GLTP-fold interaction with planar phosphatidylcholine surfaces is synergistically stimulated by phosphatidic acid and phosphatidylethanolamine

GLTP Is a Potential Prognostic Biomarker and Correlates with Immunotherapy Efficacy in Cervical Cancer

Cervical cancer (CC) is the fourth most commonly diagnosed cancer in women worldwide. The prognosis of CC patients remains poor. The objective of our study was to explore the potential of glycolipid transfer protein (GLTP) in predicting the prognosis of CC and patients' response to immunotherapy. The expression of GLTP was determined using TCGA and GEO datasets. The prognostic value of GLTP in CC patients was analyzed using Kaplan-Meier analysis and multivariate analysis. The relationships between BTBD10 and immunological checkpoints, immune checkpoint genes, and ferroptosis-related genes were analyzed to explore the impact of GLTP on CC immunotherapy. According to the dysregulated expressions of BTBD10, the IC50 distribution of various targeted medicines was studied. In this study, we found that GLTP expression was distinctly upregulated in CC specimens. However, Kaplan-Meier assays showed that CC patients with low GLTP expressions tended to exhibit a shorter overall survival. Importantly, multivariate assays revealed that GLTP expression was an independent prognostic factor for CC patients. Moreover, we observed that GLTP expression was related to CD4+ T cells, macrophages, and dendritic cells (DCs). Meanwhile, GLTP expressions were associated with those of immune checkpoints, ferroptosis-related genes, and m6A-related genes. The IC50 of Cisplatin, Docetaxel, and Paclitaxel was lower in the high-GLTP-expressing group. Taken together, GLTP was expected to be a prognostic and immunotherapeutic marker for CC.

Ceramide-1-phosphate transfer protein promotes sphingolipid reorientation needed for binding during membrane interaction

Lipid transfer proteins acquire and release their lipid cargoes by interacting transiently with source and destination biomembranes. In the GlycoLipid Transfer Protein (GLTP) superfamily, the two-layer all-α-helical GLTP-fold defines proteins that specifically target sphingolipids (SLs) containing either sugar or phosphate headgroups via their conserved but evolutionarily-modified SL recognitions centers. Despite comprehensive structural insights provided by X-ray crystallography, the conformational dynamics associated with membrane interaction and SL uptake/release by GLTP superfamily members have remained unknown. Herein, we report insights gained from molecular dynamics (MD) simulations into the conformational dynamics that enable ceramide-1-phosphate transfer proteins (CPTPs) to acquire and deliver ceramide-1-phosphate (C1P) during interaction with 1-palmitoyl-2-oleoyl phosphatidylcholine bilayers. The focus on CPTP reflects this protein's involvement in regulating pro-inflammatory eicosanoid production and autophagy-dependent inflammasome assembly that drives interleukin (IL-1β and IL-18) production and release by surveillance cells. We found that membrane penetration by CPTP involved α-6 helix and the α-2 helix N-terminal region, was confined to one bilayer leaflet, and was relatively shallow. Large-scale dynamic conformational changes were minimal for CPTP during membrane interaction or C1P uptake except for the α-3/α-4 helices connecting loop, which is located near the membrane interface and interacts with certain phosphoinositide headgroups. Apart from functioning as a shallow membrane-docking element, α-6 helix was found to adeptly reorient membrane lipids to help guide C1P hydrocarbon chain insertion into the interior hydrophobic pocket of the SL binding site.These findings support a proposed 'hydrocarbon chain-first' mechanism for C1P uptake, in contrast to the 'lipid polar headgroup-first' uptake used by most lipid-transfer proteins.

ATP-binding cassette transporters mediate differential biosynthesis of glycosphingolipid species

The cytosolic-oriented glucosylceramide (GlcCer) synthase is enigmatic, requiring nascent GlcCer translocation to the luminal Golgi membrane to access glycosphingolipid (GSL) anabolic glycosyltransferases. The mechanism by which GlcCer is flipped remains unclear. To investigate the role of GlcCer-binding partners in this process, we previously made cleavable, biotinylated, photoreactive GlcCer analogs in which the reactive nitrene was closely apposed to the GlcCer head group, while maintaining a C16-acyl chain. GlcCer-binding protein specificity was validated for both photoprobes. Using one probe, XLB, here we identified ATP-binding cassette (ABC) transporters ABCA3, ABCB4, and ABCB10 as unfractionated microsomal GlcCer-binding proteins in DU-145 prostate tumor cells. siRNA knockdown (KD) of these transporters differentially blocked GSL synthesis assessed in toto and via metabolic labeling. KD of ABCA3 reduced acid/neutral GSL levels, but increased those of LacCer, while KD of ABCB4 preferentially reduced neutral GSL levels, and KD of ABCB10 reduced levels of both neutral and acidic GSLs. Depletion of ABCA12, implicated in GlcCer transport, preferentially decreased neutral GSL levels, while ABCB1 KD preferentially reduced gangliosides, but increased neutral GSL Gb3. These results imply that multiple ABC transporters may provide distinct but overlapping GlcCer and LacCer pools within the Golgi lumen for anabolism of different GSL series by metabolic channeling. Differential ABC family member usage may fine-tune GSL biosynthesis depending on cell/tissue type. We conclude that ABC transporters provide a new tool for the regulation of GSL biosynthesis and serve as potential targets to reduce selected GSL species/subsets in diseases in which GSLs are dysregulated.

In Vitro Measurement of Sphingolipid Intermembrane Transport Illustrated by GLTP Superfamily Members

Herein, we describe methodological approaches for measuring in vitro transfer of sphingolipids (SLs) between membranes. The approaches rely on direct tracking of the lipid. Typically, direct tracking involves lipid labeling via attachment of fluorophores or introduction of radioactivity. Members of the GlycoLipid Transfer Protein (GLTP) superfamily are used to illustrate two broadly applicable methods for direct lipid tracking. One method relies on Förster resonance energy transfer (FRET) that enables continuous assessment of fluorophore-labeled SL transfer in real time between lipid donor and acceptor vesicles. The second method relies on tracking of radiolabeled SL transfer by separation of lipid donor and acceptor vesicles at discrete time points. The assays are readily adjustable for assessing lipid transfer (1) between various model membrane assemblies (vesicles, micelles, bicelles, nanodiscs), (2) involving other lipid types by other lipid transfer proteins, (3) with protein preparations that are either crudely or highly purified, and (4) that is spontaneous and occurs in the absence of protein.

How α-Helical Motifs Form Functionally Diverse Lipid-Binding Compartments

Lipids are produced site-specifically in cells and then distributed nonrandomly among membranes via vesicular and nonvesicular trafficking mechanisms. The latter involves soluble amphitropic proteins extracting specific lipids from source membranes to function as molecular solubilizers that envelope their insoluble cargo before transporting it to destination sites. Lipid-binding and lipid transfer structural motifs range from multi-β-strand barrels, to β-sheet cups and baskets covered by α-helical lids, to multi-α-helical bundles and layers. Here, we focus on how α-helical proteins use amphipathic helical layering and bundling to form modular lipid-binding compartments and discuss the functional consequences. Preformed compartments generally rely on intramolecular disulfide bridging to maintain conformation (e.g., albumins, nonspecific lipid transfer proteins, saposins, nematode polyprotein allergens/antigens). Insights into nonpreformed hydrophobic compartments that expand and adapt to accommodate a lipid occupant are few and provided mostly by the three-layer, α-helical ligand-binding domain of nuclear receptors. The simple but elegant and nearly ubiquitous two-layer, α-helical glycolipid transfer protein (GLTP)-fold now further advances understanding.

Synthesis of a novel photoactivatable glucosylceramide cross-linker

The biosynthesis of glucosylceramide (GlcCer) is a key rate-limiting step in complex glycosphingolipid (GSL) biosynthesis. To further define interacting partners of GlcCer, we have made a cleavable, biotinylated, photoreactive GlcCer analog in which the reactive nitrene is closely apposed to the GlcCer head group, by substituting the native fatty acid with d, l-2-aminohexadecanoic acid. Two amino-GlcCer diastereomer cross-linkers (XLA and XLB) were generated. XLB proved an effective lactosylceramide (LacCer) synthase substrate while XLA was inhibitory. Both probes specifically bound and cross-linked the GlcCer binding protein, glycolipid transfer protein (GLTP), but not other GSL binding proteins (Shiga toxin and cholera toxin). GlcCer inhibited GLTP cross-linking. Both GlcCer cross-linkers competed with microsomal nitrobenzoxadiazole (NBD)-GlcCer anabolism to NBD-LacCer. GLTP showed marked, ATP-dependent enhancement of cell-free intact microsomal LacCer synthesis from endogenous or exogenous liposomal GlcCer, supporting a role in the transport/membrane translocation of cytosolic and extra-Golgi GlcCer. GLTP was specifically labeled by either XLA or XLB GlcCer cross-linker during this process, together with a (the same) small subset of microsomal proteins. These cross-linkers will serve to probe physiologically relevant GlcCer-interacting cellular proteins.

Investigation of the biophysical properties of a fluorescently modified ceramide-1-phosphate

Ceramide-1-phosphate (C1P) is an important signaling sphingolipid and a metabolite of ceramide. C1P contains an anionic phosphomonoester head group and has been shown to regulate physiological and pathophysiological processes such as cell proliferation, inflammation, apoptosis, phagocytosis, and macrophage chemotaxis. Despite this mechanistic information on its role in intra- and intercellular communication, little information is available on the biophysical properties of C1P in biological membranes and how it interacts with effector proteins. Fluorescently labeled lipids have been a useful tool to understand the membrane behavior properties of lipids such as phosphatidylserine, cholesterol, and some phosphoinositides. However, to the best of our knowledge, fluorescently labeled C1P hasn't been implemented to investigate its ability to serve as a mimetic of endogenous C1P in cells or untagged C1P in in vitro experiments. Cellular and in vitro assays demonstrate TopFluor-C1P harbors a fluorescent group that is fully buried in the hydrocarbon core and fluoresces across the spectrum of physiological pH values. Moreover, TopFluor-C1P didn't affect cellular toxicity at concentrations employed, was as effective as unlabeled C1P in recruiting an established protein effector to intracellular membranes, and its subcellular localization recapitulated what is known for endogenous C1P. Notably, the diffusion coefficient of TopFluor-C1P was slower than that of TopFluor-phosphatidylserine or TopFluor-cholesterol in the plasma membrane and similar to that of other fluorescently labeled sphingolipids including ceramide and sphingomyelin. These studies demonstrate that TopFluor-C1P should be a reliable mimetic of C1P to study C1P membrane biophysical properties and C1P interactions with proteins.

Sphingolipid transfer proteins defined by the GLTP-fold

Glycolipid transfer proteins (GLTPs) originally were identified as small (~24 kDa), soluble, amphitropic proteins that specifically accelerate the intermembrane transfer of glycolipids. GLTPs and related homologs now are known to adopt a unique, helically dominated, two-layer 'sandwich' architecture defined as the GLTP-fold that provides the structural underpinning for the eukaryotic GLTP superfamily. Recent advances now provide exquisite insights into structural features responsible for lipid headgroup selectivity as well as the adaptability of the hydrophobic compartment for accommodating hydrocarbon chains of differing length and unsaturation. A new understanding of the structural versatility and evolutionary premium placed on the GLTP motif has emerged. Human GLTP-motifs have evolved to function not only as glucosylceramide binding/transferring domains for phosphoinositol 4-phosphate adaptor protein-2 during glycosphingolipid biosynthesis but also as selective binding/transfer proteins for ceramide-1-phosphate. The latter, known as ceramide-1-phosphate transfer protein, recently has been shown to form GLTP-fold while critically regulating Group-IV cytoplasmic phospholipase A2 activity and pro-inflammatory eicosanoid production.

Membranes and mammalian glycolipid transferring proteins

Glycolipids are synthesized in and on various organelles throughout the cell. Their trafficking inside the cell is complex and involves both vesicular and protein-mediated machineries. Most important for the bulk lipid transport is the vesicular system, however, lipids moved by transfer proteins are also becoming more characterized. Here we review the latest advances in the glycolipid transfer protein (GLTP) and the phosphoinositol 4-phosphate adaptor protein-2 (FAPP2) field, from a membrane point of view.