Differential localization of sphingomyelin synthase isoforms in neurons regulates sphingomyelin cluster formation

Single-Cell and Subcellular Analysis Using Ultrahigh Resolution 21 T MALDI FTICR Mass Spectrometry

The mammalian brain contains ∼20,000 distinct lipid species that contribute to its structural organization and function. The lipid profiles of cells change in response to a variety of cellular signals and environmental conditions that result in modulation of cell function through alteration of phenotype. The limited sample material combined with the vast chemical diversity of lipids makes comprehensive lipid profiling of individual cells challenging. Here, we leverage the resolving power of a 21 T Fourier-transform ion cyclotron resonance (FTICR) mass spectrometer for chemical characterization of individual hippocampal cells at ultrahigh mass resolution. The accuracy of the acquired data allowed differentiation of freshly isolated and cultured hippocampal cell populations, as well as finding differences in lipids between the soma and neuronal processes of the same cell. Differences in lipids include TG 42:2 observed solely in the cell bodies and SM 34:1;O2 found only in the cellular processes. The work represents the first mammalian single cells analyzed at ultrahigh resolution and is an advance in the performance of mass spectrometry (MS) for single-cell research.

Sphingomyelin 16:0 is a therapeutic target for neuronal death in acid sphingomyelinase deficiency

Acid sphingomyelinase deficiency (ASMD) is a lysosomal storage disorder caused by mutations in the SMPD1 gene encoding for the acid sphingomyelinase (ASM). While intravenous infusion of recombinant ASM is an effective treatment for the peripheral disease, the neurological complications of ASMD remain unaddressed. It has been shown that aberrantly high level of total brain sphingomyelin (SM) is a key pathological event leading to neurodegeneration. Using mice lacking ASM (ASMko), which mimic the disease, we here demonstrate that among the SM species, SM16:0 shows the highest accumulation and toxicity in ASMko neurons. By targeting lysosomes, SM16:0 causes permeabilization and exocytosis of these organelles and induces oxidative stress and cell death. We also show that genetic silencing of Ceramide Synthase 5, which is involved in SM16:0 synthesis and overexpressed in the ASMko brain, prevents disease phenotypes in ASMko cultured neurons and mice. The levels of SM16:0 in plasma also show a strong correlation with those in brain that is higher than in liver, even at early stages of the disease. These results identify SM16:0 both as a novel therapeutic target and potential biomarker of brain pathology in ASMD.

The Multiple Roles of Sphingomyelin in Parkinson's Disease

Advances over the past decade have improved our understanding of the role of sphingolipid in the onset and progression of Parkinson's disease. Much attention has been paid to ceramide derived molecules, especially glucocerebroside, and little on sphingomyelin, a critical molecule for brain physiopathology. Sphingomyelin has been proposed to be involved in PD due to its presence in the myelin sheath and for its role in nerve impulse transmission, in presynaptic plasticity, and in neurotransmitter receptor localization. The analysis of sphingomyelin-metabolizing enzymes, the development of specific inhibitors, and advanced mass spectrometry have all provided insight into the signaling mechanisms of sphingomyelin and its implications in Parkinson's disease. This review describes in vitro and in vivo studies with often conflicting results. We focus on the synthesis and degradation enzymes of sphingomyelin, highlighting the genetic risks and the molecular alterations associated with Parkinson's disease.

Pharmacodynamic substances in Salvia miltiorrhiza for prevention and treatment of hyperlipidemia and coronary heart disease based on lipidomics technology and network pharmacology analysis

In this study, untargeted lipidomics based on UPLC-Q/TOF-MS, network pharmacology and atomic force microscopy were used to explore the common biomarkers of hyperlipidemia and coronary heart disease, the therapeutic mechanism of the main components of Salvia miltiorrhiza as well as the action mechanism of key lipids. Firstly, the serum samples of 30 healthy people, 30 patients with coronary heart disease and 30 patients with hyperlipidemia were analyzed by using lipidomics technology to obtain biomarkers which can be used to link hyperlipidemia and coronary heart disease and to find potential targets; then, the key components and core targets of Salvia miltiorrhiza intervention in hyperlipidemia and coronary heart disease were analyzed by network pharmacology, the results were verified by atomic force microscopy. It showed that SMS2 might be the key target. And through network pharmacology and atomic force microscope analysis, it can be inferred that salvianolic acid A can combine with SMS2 to play a therapeutic role.

Impact of Intrinsic and Extrinsic Factors on Cellular Sphingomyelin Imaging with Specific Reporter Proteins

Sphingomyelin (SM) is a major sphingolipid in mammalian cells. Although SM is enriched in the outer leaflet of the cell plasma membrane, lipids are also observed in the inner leaflet of the plasma membrane and intracellular organelles such as endolysosomes, the Golgi apparatus and nuclei. SM is postulated to form clusters with glycosphingolipids (GSLs), cholesterol (Chol), and other SM molecules through hydrophobic interactions and hydrogen bonding. Thus, different clusters composed of SM, SM/Chol, SM/GSL and SM/GSL/Chol with different stoichiometries may exist in biomembranes. In addition, SM monomers may be located in the glycerophospholipid-rich areas of membranes. Recently developed SM-binding proteins (SBPs) distinguish these different SM assemblies. Here, we summarize the effects of intrinsic factors regulating the lipid-binding specificity of SBPs and extrinsic factors, such as the lipid phase and lipid density, on SM recognition by SBPs. The combination of different SBPs revealed the heterogeneity of SM domains in biomembranes.

Sphingomyelin Synthase 2 Inhibition Ameliorates Cerebral Ischemic Reperfusion Injury Through Reducing the Recruitment of Toll-Like Receptor 4 to Lipid Rafts

Background

Inflammation is recognized as an important contributor of ischemia/reperfusion (I/R) damage after ischemic stroke. Sphingomyelin synthase 2 (SMS2), the key enzyme for the biosynthesis of sphingomyelin, can function as a critical mediator of inflammation. In the present study, we investigated the role of SMS2 in a mouse model of cerebral I/R.

Methods and Results

Cerebral I/R was induced by 60‐minute transient middle cerebral artery occlusion in SMS2 knockout (SMS2^‐/‐) mice and wild‐type mice. Brain injury was determined by neurological deficits and infarct volume at 24 and 72 hours after transient middle cerebral artery occlusion. Microglia activation and inflammatory factors were detected by immunofluorescence staining, flow cytometry, western blot, and RT‐PCR. SMS2 deficiency significantly improved neurological function and minimized infarct volume at 72 hours after transient middle cerebral artery occlusion. The neuroprotective effects of SMS2 deficiency were associated with (1) suppression of microglia activation through Toll‐like receptor 4/nuclear factor kappa‐light‐chain‐enhancer of activated B cells pathway and (2) downregulation of the level of galactin‐3 and other proinflammatory cytokines. The mechanisms underlying the beneficial effects of SMS2 deficiency may include altering sphingomyelin components in lipid raft fractions, thus impairing the recruitment of Toll‐like receptor 4 to lipid rafts and subsequently reducing Toll‐like receptor 4/myeloid differentiation factor 2 complex formation on the surface of microglia.

Conclusions

SMS2 deficiency ameliorated inflammatory injury after cerebral I/R in mice, and SMS2 may be a key modulator of Toll‐like receptor 4/nuclear factor kappa‐light‐chain‐enhancer of activated B cells activation by disturbing the membrane component homeostasis during cerebral I/R.

Osteoporosis and skeletal dysplasia caused by pathogenic variants in SGMS2

Mechanisms leading to osteoporosis are incompletely understood. Genetic disorders with skeletal fragility provide insight into metabolic pathways contributing to bone strength. We evaluated 6 families with rare skeletal phenotypes and osteoporosis by next-generation sequencing. In all the families, we identified a heterozygous variant in SGMS2, a gene prominently expressed in cortical bone and encoding the plasma membrane–resident sphingomyelin synthase SMS2. Four unrelated families shared the same nonsense variant, c.148C>T (p.Arg50*), whereas the other families had a missense variant, c.185T>G (p.Ile62Ser) or c.191T>G (p.Met64Arg). Subjects with p.Arg50* presented with childhood-onset osteoporosis with or without cranial sclerosis. Patients with p.Ile62Ser or p.Met64Arg had a more severe presentation, with neonatal fractures, severe short stature, and spondylometaphyseal dysplasia. Several subjects had experienced peripheral facial nerve palsy or other neurological manifestations. Bone biopsies showed markedly altered bone material characteristics, including defective bone mineralization. Osteoclast formation and function in vitro was normal. While the p.Arg50* mutation yielded a catalytically inactive enzyme, p.Ile62Ser and p.Met64Arg each enhanced the rate of de novo sphingomyelin production by blocking export of a functional enzyme from the endoplasmic reticulum. SGMS2 pathogenic variants underlie a spectrum of skeletal conditions, ranging from isolated osteoporosis to complex skeletal dysplasia, suggesting a critical role for plasma membrane–bound sphingomyelin metabolism in skeletal homeostasis.

The identification of 6 families with childhood-onset osteoporosis with mutations in SGMS2 suggests a critical role for plasma membrane–bound sphingomyelin metabolism in skeletal homeostasis.

Role and Function of Sphingomyelin Biosynthesis in the Development of Cancer

Sphingomyelin (SM) biosynthesis represents a complex, finely regulated process, mostly occurring in vertebrates. It is intimately linked to lipid transport and it is ultimately carried out by two enzymes, SM synthase 1 and 2, selectively localized in the Golgi and plasma membrane. In the course of the SM biosynthetic reaction, various lipids are metabolized. Because these lipids have both structural and signaling functions, the SM biosynthetic process has the potential to affect diverse important cellular processes (such as cell proliferation, cell survival, and migration). Thus defects in SM biosynthesis might directly or indirectly impact the normal physiology of the cell and eventually of the organism. In this chapter, we will focus on evidence supporting a role for SM biosynthesis in specific cellular functions and how its dysregulation can affect neoplastic transformation.

Towards High-Resolution Tissue Imaging Using Nanospray Desorption Electrospray Ionization Mass Spectrometry Coupled to Shear Force Microscopy

Constant mode ambient mass spectrometry imaging (MSI) of tissue sections with high lateral resolution of better than 10 μm was performed by combining shear force microscopy with nanospray desorption electrospray ionization (nano-DESI). Shear force microscopy enabled precise control of the distance between the sample and nano-DESI probe during MSI experiments and provided information on sample topography. Proof-of-concept experiments were performed using lung and brain tissue sections representing spongy and dense tissues, respectively. Topography images obtained using shear force microscopy were comparable to the results obtained using contact profilometry over the same region of the tissue section. Variations in tissue height were found to be dependent on the tissue type and were in the range of 0-5 μm for lung tissue and 0-3 μm for brain tissue sections. Ion images of phospholipids obtained in this study are in good agreement with literature data. Normalization of nano-DESI MSI images to the signal of the internal standard added to the extraction solvent allowed us to construct high-resolution ion images free of matrix effects. Graphical Abstract ᅟ.

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.

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).

Pore-forming toxins: Properties, diversity, and uses as tools to image sphingomyelin and ceramide phosphoethanolamine

Pore-forming toxins (PFTs) represent a unique class of highly specific lipid-binding proteins. The cytotoxicity of these compounds has been overcome through crystallographic structure and mutation studies, facilitating the development of non-toxic lipid probes. As a consequence, non-toxic PFTs have been utilized as highly specific probes to visualize the diversity and dynamics of lipid nanostructures in living and fixed cells. This review is focused on the application of PFTs and their non-toxic analogs as tools to visualize sphingomyelin and ceramide phosphoethanolamine, two major phosphosphingolipids in mammalian and insect cells, respectively. This article is part of a Special Issue entitled: Pore-Forming Toxins edited by Mauro Dalla Serra and Franco Gambale.

Lipid rafts and Alzheimer's disease: protein-lipid interactions and perturbation of signaling

Lipid rafts are membrane domains, more ordered than the bulk membrane and enriched in cholesterol and sphingolipids. They represent a platform for protein-lipid and protein–protein interactions and for cellular signaling events. In addition to their normal functions, including membrane trafficking, ligand binding (including viruses), axonal development and maintenance of synaptic integrity, rafts have also been implicated in the pathogenesis of several neurodegenerative diseases including Alzheimer’s disease (AD). Lipid rafts promote interaction of the amyloid precursor protein (APP) with the secretase (BACE-1) responsible for generation of the amyloid β peptide, Aβ. Rafts also regulate cholinergic signaling as well as acetylcholinesterase and Aβ interaction. In addition, such major lipid raft components as cholesterol and GM1 ganglioside have been directly implicated in pathogenesis of the disease. Perturbation of lipid raft integrity can also affect various signaling pathways leading to cellular death and AD. In this review, we discuss modulation of APP cleavage by lipid rafts and their components, while also looking at more recent findings on the role of lipid rafts in signaling events.

Deficiency of sphingomyelin synthase-1 but not sphingomyelin synthase-2 causes hearing impairments in mice

Sphingomyelin (SM) is a sphingolipid reported to function as a structural component of plasma membranes and to participate in signal transduction. The role of SM metabolism in the process of hearing remains controversial. Here, we examined the role of SM synthase (SMS), which is subcategorized into the family members SMS1 and SMS2, in auditory function. Measurements of auditory brainstem response (ABR) revealed hearing impairment in SMS1−/− mice in a low frequency range (4–16 kHz). As a possible mechanism of this impairment, we found that the stria vascularis (SV) in these mice exhibited atrophy and disorganized marginal cells. Consequently, SMS1−/− mice exhibited significantly smaller endocochlear potentials (EPs). As a possible mechanism for EP reduction, we found altered expression patterns and a reduced level of KCNQ1 channel protein in the SV of SMS1−/− mice. These mice also exhibited reduced levels of distortion product otoacoustic emissions. Quantitative comparison of the SV atrophy, KCNQ1 expression, and outer hair cell density at the cochlear apical and basal turns revealed no location dependence, but more macrophage invasion into the SV was observed in the apical region than the basal region, suggesting a role of cochlear location-dependent oxidative stress in producing the frequency dependence of hearing loss in SMS1−/− mice. Elevated ABR thresholds, decreased EPs, and abnormal KCNQ1 expression patterns in SMS1−/− mice were all found to be progressive with age. Mice lacking SMS2, however, exhibited neither detectable hearing loss nor changes in their EPs. Taken together, our results suggest that hearing impairments occur in SMS1−/− but not SMS2−/− mice. Defects in the SV with subsequent reductions in EPs together with hair cell dysfunction may account, at least partially, for hearing impairments in SMS1−/− mice.