined how sphingomyelin synthase 2 (SMS2) affects osteoclast differentiation by osteoblasts. We knocked down the expression of SMS2 with siRNA targeting the Sgms2 gene in mouse primary osteoblasts. The effects of SMS2 knockdown in osteoblasts were examined using polymerase chain reaction and western blotting. The knockdown of SMS2 suppressed the formation of TRAP-positive multinucleated cells by co-culture of osteoblasts and bone marrow cells compared to the control. We found that receptor activator of nuclear factor κB ligand (RANKL) mRNA expression was significantly reduced by 1,25(OH)₂D₃ stimulation in SMS2 siRNA osteoblasts. The knockdown of SMS2 repressed the expression of retinoid-X-receptor-α (RXRα) regardless of 1,25(OH)₂D₃ stimulation. TRAP-positive multinucleated cell formation was significantly reduced by RXRα siRNA in osteoblasts in a co-culture system. These results suggest that SMS2 regulates osteoclast differentiation by inducing RANKL expression via RXRα.

Deficiency of sphingomyelin synthase 2 prolongs survival by the inhibition of lymphoma infiltration through ICAM-1 reduction

The tumor microenvironment (TME) formation involving host cells and cancer cells through cell adhesion molecules (CAMs) is essential for the multiple steps of cancer metastasis and growth. Sphingomyelin synthase 2 (SMS2) is involved in inflammatory diseases such as obesity and diabetes mellitus by regulation of the SM/ceramide balance. However, the involvement of SMS2 in TME formation and metastasis is largely unknown. Here, we report that SMS2-deficient (SMS2-KO) mice show suppressed the EL4 cell infiltration to liver and prolonged survival time. ICAM-1 was identified as a candidate for the inhibition of TME formation in immortalized mouse embryonic fibroblasts (tMEFs) from mRNA array analysis for CAMs. Reduced SM/ceramide balance in SMS2-KO tMEFs suppressed the attachment of EL4 cells through transcriptional reduction of ICAM-1 by the inhibition of NF-κB activation. TNF-α-induced NF-κB activation and subsequent induction of ICAM-1 were suppressed in SMS2-KO tMEFs but restored by SMS2 re-introduction. In the EL4 cell infiltration mouse model, EL4 injection increased ICAM-1 expression in WT liver but not in SMS2-KO mouse liver. Therefore, inhibition of SMS2 may be a therapeutic target to suppress the infiltration of malignant lymphoma.

Long acyl chain ceramides govern cholesterol and cytoskeleton dependence of membrane outer leaflet dynamics

The spatiotemporal dynamics of the plasma membrane is a consequence of fine-tuned interactions between membrane components. However, the precise identity of molecular factors that maintain this delicate balance, which is lost even in cell membrane derived mimics, remains elusive. Here, we use two cell lines, CHO-K1 and RBL-2H3, which show differences in outer membrane organization, dynamics, and cytoskeleton coupling, to investigate the underlying factors. To our surprise, knock-down of the cytoskeleton-interacting Immunoglobulin E receptor, which is abundant in RBL-2H3 but not in CHO-K1 cells, is not responsible for lipid confinement or cytoskeleton coupling. A subsequent lipidomic analysis of the two cell membranes revealed differences in total membrane ceramide content (C16 to C24). Analysis of the dynamics and organization of ceramide treated live cell membranes by imaging fluorescence correlation spectroscopy demonstrates that C24 and C16 saturated ceramides uniquely alter membrane dynamics by promoting the formation of cholesterol-independent domains and by elevating the inter-leaflet coupling.

The role of PS 18:0/18:1 in membrane function

Various studies have demonstrated that the two leaflets of cellular membranes interact, potentially through so-called interdigitation between the fatty acyl groups. While the molecular mechanism underlying interleaflet coupling remains to be fully understood, recent results suggest interactions between the very-long-chain sphingolipids in the outer leaflet, and phosphatidylserine PS18:0/18:1 in the inner leaflet, and an important role for cholesterol for these interactions. Here we review the evidence that cross-linking of sphingolipids may result in clustering of phosphatidylserine and transfer of signals to the cytosol. Although much remains to be uncovered, the molecular properties and abundance of PS 18:0/18:1 suggest a unique role for this lipid.

There are several lines of evidence for interactions between the two membrane leaflets in cells. In this review the authors discuss the transmembrane coupling of lipids, the involvement of phosphatidyl serine species PS 18:0/18:1, and their importance for various cellular processes.

Side-chain deuterated cholesterol as a molecular probe to determine membrane order and cholesterol partitioning

24dCho, which perfectly retains the cholesterol's membrane properties, was developed to examine cholesterol's interactions and membrane partitions using solid state ²H NMR.

Sphingolipids and lipid rafts: Novel concepts and methods of analysis

About twenty years ago, the functional lipid raft model of the plasma membrane was published. It took into account decades of research showing that cellular membranes are not just homogenous mixtures of lipids and proteins. Lateral anisotropy leads to assembly of membrane domains with specific lipid and protein composition regulating vesicular traffic, cell polarity, and cell signaling pathways in a plethora of biological processes. However, what appeared to be a clearly defined entity of clustered raft lipids and proteins became increasingly fluid over the years, and many of the fundamental questions about biogenesis and structure of lipid rafts remained unanswered. Experimental obstacles in visualizing lipids and their interactions hampered progress in understanding just how big rafts are, where and when they are formed, and with which proteins raft lipids interact. In recent years, we have begun to answer some of these questions and sphingolipids may take center stage in re-defining the meaning and functional significance of lipid rafts. In addition to the archetypical cholesterol-sphingomyelin raft with liquid ordered (Lo) phase and the liquid-disordered (Ld) non-raft regions of cellular membranes, a third type of microdomains termed ceramide-rich platforms (CRPs) with gel-like structure has been identified. CRPs are "ceramide rafts" that may offer some fresh view on the membrane mesostructure and answer several critical questions for our understanding of lipid rafts.

Prominin-1/CD133: Lipid Raft Association, Detergent Resistance, and Immunodetection

The cell surface antigen prominin‐1 (alias CD133) has gained enormous interest in the past 2 decades and given rise to debates as to its utility as a biological stem and cancer stem cell marker. Important and yet often overlooked knowledge that is pertinent to its physiological function has been generated in other systems given its more general expression beyond primitive cells. This article briefly discusses the importance of particular biochemical features of CD133 with relation to its association with membrane microdomains (lipid rafts) and proper immunodetection. It also draws attention toward the adequate use of detergents and caveats that may apply to the interpretation of the results generated. Stem Cells Translational Medicine2018;7:155–160

Membrane lipids and cell signaling

PURPOSE OF REVIEW

Reception and transmission of signals across the plasma membrane has been a function generally attributed to transmembrane proteins. In the last 3 years, however, a growing number of reports have further acknowledged important contributions played by membrane lipids in the process of signal transduction.

RECENT FINDINGS

In particular, the constituency of membrane lipids can regulate how proteins with SH2 domains and molecules like K-Ras expose their catalytic domains to the cytosol and interact with effectors and second messengers. Recent reports have also shown that the degree of saturation of phospholipids can reduce the activation of certain G-protein-coupled receptors, and signaling downstream to Toll-like receptor 4 with consequences to nuclear factor kappa B activation and inflammation. Levels of specific gangliosides in the membrane were reported to activate integrins in a cell-autonomous manner affecting tumor cell migration. Furthermore, high resolution of the association of cholesterol with the smoothened receptor has clarified its participation in sonic hedgehog signaling. These are some of the key advancements that have further propelled our understanding of the broad versatile contributions of membrane lipids in signal transduction.

SUMMARY

As we gain definitive detail regarding the impact of lipid-protein interactions and their consequences to cell function, the options for therapeutic targeting expand with the possibility of greater specificity.

The mystery of membrane organization: composition, regulation and roles of lipid rafts

Cellular plasma membranes are laterally heterogeneous, featuring a variety of distinct subcompartments that differ in their biophysical properties and composition. A large number of studies have focused on understanding the basis for this heterogeneity and its physiological relevance. The membrane raft hypothesis formalized a physicochemical principle for a subtype of such lateral membrane heterogeneity, in which the preferential associations between cholesterol and saturated lipids drive the formation of relatively packed (or ordered) membrane domains that selectively recruit certain lipids and proteins. Recent studies have yielded new insights into this mechanism and its relevance in vivo, owing primarily to the development of improved biochemical and biophysical technologies.

Sphingolipids and glycerophospholipids - The "ying and yang" of lipotoxicity in metabolic diseases

Sphingolipids in general and ceramides in particular, contribute to pathophysiological mechanisms by modifying signalling and metabolic pathways. Here, we present the available evidence for a bidirectional homeostatic crosstalk between sphingolipids and glycerophospholipids, whose dysregulation contributes to lipotoxicity induced metabolic stress. The initial evidence for this crosstalk originates from simulated models designed to investigate the biophysical properties of sphingolipids in plasma membrane representations. In this review, we reinterpret some of the original findings and conceptualise them as a sort of "ying/yang" interaction model of opposed/complementary forces, which is consistent with the current knowledge of lipid homeostasis and pathophysiology. We also propose that the dysregulation of the balance between sphingolipids and glycerophospholipids results in a lipotoxic insult relevant in the pathophysiology of common metabolic diseases, typically characterised by their increased ceramide/sphingosine pools.

Sphingomyelin generated by sphingomyelin synthase 1 is involved in attachment and infection with Japanese encephalitis virus

Japanese encephalitis virus (JEV) is a mosquito-borne RNA virus which infects target cells via the envelope protein JEV-E. However, its cellular targets are largely unknown. To investigate the role of sphingomyelin (SM) in JEV infection, we utilized SM-deficient immortalized mouse embryonic fibroblasts (tMEF) established from SM synthase 1 (SMS1)/SMS2 double knockout mice. SMS deficiency significantly reduced both intracellular and extracellular JEV levels at 48 h after infection. Furthermore, after 15 min treatment with JEV, the early steps of JEV infection such as attachment and cell entry were also diminished in SMS-deficient tMEFs. The inhibition of JEV attachment and infection were recovered by overexpression of SMS1 but not SMS2, suggesting SMS1 contributes to SM production for JEV attachment and infection. Finally, intraperitoneal injection of JEV into SMS1-deficient mice showed an obvious decrease of JEV infection and its associated pathologies, such as meningitis, lymphocyte infiltration, and elevation of interleukin 6, compared with wild type mice. These results suggest that SMS1-generated SM on the plasma membrane is related in JEV attachment and subsequent infection, and may be a target for inhibition of JEV infection.

Stressful learning paradigm precludes manifestation of cognitive ability in sphingomyelin synthase-2 knockout mice

Sphingomyelin synthases (SMSs) are enzymes converting ceramide to sphingomyelin. The behavioral phenotype attributed to their disruption has not been well described. We examined learning ability and hippocampal synaptic plasticity in mice deficient in SMS2 (SMS2 KO). In context-dependent fear learning and novel object recognition test, no difference in learning ability was detected in SMS2 KO and wild-type (WT) mice. By contrast, achievement of the Morris water maze (MWM) test was deteriorated in SMS2 KO mice. In the hippocampal CA1, while the basic synaptic transmission was normal, both short- and long-term synaptic plasticity was moderately suppressed. We interpret that the MWM test taking place in wet environment may represent learning paradigm under more stressful condition than those performed in dry conditions, and that the learning ability of SMS2 KO mice failed to manifest itself fully in stressful situations. In agreement, forced swimming induced depression-like behavior more easily in SMS2 KO mice. Mass spectrometry suggested a slightly altered species distribution of ceramide in the hippocampus of SMS2 KO mice. These findings support the proposal that altered synthesis of ceramide, which is the substrate of SMS2 and therefore expected to be modified in SMS2 KO mice, is associated with depression-like tendency in animal models and depressive disorder in humans.

Possible roles of long-chain sphingomyelines and sphingomyelin synthase 2 in mouse macrophage inflammatory response

To evaluate the precise role of sphingomyelin synthase 2 (SMS2) in sphingomyelin (SM) metabolism and their anti-inflammatory properties, we analyzed species of major SM and ceramide (Cer) (18:1, 18:0 sphingoid backbone, C14 - C26 N-acyl part) in SMS2 knockout and wild-type mouse plasma and liver using HPLC-MS. SMS2 deficiency significantly decreased very long chain SM (SM (d18:1/22:0) and SM (d18:1/24:0 or d18:0/24:1)) and increased very long chain Cer (Cer (d18:1/24:0 or d18:0/24:1) and Cer (d18:1/24:1)), but not long chain SM (SM (d18:1/16:0), SM (d18:1/18:0 or d18:0/18:1) and SM (d18:1/18:1)) in plasma. To examine the effects of SM on inflammation, we studied the role of very long chain SM in macrophage activation. Addition of SM (d18:1/24:0) strongly upregulated several macrophage activation markers, SM (d18:1/6:0) and Cer (d18:1/24:0) however, did not. It was suggested that very long chain SM but not long chain SM were decreased in SMS2-deficient mice liver and plasma. And the exogenously added very long chain SM (d18:1/24:0) could activate macrophages directly, suggesting a novel role of plasma very long chain SM in modulating macrophage activation and resulting inflammation.

Imaging Mass Spectrometry Reveals Acyl-Chain- and Region-Specific Sphingolipid Metabolism in the Kidneys of Sphingomyelin Synthase 2-Deficient Mice

Obesity was reported to cause kidney injury by excessive accumulation of sphingolipids such as sphingomyelin and ceramide. Sphingomyelin synthase 2 (SMS2) is an important enzyme for hepatic sphingolipid homeostasis and its dysfunction is considered to result in fatty liver disease. The expression of SMS2 is also high in the kidneys. However, the contribution of SMS2 on renal sphingolipid metabolism remains unclear. Imaging mass spectrometry is a powerful tool to visualize the distribution and provide quantitative data on lipids in tissue sections. Thus, in this study, we analyzed the effects of SMS2 deficiency on the distribution and concentration of sphingomyelins in the liver and kidneys of mice fed with a normal-diet or a high-fat-diet using imaging mass spectrometry and liquid chromatography/electrospray ionization-tandem mass spectrometry. Our study revealed that high-fat-diet increased C18–C22 sphingomyelins, but decreased C24-sphingomyelins, in the liver and kidneys of wild-type mice. By contrast, SMS2 deficiency decreased C18–C24 sphingomyelins in the liver. Although a similar trend was observed in the whole-kidneys, the effects were minor. Interestingly, imaging mass spectrometry revealed that sphingomyelin localization was specific to each acyl-chain length in the kidneys. Further, SMS2 deficiency mainly decreased C22-sphingomyelin in the renal medulla and C24-sphingomyelins in the renal cortex. Thus, imaging mass spectrometry can provide visual assessment of the contribution of SMS2 on acyl-chain- and region-specific sphingomyelin metabolism in the kidneys.

Cellular traffic cops: the interplay between lipids and proteins regulates vesicular formation, trafficking, and signaling in mammalian cells

Protein secretion and vesicular trafficking in mammalian cells rely on several key lipids including sphingolipids, phospholipids, and neutral lipids crucial to protein processing and other intracellular events. Proteins interact with these lipids to alter the shape of lipid bilayer, thereby playing a pivotal role in cellular sorting. Although some efforts have elucidated the role of these components, extensive studies are needed to further decipher the protein-lipid interactions along with the effect of membrane curvature and rafts in sorting of proteins. The regulatory role of proteins in subcellular localization and metabolism of lipids also needs to be described. Recent studies on the role of lipid-protein interactions in modulating membrane shape, signal transduction, and vesicular trafficking are presented in this review.