Differential expression of receptors for Shiga and Cholera toxin is regulated by the cell cycle

The Mutagenic Plasticity of the Cholera Toxin B-Subunit Surface Residues: Stability and Affinity

Mastering selective molecule trafficking across human cell membranes poses a formidable challenge in healthcare biotechnology while offering the prospect of breakthroughs in drug delivery, gene therapy, and diagnostic imaging. The cholera toxin B-subunit (CTB) has the potential to be a useful cargo transporter for these applications. CTB is a robust protein that is amenable to reengineering for diverse applications; however, protein redesign has mostly focused on modifications of the N- and C-termini of the protein. Exploiting the full power of rational redesign requires a detailed understanding of the contributions of the surface residues to protein stability and binding activity. Here, we employed Rosetta-based computational saturation scans on 58 surface residues of CTB, including the GM1 binding site, to analyze both ligand-bound and ligand-free structures to decipher mutational effects on protein stability and GM1 affinity. Complimentary experimental results from differential scanning fluorimetry and isothermal titration calorimetry provided melting temperatures and GM1 binding affinities for 40 alanine mutants among these positions. The results showed that CTB can accommodate diverse mutations while maintaining its stability and ligand binding affinity. These mutations could potentially allow modification of the oligosaccharide binding specificity to change its cellular targeting, alter the B-subunit intracellular routing, or impact its shelf-life and in vivo half-life through changes to protein stability. We anticipate that the mutational space maps presented here will serve as a cornerstone for future CTB redesigns, paving the way for the development of innovative biotechnological tools.

In vitro characterization of cells derived from a patient with the GLA variant c.376A>G (p.S126G) highlights a non-pathogenic role in Fabry disease

Fabry disease (FD) is a life-limiting disorder characterized by intracellular globotriaosylceramide (Gb3) accumulations. The underlying α-galactosidase A (α-GAL A) deficiency is caused by variants in the gene GLA. Variants of unknown significance (VUS) are frequently found in GLA and challenge clinical management. Here, we investigated a 49-year old man with cryptogenic lacunar cerebral stroke and the chance finding of the VUS S126G, who was sent to our center for diagnosis and initiation of a costly and life-long FD-specific treatment. We combined clinical examination with in vitro investigations of dermal fibroblasts (HDF), induced pluripotent stem cells (iPSC), and iPSC-derived sensory neurons. We analyzed α-GAL A activity in iPSC, Gb3 accumulation in all three cell types, and action potential firing in sensory neurons. Neurological examination and small nerve fiber assessment was normal except for reduced distal skin innervation. S126G iPSC showed normal α-GAL A activity compared to controls and no Gb3 deposits were found in all three cell types. Baseline electrophysiological characteristics of S126G neurons showed no difference compared to healthy controls as investigated by patch-clamp recordings. We pioneer multi-level cellular characterization of the VUS S126G using three cell types derived from a patient and provide further evidence for the benign nature of S126G in GLA, which is of great importance in the management of such cases in clinical practice.

Fluorescence Intensity and Fluorescence Lifetime Imaging Microscopies (FLIM) of Cell Differentiation in the Small Intestinal Organoids Using Cholera Toxin

Live cell microscopies of in vitro, ex vivo, and in vivo experimental intestinal models enable visualizing cell proliferation, differentiation, and functional cellular status in response to intrinsic and extrinsic (e.g., in the presence of microbiota) factors. While the use of transgenic animal models expressing biosensor fluorescent proteins can be laborious and not compatible with clinical samples and patient-derived organoids, the use of fluorescent dye tracers is an attractive alternative. In this protocol, we describe how the differentiation-dependent intestinal cell membrane composition can be labeled using fluorescent cholera toxin subunit B (CTX) derivatives. By using the culture of mouse adult stem cell-derived small intestinal organoids, we show that CTX can bind specific plasma membrane domains in differentiation-dependent manner. Green (Alexa Fluor 488) and red (Alexa Fluor 555) fluorescent CTX derivatives also display additional contrast in a fluorescence lifetime domain, when probed by the fluorescence lifetime imaging microscopy (FLIM), and can be used together with other fluorescent dyes and cell tracers. Importantly, CTX staining remains confined to specific regions in the organoids after fixation, which enables using it in both live cell and fixed tissue immunofluorescence microscopies.

Role of Globotriaosylceramide in Physiology and Pathology

At first glance, the biological function of globoside (Gb) clusters appears to be that of glycosphingolipid (GSL) receptors for bacterial toxins that mediate host-pathogen interaction. Indeed, certain bacterial toxin families have been evolutionarily arranged so that they can enter eukaryotic cells through GSL receptors. A closer look reveals this molecular arrangement allocated on a variety of eukaryotic cell membranes, with its role revolving around physiological regulation and pathological processes. What makes Gb such a ubiquitous functional arrangement? Perhaps its peculiarity is underpinned by the molecular structure itself, the nature of Gb-bound ligands, or the intracellular trafficking unleashed by those ligands. Moreover, Gb biological conspicuousness may not lie on intrinsic properties or on its enzymatic synthesis/degradation pathways. The present review traverses these biological aspects, focusing mainly on globotriaosylceramide (Gb3), a GSL molecule present in cell membranes of distinct cell types, and proposes a wrap-up discussion with a phylogenetic view and the physiological and pathological functional alternatives.

Dietary Isothiocyanates, Sulforaphane and 2-Phenethyl Isothiocyanate, Effectively Impair Vibrio cholerae Virulence

Vibrio cholerae represents a constant threat to public health, causing widespread infections, especially in developing countries with a significant number of fatalities and serious complications every year. The standard treatment by oral rehydration does not eliminate the source of infection, while increasing antibiotic resistance among pathogenic V. cholerae strains makes the therapy difficult. Thus, we assessed the antibacterial potential of plant-derived phytoncides, isothiocyanates (ITC), against V. cholerae O365 strain. Sulforaphane (SFN) and 2-phenethyl isothiocyanate (PEITC) ability to inhibit bacterial growth was assessed. Minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values indicate that these compounds possess antibacterial activity and are also effective against cells growing in a biofilm. Tested ITC caused accumulation of stringent response alarmone, ppGpp, which indicates induction of the global stress response. It was accompanied by bacterial cytoplasm shrinkage, the inhibition of the DNA, and RNA synthesis as well as downregulation of the expression of virulence factors. Most importantly, ITC reduced the toxicity of V. cholerae in the in vitro assays (against Vero and HeLa cells) and in vivo, using Galleria mellonella larvae as an infection model. In conclusion, our data indicate that ITCs might be considered promising antibacterial agents in V. cholerae infections.

Design of abiotic polymer ligand-decorated lipid nanoparticles for effective neutralization of target toxins in the blood

Macromolecular toxins often induce inflammatory cytokine production, multiple-organ dysfunction, and cell death. Synthetic polymer ligands (PLs) prepared with several functional monomers have the potential of neutralizing target toxins after binding to them; therefore, they are of significant interest as abiotic antidotes. Although PLs show little toxin neutralization effect in the bloodstream because of immediate elimination from there, the toxin neutralization effect is significantly improved by the direct decoration of PLs onto lipid nanoparticles (PL-LNPs). However, this direct decoration decreases PL mobility, induces LNP aggregation after capturing the target, and decreases LNP blood circulation time. We designed novel PL-LNPs to improve PL mobility, inhibit the aggregation tendency after capturing the target, and increase LNP blood circulation time in order to achieve highly effective toxin neutralization in vivo. Specifically, LNPs were modified with PLs-conjugated polyethylene glycol (PEG), and additional PEG was used to modify the PL-decorated LNPs (PL-PEG-LNPs). Histones were used as target toxins, and N-isopropylacrylamide-based PLs were used for histone capture. PEGylation increased the plasma LNP level 24 h after intravenous injection by ∼90 times and inhibited LNP aggregation after histone capture. The dissociation constant (K_d) of PL-PEG-LNPs against histone was two times smaller compared to that of PL-LNPs. Although PL-LNPs inhibited histone-platelet interaction in the bloodstream, a large amount of histone-PL-LNP complexes accumulated in the lungs because of aggregation. However, PL-PEG-LNPs inhibited both histone-platelet interaction and histone accumulation in the lungs. Importantly, PL-PEG-LNP treatment increased the survival rate of histone-treated mice compared to PL-LNPs. These results provide a platform for the development of abiotic antidote nanoparticles in vivo.

Dynamic interplay between cell membrane tension and clathrin-mediated endocytosis

Deformability of the plasma membrane, the outermost surface of metazoan cells, allows cells to be dynamic, mobile and flexible. Factors that affect this deformability, such as tension on the membrane, can regulate a myriad of cellular functions, including membrane resealing, cell motility, polarisation, shape maintenance, membrane area control and endocytic vesicle trafficking. This review focuses on mechanoregulation of clathrin-mediated endocytosis (CME). We first delineate the origins of cell membrane tension and the factors that yield to its spatial and temporal fluctuations within cells. We then review the recent literature demonstrating that tension on the membrane is a fast-acting and reversible regulator of CME. Finally, we discuss tension-based regulation of endocytic clathrin coat formation during physiological processes.

Assessing the role of glycosphingolipids in the phenotype severity of Fabry disease mouse model

Fabry disease is caused by deficient activity of α-galactosidase A, an enzyme that hydrolyzes the terminal α-galactosyl moieties from glycolipids and glycoproteins, and subsequent accumulation of glycosphingolipids, mainly globotriaosylceramide (Gb₃), globotriaosylsphingosine (lyso-Gb₃), and galabiosylceramide. However, there is no known link between these compounds and disease severity. In this study, we compared Gb₃ isoforms (various fatty acids) and lyso-Gb₃ analogs (various sphingosine modifications) in two strains of Fabry disease mouse models: a pure C57BL/6 (B6) background or a B6/129 mixed background, with the latter exhibiting more prominent cardiac and renal hypertrophy and thermosensation deficits. Total Gb₃ and lyso-Gb₃ levels in the heart, kidney, and dorsal root ganglion (DRG) were similar in the two strains. However, levels of the C20-fatty acid isoform of Gb₃ and particular lyso-Gb₃ analogs (+18, +34) were significantly higher in Fabry-B6/129 heart tissue when compared with Fabry-B6. By contrast, there was no difference in Gb₃ and lyso-Gb₃ isoforms/analogs in the kidneys and DRG between the two strains. Furthermore, using immunohistochemistry, we found that Gb₃ massively accumulated in DRG mechanoreceptors, a sensory neuron subpopulation with preserved function in Fabry disease. However, Gb₃ accumulation was not observed in nonpeptidergic nociceptors, the disease-relevant subpopulation that has remarkably increased isolectin-B4 (the marker of nonpeptidergic nociceptors) binding and enlarged cell size. These findings suggest that specific species of Gb₃ or lyso-Gb₃ may play major roles in the pathogenesis of Fabry disease, and that Gb₃ and lyso-Gb₃ are not responsible for the pathology in all tissues or cell types.

Super-Resolution Microscopy Reveals Local Accumulation of Plasma Membrane Gangliosides at Neisseria meningitidis Invasion Sites

Neisseria meningitidis (meningococcus) is a Gram-negative bacterium responsible for epidemic meningitis and sepsis worldwide. A critical step in the development of meningitis is the interaction of bacteria with cells forming the blood-cerebrospinal fluid barrier, which requires tight adhesion of the pathogen to highly specialized brain endothelial cells. Two endothelial receptors, CD147 and the β2-adrenergic receptor, have been found to be sequentially recruited by meningococci involving the interaction with type IV pilus. Despite the identification of cellular key players in bacterial adhesion the detailed mechanism of invasion is still poorly understood. Here, we investigated cellular dynamics and mobility of the type IV pilus receptor CD147 upon treatment with pili enriched fractions and specific antibodies directed against two extracellular Ig-like domains in living human brain microvascular endothelial cells. Modulation of CD147 mobility after ligand binding revealed by single-molecule tracking experiments demonstrates receptor activation and indicates plasma membrane rearrangements. Exploiting the binding of Shiga (STxB) and Cholera toxin B (CTxB) subunits to the two native plasma membrane sphingolipids globotriaosylceramide (Gb3) and raft-associated monosialotetrahexosylganglioside GM1, respectively, we investigated their involvement in bacterial invasion by super-resolution microscopy. Structured illumination microscopy (SIM) and direct stochastic optical reconstruction microscopy (dSTORM) unraveled accumulation and coating of meningococci with GM1 upon cellular uptake. Blocking of CTxB binding sites did not impair bacterial adhesion but dramatically reduced bacterial invasion efficiency. In addition, cell cycle arrest in G1 phase induced by serum starvation led to an overall increase of GM1 molecules in the plasma membrane and consequently also in bacterial invasion efficiency. Our results will help to understand downstream signaling events after initial type IV pilus-host cell interactions and thus have general impact on the development of new therapeutics targeting key molecules involved in infection.

Glycosphingolipid metabolism in cell fate specification

Glycosphingolipids (GSLs) are ubiquitous components of eukaryotic plasma membranes that consist of a ceramide backbone linked to a glycan moiety. Both the ceramide and the glycan parts of GSLs display structural variations that result in a remarkable repertoire of diverse compounds. This diversity of GSLs is exploited during embryogenesis, when different GSLs are produced at specific developmental stages and along several differentiation trajectories. Importantly, plasma membrane receptors interact with GSLs to modify their activities. Consequently, two otherwise identical cells can respond differently to the same stimulus owing to their different GSL composition. The metabolic reprograming of GSLs is in fact a necessary part of developmental programs, as its impairment results in developmental failure or tissue-specific defects. Moreover, single-cell variability is emerging as a fundamental player in development: GSL composition displays cell-to-cell variability in syngeneic cell populations owing to the regulatory gene expression circuits involved in microenvironment adaptation and in differentiation. Here, we discuss how GSLs are synthesized and classified and review the role of GSLs in the establishment and maintenance of cell identity. We further highlight the existence of the regulatory circuits that modify GSL pathways and speculate how GSL heterogeneity might contribute to developmental patterning.

Endocytosis in proliferating, quiescent and terminally differentiated cells

Endocytosis mediates nutrient uptake, receptor internalization and the regulation of cell signaling. It is also hijacked by many bacteria, viruses and toxins to mediate their cellular entry. Several endocytic routes exist in parallel, fulfilling different functions. Most studies on endocytosis have used transformed cells in culture. However, as the majority of cells in an adult body have exited the cell cycle, our understanding is biased towards proliferating cells. Here, we review the evidence for the different pathways of endocytosis not only in dividing, but also in quiescent, senescent and terminally differentiated cells. During mitosis, residual endocytosis is dedicated to the internalization of caveolae and specific receptors. In non-dividing cells, clathrin-mediated endocytosis (CME) functions, but the activity of alternative processes, such as caveolae, macropinocytosis and clathrin-independent routes, vary widely depending on cell types and functions. Endocytosis supports the quiescent state by either upregulating cell cycle arrest pathways or downregulating mitogen-induced signaling, thereby inhibiting cell proliferation. Endocytosis in terminally differentiated cells, such as skeletal muscles, adipocytes, kidney podocytes and neurons, supports tissue-specific functions. Finally, uptake is downregulated in senescent cells, making them insensitive to proliferative stimuli by growth factors. Future studies should reveal the molecular basis for the differences in activities between the different cell states.

Glycosphingolipid metabolic reprogramming drives neural differentiation

Neural development is accomplished by differentiation events leading to metabolic reprogramming. Glycosphingolipid metabolism is reprogrammed during neural development with a switch from globo- to ganglio-series glycosphingolipid production. Failure to execute this glycosphingolipid switch leads to neurodevelopmental disorders in humans, indicating that glycosphingolipids are key players in this process. Nevertheless, both the molecular mechanisms that control the glycosphingolipid switch and its function in neurodevelopment are poorly understood. Here, we describe a self-contained circuit that controls glycosphingolipid reprogramming and neural differentiation. We find that globo-series glycosphingolipids repress the epigenetic regulator of neuronal gene expression AUTS2. AUTS2 in turn binds and activates the promoter of the first and rate-limiting ganglioside-producing enzyme GM3 synthase, thus fostering the synthesis of gangliosides. By this mechanism, the globo-AUTS2 axis controls glycosphingolipid reprogramming and neural gene expression during neural differentiation, which involves this circuit in neurodevelopment and its defects in neuropathology.

Endoplasmic Reticulum-Targeted Subunit Toxins Provide a New Approach to Rescue Misfolded Mutant Proteins and Revert Cell Models of Genetic Diseases

Many germ line diseases stem from a relatively minor disturbance in mutant protein endoplasmic reticulum (ER) 3D assembly. Chaperones are recruited which, on failure to correct folding, sort the mutant for retrotranslocation and cytosolic proteasomal degradation (ER-associated degradation-ERAD), to initiate/exacerbate deficiency-disease symptoms. Several bacterial (and plant) subunit toxins, retrograde transport to the ER after initial cell surface receptor binding/internalization. The A subunit has evolved to mimic a misfolded protein and hijack the ERAD membrane translocon (dislocon), to effect cytosolic access and cytopathology. We show such toxins compete for ERAD to rescue endogenous misfolded proteins. Cholera toxin or verotoxin (Shiga toxin) containing genetically inactivated (± an N-terminal polyleucine tail) A subunit can, within 2–4 hrs, temporarily increase F508delCFTR protein, the major cystic fibrosis (CF) mutant (5-10x), F508delCFTR Golgi maturation (<10x), cell surface expression (20x) and chloride transport (2x) in F508del CFTR transfected cells and patient-derived F508delCFTR bronchiolar epithelia, without apparent cytopathology. These toxoids also increase glucocerobrosidase (GCC) in N370SGCC Gaucher Disease fibroblasts (3x), another ERAD–exacerbated misfiling disease. We identify a new, potentially benign approach to the treatment of certain genetic protein misfolding diseases.

Inhibition of Rab prenylation by statins induces cellular glycosphingolipid remodeling

Statins, which specifically inhibit HMG Co-A reductase, the rate-limiting step of cholesterol biosynthesis, are widely prescribed to reduce serum cholesterol and cardiac risk, but many other effects are seen. We now show an effect of these drugs to induce profound changes in the step-wise synthesis of glycosphingolipids (GSLs) in the Golgi. Glucosylceramide (GlcCer) was increased several-fold in all cell lines tested, demonstrating a widespread effect. Additionally, de novo or elevated lactotriaosylceramide (Lc3Cer; GlcNAcβ1-3Galβ1-4GlcCer) synthesis was observed in 70%. Western blot showed that GlcCer synthase (GCS) was elevated by statins, and GCS and Lc3Cer synthase (Lc3S) activities were increased; however, transcript was elevated for Lc3S only. Supplementation with the isoprenoid precursor, geranylgeranyl pyrophosphate (GGPP), a downstream product of HMG Co-A reductase, reversed statin-induced glycosyltransferase and GSL elevation. The Rab geranylgeranyl transferase inhibitor 3-PEHPC, but not specific inhibitors of farnesyl transferase, or geranylgeranyl transferase I, was sufficient to replicate statin-induced GlcCer and Lc3Cer synthesis, supporting a Rab prenylation-dependent mechanism. While total cholesterol was unaffected, the trans-Golgi network (TGN) cholesterol pool was dissipated and medial Golgi GCS partially relocated by statins. GSL-dependent vesicular retrograde transport of Verotoxin and cholera toxin to the Golgi/endoplasmic reticulum were blocked after statin or 3-PEHPC treatment, suggesting aberrant, prenylation-dependent vesicular traffic as a basis of glycosyltransferase increase and GSL remodeling. These in vitro studies indicate a previously unreported link between Rab prenylation and regulation of GCS activity and GlcCer metabolism.

Blood Groups in Infection and Host Susceptibility

Blood group antigens represent polymorphic traits inherited among individuals and populations. At present, there are 34 recognized human blood groups and hundreds of individual blood group antigens and alleles. Differences in blood group antigen expression can increase or decrease host susceptibility to many infections. Blood groups can play a direct role in infection by serving as receptors and/or coreceptors for microorganisms, parasites, and viruses. In addition, many blood group antigens facilitate intracellular uptake, signal transduction, or adhesion through the organization of membrane microdomains. Several blood groups can modify the innate immune response to infection. Several distinct phenotypes associated with increased host resistance to malaria are overrepresented in populations living in areas where malaria is endemic, as a result of evolutionary pressures. Microorganisms can also stimulate antibodies against blood group antigens, including ABO, T, and Kell. Finally, there is a symbiotic relationship between blood group expression and maturation of the gastrointestinal microbiome.

Bovine macrophages sense Escherichia coli Shiga toxin 1

Shiga toxin (Stx)-producing Escherichia coli (STEC) infections in cattle are asymptomatic; however, Stx impairs the initiation of an adaptive immune response by targeting bovine peripheral and intraepithelial lymphocytes. As presumptive bovine mucosal macrophages (Mø) are also sensitive to Stx, STEC may even exert immune modulatory effects by acting on steps preceding lymphocyte activation at the Mø level. We therefore studied the expression of the Stx receptor (CD77), cellular phenotype and functions after incubation of primary bovine monocyte-derived Mø with purified Stx1. A significant portion of bovine Mø expressed CD77 on their surface, with the recombinant B-subunit of Stx1 binding to >50% of the cells. Stx1 down-regulated significantly surface expression of CD14, CD172a and co-stimulatory molecules CD80 and CD86 within 4 h of incubation, while MHC-II expression remained unaffected. Furthermore, incubation of Mø with Stx1 increased significantly numbers of transcripts for IL-4, IL-6, IL-10, IFN-γ, TNF-α, IL-8 and GRO-α but not for IL-12, TGF-β, MCP-1 and RANTES. In the course of bovine STEC infections, Stx1 appears to induce in Mø a mixed response pattern reminiscent of regulatory Mø, which may amplify the direct suppressive effect of the toxin on lymphocytes.

Cell density-induced changes in lipid composition and intracellular trafficking

Cell density is one of the extrinsic factors to which cells adapt their physiology when grown in culture. However, little is known about the molecular changes which occur during cell growth and how cellular responses are then modulated. In many cases, inhibitors, drugs or growth factors used for in vitro studies change the rate of cell proliferation, resulting in different cell densities in control and treated samples. Therefore, for a comprehensive data analysis, it is essential to understand the implications of cell density on the molecular level. In this study, we have investigated how lipid composition changes during cell growth, and the consequences it has for transport of Shiga toxin. By quantifying 308 individual lipid species from 17 different lipid classes, we have found that the levels and species distribution of several lipids change during cell growth, with the major changes observed for diacylglycerols, phosphatidic acids, cholesterol esters, and lysophosphatidylethanolamines. In addition, there is a reduced binding and retrograde transport of Shiga toxin in high density cells which lead to reduced intoxication by the toxin. In conclusion, our data provide novel information on how lipid composition changes during cell growth in culture, and how these changes can modulate intracellular trafficking.

Fast structural responses of gap junction membrane domains to AB5 toxins

Gap junctions (GJs) represent connexin-rich membrane domains that connect interiors of adjoining cells in mammalian tissues. How fast GJs can respond to bacterial pathogens has not been known previously. Using Bessel beam plane illumination and confocal spinning disk microscopy, we found fast (~500 ms) formation of connexin-depleted regions (CDRs) inside GJ plaques between cells exposed to AB5 toxins. CDR formation appears as a fast redistribution of connexin channels within GJ plaques with minor changes in outline or geometry. CDR formation does not depend on membrane trafficking or submembrane cytoskeleton and has no effect on GJ conductance. However, CDR responses depend on membrane lipids, can be modified by cholesterol-clustering agents and extracellular K(+) ion concentration, and influence cAMP signaling. The CDR response of GJ plaques to bacterial toxins is a phenomenon observed for all tested connexin isoforms. Through signaling, the CDR response may enable cells to sense exposure to AB5 toxins. CDR formation may reflect lipid-phase separation events in the biological membrane of the GJ plaque, leading to increased connexin packing and lipid reorganization. Our data demonstrate very fast dynamics (in the millisecond-to-second range) within GJ plaques, which previously were considered to be relatively stable, long-lived structures.

Alcohol induces Golgi fragmentation in differentiated PC12 cells by deregulating Rab1-dependent ER-to-Golgi transport

In the present study, we analyze the effects of ethanol on the Golgi structure and membrane transport in differentiated PC12 cells, which are used as a model of neurons. Chronic exposure to moderate doses of ethanol induces Golgi fragmentation, a common characteristic of many neurodegenerative diseases. Alcohol impaired the lateral linking of stacks without causing microtubule damage. Extensive immunocytochemical and western blot analyses of representative Golgi proteins showed that few, but important, proteins are significantly affected. Thus, alcohol exposure induced a significant ER-to-Golgi transport delay, the retention of the GTPase Rab1 in the Golgi membranes and the accumulation of tethering factor p115 in the cytosol. These modifications would explain the observed fragmentation. The amount of p115 and the stacking protein GRASP65 increased in alcohol-treated cells, which might be a mechanism to reverse Golgi damage. Importantly, the overexpression of GTP-tagged Rab1 but not of a dominant-negative Rab1 mutant, restored the Golgi morphology, suggesting that this protein is the main target of alcohol. Taken together, our results support the view that alcohol and neurodegenerative diseases such as Parkinson have similar effects on intracellular trafficking and provide new clues on the neuropathology of alcoholism.

Chemoenzymatic site-specific labeling of influenza glycoproteins as a tool to observe virus budding in real time

The influenza virus uses the hemagglutinin (HA) and neuraminidase (NA) glycoproteins to interact with and infect host cells. While biochemical and microscopic methods allow examination of the early steps in flu infection, the genesis of progeny virions has been more difficult to follow, mainly because of difficulties inherent in fluorescent labeling of flu proteins in a manner compatible with live cell imaging. We here apply sortagging as a chemoenzymatic approach to label genetically modified but infectious flu and track the flu glycoproteins during the course of infection. This method cleanly distinguishes influenza glycoproteins from host glycoproteins and so can be used to assess the behavior of HA or NA biochemically and to observe the flu glycoproteins directly by live cell imaging.

Enveloped viruses such as the influenza virus cause significant disease in humans. In order to cause a productive infection, the virus particle must interact with the host cell using the viral proteins encoded within its genome. For many such viruses, it is possible to directly observe the early steps in infection, yet for technical reasons it has been extremely difficult to study the genesis of daughter virions as they bud off of infected host cells. Here we devised a chemoenzymatic labeling strategy to site-specifically append probes to the influenza hemagglutinin (HA) and neuraminidase (NA) proteins using the bacterial sortase A enzyme. Because labeling is confined to surface exposed HA and NA in the context of live, infected cells, it is possible to study budding biochemically and microscopically in real-time. Using this system, we can observe budding of flu virions from discrete sites at the cell surface. Our work will enable detailed investigation into the birth of viruses from infected host cells and can likely be applied to viruses other than influenza that have been similarly resistant to real-time microscopic observation during budding.

Structure-dependent pseudoreceptor intracellular traffic of adamantyl globotriaosyl ceramide mimics

The verotoxin (VT) (Shiga toxin) receptor globotriaosyl ceramide (Gb(3)), mediates VT1/VT2 retrograde transport to the endoplasmic reticulum (ER) for cytosolic A subunit access to inhibit protein synthesis. Adamantyl Gb(3) is an amphipathic competitive inhibitor of VT1/VT2 Gb(3) binding. However, Gb(3)-negative VT-resistant CHO/Jurkat cells incorporate adaGb(3) to become VT1/VT2-sensitive. CarboxyadaGb(3), urea-adaGb(3), and hydroxyethyl adaGb(3), preferentially bound by VT2, also mediate VT1/VT2 cytotoxicity. VT1/VT2 internalize to early endosomes but not to Golgi/ER. AdabisGb(3) (two deacyl Gb(3)s linked to adamantane) protects against VT1/VT2 more effectively than adaGb(3) without incorporating into Gb(3)-negative cells. AdaGb(3) (but not hydroxyethyl adaGb(3)) incorporation into Gb(3)-positive Vero cells rendered punctate cell surface VT1/VT2 binding uniform and subverted subsequent Gb(3)-dependent retrograde transport to Golgi/ER to render cytotoxicity (reduced for VT1 but not VT2) brefeldin A-resistant. VT2-induced vacuolation was maintained in adaGb(3)-treated Vero cells, but vacuolar membrane VT2 was lost. AdaGb(3) destabilized membrane cholesterol and reduced Gb(3) cholesterol stabilization in phospholipid liposomes. Cholera toxin GM1-mediated Golgi/ER targeting was unaffected by adaGb(3). We demonstrate the novel, lipid-dependent, pseudoreceptor function of Gb(3) mimics and their structure-dependent modulation of endogenous intracellular Gb(3) vesicular traffic.

Cell cycle-dependent subcellular distribution of ClC-3 in HeLa cells

Chloride channel-3 (ClC-3) is suggested to be a component and/or a regulator of the volume-activated Cl(-) channel in the plasma membrane. However, ClC-3 is predominantly located inside cells and the role of intracellular ClC-3 in tumor growth is unknown. In this study, we found that the subcellular distribution of endogenous ClC-3 varied in a cell cycle-dependent manner in HeLa cells. During interphase, ClC-3 was distributed throughout the cell and it accumulated at various positions in different stages. In early G1, ClC-3 was mainly located in the nucleus. In middle G1, ClC-3 gathered around the nuclear periphery as a ring. In late G1, ClC-3 moved back into the nucleus, where it remained throughout S phase. In G2, ClC-3 was concentrated in the cytoplasm. When cells progressed from G2 to the prophase of mitosis, ClC-3 from the cytoplasm translocated into the nucleus. During metaphase and anaphase, ClC-3 was distributed throughout the cell except for around the chromosomes and was aggregated at the spindle poles and in between two chromosomes, respectively. ClC-3 was then again concentrated in the nucleus upon the progression from telophase to cytokinesis. These results reveal a cell cycle-dependent change of the subcellular distribution of ClC-3 and strongly suggest that ClC-3 has nucleocytoplasmic shuttling dynamics that may play key regulatory roles during different stages of the cell cycle in tumor cells.

Identification of host cell factors required for intoxication through use of modified cholera toxin

We describe a novel labeling strategy to site-specifically attach fluorophores, biotin, and proteins to the C terminus of the A1 subunit (CTA1) of cholera toxin (CTx) in an otherwise correctly assembled and active CTx complex. Using a biotinylated N-linked glycosylation reporter peptide attached to CTA1, we provide direct evidence that ~12% of the internalized CTA1 pool reaches the ER. We also explored the sortase labeling method to attach the catalytic subunit of diphtheria toxin as a toxic warhead to CTA1, thus converting CTx into a cytolethal toxin. This new toxin conjugate enabled us to conduct a genetic screen in human cells, which identified ST3GAL5, SLC35A2, B3GALT4, UGCG, and ELF4 as genes essential for CTx intoxication. The first four encode proteins involved in the synthesis of gangliosides, which are known receptors for CTx. Identification and isolation of the ST3GAL5 and SLC35A2 mutant clonal cells uncover a previously unappreciated differential contribution of gangliosides to intoxication by CTx.

The effects of cholera toxin on cellular energy metabolism

Multianalyte microphysiometry, a real-time instrument for simultaneous measurement of metabolic analytes in a microfluidic environment, was used to explore the effects of cholera toxin (CTx). Upon exposure of CTx to PC-12 cells, anaerobic respiration was triggered, measured as increases in acid and lactate production and a decrease in the oxygen uptake. We believe the responses observed are due to a CTx-induced activation of adenylate cyclase, increasing cAMP production and resulting in a switch to anaerobic respiration. Inhibitors (H-89, brefeldin A) and stimulators (forskolin) of cAMP were employed to modulate the CTx-induced cAMP responses. The results of this study show the utility of multianalyte microphysiometry to quantitatively determine the dynamic metabolic effects of toxins and affected pathways.

Novel cell-based method to detect Shiga toxin 2 from Escherichia coli O157:H7 and inhibitors of toxin activity

Escherichia coli O157:H7 is a leading cause of food-borne illness. This human pathogen produces Shiga toxins (Stx1 and Stx2) which inhibit protein synthesis by inactivating ribosome function. The present study describes a novel cell-based assay to detect Stx2 and inhibitors of toxin activity. A Vero cell line harboring a destabilized variant (half-life, 2 h) of the enhanced green fluorescent protein (d2EGFP) was used to monitor the toxin-induced inhibition of protein synthesis. This Vero-d2EGFP cell line produced a fluorescent signal which could be detected by microscopy or with a plate reader. However, a greatly attenuated fluorescent signal was detected in Vero-d2EGFP cells that had been incubated overnight with either purified Stx2 or a cell-free culture supernatant from Stx1- and Stx2-producing E. coli O157:H7. Dose-response curves demonstrated that the Stx2-induced inhibition of enhanced green fluorescent protein fluorescence mirrored the Stx2-induced inhibition of overall protein synthesis and identified a picogram-per-milliliter threshold for toxin detection. To establish our Vero-d2EGFP assay as a useful tool for the identification of toxin inhibitors, we screened a panel of plant compounds for antitoxin activities. Fluorescent signals were maintained when Vero-d2EGFP cells were exposed to Stx1- and Stx2-containing medium in the presence of either grape seed or grape pomace extract. The antitoxin properties of the grape extracts were confirmed with an independent toxicity assay that monitored the overall level of protein synthesis in cells treated with purified Stx2. These results indicate that the Vero-d2EGFP fluorescence assay is an accurate and sensitive method to detect Stx2 activity and can be utilized to identify toxin inhibitors.

Efficient antiproliferative and antiangiogenic effects on human ovarian cancer growth by gene transfer of attenuated mutants of Shiga-like toxin I

To evaluate the potential effect of anticancer and antiangiogenesis of Stx1(W203F) and Stx1(R170H), two attenuated mutants of Shiga-like toxin I (Stx1), in cancer gene therapy. Antiproliferative effects of these Stx1 mutants were tested in human ovarian carcinoma cell line SKOV3 and human umbilical vein endothelial cells (HUVECs) in vitro. Effect of these Stx1 mutants on inducing cell death and cell cycle arrest was analyzed in SKOV3 cells. Influence of these Stx1 mutants on endothelial cell function was analyzed in HUVECs. In vivo therapeutic effect of these Stx1 mutants on SKOV3 was explored using xenograft models in nude mice. These Stx1 mutants can inhibit the growth of SKOV3 or HUVECs and this effect can be abrogated by antibody specific for Stx1. They caused considerable cell death of SKOV3 cells in 24 h; neither caspase activity nor DNA fragmentation was observed, and necrosis is the major mode of cell death. These Stx1 mutants can induce cell cycle arrest of SKOV3 cells in G(2)-M or S phase depending on the dosage of gene transfer. Furthermore, they significantly decreased migration and capillary tube formation of HUVECs at low dose. In vivo study showed that Stx1(W203F) but not Stx1(R170H) significantly suppressed transplanted SKOV3 tumor growth in nude mice model. Interestingly, the microvessel densities of tumor treated with Stx1(W203F) and Stx1(R170H) were significantly reduced. This study suggests that genes encoding attenuated Stx1 can be selected as good candidates for the gene therapy of ovarian carcinoma because of their antiproliferative and antiangiogenic effects.

Herpes simplex virus type 1 mediates fusion through a hemifusion intermediate by sequential activity of glycoproteins D, H, L, and B

Virus-induced membrane fusion can be subdivided into three phases defined by studies of class I and class II fusion proteins. During Phase I, two membranes are brought into close apposition. Phase II marks the mixing of the outer membrane leaflets leading to formation of a hemifusion intermediate. A fusion pore stably forms and expands in Phase III, thereby completing the fusion process. Herpes simplex virus type 1 (HSV-1) requires four glycoproteins to complete membrane fusion, but none has been defined as class I or II. Therefore, we investigated whether HSV-1-induced membrane fusion occurred following the same general phases as those described for class I and II proteins. In this study we demonstrate that glycoprotein D (gD) and the glycoprotein H and glycoprotein L complex (gHL) mediated lipid mixing indicative of hemifusion. However, content mixing and full fusion required glycoprotein B (gB) to be present along with gD and gHL. Our results indicate that, like class I and II fusion proteins, fusion mediated by HSV-1 glycoproteins occurred through a hemifusion intermediate. In addition, both gB and gHL are probably directly involved in the fusion process. From this, we propose a sequential model for fusion via HSV-1 glycoproteins whereby gD is required for Phase I, gHL is required for Phase II, and gB is required for Phase III. We further propose that glycoprotein H and gB are likely to function sequentially to promote membrane fusion in other herpesviruses such as Epstein-Barr virus and human herpesvirus 8.

Compensatory renal growth protects mice against Shiga toxin 2-induced toxicity

Uninephrectomy (Unx) is followed by the compensatory renal growth (CRG) of the remaining kidney. Previous evidence has shown that during CRG, renal tissue is resistant to a variety of pathologies. We tested the hypothesis that the functional changes that take place during CRG could attenuate Shiga toxin (Stx) toxicity in a mouse model of Stx2-induced hemolytic uremic syndrome (HUS). The participation of nitric oxide (NO) was analyzed. After CRG induction with Unx, mice were exposed to a lethal dose of Stx2, and the degree of renal damage and mortality was measured. Stx2 effects on the growth, renal blood flow (RBF) and NO synthase (NOS) intrarenal expression in the remaining kidney were then studied. The induction of CRG strongly prevented Stx2-mediated mortality and renal damage. Administration of the NOS inhibitor NG-nitro-L-arginine methyl ester (L-NAME) during CRG partially impaired the protection. Both Stx2 and L-NAME interfered with the hypertrophic and hyperplastic responses to Unx, as well as with the increase in RBF. In intact mice, Stx2 decreased renal perfusion, inhibited endothelial NOS basal expression and enhanced inducible NOS expression; all of these effects were attenuated by prior Unx. It is concluded that during CRG mice are highly protected against Stx2 toxicity and lethality. The protective capacity of CRG could be related to the enhancement of renal perfusion and preservation of eNOS renal expression, counterbalancing two major pathogenic mechanisms of Stx2.

Dynamic Green Fluorescent Protein Sensors for High‐Content Analysis of the Cell Cycle

We have developed two dynamic sensors that report cell cycle position in living mammalian cells. The sensors use well-characterized components from proteins that are spatially and temporally regulated through the cell cycle. Coupling of these components to Enhanced Green Fluorescent Protein (EGFP) has been used to engineer fusion proteins that report G1/S and G2/M transitions during the cell cycle without perturbing cell cycle progression. Expression of these sensors in stable cell lines allows high content analysis of the effects of drugs and gene knockdown on the cell cycle using automated image analysis to determine cell cycle position and to abstract correlative data from multiplexed sensors and morphological analysis.

Chemokines bind to sulfatides as revealed by surface plasmon resonance

Chemokines bind to sulfated cell surface glycosaminoglycans and thereby modulate signaling mediated by G-protein-coupled seven-transmembrane domain chemokine receptors. Similar to glycosaminoglycans, sulfated oligosaccharides are also exposed on the cell surface by sulfatides, a class of glycosphingolipids. We have now identified sulfated glycosphingolipids (sulfatides) as novel binding partners for chemokines. Using surface plasmon resonance (SPR), the binding of proinflammatory and homeostatic chemokines to glycosphingolipids, in particular sulfatides, was investigated. Chemokines were immobilized while glycosphingolipids or additional phospholipids incorporated into liposomes were applied as soluble analytes. A specific affinity of the chemokines MCP-1/CCL2, IL-8/CXCL8, SDF-1alpha/CXCL12, MIP-1alpha/CCL3 and MIP-1beta/CCL4 to the sulfatides SM4s, SM3, SM2a and SB2, SB1a was detected. No significant interactions with the chemokines were observed for gangliosides, neutral glycosphingolipids or phospholipids. Chemokine receptors have been associated with the detergent-insoluble fraction supposed to contain 'rafts', i.e., glycosphingolipid enriched microdomains of the cell surface. Accordingly, the data suggest that early chemokine receptor signaling may take place in the vicinity of sulfated glycosphingolipids on the cell surface, whereby these sulfatides could modulate the chemokine receptor-mediated cell activation signal.

Internalization of the dengue virus is cell cycle modulated in HepG2, but not Vero cells

While many studies have investigated the relationship between cell type and dengue virus infection, no study to date has examined the effect of cell physiology on permissiveness to infection. Unsynchronized and artificially synchronized cell populations at different stages of the cell cycle of two cell types (Vero and HepG2) were examined for permissiveness to infection by two dengue virus serotypes (serotypes 2 and 3) by determining both the levels of virus produced as well as the percentage of cells infected. Vero cells showed no significant differences between either viral production or percentage of cells infected as compared to unsynchronized cells for any of the phases investigated, although production of virus (for both serotypes 2 and 3) was somewhat lower for cells infected during S phase. In contrast, HepG2 cells were significantly more permissive for both infection and virus production in the G(2) phase as compared to other phases examined and serotype differences in permissiveness to infection were noted with cells in the M phase of the cell cycle. These results suggest that the cell cycle may be a mediator of cell permissiveness in some cell types.

Analysing the action of bacterial toxins in living cells with fluorescence resonance energy transfer (FRET)

Bacterial toxins represent small molecules produced by microorganisms. Different toxins act on specific target molecules in mammalian cells. Once discovered, bacterial toxins have been providing tools to study cellular functions and often helped the dissection of complex cellular pathways, e.g. endocytic or secretory trafficking or signal transduction, by virtue of the fact that they either block or activate their specific cellular target molecules. Purified bacterial toxins have also allowed to address many basic biological questions and have provided tools for in vitro and in vivo experimental approaches in many fields of modern biology. The understanding of how bacterial toxins act in living cells often depends on our ability to visualize the trafficking and signaling pathways of these molecules. Fluorescence microscopy and other imaging tools are essential to provide insights into the functional changes induced by these pathogens at the level of individual host cells or single target proteins. Inside a single cell we can measure and quantify the effects of bacterial toxins on specific cellular proteins by microscopic and spectroscopic techniques. Fluorescence resonance energy transfer (FRET) is a high-resolution technique that allows to study protein-protein interactions. FRET can provide distance information in the range of 3- 7 nm between fluorescently labeled bacterial proteins in the live cell and cellular target proteins expressed as chimeras with green fluorescent protein (GFP), or spectrally shifted variants thereof. The purpose of this review is to introduce readers to the main experimental setups for analyses of protein-protein interactions using FRET as well as some applications.

Role of multiple drug resistance protein 1 in neutral but not acidic glycosphingolipid biosynthesis

Transfection studies have implicated the multiple drug resistance pump, MDR1, as a glucosyl ceramide translocase within the Golgi complex (Lala, P., Ito, S., and Lingwood, C. A. (2000) J. Biol. Chem. 275, 6246-6251). We now show that MDR1 inhibitors, cyclosporin A or ketoconazole, inhibit neutral glycosphingolipid biosynthesis in 11 of 12 cell lines tested. The exception, HeLa cells, do not express MDR1. Microsomal lactosyl ceramide and globotriaosyl ceramide synthesis from endogenous or exogenously added liposomal glucosyl ceramide was inhibited by cyclosporin A, consistent with a direct role for MDR1/glucosyl ceramide translocase activity in their synthesis. In contrast, cellular ganglioside synthesis in the same cells, was unaffected by MDR1 inhibition, suggesting neutral and acid glycosphingolipids are synthesized from distinct precursor glycosphingolipid pools. Metabolic labeling in wild type and knock-out (MDR1a, 1b, MRP1) mouse fibroblasts showed the same loss of neutral glycosphingolipid (glucosyl ceramide, lactosyl ceramide) but not ganglioside (GM3) synthesis, confirming the proposed role for MDR1 translocase activity. Cryo-immunoelectron microscopy showed MDR1 was predominantly intracellular, largely in rab6-containing Golgi vesicles and Golgi cisternae, the site of glycosphingolipid synthesis. These studies identify MDR1 as the major glucosyl ceramide flippase required for neutral glycosphingolipid anabolism and demonstrate a previously unappreciated dichotomy between neutral and acid glycosphingolipid synthesis.

Forssman synthetase expression results in diminished shiga toxin susceptibility: a role for glycolipids in determining host-microbe interactions

Forssman glycolipid (FG), the product of Forssman synthetase (FS), is widely expressed among nonprimate mammalian species. Here, we describe a molecular and genetic relationship between FG expression and Shiga toxin (Stx) susceptibility. We have isolated the FS cDNA from human, canine, and murine cells. Whereas the murine and canine FS genes express a functional enzyme, the human FS cDNA was found to express a protein that lacks FS activity, despite a high degree of sequence identity with the enzymatically active murine and canine FS genes. In order to examine the relationship between FG expression and Stx susceptibility, Vero cells were transfected with the three FS orthologues or a vector control. Complementation with the human FS cDNA had no effect on Stx susceptibility, whereas stable expression of the canine and murine FS resulted in markedly decreased susceptibility to toxin. Among individual cells, an inverse correlation between FG expression and Stx binding was demonstrated. Moreover, only strongly FG-reactive cells were capable of growing in the presence of Stx. These cells were found to have high levels of FG expression and a correspondingly diminished GbO(3) content. We conclude that expression of a functionally active FS modifies Stx receptor glycolipids to FG and results in markedly decreased susceptibility to toxin. We speculate that inactivation of the FS gene during primate evolution may account, at least in part, for the marked susceptibility of human cells to Stx.

Verotoxin 1 binding to intestinal crypt epithelial cells results in localization to lysosomes and abrogation of toxicity

Verotoxins (VTs) are important virulence factors of enterohaemorrhagic Escherichia coli (EHEC), a group of bacteria associated with severe disease sequelae in humans. The potent cytotoxic activity of VTs is important in pathogenicity, resulting in the death of cells expressing receptor Gb3 (globotriaosylceramide). EHEC, particularly serotype O157:H7, frequently colonize reservoir hosts (such as cattle) in the absence of disease, however, the basis to avirulence in this host has been unclear. The objective of this study was assessment of interaction between VT and intestinal epithelium, which represents the major interface between the host and enteric organisms. Bovine intestinal epithelial cells expressed Gb3 in vitro in primary cell cultures, localizing specifically to proliferating crypt cells in corroboration with in situ immunohistological observations on intestinal mucosa. Expression of receptor by these cells contrasts with the absence of Gb3 on human intestinal epithelium in vivo. Despite receptor expression, VT exhibited no cytotoxic activity against bovine epithelial cells. Sub-cellular localization of VT indicated that this toxin was excluded from endoplasmic reticulum but localized to lysosomes, corresponding with abrogation of cytotoxicity. VT intracellular trafficking was unaffected by treatment of primary cell cultures with methyl-beta-cyclodextrin, indicating that Gb3 in these cells is not associated with lipid rafts but is randomly distributed in the membrane. The combination of Gb3 isoform, membrane distribution and VT trafficking correlate with observations of other receptor-positive cells that resist verocytotoxicity. These studies demonstrate that intestinal epithelium is an important determinant in VT interaction with major implications for the differential consequences of EHEC infection in reservoir hosts and humans.