Trafficking of Acetyl-C16-Ceramide-NBD with Long-Term Stability and No Cytotoxicity into the Golgi Complex

Organelle-Targeted Laurdans Measure Heterogeneity in Subcellular Membranes and Their Responses to Saturated Lipid Stress

Organelles feature characteristic lipid compositions that lead to differences in membrane properties. In cells, membrane ordering and fluidity are commonly measured using the solvatochromic dye Laurdan, whose fluorescence is sensitive to lipid packing. As a general lipophilic dye, Laurdan stains all hydrophobic environments in cells; therefore, it is challenging to characterize membrane properties in specific organelles or assess their responses to pharmacological treatments in intact cells. Here, we describe the synthesis and application of Laurdan-derived probes that read out the membrane packing of individual cellular organelles. The set of organelle-targeted Laurdans (OTL) localizes to the ER, mitochondria, lysosomes, and Golgi compartments with high specificity while retaining the spectral resolution needed to detect biological changes in membrane ordering. We show that ratiometric imaging with OTLs can resolve membrane heterogeneity within organelles as well as changes in lipid packing resulting from inhibition of trafficking or bioenergetic processes. We apply these probes to characterize organelle-specific responses to saturated lipid stress. While the ER and lysosomal membrane fluidity is sensitive to exogenous saturated fatty acids, that of mitochondrial membranes is protected. We then use differences in ER membrane fluidity to sort populations of cells based on their fatty acid diet, highlighting the ability of organelle-localized solvatochromic probes to distinguish between cells based on their metabolic state. These results expand the repertoire of targeted membrane probes and demonstrate their application in interrogating lipid dysregulation.

Organelle-targeted Laurdans measure heterogeneity in subcellular membranes and their responses to saturated lipid stress

Cell organelles feature characteristic lipid compositions that lead to differences in membrane properties. In living cells, membrane ordering and fluidity are commonly measured using the solvatochromic dye Laurdan, whose fluorescence is sensitive to membrane packing. As a general lipophilic dye, Laurdan stains all hydrophobic environments in cells, so it is challenging to characterize membrane properties in specific organelles or assess their responses to pharmacological treatments in intact cells. Here, we describe the synthesis and application of Laurdan-derived probes that read out membrane packing of individual cellular organelles. The set of Organelle-targeted Laurdans (OTL) localizes to the ER, mitochondria, lysosomes and Golgi compartments with high specificity, while retaining the spectral resolution needed to detect biological changes in membrane packing. We show that ratiometric imaging with OTL can resolve membrane heterogeneity within organelles, as well as changes in membrane packing resulting from inhibition of lipid trafficking or bioenergetic processes. We apply these probes to characterize organelle-specific responses to saturated lipid stress. While ER and lysosomal membrane fluidity is sensitive to exogenous saturated fatty acids, that of mitochondrial membranes is protected. We then use differences in ER membrane fluidity to sort populations of cells based on their fatty acid diet, highlighting the ability of organelle-localized solvatochromic probes to distinguish between cells based on their metabolic state. These results expand the repertoire of targeted membrane probes and demonstrate their application to interrogating lipid dysregulation.

Palmitoylation-Dependent Small-Molecule Fluorescent Probes for Live-Cell Golgi Imaging

Small-molecule fluorescent probes enabling visualization of the Golgi apparatus in living cells are essential tools for studying Golgi-associated biological processes and diseases. So far, several fluorescent Golgi stains have been developed by linking ceramide lipids to fluorophores. However, ceramide-based probes suffer from cumbersome staining procedures and low Golgi specificity. Here, we introduce fluorescent Golgi-staining probes based on the tri-N-methylated myristoyl-Gly-Cys (myrGC^3Me) motif. The cell-permeable myrGC^3Me motif localizes to the Golgi membrane upon S-palmitoylation. By modularly conjugating the myrGC^3Me motif to fluorophores, we developed blue, green, and red fluorescent Golgi probes, all of which allowed simple and rapid staining of the Golgi in living cells with high specificity and no cytotoxicity. The probe was also applicable to the visualization of dynamic changes of the Golgi morphology induced by drug treatments and during cell division. The present work provides an entirely new series of live-cell Golgi probes useful for cell biological and diagnostic applications.

Polyzwitterionic Coating of Porous Adsorbents for Therapeutic Apheresis

Adsorbents for whole blood apheresis need to be highly blood compatible to minimize the activation of blood cells on the biomaterial surface. Here, we developed blood-compatible matrices by surface modification with polyzwitterionic polysulfobetainic and polycarboxybetainic coatings. Photoreactive zwitterionic terpolymers were synthesized by free-radical polymerization of zwitterionic, photoreactive, and fluorescent monomers. Upon UV irradiation, the terpolymers were photodeposited and mutually crosslinked on the surface of hydrophobic polystyrene-co-divinylbenzene and hydrophilic polyacrylamide-co-polyacrylate (DALI) beads. Fluorescent microscopy revealed coatings with an average thickness of 5 µm, which were limited to the bead surface. Blood compatibility was assessed based on polymer-induced hemolysis, coagulation parameters, and in vitro tests. The maintenance of the adsorption capacity after coating was studied in human whole blood with cytokines for polystyrene beads (remained capacity 25-67%) and with low-density lipoprotein (remained capacity 80%) for polyacrylate beads. Coating enhanced the blood compatibility of hydrophobic, but not of hydrophilic adsorbents. The most prominent effect was observed on coagulation parameters (e.g., PT, aPTT, TT, and protein C) and neutrophil count. Polycarboxybetaine with a charge spacer of five carbons was the most promising polyzwitterion for the coating of adsorbents for whole blood apheresis.

Turn-on, photostable, nontoxic and specific, iron(II) sensor

The pressing need to develop a specific analytical sensor that can identify and quantify Fe(II) without a cytotoxic response was the major motivation drive in this work. The turn-on fluorescent sensor here described can successfully detect Fe(II) and discriminate this ion from other analytes that commonly act as interferents in biological media. Moreover, this reduced fluoresceinamine-based sensor has a high photostability and high dissociation constant, which is an indication that the complex obtained between reduced fluoresceinamine (RFL) and Fe(II) is highly stable. This fluorescence-based sensor has a binding mechanism of 1:1 and a positive cooperativity was found between analyte and sensor. The detection, quantification and sensitivity parameters of the sensor were determined: 21.6 ± 0.1 μM; 65.6 ± 0.1 μM and 48 ± 3 (×10⁷) μM, respectively. To evaluate a possible cytotoxicity effect an erythrocyte assay was performed and the obtained data were evaluated considering CdTe Quantum Dots (QDs) passivated with mercaptoacetic acid has experimental control. According to the resulting data RFL is not cytotoxic even when used in high concentrations, 660 mM. On the other hand QDs are quite different. Indeed it was proven that these heavy metal-based nanoparticles are responsible for 40% erytrocytes hemolysis in concentrations of 600 mM.

Sphingolipid metabolism, transport, and functions in plants: Recent progress and future perspectives

Sphingolipids, which comprise membrane systems together with other lipids, are ubiquitous in cellular organisms. They show a high degree of diversity across plant species and vary in their structures, properties, and functions. Benefiting from the development of lipidomic techniques, over 300 plant sphingolipids have been identified. Generally divided into free long-chain bases (LCBs), ceramides, glycosylceramides (GlcCers) and glycosyl inositol phosphoceramides (GIPCs), plant sphingolipids exhibit organized aggregation within lipid membranes to form raft domains with sterols. Accumulating evidence has revealed that sphingolipids obey certain trafficking and distribution rules and confer unique properties to membranes. Functional studies using sphingolipid biosynthetic mutants demonstrate that sphingolipids participate in plant developmental regulation, stimulus sensing, and stress responses. Here, we present an updated metabolism/degradation map and summarize the structures of plant sphingolipids, review recent progress in understanding the functions of sphingolipids in plant development and stress responses, and review sphingolipid distribution and trafficking in plant cells. We also highlight some important challenges and issues that we may face during the process of studying sphingolipids.

[Novel Tyrosine Phosphorylation Signals in the Nucleus and on Mitotic Spindle Fibers and Lysosomes Revealed by Strong Inhibition of Tyrosine Dephosphorylation]

Protein-tyrosine phosphorylation is one of the posttranslational modifications and plays critical roles in regulating a wide variety of cellular processes, such as cell proliferation, differentiation, adhesion, migration, survival, and apoptosis. Protein-tyrosine phosphorylation is reversibly regulated by protein-tyrosine kinases and protein-tyrosine phosphatases. Strong inhibition of protein-tyrosine phosphatase activities is required to undoubtedly detect tyrosine phosphorylation. Our extremely careful usage of Na₃VO₄, a potent protein-tyrosine phosphatase inhibitor, has revealed not only the different intracellular trafficking pathways of Src-family tyrosine kinase members but also novel tyrosine phosphorylation signals in the nucleus and on mitotic spindle fibers and lysosomes. Furthermore, despite that the first identified oncogene product v-Src is generally believed to induce transformation through continuous stimulation of proliferation signaling by its strong tyrosine kinase activity, v-Src-driven transformation was found to be caused not by continuous proliferation signaling but by v-Src tyrosine kinase activity-dependent stochastic genome alterations. Here, I summarize our findings regarding novel tyrosine phosphorylation signaling in a spatiotemporal sense and highlight the significance of the roles of tyrosine phosphorylation in transcriptional regulation inside the nucleus and chromosome dynamics.

Down-regulation of ceramide kinase via proteasome and lysosome pathways in PC12 cells by serum withdrawal: Its protection by nerve growth factor and role in exocytosis

Ceramide kinase (CerK) phosphorylates ceramide to ceramide-1-phosphate (C1P). CerK is highly expressed in the brain, and its association with the neuronal function has been reported. Previous reports showed that the activity of CerK is regulated by post-translational modifications including phosphorylation, whereas the cellular fate of CerK protein and its role in neuronal functions have not been clearly elucidated. Therefore, we investigated these issues in PC12 cells. Treatment with nerve growth factor (NGF) for 6 h increased the formation of C1P but not CerK mRNA. Knockdown of CerK and overexpression of HA-tagged CerK down- and up-regulated the formation of C1P, respectively. In PC12-CerK-HA cells, serum withdrawal caused ubiquitination of CerK-HA protein and down-regulated both CerK-HA protein and C1P formation within 6 h, and these down-regulations were abolished by co-treatments with NGF or proteasome inhibitors such as MG132 and clasto-lactacystin. Microscopic analysis showed that treatment with the proteasome inhibitors increased CerK-HA in puncture structures, possibly endosomes and/or vesicles, in cells. Treatment with the lysosome inhibitors reduced serum withdrawal-induced down-regulation of CerK-HA protein but not C1P formation. When knockdown or overexpression of CerK was performed, Ca²⁺-induced release of [³H] noradrenaline was reduced or enhanced, respectively, but neurite extension was not modified. There was a positive correlation between noradrenaline release and formation of C1P and/or CerK-HA levels in NGF- and clasto-lactacystin-treated cells. These results suggest that levels of CerK were down-regulated by the ubiquitin/proteasome and lysosome pathways and the former pathway-sensitive pool of CerK was suggested to be linked with exocytosis in PC12 cells.

Click reactions with functional sphingolipids

Sphingolipids and glycosphingolipids can regulate cell recognition and signalling. Ceramide and sphingosine-1-phosphate are major players in the sphingolipid pathways and are involved in the initiation and regulation of signalling, apoptosis, stress responses and infection. Specific chemically synthesised sphingolipid derivatives containing small functionalities like azide or alkyne can mimic the biological properties of natural lipid species, which turns them into useful tools for the investigation of the highly complex sphingolipid metabolism by rapid and selective 'click chemistry' using sensitive tags like fluorophores. Subsequent analysis by various fluorescence microscopy techniques or mass spectrometry allows the identification and quantification of the corresponding sphingolipid metabolites as well as the research of associated enzymes. Here we present an overview of recent advances in the synthesis of ceramide and sphingosine analogues for bioorthogonal click reactions to study biosynthetic pathways and localization of sphingolipids for the development of novel therapeutics against lipid-dependent diseases.

Development of a new doubly-labeled fluorescent ceramide probe for monitoring the metabolism of sphingolipids in living cells

A new ceramide analog, 1, containing two fluorescent dyes, NBD in the N-acyl part and KFL5 in the alkyl part, was synthesized. The fluorescence from both NBD and KFL5 was detected in living cells in a time-dependent manner. A multi-wavelength fluorescence detector was used to detect ceramide metabolites including sphingosine, sphingosine-1-phosphate, glucosylceramide, and sphingomyelin, which are connected to the fluorescent dyes, simultaneously in a single TLC plate.

A series of ceramide analogs modified at the 1-position with potent activity against the intracellular growth of Chlamydia trachomatis

BACKGROUND

Chlamydia trachomatis is an intracellular pathogen that requires different nutrients, including sphingolipids, for survival. Components for the transport and biosynthesis of sphingolipids thus may have a potential as antichlamydial targets.

RESULTS

In this study, we synthesized a collection of 24 ceramide derivatives. Three derivatives show pronounced activity with submicromolar IC50. The potency of these compounds was one order of magnitude higher than that of the antibiotic chloramphenicol. We show a detailed structure-activity relationship study for this novel compound class exhibiting a significant effect on the growth of C. trachomatis L2 without penetrating the bacteria itself.

CONCLUSION

The structure-activity relationship presented here defines an important step toward the molecular target of this compound class, which is still elusive.

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.