Preferential expression of phosphatidylglucoside along neutrophil differentiation pathway

Phosphate-Containing Glycolipids: A Review on Synthesis and Bioactivity

Phosphate-containing glycolipids (PcGL) are scarcer than the better understood glycolipids. They are composed of arrangements of phosphate, carbohydrates and glycerol units and are always found associated with lipids. PcGL are often found associated with cell membranes, suggesting they play roles in cell membrane structure and intercellular interactions. This article aims to provide an up-to-date overview of the existing knowledge and research on PcGL, emphasizing their synthesis and wide range of biological activities. When it comes to the synthesis of PcGL compounds, the strategies for glycosylation mainly rely on the thioglycoside donor, the trichloroacetamidate donor and halide donor strategies, while phosphorylation is stapled and falls on either phosphite chemistry or phosphoryl chloride chemistry. Certain bacteria utilize PcGLs in their pathogenicity, triggering an inflammatory response within the host's defense mechanisms. The best-known examples of these structures are teichoic acids, lipopolysaccharide and the capsular polysaccharide found in bacteria, all of which are frequently implicated in bacterial infections. Given the degree of variability within PcGL structures, they were found to display a wide range of bioactivities. PcGL compounds were found to: (1) have anti-metastatic properties, (2) behave as agonists or antagonists of platelet aggregation, (3) be mostly pro-inflammatory, (4) display antifungal and antibiotic activity and (5) have neurogenic activity.

Structures, functions, and syntheses of glycero-glycophospholipids

Biological membranes consist of integral and peripheral protein-associated lipid bilayers. Although constituent lipids vary among cells, membrane lipids are mainly classified as phospholipids, glycolipids, and sterols. Phospholipids are further divided into glycerophospholipids and sphingophospholipids, whereas glycolipids are further classified as glyceroglycolipids and sphingoglycolipids. Both glycerophospholipids and glyceroglycolipids contain diacylglycerol as the common backbone, but their head groups differ. Most glycerolipids have polar head groups containing phosphate esters or sugar moieties. However, trace components termed glycero-glycophospholipids, each possessing both a phosphate ester and a sugar moiety, exist in membranes. Recently, the unique biological activities of glycero-glycophospholipids have attracted considerable attention. In this review, we describe the structure, distribution, function, biosynthesis, and chemical synthetic approaches of representative glycero-glycophospholipids-phosphatidylglucoside (PtdGlc) and enterobacterial common antigen (ECA). In addition, we introduce our recent studies on the rare glycero-glyco"pyrophospho"lipid, membrane protein integrase (MPIase), which is involved in protein translocation across biomembranes.

Lysophosphatidylglucoside/GPR55 signaling promotes foam cell formation in human M2c macrophages

Atherosclerosis is a major cause of cerebral and cardiovascular diseases. Intravascular plaques, a well-known pathological finding of atherosclerosis, have a necrotic core composed of macrophages and dead cells. Intraplaque macrophages, which are classified into various subtypes, play key roles in maintenance of normal cellular microenvironment. Excessive uptake of oxidized low-density lipoprotein causes conversion of macrophages to foam cells, and consequent progression/exacerbation of atherosclerosis. G-protein-coupled receptor 55 (GPR55) signaling has been reported to associate with atherosclerosis progression. We demonstrated recently that lysophosphatidylglucoside (lysoPtdGlc) is a specific ligand of GPR55, although in general physiological ligands of GPR55 are poorly understood. Phosphatidylglucoside is expressed on human monocytes and can be converted to lysoPtdGlc. In the present study, we examined possible involvement of lysoPtdGlc/GPR55 signaling in foam cell formation. In monocyte-derived M2c macrophages, lysoPtdGlc/GPR55 signaling inhibited translocation of ATP binding cassette subfamily A member 1 to plasma membrane, and cholesterol efflux. Such inhibitory effect was reversed by GPR55 antagonist ML193. LysoPtdGlc/GPR55 signaling in M2c macrophages was involved in excessive lipid accumulation, thereby promoting foam cell formation. Our findings suggest that lysoPtdGlc/GPR55 signaling is a potential therapeutic target for inhibition of atherosclerosis progression.

Distribution of Glycolipids in the Plasma Membrane Monitored by Specific Probes in Combination with Sodium Dodecyl Sulfate-Digested Freeze-Fracture Replica Labeling (SDS-FRL)

Glycolipids are mainly distributed in the outer leaflet of the plasma membrane and are involved in cellular signaling by modulating the activity of cell surface receptor proteins. Glycolipids themselves also work as cell surface receptors of bacterial toxins. Anti-glycolipid antibodies are associated with various pathological conditions. The cellular distribution of glycolipids has been studied using specific toxins or antibodies. However, these proteins are multivalent and thus potentially induce the artificial aggregation of glycolipids. Since chemical fixative such as paraformaldehyde does not fix glycolipids, an alternative methodology is required to localize glycolipids with multivalent probes. Sodium dodecyl sulfate-digested freeze-fracture replica labeling (SDS-FRL) physically fixes glycolipids on the cast after quick freezing. Thus, SDS-FRL provides the opportunity to observe the natural distribution of glycolipids using multivalent probes. Here, we describe the application of SDS-FRL on the cell surface distribution of phosphatidylglucoside.

Multiplicity of Glycosphingolipid-Enriched Microdomain-Driven Immune Signaling

Glycosphingolipids (GSLs), together with cholesterol, sphingomyelin (SM), and glycosylphosphatidylinositol (GPI)-anchored and membrane-associated signal transduction molecules, form GSL-enriched microdomains. These specialized microdomains interact in a cis manner with various immune receptors, affecting immune receptor-mediated signaling. This, in turn, results in the regulation of a broad range of immunological functions, including phagocytosis, cytokine production, antigen presentation and apoptosis. In addition, GSLs alone can regulate immunological functions by acting as ligands for immune receptors, and exogenous GSLs can alter the organization of microdomains and microdomain-associated signaling. Many pathogens, including viruses, bacteria and fungi, enter host cells by binding to GSL-enriched microdomains. Intracellular pathogens survive inside phagocytes by manipulating intracellular microdomain-driven signaling and/or sphingolipid metabolism pathways. This review describes the mechanisms by which GSL-enriched microdomains regulate immune signaling.

Lysophosphatidylglucoside is a GPR55 -mediated chemotactic molecule for human monocytes and macrophages

Neutrophils undergo spontaneous apoptosis within 24-48 h after leaving bone marrow. Apoptotic neutrophils are subsequently phagocytosed and cleared by macrophages, thereby maintaining neutrophil homeostasis. Previous studies have demonstrated involvement of lysophosphatidylglucoside (lysoPtdGlc), a degradation product of PtdGlc, in modality-specific repulsive guidance of spinal sensory axons, via its specific receptor GPR55. In the present study, using human monocytic cell line THP-1 as a model, we demonstrated that lysoPtdGlc induces monocyte/macrophage migration with typical bell-haped curve and a peak at concentration 10^-9 M. Lysophosphatidylinositol (lysoPtdIns), a known GPR55 ligand, induced migration at higher concentration (10^-7 M). LysoPtdGlc-treated cells had a polarized shape, whereas lysoPtdIns-treated cells had a spherical shape. In EZ-TAXIScan (chemotaxis) assay, lysoPtdGlc induced chemotactic migration activity of THP-1 cells, while lysoPtdIns induced random migration activity. GPR55 antagonist ML193 inhibited lysoPtdGlc-induced THP-1 cell migration, whereas lysoPtdIns-induced migration was inhibited by CB₂-receptor inverse agonist. SiRNA experiments showed that GPR55 mediated lysoPtdGlc-induced migration, while lysoPtdIns-induced migration was mediated by CB₂ receptor. Our findings, taken together, suggest that lysoPtdGlc functions as a chemotactic molecule for human monocytes/macrophages via GPR55 receptor, while lysoPtdIns induces random migration activity via CB₂ receptor.

Glycolipids: Linchpins in the Organization and Function of Membrane Microdomains

Membrane microdomains, also called lipid rafts, are areas on membrane enriched in glycolipids, sphingolipids, and cholesterol. Although membrane microdomains are thought to play key roles in many cellular functions, their structures, properties, and biological functions remain obscure. Cellular membranes contain several types of glycoproteins, glycolipids, and other lipids, including cholesterol, glycerophospholipids, and sphingomyelin. Depending on their physicochemical properties, especially the characteristics of their glycolipids, various microdomains form on these cell membranes, providing structural or functional contextures thought to be essential for biological activities. For example, the plasma membranes of human neutrophils are enriched in lactosylceramide (LacCer) and phosphatidylglucoside (PtdGlc), each of which forms different membrane microdomains with different surrounding molecules and is involved in different functions of neutrophils. Specifically, LacCer forms Lyn-coupled lipid microdomains, which mediate neutrophil chemotaxis, phagocytosis, and superoxide generation, whereas PtdGlc-enriched microdomains mediate neutrophil differentiation and spontaneous apoptosis. However, the mechanisms by which these glycolipids form different nano/meso microdomains and mediate their specialized functions remain incompletely understood. This review describes current understanding of the roles of glycolipids and sphingolipids in their enriched contextures on cellular membranes, including their mechanisms of facilitation and regulation of intracellular signaling. This review also introduces new concepts about the roles of glycolipid and sphingolipid-dependent contextures in immunological functions.

Organization and functions of glycolipid-enriched microdomains in phagocytes

Populations of glycolipids change markedly during leukocyte differentiation, suggesting that these molecules are involved in biological functions. About 70% of the glycosphingolipids in human neutrophils are lactosylceramide, a molecule also expressed on monocytes and dendritic cells, but not on lymphocytes. In contrast, phosphatidylglucoside is mainly expressed on neutrophils. STED microscopic analysis showed that phosphatidylglucoside and lactosylceramide form different domains on plasma membranes of neutrophils, with phosphatidylglucoside preferentially expressed along the neutrophil differentiation pathway. Phosphatidylglucoside was found to mediate the differentiation of HL-60 cells into the neutrophilic lineage, and to be involved in FAS-dependent neutrophil apoptosis. In contrast, lactosylceramide was only expressed on mature neutrophils. Complexes of lactosylceramide and the Src family kinase Lyn form membrane microdomains. LacCer-enriched membrane microdomains mediate neutrophil innate immune responses; e.g. chemotaxis, phagocytosis, and superoxide generation. C24 fatty acid chains of LacCer are indispensable for the formation of LacCer-Lyn complexes and for LacCer-dependent functions. Moreover, Lyn-coupled LacCer-enriched microdomains serve as signal transduction platforms for αMβ2 integrin-mediated phagocytosis. This review describes the organization and potential functions of glycolipids in phagocytes, as well as the roles of both phosphatidylglucoside and lactosylceramide in neutrophils. This article is part of a Special Issue entitled Linking transcription to physiology in lipidomics.

New insights on glucosylated lipids: metabolism and functions

Ceramide, cholesterol, and phosphatidic acid are major basic structures for cell membrane lipids. These lipids are modified with glucose to generate glucosylceramide (GlcCer), cholesterylglucoside (ChlGlc), and phosphatidylglucoside (PtdGlc), respectively. Glucosylation dramatically changes the functional properties of lipids. For instance, ceramide acts as a strong tumor suppressor that causes apoptosis and cell cycle arrest, while GlcCer has an opposite effect, downregulating ceramide activities. All glucosylated lipids are enriched in lipid rafts or microdomains and play fundamental roles in a variety of cellular processes. In this review, we discuss the biological functions and metabolism of these three glucosylated lipids.

A world of sphingolipids and glycolipids in the brain--novel functions of simple lipids modified with glucose

Glycosphingolipids (GSLs) are present on cell surface membranes and are particularly abundant in the brain. Since over 300-400 GSLs are synthesized from glucosylceramide (GlcCer), GlcCer is believed to only serve as the source of most GSLs, including sialic acid-containing GSLs or gangliosides, in the brain. Recent studies, however, suggest that GlcCer itself plays a role in the heat stress response, as it functions as a glucose donor for the synthesis of cholesterylglucoside, a lipid mediator in heat stress responses in animals. GlcCer in adipose tissues is also thought to be involved in mechanisms that regulate energy (sugar and lipid) metabolism. Our extensive structural study revealed an additional novel glucosylated membrane lipid, called phosphatidylglucoside, in developing rodent brains and human neutrophils. These lipids, all modified with glucose, are enriched in lipid rafts and play important roles in basic cellular processes. Here, I summarize the recent progress regarding these glucosylated lipids and their biosynthesis and regulation in the central nervous system (CNS).