1. Introduction
Biological membranes are mainly composed of phospholipids, sphingolipids, cholesterol, and membrane-associated proteins. These molecules are nonhomogeneously distributed in membranes and can rearrange, leading to the formation of membrane “domains” with highly differentiated molecular compositions and supramolecular architectures, which are stabilized by lateral interactions among the membrane components. Although glycosphingolipids (GSLs) were originally thought to be structural components of plasma membranes [ 1 ], several experiments suggested that GSLs are involved in the regulation of numerous cellular functions [ 2 ]. The membrane lipid bilayer is a stable structure, constituting a physical boundary between intra- and extracellular environments. GSLs are expressed on the surface of cellular membranes. Based on their physicochemical properties, especially their many hydroxyl and acetamide groups, which can act as hydrogen bond donors and acceptors, GSLs form clusters through cis interactions [ 2 ]. There is a general consensus on the roles played by the ceramide moiety of GSLs in promoting the formation and stabilization of membrane lipid domains. In addition, ceramide was also shown to be involved in GSL-mediated functions and several biological activities [ 3 , 4 ]. Ceramide is composed of sphingosine and fatty acid chains. We recently showed that very long fatty acid chains of ceramide, such as C24:0 and C24:1, are responsible for the direct connection between lactosylceramide (LacCer, CDw17) and palmitoylated signal transducer molecules [ 5 ]. Moreover, the phosphorylated product of sphingosine, sphingosine-1-phosphate (S1P), was shown to be important in immunological, especially inflammatory reactions [ 4 , 6 ].
This review describes the role of the fatty acid chains of ceramide in GSL-mediated outside-in signaling in promoting GSL-enriched domain-mediated cellular functions, as well as the activities of S1P in inflammatory reactions of keratinocytes in human.
2. Organization of GSL-Enriched Lipid Microdomains
GSLs on biological membranes tend to form specific domains with several types of molecules. The most studied GSL-enriched domains are membrane lipid microdomains, called lipid rafts, defined by their GSL- and cholesterol-rich nature, enrichment in GPI-anchored proteins and membrane-anchored signaling molecules, and cytoskeletal association [ 7 , 8 ]. As shown in artificial membrane models, GSLs tend to form clusters [ 9 ], with this cluster formation confirmed in intact cells by immunoelectron microscopy [ 10 – 12 ]. The GSL-enriched microdomains on plasma membranes have a diameter of 50–100 nm and include signal transducer molecules, such as Src family kinases [ 11 , 12 ]. GSLs that contain saturated fatty acid chains with higher transition temperatures [ 13 ] show ordered, less fluid, liquid phase. Cholesterol is composed of a highly hydrophobic sterol-ring system and 3-hydroxy moiety, the only hydrophilic part of the molecule. The small cholesterol sterol-ring system and the ceramide moiety of sphingolipids are thought to interact via hydrogen bonds and hydrophobic van der Waal's interactions [ 14 ]. In addition, hydrophilic interactions between sugar moieties of GSLs promote the lateral association of GSLs and cholesterol. In contrast, phospholipids have low acyl chain melting temperatures and unsaturated acyl chains. Phospholipids tend to be loosely packaged in bilayers, resulting in the formation of liquid-disordered membranes that allow rapid lateral and rotational movement of lipids [ 15 ]. These interactions result in the separation of GSL- and cholesterol-enriched lipid microdomains from other phospholipids in the cell membrane and the formation of distinct domains.
Electron microscopy using labeled anti-GSL antibodies has revealed GSL clusters on the surface of glycosphingolipid/phosphatidylcholine (PC) liposomes, even in the absence of sphingomyelin (SM) and cholesterol [ 2 ]. LacCer forms clusters, consisting of LacCer-enriched microdomains, on plasma membranes [ 11 ]. The anti-LacCer mAbs T5A7 and Huly-m13 recognized LacCer on human neutrophils, but only T5A7 recognized LacCer on mouse neutrophils. Interestingly, Huly-m13 but not T5A7 can be used for immunoprecipitation [ 16 ], suggesting a difference in binding and/or cluster formation of Huly-m13 and T5A7 to LacCer-enriched microdomains. Indeed, stimulated emission depletion (STED) superresolution microscopy showed that T5A7 and Huly-m13 bind to different regions of the same LacCer/dioleoylphosphatidylcholine (DOPC) liposomes ( Figure 1 ) [ 17 ]. LacCer-enriched microdomains are composed of LacCer, SM, phospholipids, and cholesterol. Surface plasmon resonance analysis showed that reduction of the LacCer content in the DOPC/cholesterol/LacCer/SM lipid layer markedly decreased Huly-m13 but not T5A7 binding to LacCer [ 17 ], suggesting that the content of LacCer in LacCer-enriched microdomains affects the binding avidity of Huly-m13 to LacCer. In contrast, the molecular species of PC, including DOPC, dipalmitoylphosphatidylcholine (DPPC), and palmitoyl-oleoyl-phosphatidylcholine (POPC), did not affect the binding avidity of Huly-m13 to LacCer-coated plastic wells. Lactose inhibited the binding of Huly-m13 to LacCer/DOPC liposome-coated and DOPC/LacCer mixture-coated plastic wells, suggesting that Huly-m13 binds only to LacCer clusters in LacCer-enriched microdomains. In contrast, the binding avidity of T5A7 to LacCer-coated plastic wells was much weaker than its binding avidity to DOPC/LacCer-, POPC/LacCer-, and DPPC/LacCer mixture-coated wells, suggesting that the binding of T5A7 to LacCer is affected by PC. The ability of lactose to inhibit the binding of T5A7 to DOPC/LacCer liposome-coated plastic wells was similar to its ability to inhibit the binding of Huly-m13 [ 17 ]. In contrast, lactose inhibition of T5A7 binding to DOPC/LacCer mixture-coated plastic wells was significantly lower than its inhibition of Huly-m13 binding, suggesting that T5A7 recognizes the PC-enhanced three-dimensional structure of LacCer clusters. Thus, Huly-m13 may bind to the core region of lactose clusters in LacCer-enriched domains, while T5A7 binds to “dispersed” LacCer clusters in the phase boundary regions of these microdomains. These findings suggest that the specificities of these antibodies against the same GSLs are dependent on the organizations of the GSLs and molecules surrounding the GSL-enriched domains.
3. GSL Metabolism Diseases
Disorders of the degradation of GSLs sometimes cause human diseases [ 18 , 19 ]. For degradation, GSLs are endocytosed and reach endosomes and other organelles. Then, those molecules are constitutively degraded by their suitable catabolic enzymes. When the activities of lysosomal enzymes are impaired, degradation is not able to proceed normally and undegraded molecules accumulate in the organelle and intracellular membranes, causing several metabolism diseases. For instance, genetic disorder of glucocerebrosidase (GBA) (EC 3.2.1.45; [ 20 ]), Gaucher disease, results in accumulation of GlcCer and its deacetylated form glucosylsphingosine is caused by abnormality of GBA. Gaucher disease is a multisystem disorder whose features include peripheral blood cytopenias, hepatosplenomegaly, bone disease, and neurological manifestations in some cases [ 21 ]. The form of intravenous enzyme replacement therapy in the 1990s has been developed and resulted in dramatic improvements in haematological and visceral disease [ 22 ]. Recognition of complications, including multiple myeloma and Parkinson disease, has challenged the traditional macrophage-centric view of the pathophysiology of this disorder. However, the pathways by which enzyme deficiency results in the clinical manifestations of this disorder remain obscure. In spinal cords of amyotrophic lateral sclerosis (ALS) patients, levels of GM1, GM3, LacCer, GlcCer, GalCer, and ceramide were significantly elevated [ 23 ]. Furthermore, glucocerebrosidase-1, glucocerebrosidase-2, hexosaminidase, galactosylceramidase, α -galactosidase, and β -galactosidase activities were also elevated in those patients. Inhibition of glucosylceramide synthesis accelerated disease course in ALS model mice, whereas infusion of exogenous GM3 significantly slowed the onset of paralysis and increased survival. These observations suggest that GSLs and their metabolism are likely important participants in pathogenesis of ALS. Further studies about GSL metabolism pathways in GSL-related disease will serve to advance our understanding of other associated disorders.
4. GSL- and Ceramide-Enriched Membrane Microdomains Are Binding Targets for Pathogenic Microorganisms
Over the last 30 years, many studies have indicated that GSLs expressed on the cell surface may act as binding sites for microorganisms. The binding avidities of microorganisms to several types of GSL [ 24 – 27 ] suggest that GSLs are involved in host-pathogen interactions. Indeed, microorganisms have been shown to recognize and enter host cells via GSL-enriched membrane microdomains on the cells [ 28 ]. Among GSLs, LacCer has been well described to bind to several kinds of microorganisms, including viruses and fungi [ 27 ]. For instance, Candida albicans specifically bind to LacCer though the binding of β -1,6-long glucosyl side-chain-branched β -1,3-glucan to LacCer-enriched domains [ 26 , 29 ]. It is also well known that microorganisms-derived toxins, such as Shiga toxin, specifically bind to GSLs [ 30 – 32 ]. Furthermore, a sphingolipid metabolite, ceramide, has been demonstrated to play a crucial role in pulmonary infection and inflammation [ 33 ].Ceramide, which is degraded product of GSLs and sphingomyelin, has been reported to form ceramide-rich membrane platforms and involve uptake of several microorganisms including Pseudomonas aeruginosa . Abnormal amounts of enzymes involved in the synthesis of ceramide have been demonstrated in emphysematic smokers and in patients with severe sepsis [ 34 ]. Therefore, GSLs and their metabolites play important roles in infection and inflammation.
5. Fatty Acid Chains of Ceramide Are Indispensable for GSL-Mediated Signaling
GSLs have been reported to interact with membrane proteins and modulate the properties of these proteins [ 2 , 25 ]. In addition, certain proteins, including glycosylphosphatidylinositol- (GPI-) anchored and palmitoylated proteins, tend to enter GSL-enriched membrane microdomains [ 13 ]. These observations suggested that GSLs may be involved in transferring information across membranes. However, the mechanism by which GSLs interact with proteins and mediate outside-in signaling is unclear. The ceramide moiety consists of a long chain base linked to a fatty acid chain. Sphingosine containing C18 carbons [(2 S ,3 R ,4 E )-2-amino-1,3-dihydroxy-octadecene] is generally the main structure in mammals, but a structure containing 20 carbons is relatively abundant in neurons. However, the fatty acid content of GSL ceramide is highly heterogeneous [ 35 ]. Ceramide is synthesized by ceramide synthases (CerS) 1–6, each of which uses a restricted subset of fatty acyl-CoAs for N-acylation of the sphingoid long chain base [ 36 ]. The expression levels of genes encoding CerS are tissue specific, suggesting that the molecular varieties and expression patterns of GSLs are associated with the functions of these cells [ 37 ].
Although GSL-enriched microdomains have been implicated in a number of important membrane events [ 2 , 38 , 39 ], the molecular mechanisms responsible for GSL-mediated cell functions are still unclear. One of the main issues centers around the association of GSLs with signal transducer molecules localized on the cytosolic side. However, we recently analyzed LacCer-enriched microdomains in human neutrophilic lineage cells [ 38 ]. LacCer, along with the Src family kinase Lyn, forms lipid microdomains on the plasma membranes of human neutrophils and is involved in several cellular functions, including chemotaxis, phagocytosis, and superoxide generation, highly dependent on Lyn [ 16 , 29 , 38 ]. HL-60 cells differentiated into neutrophilic lineage cells by DMSO (D-HL-60) were found to acquire superoxide generating activity, but not through LacCer, despite their expression of LacCer on plasma membranes [ 38 ]. Most LacCer and Lyn were recovered in the microdomain fractions of neutrophils and D-HL-60 cells. Lipidomics analysis revealed that LacCer in the neutrophil plasma membrane was mainly composed of molecular species containing C16:0, C24:1, and C24:0 fatty acid chains, whereas over 70% of LacCer in the plasma membranes of D-HL-60 cells contained C16:0 fatty acid chains, but only about 14% were C24:1 and C24:0 [ 11 ]. Lyn was immunoprecipitated by anti-LacCer antibody in neutrophils but not D-HL-60 cells. Importantly, Lyn was coimmunoprecipitated by anti-LacCer antibody from the detergent resistant membrane (DRM) fraction of plasma membranes from C24:0 and C24:1, but not C16:0 or C22:0, LacCer-loaded D-HL-60 cells. Anti-LacCer antibody induced superoxide generation from D-HL-60 cells loaded with C24:0-LacCer, but not C16:0-LacCer. Lyn colocalized with LacCer-enriched domains of D-HL-60 cells loaded with C24:0-LacCer, but not C16:0-LacCer. These results suggested that the C24 fatty acid chain of LacCer is indispensable for connecting Lyn with LacCer-enriched microdomains. Knockdown of Lyn molecules by human Lyn-specific short interfering RNA (siRNA) in D-HL-60 cells completely abolished the effects of C24:1-LacCer loading function [ 11 ], suggesting that Lyn is crucial for C24-LacCer-mediated neutrophil function. Experiments using azide-photoactivatable tritium-labeled C24- and C16-LacCer revealed that C24- but not C16-LacCer directly associated with Lyn and a heterotrimeric G protein subunit G α i. These results confirm a specific direct interaction between C24-LacCer and the signal transduction molecules Lyn and G α i, which are associated with the cytoplasmic layer via palmitic acid chains ( Figure 2 ). LacCer species with very long fatty acids are indispensable for Lyn-coupled LacCer-enriched membrane microdomain-mediated neutrophil functions.
GPI-anchored proteins are composed of glycerol phospholipids, which do not have C24 fatty acid chains, suggesting that GPI-anchored proteins are not able to form large clusters by themselves and cannot directly connect with signal transduction molecules through fatty acid chains. To mediate cell functions, GPI-anchored proteins require signal transduction molecule-coupled transmembrane proteins or GSL-enriched domains, such as LacCer-enriched domains [ 40 ]. Further studies are required to determine the organization and signaling mechanisms of membrane microdomains.
6. Role of Ceramide Metabolite Sphingosine-1-phosphate in Immunological Reactions of Human Keratinocytes
The epidermis consists of a single layer of proliferating undifferentiated keratinocytes, the stratum basale, and several superficial layers of the stratum spinosum and stratum granulosum (SG), which form the stratum corneum (SC). The SC acts as an air-liquid interface barrier to avoid drying of tissues in contact with air. Ceramide is the main component of SC and is important for the water retention and permeability barrier functions of SC. Ceramides account for 30–40% of SC lipids [ 41 , 42 ]. All ceramide molecules in the SC are derived from GlcCer and SM [ 43 ]. CDase hydrolyzed ceramide to yield sphingosine and fatty acids. Sphingosine can be phosphorylated by sphingosine kinase to form S1P, a molecule involved in a wide range of cellular functions, including growth, differentiation, survival, chemotaxis, angiogenesis, and embryogenesis, in various types of cells [ 44 , 45 ]. S1P was shown to inhibit keratinocyte proliferation, to promote corneocyte differentiation [ 46 ], and to chemoattract keratinocytes. Roles of S1P in skin immunological functions have been demonstrated in mouse models [ 45 , 47 – 51 ]. Mice are the good experimental tool of choice for the majority of immunologists, and the study of immune responses in mice has provided considerable insight into human immune system function. However, there are significant differences in immunological reactions between mice and human [ 52 ]. Little is known, however, about the role of ceramide metabolites in the immunological functions of differentiating keratinocytes.
A neutral CDase from Pseudomonas aeruginosa AN17 (PaCDase) isolated from a patient with atopic dermatitis (AD) was shown to require detergents to hydrolyze ceramide [ 53 ]. Staphylococcus aureus -derived lipids, which consist primarily of cardiolipin and phosphatidylglycerol, enhanced the PaCDase hydrolysis of normal ceramide and of human skin-specific omega-hydroxyacyl ceramide in the absence of detergents [ 11 ]. A three-dimensionally cultured human primary keratinocyte (3D keratinocyte) culture system has been utilized to simulate epidermal differentiation at its air-liquid interface, resulting in the generation of basal, spinous, and granular layers and an SC, with the latter displaying permeability barrier functions [ 54 ]. Treatment of 3D keratinocytes with PaCDase and water-soluble stimulants of keratinocytes, including trypsin, Dermatophagoides pteronyssinus class 1 allergen (Der p1), and Dermatophagoides farinae allergen (Der f1) had no effect on the expression of any of the genes in our DNA microarray analysis [ 55 ], indicating that the SC of the 3D keratinocyte culture acts as a permeability barrier. Triton X-100 is a detergent that reduces permeability barrier functions, thereby moderately increasing transepidermal water loss and the production of erythema on human skin [ 56 ]. In the presence of 0.1% Triton X-100, PaCDase markedly enhanced TNF- α mRNA expression in 3D keratinocytes, an increase not observed in cells treated with Triton X-100 alone [ 55 ]. TNF- α mRNA expression was not enhanced by heat-inactivated or mutant PaCDase, suggesting that ceramide metabolites induce TNF- α mRNA expression in keratinocytes. TNF- α , a critical cytokine in several dermatological diseases [ 57 ], is secreted by keratinocytes [ 58 ] and shown to be involved in the progression of atopic dermatitis (AD) [ 59 ]. Among the metabolites of ceramide, only sphingosine and S1P enhanced TNF- α mRNA levels in 3D keratinocytes. S1P is synthesized from sphingosine by sphingosine kinase (SphK) and stimulates 3D keratinocytes through specific receptors [ 60 ]. Both the specific SphK inhibitor CAS 1177741-83-1 and the S1P receptor antagonist VPC 23019 suppressed the PaCDase-induced expression of TNF- α mRNA in 3D keratinocytes. S1P is generally considered to stimulate cells through plasma membrane G protein-coupled receptors, for example, S1P1–S1P5 [ 61 ]. S1P was recently shown to activate NF- κ B [ 62 ] independently of S1P receptors [ 63 ]. However, VPC2301, a competitive antagonist for S1P1 and S1P3 receptors, inhibited the PaCDase-enhanced gene expression not only of TNF- α but also of endothelin-1 and IL-8 [ 55 ]. Thus, the S1P-induced production of these inflammatory mediators is mediated by S1P receptors in human primary keratinocytes in a 3D culture system. cDNA microarray analysis showed that S1P strongly upregulated the expression of endothelin-1, CXCL1, TNF- α , β -defensin 5, IL-8, CXCL2, interferon regulatory factor 1, GADD45 gamma, and IL-23 α subunit mRNAs [ 55 ]. IL-8, CXCL1, and CXCL2 have been reported to be upregulated in the lesional skin of patients with AD and psoriasis [ 64 ]. S1P also enhanced the expression of claudin-4 mRNA, which has been observed in more layers of psoriatic than normal epidermis [ 65 ]. TNF- α can induce the production of endothelin-1 and IL-8 by human keratinocytes [ 66 , 67 ]. Epidermal keratinocytes produce and respond to TNF- α via TNFR1 [ 68 ]. Infliximab, a chimeric IgG1 κ monoclonal antibody against human TNF- α , inhibits the TNF- α -mediated production of IL-8 by keratinocytes [ 69 ]. PaCDase-induced phosphorylation of NF- κ B p65 was markedly suppressed by infliximab [ 55 ]. The NF- κ B inhibitor curcumin inhibited PaCDase-induced expression of IL-8 and endothelin-1 mRNAs but not of TNF- α mRNA. TNF- α induces IL-8 production via NF- κ B [ 70 ]. Therefore, it is likely that S1P induces TNF- α production and release from 3D keratinocytes via S1P receptors, resulting in TNF- α induction of cytokine production through NF- κ B-mediated signal transduction ( Figure 3 ). TNF- α is a critical cytokine in psoriatic immunopathology, and the development of an effective strategy is required to counteract its effects [ 57 ]. Infliximab, which is used to treat patients with plaque psoriasis, psoriatic arthritis, pustular psoriasis (excluding localized type), and psoriatic erythroderma [ 71 , 72 ], downregulates antiapoptotic proteins in regressing psoriatic skin [ 72 ]. The effects of infliximab have also been evaluated in other inflammatory dermatoses and in systemic diseases involving the skin, pityriasis rubra pilaris, pyoderma gangrenosum, and cutaneous sarcoidosis [ 73 ]. AD is characterized by a marked reduction in ceramides in the SC of lesional and nonlesional forearms [ 74 , 75 ] and by increased activities of the enzymes ceramidase (CDase). The metabolic conversion of ceramide to S1P has been found to protect keratinocytes against UVB-induced, ceramide-mediated apoptosis [ 76 ]. These observations suggest that ceramide metabolites, especially S1P, are involved in AD. AD is a common pruritic, inflammatory skin disorder [ 77 ]. Chronic, localized, or even generalized pruritus is the diagnostic hallmark of AD. Histamine H1-receptor blockers are used to treat all types of itch resulting from serious skin diseases, such as AD, as well as from renal and liver diseases. However, they often lack efficacy in chronic itch, a profound clinical problem that decreases quality of life [ 78 ]. Nerve density in the epidermis is partly involved in itch sensitization in pruritic skin diseases, such as AD [ 79 ]. Endothelin-1 has been shown to elicit itch in humans [ 80 – 82 ]. The molecular pathways that contribute to the transduction of itch responses to endothelin-1 do not require either PLC β 3 or TRPV1 of neurons, which mediate histamine- and serotonin-induced itch responses, respectively [ 83 ]. Thus, keratinocyte-produced S1P may be involved in endothelin-1-mediated pruritus in AD. Therefore, atopic dermatitis may be exacerbated by treatment with S1P analogue FTY720.
7. Conclusion
Several reports have described the roles of ceramide metabolites in immunological and inflammatory diseases [ 84 – 88 ]. However, the physiological roles of GSL-enriched microdomains are largely undetermined, although much is known about the organization and functions of LacCer-enriched microdomains [ 24 , 89 , 90 ]. The analogous patterns of GSLs and motifs of PAMPs result in the generation of autoantibodies against these GSLs, inducing severe autoimmune inflammatory diseases [ 91 ]. Antibodies against neuronal tissues are involved in immune-mediated neurological disorders, with expression of several of these antibodies found to correlate with the pathophysiology of these diseases [ 92 , 93 ]. Therefore, elucidation of their organization and structural specificities, based on interactions between GSLs and surrounding molecules, are important for understanding the physiological functions of GLS-enriched microdomains and their related diseases.
Acknowledgments
This study was supported in part by a Grant-in-Aid (S1201013, S1311011) from the Foundation of Strategic Research Projects in Private Universities and by a Grant-in-Aid for Exploratory Research (24659293) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan.
Abbreviations
Glycosphingolipid
Pseudomonas aeruginosa -derived neutral ceramidase
Sphingosine-1-phosphate
Three-dimensionally cultured human primary keratinocytes
Sphingosine kinase.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
1 Ogretmen B. Hannun Y. A. Biologically active sphingolipids in cancer pathogenesis and treatment Nature Reviews Cancer 2004 4 8 604 616 10.1038/nrc1411 2-s2.0-3543114272 15286740
2 Hakomori S. Structure, organization, and function of glycosphingolipids in membrane Current Opinion in Hematology 2003 10 1 16 24 10.1097/00062752-200301000-00004 2-s2.0-0037215494 12483107
3 Iwabuchi K. Nakayama H. Iwahara C. Takamori K. Significance of glycosphingolipid fatty acid chain length on membrane microdomain-mediated signal transduction FEBS Letters 2010 584 9 1642 1652 10.1016/j.febslet.2009.10.043 2-s2.0-77951932472 19852959
4 Snider A. J. Orr Gandy K. A. Obeid L. M. Sphingosine kinase: role in regulation of bioactive sphingolipid mediators in inflammation Biochimie 2010 92 6 707 715 10.1016/j.biochi.2010.02.008 2-s2.0-77953293446 20156522 PMC2878898
5 Chiricozzi E. Ciampa M. G. Brasile G. Direct interaction, instrumental for signaling processes, between LacCer and Lyn in the lipid rafts of neutrophil-like cells Journal of Lipid Research 2015 56 1 129 141 10.1194/jlr.m055319 2-s2.0-84920651531 25418321 PMC4274061
6 Gonzalez-Cabrera P. J. Brown S. Studer S. M. Rosen H. S1P signaling: new therapies and opportunities F1000Prime Reports 2014 6, article 109 10.12703/p6-109 2-s2.0-84914809927 PMC4251414 25580263
7 Simons K. Ikonen E. Functional rafts in cell membranes Nature 1997 387 6633 569 572 10.1038/42408 2-s2.0-0030949124 9177342
8 Pike L. J. Lipid rafts: bringing order to chaos Journal of Lipid Research 2003 44 4 655 667 10.1194/jlr.r200021-jlr200 2-s2.0-0041494100 12562849
9 Prinetti A. Loberto N. Chigorno V. Sonnino S. Glycosphingolipid behaviour in complex membranes Biochimica et Biophysica Acta 2009 1788 1 184 193 10.1016/j.bbamem.2008.09.001 2-s2.0-58149201134 18835549
10 Murate M. Abe M. Kasahara K. Iwabuchi K. Umeda M. Kobayashi T. Transbilayer distribution of lipids at nano scale Journal of Cell Science 2015 128 8 1627 1638 10.1242/jcs.163105 25673880
11 Iwabuchi K. Prinetti A. Sonnino S. Involvement of very long fatty acid-containing lactosylceramide in lactosylceramide-mediated superoxide generation and migration in neutrophils Glycoconjugate Journal 2008 25 4 357 374 10.1007/s10719-007-9084-6 2-s2.0-41149105450 18041581
12 Fujita A. Cheng J. Fujimoto T. Segregation of GM1 and GM3 clusters in the cell membrane depends on the intact actin cytoskeleton Biochimica et Biophysica Acta: Molecular and Cell Biology of Lipids 2009 1791 5 388 396 10.1016/j.bbalip.2009.01.008 2-s2.0-64749090227 19830910
13 Sonnino S. Prinetti A. Mauri L. Chigorno V. Tettamanti G. Dynamic and structural properties of sphingolipids as driving forces for the formation of membrane domains Chemical Reviews 2006 106 6 2111 2125 10.1021/cr0100446 2-s2.0-33745753441 16771445
14 Mukherjee S. Maxfield F. R. Membrane domains Annual Review of Cell and Developmental Biology 2004 20 839 866 10.1146/annurev.cellbio.20.010403.095451 2-s2.0-7944236082 15473862
15 Brown D. A. London E. Structure and function of sphingolipid- and cholesterol-rich membrane rafts Journal of Biological Chemistry 2000 275 23 17221 17224 10.1074/jbc.r000005200 2-s2.0-0034625373 10770957
16 Nakayama H. Yoshizaki F. Prinetti A. Lyn-coupled LacCer-enriched lipid rafts are required for CD11b/CD18-mediated neutrophil phagocytosis of nonopsonized microorganisms Journal of Leukocyte Biology 2008 83 3 728 741 10.1189/jlb.0707478 2-s2.0-43349085988 18055569
17 Iwabuchi K. Masuda H. Kaga N. Properties and functions of lactosylceramide from mouse neutrophils Glycobiology 2015 25 6 655 668 10.1093/glycob/cwv008 25595946
18 Sandhoff K. Harzer K. Gangliosides and gangliosidoses: principles of molecular and metabolic pathogenesis The Journal of Neuroscience 2013 33 25 10195 10208 10.1523/jneurosci.0822-13.2013 2-s2.0-84879143450 23785136 PMC6618597
19 Platt F. M. Sphingolipid lysosomal storage disorders Nature 2014 510 7503 68 75 10.1038/nature13476 2-s2.0-84901950458 24899306
20 Brady R. O. Gal A. E. Kanfer J. N. Bradley R. M. The metabolism of glucocerebrosides. 3. Purification and properties of a glucosyl- and galactosylceramide-cleaving enzyme from rat intestinal tissue The Journal of Biological Chemistry 1965 240 10 3766 3770 2-s2.0-0013805379 5320641
21 Cox T. M. Schofield J. P. Gaucher's disease: clinical features and natural history Bailliere's Clinical Haematology 1997 10 4 657 689 10.1016/s0950-3536(97)80033-9 2-s2.0-0031436478 9497857
22 Thomas A. S. Mehta A. Hughes D. A. Gaucher disease: haematological presentations and complications British Journal of Haematology 2014 165 4 427 440 10.1111/bjh.12804 2-s2.0-84899475987 24588457
23 Dodge J. C. Treleaven C. M. Pacheco J. Glycosphingolipids are modulators of disease pathogenesis in amyotrophic lateral sclerosis Proceedings of the National Academy of Sciences of the United States of America 2015 112 26 8100 8105 10.1073/pnas.1508767112 26056266 PMC4491749
24 Nakayama H. Ogawa H. Takamori K. Iwabuchi K. GSL-enriched membrane microdomains in innate immune responses Archivum Immunologiae et Therapiae Experimentalis 2013 61 3 217 228 10.1007/s00005-013-0221-6 2-s2.0-84878014939 23456206
25 Hakomori S.-I. Handa K. Iwabuchi K. Yamamura S. Prinetti A. New insights in glycosphingolipid function: ‘glycosignaling domain,’ a cell surface assembly of glycosphingolipids with signal transducer molecules, involved in cell adhesion coupled with signaling Glycobiology 1998 8 10 12 18 10.1093/oxfordjournals.glycob.a018822 9840984
26 Jimenez-Lucho V. Ginsburg V. Krivan H. C. Cryptococcus neoformans , Candida albicans, and other fungi bind specifically to the glycosphingolipid lactosylceramide (GAl β 1-4Glc β 1-1Cer), a possible adhesion receptor for yeasts Infection and Immunity 1990 58 7 2085 2090 2-s2.0-0025294374 2194958 10.1128/iai.58.7.2085-2090.1990 PMC258780
27 Karlsson K.-A. Animal glycolipids as attachment sites for microbes Chemistry and Physics of Lipids 1986 42 1–3 153 172 10.1016/0009-3084(86)90050-2 2-s2.0-0023050967 2435422
28 Mañes S. del Real G. Martínez-A C. Pathogens: raft hijackers Nature Reviews Immunology 2003 3 7 557 568 10.1038/nri1129 2-s2.0-0142197614 12876558
29 Sato T. Iwabuchi K. Nagaoka I. Induction of human neutrophil chemotaxis by Candida albicans -derived beta-1,6-long glycoside side-chain-branched beta-glucan Journal of Leukocyte Biology 2006 80 1 204 211 10.1189/jlb.0106069 2-s2.0-33751530922 16670126
30 Zoja C. Buelli S. Morigi M. Shiga toxin-associated hemolytic uremic syndrome: pathophysiology of endothelial dysfunction Pediatric Nephrology 2010 25 11 2231 2240 10.1007/s00467-010-1522-1 2-s2.0-77957272189 20424866
31 Yamazaki Y. Horibata Y. Magatsuka Y. Hirabayashi Y. Hashikawa T. Fucoganglioside α -fucosyl( α -galactosyl)-GM1: a novel member of lipid membrane microdomain components involved in PC12 cell neuritogenesis Biochemical Journal 2007 407 1 31 40 10.1042/bj20070090 2-s2.0-35148846513 17608628 PMC2267403
32 Lingwood C. A. Role of verotoxin receptors in pathogenesis Trends in Microbiology 1996 4 4 147 153 10.1016/0966-842x(96)10017-2 2-s2.0-0029916780 8728608
33 Seitz A. P. Grassmé H. Edwards M. J. Pewzner-Jung Y. Gulbins E. Ceramide and sphingosine in pulmonary infections Biological Chemistry 2015 396 6-7 611 620 10.1515/hsz-2014-0285 25720061
34 Esen M. Schreiner B. Jendrossek V. Mechanisms of Staphylococcus aureus induced apoptosis of human endothelial cells Apoptosis 2001 6 6 431 439 10.1023/a:1012445925628 2-s2.0-0035175328 11595832
35 Kaga N. Kazuno S. Taka H. Iwabuchi K. Murayama K. Isolation and mass spectrometry characterization of molecular species of lactosylceramides using liquid chromatography-electrospray ion trap mass spectrometry Analytical Biochemistry 2005 337 2 316 324 10.1016/j.ab.2004.11.003 2-s2.0-13244296919 15691512
36 Levy M. Futerman A. H. Mammalian ceramide synthases IUBMB Life 2010 62 5 347 356 10.1002/iub.319 2-s2.0-77953561361 20222015 PMC2858252
37 Tidhar R. Ben-Dor S. Wang E. Kelly S. Merrill A. H. Jr. Futerman A. H. Acyl chain specificity of ceramide synthases is determined within a region of 150 residues in the tram-lag-CLN8 (TLC) domain The Journal of Biological Chemistry 2012 287 5 3197 3206 10.1074/jbc.m111.280271 2-s2.0-84856305649 22144673 PMC3270974
38 Iwabuchi K. Nagaoka I. Lactosylceramide-enriched glycosphingolipid signaling domain mediates superoxide generation from human neutrophils Blood 2002 100 4 1454 1464 2-s2.0-0037103213 12149231
39 Iwabuchi K. Yamamura S. Prinetti A. Handa K. Hakomori S.-I. GM3-enriched microdomain involved in cell adhesion and signal transduction through carbohydrate-carbohydrate interaction in mouse melanoma B16 cells Journal of Biological Chemistry 1998 273 15 9130 9138 10.1074/jbc.273.15.9130 2-s2.0-0032502722 9535903
40 Kusumi A. Shirai Y. M. Koyama-Honda I. Suzuki K. G. N. Fujiwara T. K. Hierarchical organization of the plasma membrane: investigations by single-molecule tracking vs. fluorescence correlation spectroscopy FEBS Letters 2010 584 9 1814 1823 10.1016/j.febslet.2010.02.047 2-s2.0-77951923713 20178787
41 Hamanaka S. Hara M. Nishio H. Otsuka F. Suzuki A. Uchida Y. Human epidermal glucosylceramides are major precursors of stratum corneum ceramides Journal of Investigative Dermatology 2002 119 2 416 423 10.1046/j.1523-1747.2002.01836.x 2-s2.0-0036038643 12190865
42 Behne M. Uchida Y. Seki T. Ortiz De Montellano P. Elias P. M. Holleran W. M. Omega-hydroxyceramides are required for corneocyte lipid envelope (CLE) formation and normal epidermal permeability barrier function Journal of Investigative Dermatology 2000 114 1 185 192 10.1046/j.1523-1747.2000.00846.x 2-s2.0-0034081229 10620136
43 Katoh M. Hamajima F. Ogasawara T. Hata K.-I. Assessment of human epidermal model LabCyte EPI-MODEL for in vitro skin irritation testing according to European Centre for the Validation of Alternative Methods (ECVAM)-validated protocol Journal of Toxicological Sciences 2009 34 3 327 334 10.2131/jts.34.327 2-s2.0-67649236933 19483386
44 Xia P. Wadham C. Sphingosine 1-phosphate, a key mediator of the cytokine network: juxtacrine signaling Cytokine and Growth Factor Reviews 2011 22 1 45 53 10.1016/j.cytogfr.2010.09.004 2-s2.0-79952250059 21051273
45 Herzinger T. Kleuser B. Schäfer-Korting M. Korting H. C. Sphingosine-1-phosphate signaling and the skin American Journal of Clinical Dermatology 2007 8 6 329 336 10.2165/00128071-200708060-00002 2-s2.0-36448996134 18039015
46 Vogler R. Sauer B. Kim D.-S. Schäfer-Korting M. Kleuser B. Sphingosine-1-phosphate and its potentially paradoxical effects on critical parameters of cutaneous wound healing Journal of Investigative Dermatology 2003 120 4 693 700 10.1046/j.1523-1747.2003.12096.x 2-s2.0-0037378439 12648236
47 Arlt O. Schwiebs A. Japtok L. Sphingosine-1-phosphate modulates dendritic cell function: focus on non-migratory effects in vitro and in vivo Cellular Physiology and Biochemistry 2014 34 1 27 44 10.1159/000362982 2-s2.0-84903983388 24977479
48 Schaper K. Dickhaut J. Japtok L. Sphingosine-1-phosphate exhibits anti-proliferative and anti-inflammatory effects in mouse models of psoriasis Journal of Dermatological Science 2013 71 1 29 36 10.1016/j.jdermsci.2013.03.006 2-s2.0-84879028625 23643308
49 Tsuji T. Yoshida Y. Iwatsuki R. Inoue M. Fujita T. Kohno T. Therapeutic approach to steroid-resistant dermatitis using novel immunomodulator FTY720 (Fingolimod) in combination with betamethasone ointment in NC/Nga mice Biological and Pharmaceutical Bulletin 2012 35 8 1314 1319 10.1248/bpb.b12-00229 2-s2.0-84864919405 22863931
50 Bäumer W. Robach K. Mischke R. Decreased concentration and enhanced metabolism of sphingosine-1-phosphate in lesional skin of dogs with atopic dermatitis: disturbed sphingosine-1-phosphate homeostasis in atopic dermatitis Journal of Investigative Dermatology 2011 131 1 266 268 10.1038/jid.2010.252 2-s2.0-78650264894 20811395
51 Reines I. Kietzmann M. Mischke R. Topical application of sphingosine-1-phosphate and FTY720 attenuate allergic contact dermatitis reaction through inhibition of dendritic cell migration Journal of Investigative Dermatology 2009 129 8 1954 1962 10.1038/jid.2008.454 2-s2.0-67651098683 19194476
52 Mestas J. Hughes C. C. W. Of mice and not men: differences between mouse and human immunology The Journal of Immunology 2004 172 5 2731 2738 10.4049/jimmunol.172.5.2731 2-s2.0-1342303482 14978070
53 Voegeli T. S. Currie R. W. SiRNA knocks down Hsp27 and increases angiotensin II-induced phosphorylated NF- κ B p65 levels in aortic smooth muscle cells Inflammation Research 2009 58 6 336 343 10.1007/s00011-009-8166-2 2-s2.0-67649551340 19247578
54 Spöler F. Först M. Marquardt Y. High-resolution optical coherence tomography as a non-destructive monitoring tool for the engineering of skin equivalents Skin Research and Technology 2006 12 4 261 267 10.1111/j.0909-752x.2006.00163.x 2-s2.0-33745702120 17026657
55 Oizumi A. Nakayama H. Okino N. Pseudomonas-derived ceramidase induces production of inflammatory mediators from human keratinocytes via sphingosine-1-phosphate PLoS ONE 2014 9 2 e89402 10.1371/journal.pone.0089402 2-s2.0-84896077471 PMC3934885 24586752
56 Kita K. Okino N. Ito M. Reverse hydrolysis reaction of a recombinant alkaline ceramidase of Pseudomonas aeruginosa Biochimica et Biophysica Acta 2000 1485 2-3 111 120 10.1016/s1388-1981(00)00029-9 2-s2.0-0034738022 10832092
57 Cho J.-W. Lee K.-S. Kim C.-W. Curcumin attenuates the expression of IL-1beta, IL-6, and TNF-alpha as well as cyclin E in TNF-alpha-treated HaCaT cells; NF-kappaB and MAPKs as potential upstream targets International Journal of Molecular Medicine 2007 19 3 469 474 2-s2.0-33947532054 17273796
58 Köck A. Schwarz T. Kirnbauer R. Human keratinocytes are a source for tumor necrosis factor α : evidence for synthesis and release upon stimulation with endotoxin or ultraviolet light The Journal of Experimental Medicine 1990 172 6 1609 1614 10.1084/jem.172.6.1609 2-s2.0-0025677334 2258696 PMC2188768
59 Boguniewicz M. Leung D. Y. M. Atopic dermatitis: a disease of altered skin barrier and immune dysregulation Immunological Reviews 2011 242 1 233 246 10.1111/j.1600-065x.2011.01027.x 2-s2.0-79959349600 21682749 PMC3122139
60 Lichte K. Rossi R. Danneberg K. Lysophospholipid receptor-mediated calcium signaling in human keratinocytes Journal of Investigative Dermatology 2008 128 6 1487 1498 10.1038/sj.jid.5701207 2-s2.0-43749120521 18172456
61 Chi H. Sphingosine-1-phosphate and immune regulation: trafficking and beyond Trends in Pharmacological Sciences 2011 32 1 16 24 10.1016/j.tips.2010.11.002 2-s2.0-78650881166 21159389 PMC3017656
62 Siehler S. Wang Y. Fan X. Windh R. T. Manning D. R. Sphingosine 1-phosphate activates nuclear factor- κ B through Edg receptors. Activation through Edg-3 and Edg-5, but not Edg-1, in human embryonic kidney 293 cells Journal of Biological Chemistry 2001 276 52 48733 48739 10.1074/jbc.m011072200 2-s2.0-0035966092 11673450
63 Alvarez S. E. Harikumar K. B. Hait N. C. Sphingosine-1-phosphate is a missing cofactor for the E3 ubiquitin ligase TRAF2 Nature 2010 465 7301 1084 1088 10.1038/nature09128 2-s2.0-77953923379 20577214 PMC2946785
64 Fujita H. Shemer A. Suárez-Fariñas M. Lesional dendritic cells in patients with chronic atopic dermatitis and psoriasis exhibit parallel ability to activate T-cell subsets The Journal of Allergy and Clinical Immunology 2011 128 3 574.e12 582.e12 10.1016/j.jaci.2011.05.016 2-s2.0-80052259758 21704361
65 Kirschner N. Poetzl C. von den Driesch P. Alteration of tight junction proteins is an early event in psoriasis: putative involvement of proinflammatory cytokines The American Journal of Pathology 2009 175 3 1095 1106 10.2353/ajpath.2009.080973 2-s2.0-70349257315 19661441 PMC2731128
66 Ahn G. Y. Butt K. I. Jindo T. Yaguchi H. Tsuboi R. Ogawa H. The expression of endothelin-1 and its binding sites in mouse skin increased after ultraviolet B irradiation or local injection of tumor necrosis factor alpha Journal of Dermatology 1998 25 2 78 84 10.1111/j.1346-8138.1998.tb02354.x 2-s2.0-6844250999 9563273
67 Lee C. S. Ko H. H. Seo S. J. Diarylheptanoid hirsutenone prevents tumor necrosis factor-alpha-stimulated production of inflammatory mediators in human keratinocytes through NF-kappaB inhibition International Immunopharmacology 2009 9 9 1097 1104 10.1016/j.intimp.2009.05.006 2-s2.0-67649400315 19464389
68 Lisby S. Faurschou A. Gniadecki R. The autocrine TNFalpha signalling loop in keratinocytes requires atypical PKC species and NF-kappaB activation but is independent of cholesterol-enriched membrane microdomains Biochemical Pharmacology 2007 73 4 526 533 10.1016/j.bcp.2006.11.006 2-s2.0-33845933899 17145042
69 Faurschou A. Gniadecki R. Wulf H. C. Infliximab inhibits DNA repair in ultraviolet B-irradiated premalignant keratinocytes Experimental Dermatology 2008 17 11 933 938 10.1111/j.1600-0625.2008.00727.x 2-s2.0-54049122989 18557931
70 Barker J. N. W. N. Jones M. L. Mitra R. S. Modulation of keratinocyte-derived interleukin-8 which is chemotactic for neutrophils and T lymphocytes The American Journal of Pathology 1991 139 4 869 876 2-s2.0-0025986985 1681733 PMC1886319
71 Torii H. Nakagawa H. Long-term study of infliximab in Japanese patients with plaque psoriasis, psoriatic arthritis, pustular psoriasis and psoriatic erythroderma Journal of Dermatology 2011 38 4 321 334 10.1111/j.1346-8138.2010.00971.x 2-s2.0-79961179812 21544940
72 Kokolakis G. Giannikaki E. Stathopoulos E. Avramidis G. Tosca A. D. Krüger-Krasagakis S. Infliximab restores the balance between pro- and anti-apoptotic proteins in regressing psoriatic lesions British Journal of Dermatology 2012 166 3 491 497 10.1111/j.1365-2133.2011.10689.x 2-s2.0-84857639526 21985184
73 Rigopoulos D. Korfitis C. Gregoriou S. Katsambas A. D. Infliximab in dermatological treatment: beyond psoriasis Expert Opinion on Biological Therapy 2008 8 1 123 133 10.1517/14712598.8.1.123 2-s2.0-38949202414 18081542
74 Imokawa G. Abe A. Jin K. Higaki Y. Kawashima M. Hidano A. Decreased level of ceramides in stratum corneum of atopic dermatitis: an etiologic factor in atopic dry skin? Journal of Investigative Dermatology 1991 96 4 523 526 10.1111/1523-1747.ep12470233 2-s2.0-0025819102 2007790
75 Spiekstra S. W. Dos Santos G. G. Scheper R. J. Gibbs S. Potential method to determine irritant potency in vitro—comparison of two reconstructed epidermal culture models with different barrier competency Toxicology in Vitro 2009 23 2 349 355 10.1016/j.tiv.2008.12.010 19135141
76 Moonen R. M. J. Reyes I. Cavallaro G. González-Luis G. Bakker J. A. Villamor E. The T1405N carbamoyl phosphate synthetase polymorphism does not affect plasma arginine concentrations in preterm infants PLoS ONE 2010 5 5 e10792 10.1371/journal.pone.0010792 2-s2.0-77956289712 PMC2876028 20520828
77 Ghazvini P. Pagan L. C. Rutledge T. K. Goodman H. S. Jr. Atopic dermatitis Journal of Pharmacy Practice 2010 23 2 110 116 10.1177/0897190009360634 2-s2.0-77953673949 21507803
78 Tominaga M. Takamori K. An update on peripheral mechanisms and treatments of itch Biological and Pharmaceutical Bulletin 2013 36 8 1241 1247 10.1248/bpb.b13-00319 2-s2.0-84883199785 23902967
79 Tominaga M. Takamori K. Itch and nerve fibers with special reference to atopic dermatitis: therapeutic implications Journal of Dermatology 2014 41 3 205 212 10.1111/1346-8138.12317 2-s2.0-84896321226 24628070
80 Ferreira S. H. Romitelli M. de Nucci G. Endothelin-1 participation in overt and inflammatory pain Journal of Cardiovascular Pharmacology 1989 13 supplement 5 S220 S222 10.1097/00005344-198900135-00065 2-s2.0-0024514405 2473319
81 Katugampola R. Church M. K. Clough G. F. The neurogenic vasodilator response to endothelin-1: a study in human skin in vivo Experimental Physiology 2000 85 6 839 846 10.1111/j.1469-445x.2000.02089.x 2-s2.0-0033670389 11187978
82 Kido-Nakahara M. Buddenkotte J. Kempkes C. Neural peptidase endothelin-converting enzyme 1 regulates endothelin 1-induced pruritus Journal of Clinical Investigation 2014 124 6 2683 2695 10.1172/jci67323 2-s2.0-84902166170 24812665 PMC4038561
83 Imamachi N. Goon H. P. Lee H. TRPV1-expressing primary afferents generate behavioral responses to pruritogens via multiple mechanisms Proceedings of the National Academy of Sciences of the United States of America 2009 106 27 11330 11335 10.1073/pnas.0905605106 2-s2.0-67650494652 19564617 PMC2708751
84 Yamaji T. Hanada K. Sphingolipid metabolism and interorganellar transport: localization of sphingolipid enzymes and lipid transfer proteins Traffic 2015 16 2 101 122 10.1111/tra.12239 2-s2.0-84921439181 25382749
85 Tang X. Benesch M. G. Brindley D. N. Lipid phosphate phosphatases and their roles in mammalian physiology and pathology Journal of Lipid Research 2015 10.1194/jlr.r058362 PMC4617392 25814022
86 Rivera I. G. Ordonez M. Presa N. Sphingomyelinase D/ceramide 1-phosphate in cell survival and inflammation Toxins 2015 7 5 1457 1466 10.3390/toxins7051457 25938271 PMC4448157
87 Nakamura H. Murayama T. The role of sphingolipids in arachidonic acid metabolism Journal of Pharmacological Sciences 2014 124 3 307 312 10.1254/jphs.13R18CP 2-s2.0-84897391468 24599139
88 MacEyka M. Spiegel S. Sphingolipid metabolites in inflammatory disease Nature 2014 510 7503 58 67 10.1038/nature13475 2-s2.0-84901936622 24899305 PMC4320971
89 Iwabuchi K. Involvement of glycosphingolipid-enriched lipid rafts in inflammatory responses Frontiers in Bioscience 2015 20 2 325 334 10.2741/4312 2-s2.0-84919725474 25553454
90 Ekyalongo R. C. Nakayama H. Kina K. Kaga N. Iwabuchi K. Organization and functions of glycolipid-enriched microdomains in phagocytes Biochimica et Biophysica Acta 2015 1851 1 90 97 10.1016/j.bbalip.2014.06.009 2-s2.0-84911905744 24968752
91 Yuki N. Guillain-Barré syndrome and anti-ganglioside antibodies: a clinician-scientist's journey Proceedings of the Japan Academy—Series B: Physical and Biological Sciences 2012 88 7 299 326 10.2183/pjab.88.299 2-s2.0-84867034919 PMC3422685 22850724
92 Diamond B. Honig G. Mader S. Brimberg L. Volpe B. T. Brain-reactive antibodies and disease Annual Review of Immunology 2013 31 345 385 10.1146/annurev-immunol-020711-075041 2-s2.0-84875531665 PMC4401150 23516983
93 Chavada G. Willison H. J. Autoantibodies in immune-mediated neuropathies Current Opinion in Neurology 2012 25 5 550 555 10.1097/wco.0b013e328357a77f 2-s2.0-84866319049 22941260