Lipid sorting by ceramide structure from plasma membrane to ER for the cholera toxin receptor ganglioside GM1

Ganglioside Metabolism and Its Inherited Diseases

Gangliosides are sialic acid containing glycosphingolipids, which are abundant in mammalian brain tissue. Several fatal human diseases are caused by defects in glycolipid metabolism. Defects in their degradation lead to an accumulation of metabolites upstream of the defective reactions, whereas defects in their biosynthesis lead to diverse problems in a large number of organs.Gangliosides are primarily positioned with their ceramide anchor in the neuronal plasma membrane and the glycan head group exposed on the cell surface. Their biosynthesis starts in the endoplasmic reticulum with the formation of the ceramide anchor, followed by sequential glycosylation reactions, mainly at the luminal surface of Golgi and TGN membranes, a combinatorial process, which is catalyzed by often promiscuous membrane-bound glycosyltransferases.Thereafter, the gangliosides are transported to the plasma membrane by exocytotic membrane flow. After endocytosis, they are degraded within the endolysosomal compartments by a complex machinery of degrading enzymes, lipid-binding activator proteins, and negatively charged lipids.

Nanoscale Membrane Budding Induced by CTxB and Detected via Polarized Localization Microscopy

For endocytosis and exocytosis, membranes transition among planar, budding, and vesicular topographies through nanoscale reorganization of lipids, proteins, and carbohydrates. However, prior attempts to understand the initial stages of nanoscale bending have been limited by experimental resolution. Through the implementation of polarized localization microscopy, this article reports the inherent membrane bending capability of cholera toxin subunit B (CTxB) in quasi-one-component-supported lipid bilayers. Membrane buds were first detected with <50 nm radius, grew to >200 nm radius, and extended into longer tubules with dependence on the membrane tension and CTxB concentration. Compared to the concentration of the planar-supported lipid bilayers, CTxB was (12 ± 4)× more concentrated on the positive curvature top and (26 ± 11)× more concentrated on the negative Gaussian curvature neck of the nanoscale membrane buds. CTxB is frequently used as a marker for liquid-ordered lipid phases; however, the coupling between CTxB and membrane bending provides an alternate understanding of CTxB-induced membrane reorganization. These findings allow for the reinterpretation of prior observations by correlating CTxB clustering and diffusion to CTxB-induced membrane bending. Single-particle tracking was performed on single lipids and CTxB to reveal the correlations among single-molecule diffusion, CTxB accumulation, and membrane topography. Slowed lipid and CTxB diffusion was observed at the nanoscale bud locations, suggesting a local increase in the effective membrane viscosity or molecular crowding upon membrane bending. These results suggest inherent CTxB-induced membrane bending as a mechanism for initiating CTxB internalization in cells that could be independent of clathrin, caveolin, actin, and lipid phase separation.

Revealing the Raft Domain Organization in the Plasma Membrane by Single-Molecule Imaging of Fluorescent Ganglioside Analogs

Gangliosides have been implicated in a variety of physiological processes, particularly in the formation and function of raft domains in the plasma membrane. However, the scarcity of suitable fluorescent ganglioside analogs had long prevented us from determining exactly how gangliosides perform their functions in the live-cell plasma membrane. With the development of new fluorescent ganglioside analogs, as described by Komura et al. (2017), this barrier has been broken. We can now address the dynamic behaviors of gangliosides in the live-cell plasma membrane, using fluorescence microscopy, particularly by single-fluorescent molecule imaging and tracking. Single-molecule tracking of fluorescent GM1 and GM3 revealed that these molecules are transiently and dynamically recruited to monomers (monomer-associated rafts) and homodimer rafts of the raftophilic GPI-anchored protein CD59 in quiescent cells, with exponential residency times of 12 and 40ms, respectively, in a manner dependent on raft-lipid interactions. Upon CD59 stimulation, which induces CD59-cluster signaling rafts, the fluorescent GM1 and GM3 analogs were recruited to the signaling rafts, with a lifetime of 48ms. These results represent the first direct evidence that GPI-anchored receptors and gangliosides interact in a cholesterol-dependent manner. Furthermore, they show that gangliosides continually move in and out of rafts that contain CD59 in an extremely dynamic manner, with much higher frequency than expected previously. Such studies would not have been possible without fluorescent ganglioside probes, which exhibit native-like behavior and single-molecule tracking. In this chapter, we review the methods for single-molecule tracking of fluorescent ganglioside analogs and the results obtained by applying these methods.

Membrane dynamics and organelle biogenesis-lipid pipelines and vesicular carriers

Discoveries spanning several decades have pointed to vital membrane lipid trafficking pathways involving both vesicular and non-vesicular carriers. But the relative contributions for distinct membrane delivery pathways in cell growth and organelle biogenesis continue to be a puzzle. This is because lipids flow from many sources and across many paths via transport vesicles, non-vesicular transfer proteins, and dynamic interactions between organelles at membrane contact sites. This forum presents our latest understanding, appreciation, and queries regarding the lipid transport mechanisms necessary to drive membrane expansion during organelle biogenesis and cell growth.

Shiga Toxin Glycosphingolipid Receptors in Human Caco-2 and HCT-8 Colon Epithelial Cell Lines

Shiga toxins (Stxs) released by enterohemorrhagic Escherichia coli (EHEC) into the human colon are the causative agents for fatal outcome of EHEC infections. Colon epithelial Caco-2 and HCT-8 cells are widely used for investigating Stx-mediated intestinal cytotoxicity. Only limited data are available regarding precise structures of their Stx receptor glycosphingolipids (GSLs) globotriaosylceramide (Gb3Cer) and globotetraosylceramide (Gb4Cer), and lipid raft association. In this study we identified Gb3Cer and Gb4Cer lipoforms of serum-free cultivated Caco-2 and HCT-8 cells, chiefly harboring ceramide moieties composed of sphingosine (d18:1) and C16:0, C22:0 or C24:0/C24:1 fatty acid. The most significant difference between the two cell lines was the prevalence of Gb3Cer with C16 fatty acid in HCT-8 and Gb4Cer with C22–C24 fatty acids in Caco-2 cells. Lipid compositional analysis of detergent-resistant membranes (DRMs), which were used as lipid raft-equivalents, indicated slightly higher relative content of Stx receptor Gb3Cer in DRMs of HCT-8 cells when compared to Caco-2 cells. Cytotoxicity assays revealed substantial sensitivity towards Stx2a for both cell lines, evidencing little higher susceptibility of Caco-2 cells versus HCT-8 cells. Collectively, Caco-2 and HCT-8 cells express a plethora of different receptor lipoforms and are susceptible towards Stx2a exhibiting somewhat lower sensitivity when compared to Vero cells.

Gb3 Glycosphingolipids with Fluorescent Oligoene Fatty Acids: Synthesis and Phase Behavior in Model Membranes

Glycosphingolipids are involved in a number of physiological and pathophysiological processes, and they serve as receptors for a variety of bacterial toxins and viruses. To investigate their function in lipid membranes, fluorescently labeled glycosphingolipids are highly desirable. Herein, a synthetic route to access Gb₃ glycosphingolipids with fluorescently labeled fatty acids, consisting of pentaene and hexaene moieties either at the terminus or in the middle of the acyl chain, has been developed. The fluorescent properties of the Gb₃ derivatives were investigated in small unilamellar vesicles composed of a raft-like mixture. Phase-separated giant unilamellar vesicles (GUVs) allowed the quantification of the apparent partitioning coefficients of the Gb₃ compounds by means of confocal fluorescence laser scanning microscopy. The determined partition coefficients demonstrate that the Gb₃ derivatives are preferentially localized in the liquid-disordered (l_d ) phase. To analyze whether the compounds behave like their physiological counterparts, Cy3-labeled (Cy: cyanine) Shiga toxin B subunits (STxB) were specifically bound to Gb₃ -doped GUVs. However, the protein was favorably localized in the l_d phase, in contrast to results reported for STxB bound to naturally occurring Gb₃ , which is discussed in terms of the packing density of the lipids in the liquid-ordered (l_o ) phase.

Membrane Transport across Polarized Epithelia

Polarized epithelial cells line diverse surfaces throughout the body forming selective barriers between the external environment and the internal milieu. To cross these epithelial barriers, large solutes and other cargoes must undergo transcytosis, an endocytic pathway unique to polarized cell types, and significant for the development of cell polarity, uptake of viral and bacterial pathogens, transepithelial signaling, and immunoglobulin transport. Here, we review recent advances in our knowledge of the transcytotic pathway for proteins and lipids. We also discuss briefly the promise of harnessing the molecules that undergo transcytosis as vehicles for clinical applications in drug delivery.

Identification of Rab18 as an Essential Host Factor for BK Polyomavirus Infection Using a Whole-Genome RNA Interference Screen

Polyomaviruses bind to a group of specific gangliosides on the plasma membrane of the cell prior to being endocytosed. They then follow a retrograde trafficking pathway to reach the endoplasmic reticulum (ER). The viruses begin to disassemble in the ER and then exit the ER and move to the nucleus. However, the details of intracellular trafficking between the endosome and the ER are largely unknown. By implementing a whole human genome small interfering RNA screen, we identified Rab18, syntaxin 18, and the NRZ complex as key components in endosome-ER trafficking of the human polyomavirus BKPyV. These results serve to further elucidate the route BKPyV takes from outside the cell to its site of replication in the nucleus.

BK polyomavirus (BKPyV) is a human pathogen first isolated in 1971. BKPyV infection is ubiquitous in the human population, with over 80% of adults worldwide being seropositive for BKPyV. BKPyV infection is usually asymptomatic; however, BKPyV reactivation in immunosuppressed transplant patients causes two diseases, polyomavirus-associated nephropathy and hemorrhagic cystitis. To establish a successful infection in host cells, BKPyV must travel in retrograde transport vesicles to reach the nucleus. To make this happen, BKPyV requires the cooperation of host cell proteins. To further identify host factors associated with BKPyV entry and intracellular trafficking, we performed a whole-genome small interfering RNA screen on BKPyV infection of primary human renal proximal tubule epithelial cells. The results revealed the importance of Ras-related protein Rab18 and syntaxin 18 for BKPyV infection. Our subsequent experiments implicated additional factors that interact with this pathway and suggest a more detailed model of the intracellular trafficking process, indicating that BKPyV reaches the endoplasmic reticulum (ER) lumen through a retrograde transport pathway between the late endosome and the ER.

IMPORTANCE Polyomaviruses bind to a group of specific gangliosides on the plasma membrane of the cell prior to being endocytosed. They then follow a retrograde trafficking pathway to reach the endoplasmic reticulum (ER). The viruses begin to disassemble in the ER and then exit the ER and move to the nucleus. However, the details of intracellular trafficking between the endosome and the ER are largely unknown. By implementing a whole human genome small interfering RNA screen, we identified Rab18, syntaxin 18, and the NRZ complex as key components in endosome-ER trafficking of the human polyomavirus BKPyV. These results serve to further elucidate the route BKPyV takes from outside the cell to its site of replication in the nucleus.

Development of new ganglioside probes and unraveling of raft domain structure by single-molecule imaging

Gangliosides are involved in a variety of biological roles and are a component of lipid rafts found in cell plasma membranes (PMs). Gangliosides are especially abundant in neuronal PMs and are essential to their physiological functions. However, the dynamic behaviors of gangliosides have not been investigated in living cells due to a lack of fluorescent probes that behave like their parental molecules. We have recently developed, using an entirely chemical method, four new ganglioside probes (GM1, GM2, GM3, and GD1b) that act similarly to their parental molecules in terms of raft partitioning and binding affinity. Using single fluorescent-molecule imaging, we have found that ganglioside probes dynamically enter and leave rafts featuring CD59, a GPI-anchored protein. This occurs both before and after stimulation. The residency time of our ganglioside probes in rafts with CD59 oligomers was 48ms, after stimulation. The residency times in CD59 homodimer and monomer rafts were 40ms and 12ms, respectively. In this review, we introduce an entirely chemical-based ganglioside analog synthesis method and describe its application in single-molecule imaging and for the study of the dynamic behavior of gangliosides in cell PMs. Finally, we discuss how raft domains are formed, both before and after receptor engagement. This article is part of a Special Issue entitled Neuro-glycoscience, edited by Kenji Kadomatsu and Hiroshi Kitagawa.

Syntheses of Fluorescent Gangliosides for the Studies of Raft Domains

Gangliosides, glycosphingolipids containing one or more sialic acids in the glycan chain, are involved in various important biological processes in cell plasma membranes (PMs). However, the behaviors and functions of gangliosides are poorly understood, primarily because of the lack of fluorescent analogs that are equivalent to native gangliosides that can be used as chemical and physical probes. In this study, we developed entirely chemical methods to synthesize fluorescent gangliosides (GM3, GM2, GM1, and GD1b) in which the glycan components are site-specifically labeled with various fluorescent dyes. The functional evaluations of the synthesized fluorescent gangliosides demonstrated the great influence of fluorescent dye on the physical properties of gangliosides in PMs and revealed the fluorescent ganglioside analogs which show similar behaviors to the native gangliosides.

Diffusion of lipids and GPI-anchored proteins in actin-free plasma membrane vesicles measured by STED-FCS

Diffusion and interaction dynamics of molecules at the plasma membrane play an important role in cellular signaling and are suggested to be strongly associated with the actin cytoskeleton. Here we use superresolution STED microscopy combined with fluorescence correlation spectroscopy (STED-FCS) to access and compare the diffusion characteristics of fluorescent lipid analogues and GPI-anchored proteins (GPI-APs) in the live-cell plasma membrane and in actin cytoskeleton-free, cell-derived giant plasma membrane vesicles (GPMVs). Hindered diffusion of phospholipids and sphingolipids is abolished in the GPMVs, whereas transient nanodomain incorporation of ganglioside lipid GM1 is apparent in both the live-cell membrane and GPMVs. For GPI-APs, we detect two molecular pools in living cells; one pool shows high mobility with transient incorporation into nanodomains, and the other pool forms immobile clusters, both of which disappear in GPMVs. Our data underline the crucial role of the actin cortex in maintaining hindered diffusion modes of many but not all of the membrane molecules and highlight a powerful experimental approach to decipher specific influences on molecular plasma membrane dynamics.

Diffusion of lipids and GPI-anchored proteins in actin-free plasma membrane vesicles measured by STED-FCS

Diffusion and interaction dynamics of molecules at the plasma membrane play an important role in cellular signaling and are suggested to be strongly associated with the actin cytoskeleton. Here we use superresolution STED microscopy combined with fluorescence correlation spectroscopy (STED-FCS) to access and compare the diffusion characteristics of fluorescent lipid analogues and GPI-anchored proteins (GPI-APs) in the live-cell plasma membrane and in actin cytoskeleton-free, cell-derived giant plasma membrane vesicles (GPMVs). Hindered diffusion of phospholipids and sphingolipids is abolished in the GPMVs, whereas transient nanodomain incorporation of ganglioside lipid GM1 is apparent in both the live-cell membrane and GPMVs. For GPI-APs, we detect two molecular pools in living cells; one pool shows high mobility with transient incorporation into nanodomains, and the other pool forms immobile clusters, both of which disappear in GPMVs. Our data underline the crucial role of the actin cortex in maintaining hindered diffusion modes of many but not all of the membrane molecules and highlight a powerful experimental approach to decipher specific influences on molecular plasma membrane dynamics.

Gb3-binding lectins as potential carriers for transcellular drug delivery

OBJECTIVES

Epithelial cell layers as well as endothelia forming the blood-brain barrier can drastically reduce the efficiency of drug targeting. Our goal was to investigate lectins recognizing the glycosphingolipid globotriaosylceramide (Gb3) for their potential as carriers for transcytotic drug delivery.

METHODS

We utilized an in vitro model based on Madin-Darby canine kidney cells transfected with Gb3 synthase to characterize transcytosis of the Gb3-binding lectins LecA from Pseudomonas aeruginosa and the B-subunit of Shiga toxin (StxB).

RESULTS

Both lectins were rapidly transcytosed from the apical to the basolateral plasma membrane and vice versa. Whereas StxB proceeded on retrograde and transcytotic routes, LecA avoided retrograde transport. This differential trafficking could be explained by our observation that LecA and StxB segregated into different domains during endocytosis. Furthermore, inhibiting the small GTPase Rab11a, which organizes trafficking through apical recycling endosomes, blocked basolateral to apical transcytosis of both lectins.

CONCLUSIONS

Gb3-binding lectins are promising candidates for transcytotic drug delivery. Our findings highlight that LecA and StxB, which both bind Gb3 but exhibit dissimilar valence and molecular structures of their carbohydrate binding sites and can take divergent intracellular trafficking routes. This opens up the possibility of developing tailor-made glycosphingolipid-binding carrier lectins, which take optimized trafficking pathways.

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.

Glycolipid Crosslinking Is Required for Cholera Toxin to Partition Into and Stabilize Ordered Domains

Current models of lipid rafts propose that lipid domains exist as nanoscale compositional fluctuations and these fluctuations can potentially be stabilized into larger domains, consequently better compartmentalizing cellular functions. However, the mechanisms governing stabilized raft assembly and function remain unclear. Here, we test the role of glycolipid crosslinking as a raft targeting and ordering mechanism using the well-studied raft marker cholera toxin B pentamer (CTxB) that binds up to five GM1 glycosphingolipids to enter host cells. We show that when applied to cell-derived giant plasma membrane vesicles, a variant of CTxB containing only a single functional GM1 binding site exhibits significantly reduced partitioning to the ordered phase compared to wild-type CTxB with five binding sites. Moreover, monovalent CTxB does not stabilize membrane domains, unlike wild-type CTxB. These results support the long-held hypothesis that CTxB stabilizes raft domains via a lipid crosslinking mechanism and establish a role for crosslinking in the partitioning of CTxB to ordered domains.

Oral absorption of peptides and nanoparticles across the human intestine: Opportunities, limitations and studies in human tissues

In this contribution, we review the molecular and physiological barriers to oral delivery of peptides and nanoparticles. We discuss the opportunities and predictivity of various in vitro systems with special emphasis on human intestine in Ussing chambers. First, the molecular constraints to peptide absorption are discussed. Then the physiological barriers to peptide delivery are examined. These include the gastric and intestinal environment, the mucus barrier, tight junctions between epithelial cells, the enterocytes of the intestinal epithelium, and the subepithelial tissue. Recent data from human proteome studies are used to provide information about the protein expression profiles of the different physiological barriers to peptide and nanoparticle absorption. Strategies that have been employed to increase peptide absorption across each of the barriers are discussed. Special consideration is given to attempts at utilizing endogenous transcytotic pathways. To reliably translate in vitro data on peptide or nanoparticle permeability to the in vivo situation in a human subject, the in vitro experimental system needs to realistically capture the central aspects of the mentioned barriers. Therefore, characteristics of common in vitro cell culture systems are discussed and compared to those of human intestinal tissues. Attempts to use the cell and tissue models for in vitro-in vivo extrapolation are reviewed.

Complement pathway amplifies caspase-11-dependent cell death and endotoxin-induced sepsis severity

Cell death and release of proinflammatory mediators contribute to mortality during sepsis. Specifically, caspase-11-dependent cell death contributes to pathology and decreases in survival time in sepsis models. Priming of the host cell, through TLR4 and interferon receptors, induces caspase-11 expression, and cytosolic LPS directly stimulates caspase-11 activation, promoting the release of proinflammatory cytokines through pyroptosis and caspase-1 activation. Using a CRISPR-Cas9-mediated genome-wide screen, we identified novel mediators of caspase-11-dependent cell death. We found a complement-related peptidase, carboxypeptidase B1 (Cpb1), to be required for caspase-11 gene expression and subsequent caspase-11-dependent cell death. Cpb1 modifies a cleavage product of C3, which binds to and activates C3aR, and then modulates innate immune signaling. We find the Cpb1-C3-C3aR pathway induces caspase-11 expression through amplification of MAPK activity downstream of TLR4 and Ifnar activation, and mediates severity of LPS-induced sepsis (endotoxemia) and disease outcome in mice. We show C3aR is required for up-regulation of caspase-11 orthologues, caspase-4 and -5, in primary human macrophages during inflammation and that c3aR1 and caspase-5 transcripts are highly expressed in patients with severe sepsis; thus, suggesting that these pathways are important in human sepsis. Our results highlight a novel role for complement and the Cpb1-C3-C3aR pathway in proinflammatory signaling, caspase-11 cell death, and sepsis severity.

Synthetic Glycosphingolipids for Live-Cell Labeling

Glycosphingolipids are an important component of cell membranes that are involved in many biological processes. Fluorescently labeled glycosphingolipids are frequently used to gain insight into their localization. However, the attachment of a fluorophore to the glycan part or-more commonly-to the lipid part of glycosphingolipids is known to alter the biophysical properties and can perturb the biological function of the probe. Presented here is the synthesis of novel glycosphingolipid probes with mono- and disaccharide head groups and ceramide moieties containing fatty acids of varying chain length (C4 to C20). These glycosphingolipids bear an azide or an alkyne group as chemical reporter to which a fluorophore can be attached through a bioorthogonal ligation reaction. The fluorescent tag and any linker connected to it can be chosen in a flexible manner. We demonstrate the suitability of the probes by selective visualization of the plasma membrane of living cells by confocal microscopy techniques. Whereas the derivatives with the shorter fatty acids can be directly applied to HEK 293T cells, the hydrophobic glycosphingolipids with longer fatty acids can be delivered to cells using fusogenic liposomes.

Intracellular trafficking of bacterial toxins

Bacterial toxins often translocate across a cellular membrane to gain access into the host cytosol, modifying cellular components in order to exert their toxic effects. To accomplish this feat, these toxins traffic to a membrane penetration site where they undergo conformational changes essential to eject the toxin's catalytic subunit into the cytosol. In this brief review, we highlight recent findings that elucidate both the trafficking pathways and membrane translocation mechanisms of toxins that cross the plasma, endosomal, or endoplasmic reticulum (ER) membrane. These findings not only illuminate the specific nature of the host-toxin interactions during entry, but should also provide additional therapeutic strategies to prevent or alleviate the bacterial toxin-induced diseases.

Raft-based interactions of gangliosides with a GPI-anchored receptor

Gangliosides, glycosphingolipids containing one or more sialic acid(s) in the glyco-chain, are involved in various important physiological and pathological processes in the plasma membrane. However, their exact functions are poorly understood, primarily because of the scarcity of suitable fluorescent ganglioside analogs. Here, we developed methods for systematically synthesizing analogs that behave like their native counterparts in regard to partitioning into raft-related membrane domains or preparations. Single-fluorescent-molecule imaging in the live-cell plasma membrane revealed the clear but transient colocalization and codiffusion of fluorescent ganglioside analogs with a fluorescently labeled glycosylphosphatidylinisotol (GPI)-anchored protein, human CD59, with lifetimes of 12 ms for CD59 monomers, 40 ms for CD59's transient homodimer rafts in quiescent cells, and 48 ms for engaged-CD59-cluster rafts, in cholesterol- and GPI-anchoring-dependent manners. The ganglioside molecules were always mobile in quiescent cells. These results show that gangliosides continually and dynamically exchange between raft domains and the bulk domain, indicating that raft domains are dynamic entities.

Chemically modified peptides and proteins - critical considerations for oral delivery

Numerous approaches have been explored to date in the pursuit of delivering peptides or proteins via the oral route. One such example is chemical modification, whereby the native structure of a peptide or protein is tailored to provide a more efficient uptake across the epithelial barrier of the gastrointestinal tract via incorporation of a chemical motif or moiety. In this regard, a diverse array of concepts have been reported, ranging from the exploitation of endogenous transport mechanisms to incorporation of physicochemical modifications in the molecule, which promote more favorable interactions with the absorptive membrane at the cell surface. This review provides an overview of the modification technologies described in the literature and offers insights into some pragmatic considerations pertaining to their translation into clinically viable concepts.

Caveolin- and clathrin-independent entry of BKPyV into primary human proximal tubule epithelial cells

BK polyomavirus (BKPyV) is a human pathogen that causes polyomavirus-associated nephropathy and hemorrhagic cystitis in transplant patients. Gangliosides and caveolin proteins have previously been reported to be required for BKPyV infection in animal cell models. Recent studies from our lab and others, however, have indicated that the identity of the cells used for infection studies can greatly influence the behavior of the virus. We therefore wished to re-examine BKPyV entry in a physiologically relevant primary cell culture model, human renal proximal tubule epithelial cells. Using siRNA knockdowns, we interfered with expression of UDP-glucose ceramide glucosyltransferase (UGCG), and the endocytic vesicle coat proteins caveolin 1, caveolin 2, and clathrin heavy chain. The results demonstrate that while BKPyV does require gangliosides for efficient infection, it can enter its natural host cells via a caveolin- and clathrin-independent pathway. The results emphasize the importance of studying viruses in a relevant cell culture model.

Recent progress on lipid lateral heterogeneity in plasma membranes: From rafts to submicrometric domains

The concept of transient nanometric domains known as lipid rafts has brought interest to reassess the validity of the Singer-Nicolson model of a fluid bilayer for cell membranes. However, this new view is still insufficient to explain the cellular control of surface lipid diversity or membrane deformability. During the past decades, the hypothesis that some lipids form large (submicrometric/mesoscale vs nanometric rafts) and stable (>min vs s) membrane domains has emerged, largely based on indirect methods. Morphological evidence for stable submicrometric lipid domains, well-accepted for artificial and highly specialized biological membranes, was further reported for a variety of living cells from prokaryot es to yeast and mammalian cells. However, results remained questioned based on limitations of available fluorescent tools, use of poor lipid fixatives, and imaging artifacts due to non-resolved membrane projections. In this review, we will discuss recent evidence generated using powerful and innovative approaches such as lipid-specific toxin fragments that support the existence of submicrometric domains. We will integrate documented mechanisms involved in the formation and maintenance of these domains, and provide a perspective on their relevance on membrane deformability and regulation of membrane protein distribution.

Functions of cholera toxin B-subunit as a raft cross-linker

Lipid rafts are putative complexes of lipids and proteins in cellular membranes that are proposed to function in trafficking and signalling events. CTxB (cholera toxin B-subunit) has emerged as one of the most studied examples of a raft-associated protein. Consisting of the membrane-binding domain of cholera toxin, CTxB binds up to five copies of its lipid receptor on the plasma membrane of the host cell. This multivalency of binding gives the toxin the ability to reorganize underlying membrane structure by cross-linking otherwise small and transient lipid rafts. CTxB thus serves as a useful model for understanding the properties and functions of protein-stabilized domains. In the present chapter, we summarize current evidence that CTxB associates with and cross-links lipid rafts, discuss how CTxB binding modulates the architecture and dynamics of membrane domains, and describe the functional consequences of this cross-linking behaviour on toxin uptake into cells via endocytosis.

Carbohydrate conformation and lipid condensation in monolayers containing glycosphingolipid Gb3: influence of acyl chain structure

Globotriaosylceramide (Gb3), a glycosphingolipid found in the plasma membrane of animal cells, is the endocytic receptor of the bacterial Shiga toxin. Using x-ray reflectivity (XR) and grazing incidence x-ray diffraction (GIXD), lipid monolayers containing Gb3 were investigated at the air-water interface. XR probed Gb3 carbohydrate conformation normal to the interface, whereas GIXD precisely characterized Gb3's influence on acyl chain in-plane packing and area per molecule (APM). Two phospholipids, 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE), were used to study Gb3 packing in different lipid environments. Furthermore, the impact on monolayer structure of a naturally extracted Gb3 mixture was compared to synthetic Gb3 species with uniquely defined acyl chain structures. XR results showed that lipid environment and Gb3 acyl chain structure impact carbohydrate conformation with greater solvent accessibility observed for smaller phospholipid headgroups and long Gb3 acyl chains. In general, GIXD showed that Gb3 condensed phospholipid packing resulting in smaller APM than predicted by ideal mixing. Gb3's capacity to condense APM was larger for DSPC monolayers and exhibited different dependencies on acyl chain structure depending on the lipid environment. The interplay between Gb3-induced changes in lipid packing and the lipid environment's impact on carbohydrate conformation has broad implications for glycosphingolipid macromolecule recognition and ligand binding.

How synthetic membrane systems contribute to the understanding of lipid-driven endocytosis

Synthetic membrane systems, such as giant unilamellar vesicles and solid supported lipid bilayers, have widened our understanding of biological processes occurring at or through membranes. Artificial systems are particularly suited to study the inherent properties of membranes with regard to their components and characteristics. This review critically reflects the emerging molecular mechanism of lipid-driven endocytosis and the impact of model membrane systems in elucidating the complex interplay of biomolecules within this process. Lipid receptor clustering induced by binding of several toxins, viruses and bacteria to the plasma membrane leads to local membrane bending and formation of tubular membrane invaginations. Here, lipid shape, and protein structure and valency are the essential parameters in membrane deformation. Combining observations of complex cellular processes and their reconstitution on minimal systems seems to be a promising future approach to resolve basic underlying mechanisms. This article is part of a Special Issue entitled: Mechanobiology.

A facile method for controlling the reaction equilibrium of sphingolipid ceramide N-deacylase for lyso-glycosphingolipid production

Lyso-glycosphingolipids (lyso-GSLs), the N-deacylated forms of glycosphingolipids (GSLs), are important synthetic intermediates for the preparation of GSL analogs. Although lyso-GSLs can be produced by hydrolyzing natural GSLs using sphingolipid ceramide N-deacylase (SCDase), the yield for this reaction is usually low because SCDase also catalyzes the reverse reaction, ultimately establishing an equilibrium between hydrolysis and synthesis. In the present study, we developed an efficient method for controlling the reaction equilibrium by introducing divalent metal cation and detergent in the enzymatic reaction system. In the presence of both Ca(2+) and taurodeoxycholate hydrate, the generated fatty acids were precipitated by the formation of insoluble stearate salts and pushing the reaction equilibrium toward hydrolysis. The yield of GM1 hydrolysis can be achieved as high as 96%, with an improvement up to 45% compared with the nonoptimized condition. In preparative scale, 75 mg of lyso-GM1 was obtained from 100 mg of GM1 with a 90% yield, which is the highest reported yield to date. The method can also be used for the efficient hydrolysis of a variety of GSLs and sphingomyelin. Thus, this method should serve as a facile, easily scalable, and general tool for lyso-GSL production to facilitate further GSL research.

Microtubule motors power plasma membrane tubulation in clathrin-independent endocytosis

How the plasma membrane is bent to accommodate clathrin-independent endocytosis remains uncertain. Recent studies suggest Shiga and cholera toxin induce membrane curvature required for their uptake into clathrin-independent carriers by binding and cross-linking multiple copies of their glycosphingolipid receptors on the plasma membrane. But it remains unclear if toxin-induced sphingolipid crosslinking provides sufficient mechanical force for deforming the plasma membrane, or if host cell factors also contribute to this process. To test this, we imaged the uptake of cholera toxin B-subunit into surface-derived tubular invaginations. We found that cholera toxin mutants that bind to only one glycosphingolipid receptor accumulated in tubules, and that toxin binding was entirely dispensable for membrane tubulations to form. Unexpectedly, the driving force for tubule extension was supplied by the combination of microtubules, dynein and dynactin, thus defining a novel mechanism for generating membrane curvature during clathrin-independent endocytosis.

The nucleotide exchange factors Grp170 and Sil1 induce cholera toxin release from BiP to enable retrotranslocation

Cholera toxin (CT) intoxicates cells by trafficking from the cell surface to the endoplasmic reticulum (ER), where the catalytic CTA1 subunit hijacks components of the ER-associated degradation (ERAD) machinery to retrotranslocate to the cytosol and induce toxicity. In the ER, CT targets to the ERAD machinery composed of the E3 ubiquitin ligase Hrd1-Sel1L complex, in part via the activity of the Sel1L-binding partner ERdj5. This J protein stimulates BiP's ATPase activity, allowing BiP to capture the toxin. Presumably, toxin release from BiP must occur before retrotranslocation. Here, using loss-and gain-of-function approaches coupled with binding studies, we demonstrate that the ER-resident nucleotide exchange factors (NEFs) Grp170 and Sil1 induce CT release from BiP in order to promote toxin retrotranslocation. In addition, we find that after NEF-dependent release from BiP, the toxin is transferred to protein disulfide isomerase; this ER redox chaperone is known to unfold CTA1, which allows the toxin to cross the Hrd1-Sel1L complex. Our data thus identify two NEFs that trigger toxin release from BiP to enable successful retrotranslocation and clarify the fate of the toxin after it disengages from BiP.

The effects of globotriaosylceramide tail saturation level on bilayer phases

Globotriaosylceramide (Gb3) is a glycosphingolipid present in the plasma membrane that is the natural receptor of the bacterial Shiga toxin. The unsaturation level of Gb3 acyl chains has a drastic impact on lipid bilayer properties and phase behaviour, and on many Gb3-related cellular processes. For example: the Shiga toxin B subunit forms tubular invaginations in the presence of Gb3 with an unsaturated acyl chain (U-Gb3), while in the presence of Gb3 with a saturated acyl chain (S-Gb3) such invagination does not occur. We have used all-atom molecular dynamics simulations to investigate the effects of the Gb3 concentration and its acyl chain saturation on the phase behaviour of a mixed bilayer of dioleoylphosphatidylcholine and Gb3. The simulation results show that: (1) the Gb3 acyl chains (longer tails) from one leaflet interdigitate into the opposing leaflet and lead to significant bilayer rigidification and immobilisation of the lipid tails. S-Gb3 can form a highly ordered, relatively immobile phase which is resistant to bending while these changes for U-Gb3 are not significant. (2) At low concentrations of Gb3, U-Gb3 and S-Gb3 have a similar impact on the bilayer reminiscent of the effect of sphingomyelin lipids and (3) At higher Gb3 concentrations, U-Gb3 mixes better with dioleoylphosphatidylcholine than S-Gb3. Our simulations also provide the first molecular level structural model of Gb3 in membranes.

Plasma membrane reorganization: A glycolipid gateway for microbes

Ligand-receptor interactions, which represent the core for cell signaling and internalization processes are largely affected by the spatial configuration of host cell receptors. There is a growing piece of evidence that receptors are not homogeneously distributed within the plasma membrane, but are rather pre-clustered in nanodomains, or clusters are formed upon ligand binding. Pathogens have evolved many strategies to evade the host immune system and to ensure their survival by hijacking plasma membrane receptors that are most often associated with lipid rafts. In this review, we discuss the early stage molecular and physiological events that occur following ligand binding to host cell glycolipids. The ability of various biological ligands (e.g. toxins, lectins, viruses or bacteria) that bind to glycolipids to induce their own uptake into mammalian cells by creating negative membrane curvature and membrane invaginations is explored. We highlight recent trends in understanding nanoscale plasma membrane (re-)organization and present the benefits of using synthetic membrane systems. This article is part of a Special Issue entitled: Nanoscale membrane organisation and signalling.

Stable cell surface expression of GPI-anchored proteins, but not intracellular transport, depends on their fatty acid structure

Glycosylphosphatidylinositol-anchored proteins (GPI-APs) are a class of lipid anchored proteins expressed on the cell surface of eukaryotes. The potential interaction of GPI-APs with ordered lipid domains enriched in cholesterol and sphingolipids has been proposed to function in the intracellular transport of these lipid anchored proteins. Here, we examined the biological importance of two saturated fatty acids present in the phosphatidylinositol moiety of GPI-APs. These fatty acids are introduced by the action of lipid remodeling enzymes and required for the GPI-AP association within ordered lipid domains. We found that the fatty acid remodeling is not required for either efficient Golgi-to-plasma membrane transport or selective endocytosis via GPI-enriched early endosomal compartment (GEEC)/ clathrin-independent carrier (CLIC) pathway, whereas cholesterol depletion significantly affects both pathways independent of their fatty acid structure. Therefore, the mechanism of cholesterol dependence does not appear to be related to the interaction with ordered lipid domains mediated by two saturated fatty acids. Furthermore, cholesterol extraction drastically releases the unremodeled GPI-APs carrying an unsaturated fatty acid from the cell surface, but not remodeled GPI-APs carrying two saturated fatty acids. This underscores the essential role of lipid remodeling to ensure a stable membrane association of GPI-APs particularly under potential membrane lipid perturbation.

High-speed single-particle tracking of GM1 in model membranes reveals anomalous diffusion due to interleaflet coupling and molecular pinning

The biological functions of the cell membrane are influenced by the mobility of its constituents, which are thought to be strongly affected by nanoscale structure and organization. Interactions with the actin cytoskeleton have been proposed as a potential mechanism with the control of mobility imparted through transmembrane "pickets" or GPI-anchored lipid nanodomains. This hypothesis is based on observations of molecular mobility using various methods, although many of these lack the spatiotemporal resolution required to fully capture all the details of the interaction dynamics. In addition, the validity of certain experimental approaches, particularly single-particle tracking, has been questioned due to a number of potential experimental artifacts. Here, we use interferometric scattering microscopy to track molecules labeled with 20-40 nm scattering gold beads with simultaneous <2 nm spatial and 20 μs temporal precision to investigate the existence and mechanistic origin of anomalous diffusion in bilayer membranes. We use supported lipid bilayers as a model system and demonstrate that the label does not influence time-dependent diffusion in the small particle limit (≤40 nm). By tracking the motion of the ganglioside lipid GM1 bound to the cholera toxin B subunit for different substrates and lipid tail properties, we show that molecular pinning and interleaflet coupling between lipid tail domains on a nanoscopic scale suffice to induce transient immobilization and thereby anomalous subdiffusion on the millisecond time scale.

Relative roles of GM1 ganglioside, N-acylneuraminic acids, and α2β1 integrin in mediating rotavirus infection

N-acetyl- and N-glycolylneuraminic acids (Sia) and α2β1 integrin are frequently used by rotaviruses as cellular receptors through recognition by virion spike protein VP4. The VP4 subunit VP8*, derived from Wa rotavirus, binds the internal N-acetylneuraminic acid on ganglioside GM1. Wa infection is increased by enhanced internal Sia access following terminal Sia removal from main glycan chains with sialidase. The GM1 ligand cholera toxin B (CTB) reduces Wa infectivity. Here, we found sialidase treatment increased cellular GM1 availability and the infectivity of several other human (including RV-3) and animal rotaviruses, typically rendering them susceptible to methyl α-d-N-acetylneuraminide treatment, but did not alter α2β1 usage. CTB reduced the infectivity of these viruses. Aceramido-GM1 inhibited Wa and RV-3 infectivity in untreated and sialidase-treated cells, and GM1 supplementation increased their infectivity, demonstrating the importance of GM1 for infection. Wa recognition of α2β1 and internal Sia were at least partially independent. Rotavirus usage of GM1 was mapped to VP4 using virus reassortants, and RV-3 VP8* bound aceramido-GM1 by saturation transfer difference nuclear magnetic resonance (STD NMR). Most rotaviruses recognizing terminal Sia did not use GM1, including RRV. RRV VP8* interacted minimally with aceramido-GM1 by STD NMR. Unusually, TFR-41 rotavirus infectivity depended upon terminal Sia and GM1. Competition of CTB, Sia, and/or aceramido-GM1 with cell binding by VP8* from representative rotaviruses showed that rotavirus Sia and GM1 preferences resulted from VP8*-cell binding. Our major finding is that infection by human rotaviruses of commonly occurring VP4 serotypes involves VP8* binding to cell surface GM1 glycan, typically including the internal N-acetylneuraminic acid.

IMPORTANCE

Rotaviruses, the major cause of severe infantile gastroenteritis, recognize cell surface receptors through virus spike protein VP4. Several animal rotaviruses are known to bind sialic acids at the termini of main carbohydrate chains. Conversely, only a single human rotavirus is known to bind sialic acid. Interestingly, VP4 of this rotavirus bound to sialic acid that forms a branch on the main carbohydrate chain of the GM1 ganglioside. Here, we use several techniques to demonstrate that other human rotaviruses exhibit similar GM1 usage properties. Furthermore, binding by VP4 to cell surface GM1, involving branched sialic acid recognition, is shown to facilitate infection. In contrast, most animal rotaviruses that bind terminal sialic acids did not utilize GM1 for VP4 cell binding or infection. These studies support a significant role for GM1 in mediating host cell invasion by human rotaviruses.

Lipid requirements for entry of protein toxins into cells

The plant toxin ricin and the bacterial toxin Shiga toxin both belong to a group of protein toxins having one moiety that binds to the cell surface, and another, enzymatically active moiety, that enters the cytosol and inhibits protein synthesis by inactivating ribosomes. Both toxins travel all the way from the cell surface to endosomes, the Golgi apparatus and the ER before the ribosome-inactivating moiety enters the cytosol. Shiga toxin binds to the neutral glycosphingolipid Gb3 at the cell surface and is therefore dependent on this lipid for transport into the cells, whereas ricin binds both glycoproteins and glycolipids with terminal galactose. The different steps of transport used by these toxins have specific requirements for lipid species, and with the recent developments in mass spectrometry analysis of lipids and microscopical and biochemical dissection of transport in cells, we are starting to see the complexity of endocytosis and intracellular transport. In this article we describe lipid requirements and the consequences of lipid changes for the entry and intoxication with ricin and Shiga toxin. These toxins can be a threat to human health, but can also be exploited for diagnosis and therapy, and have proven valuable as tools to study intracellular transport.

Unsaturated glycoceramides as molecular carriers for mucosal drug delivery of GLP-1

The incretin hormone Glucagon-like peptide 1 (GLP-1) requires delivery by injection for the treatment of Type 2 diabetes mellitus. Here, we test if the properties of glycosphingolipid trafficking in epithelial cells can be applied to convert GLP-1 into a molecule suitable for mucosal absorption. GLP-1 was coupled to the extracellular oligosaccharide domain of GM1 species containing ceramides with different fatty acids and with minimal loss of incretin bioactivity. When applied to apical surfaces of polarized epithelial cells in monolayer culture, only GLP-1 coupled to GM1-ceramides with short- or cis-unsaturated fatty acids trafficked efficiently across the cell to the basolateral membrane by transcytosis. In vivo studies showed mucosal absorption after nasal administration. The results substantiate our recently reported dependence on ceramide structure for trafficking the GM1 across polarized epithelial cells and support the idea that specific glycosphingolipids can be harnessed as molecular vehicles for mucosal delivery of therapeutic peptides.

Raft-like membranes from the trans-Golgi network and endosomal compartments

This article describes a procedure to prepare a raft-like intracellular membrane fraction enriched for the trans-Golgi network (TGN) and endosomal compartments. The initial step in this technique involves cell disruption by homogenization, followed by clearance of the plasma membrane, late endosomes, mitochondria and the endoplasmic reticulum by differential sedimentation. Carbonate treatment, sonication and sucrose density-gradient ultracentrifugation are subsequently used to isolate the target membranes. The isolated subcellular fraction contains less than 1% of the total cellular proteins, but it is highly enriched for syntaxin-6 and Rab11. Typically, 40-60% of the cellular pool of GM1 glycosphingolipid and 10-20% of the total cellular cholesterol cofractionate with this buoyant membrane fraction. Given the role of GM1 as a cell-surface receptor for the cholera toxin and that levels of both GM1 and cholesterol in the TGN-endosomal compartment are upregulated in some inherited diseases, this protocol can potentially be applied to the analysis of disease-associated changes to GM1-enriched intracellular membranes. The isolated membranes are very well separated from caveolin-rich domains of the plasma membrane, the TGN and recycling endosomes. The entire protocol can be completed in as little as 1 d.

Establishment of an in vitro transport assay that reveals mechanistic differences in cytosolic events controlling cholera toxin and T-cell receptor α retro-translocation

Following retrograde trafficking to the endoplasmic reticulum (ER), cholera toxin A1 (CTA1) subunit hijacks ER-associated degradation (ERAD) machinery and retro-translocates into the cytosol to induce toxicity. We previously established a cell-based in vivo assay to identify ER components that regulate this process. However, elucidating cytosolic events that govern CTA1 retro-translocation using this assay is difficult as manipulating cytosolic factors often perturbs toxin retrograde transport to the ER. To circumvent this problem, we developed an in vitro assay in semi-permeabilized cells that directly monitors CTA1 release from the ER into the cytosol. We demonstrate CTA1 is released into the cytosol as a folded molecule in a p97- and proteasome-independent manner. Release nonetheless involves a GTP-dependent reaction. Upon extending this assay to the canonical ERAD substrate T-cell receptor α (TCRα), we found the receptor is unfolded when released into the cytosol and degraded by membrane-associated proteasome. In this reaction, p97 initially extracts TCRα from the ER membrane, followed by TCRα discharge into the cytosol that requires additional energy-dependent cytosolic activities. Our results reveal mechanistic insights into cytosolic events controlling CTA1 and TCRα retro-translocation, and provide a reliable tool to further probe this process.

Ganglioside GM1-mediated transcytosis of cholera toxin bypasses the retrograde pathway and depends on the structure of the ceramide domain

Cholera toxin causes diarrheal disease by binding ganglioside GM1 on the apical membrane of polarized intestinal epithelial cells and trafficking retrograde through sorting endosomes, the trans-Golgi network (TGN), and into the endoplasmic reticulum. A fraction of toxin also moves from endosomes across the cell to the basolateral plasma membrane by transcytosis, thus breeching the intestinal barrier. Here we find that sorting of cholera toxin into this transcytotic pathway bypasses retrograde transport to the TGN. We also find that GM1 sphingolipids can traffic from apical to basolateral membranes by transcytosis in the absence of toxin binding but only if the GM1 species contain cis-unsaturated or short acyl chains in the ceramide domain. We found previously that the same GM1 species are needed to efficiently traffic retrograde into the TGN and endoplasmic reticulum and into the recycling endosome, implicating a shared mechanism of action for sorting by lipid shape among these pathways.

Ganglioside influence on phospholipid films investigated with single molecule fluorescence measurements

Single molecule fluorescence measurements are used to probe the effects of GM1 in DPPC monolayers. Langmuir-Blodgett films of GM1 and DPPC were doped with ~10(-8) mol % of the fluorescent lipid probe, BODIPY-PC, and transferred onto glass substrates at 23 mN/m. As shown previously, the individual orientation of each BODIPY-PC probe in the membrane can be measured using defocused polarized total internal reflection fluorescence microscopy, revealing changes in film properties at the molecular level. Here, BODIPY-PC tilt angle histograms are used to characterize the effects of GM1 in DPPC films from 0.05 to 100 mol % GM1. At high GM1 levels (>5 mol % GM1), trends in the single molecule measurements agree with previous bulk measurements showing the turnover from condensing to expanding influence of GM1 at 15-20 mol %, thus validating the single molecule approach. At biologically relevant, low concentrations of GM1 (<5 mol % GM1), where bulk fluorescence measurements are less informative, the single molecule measurements reveal a marked influence of GM1 on film properties. The addition of trace amounts of GM1 to DPPC films leads to an expansion of the film which continues to 0.10 mol % GM1, above which the trend reverses and the condensing effect previously noted is observed.

Shiga toxin-binding site for host cell receptor GPP130 reveals unexpected divergence in toxin-trafficking mechanisms

Manganese is specifically effective against Shiga toxin (STx) and STx1. STx2 does not bind the manganese-sensitive host cell receptor GPP130, because a histidine/asparagine pair that constitutes the binding site for GPP130 is not conserved. This reveals an unexpected and significant functional divergence in Shiga toxin evolution.

Shiga toxicosis is caused by retrograde trafficking of one of three types of Shiga toxin (STx), STx, STx1, or STx2. Trafficking depends on the toxin B subunits, which for STx and STx1 are identical and bind GPP130, a manganese (Mn)-sensitive intracellular trafficking receptor. Elevated Mn down-regulates GPP130, rendering STx/STx1 harmless. Its effectiveness against STx2, however, which is a serious concern in the developed world, is not known. Here we show that Mn-induced GPP130 down-regulation fails to block STx2 trafficking. To shed light on this result, we tested the purified B subunit of STx2 for binding to GPP130 and found that it failed to interact. We then mapped residues at the interface of the GPP130-STx/STx1 complex. In GPP130, binding mapped to a seven-residue stretch in its lumenal stem domain next to the transmembrane domain. This stretch was required for STx/STx1 transport. In STx/STx1, binding mapped to a histidine–asparagine pair on a surface-exposed loop of the toxin B subunit. Significantly, these residues are not conserved in STx2, explaining the lack of effectiveness of Mn against STx2. Together our results imply that STx2 uses an evolutionarily distinct trafficking mechanism and that Mn as a potential therapy should be focused on STx/STx1 outbreaks, which account for the vast majority of cases worldwide.