Cholera toxin is found in detergent-insoluble rafts/domains at the cell surface of hippocampal neurons but is internalized via a raft-independent mechanism

Entry of nanoparticles into cells and tissues: status and challenges

In this article we discuss how nanoparticles (NPs) of different compositions may interact with and be internalized by cells, and the consequences of that for cellular functions. A large number of NPs are made with the intention to improve cancer treatment, the goal being to increase the fraction of injected drug delivered to the tumor and thereby improve the therapeutic effect and decrease side effects. Thus, we discuss how NPs are delivered to tumors and some challenges related to investigations of biodistribution, pharmacokinetics, and excretion. Finally, we discuss requirements for bringing NPs into clinical use and aspects when it comes to usage of complex and slowly degraded or nondegradable NPs.

The role of lipid rafts in vesicle formation

The formation of membrane vesicles is a common feature in all eukaryotes. Lipid rafts are the best-studied example of membrane domains for both eukaryotes and prokaryotes, and their existence also is suggested in Archaea membranes. Lipid rafts are involved in the formation of transport vesicles, endocytic vesicles, exocytic vesicles, synaptic vesicles and extracellular vesicles, as well as enveloped viruses. Two mechanisms of how rafts are involved in vesicle formation have been proposed: first, that raft proteins and/or lipids located in lipid rafts associate with coat proteins that form a budding vesicle, and second, vesicle budding is triggered by enzymatic generation of cone-shaped ceramides and inverted cone-shaped lyso-phospholipids. In both cases, induction of curvature is also facilitated by the relaxation of tension in the raft domain. In this Review, we discuss the role of raft-derived vesicles in several intracellular trafficking pathways. We also highlight their role in different pathways of endocytosis, and in the formation of intraluminal vesicles (ILVs) through budding inwards from the multivesicular body (MVB) membrane, because rafts inside MVB membranes are likely to be involved in loading RNA into ILVs. Finally, we discuss the association of glycoproteins with rafts via the glycocalyx.

Computational Approaches to Explore Bacterial Toxin Entry into the Host Cell

Many bacteria secrete toxic protein complexes that modify and disrupt essential processes in the infected cell that can lead to cell death. To conduct their action, these toxins often need to cross the cell membrane and reach a specific substrate inside the cell. The investigation of these protein complexes is essential not only for understanding their biological functions but also for the rational design of targeted drug delivery vehicles that must navigate across the cell membrane to deliver their therapeutic payload. Despite the immense advances in experimental techniques, the investigations of the toxin entry mechanism have remained challenging. Computer simulations are robust complementary tools that allow for the exploration of biological processes in exceptional detail. In this review, we first highlight the strength of computational methods, with a special focus on all-atom molecular dynamics, coarse-grained, and mesoscopic models, for exploring different stages of the toxin protein entry mechanism. We then summarize recent developments that are significantly advancing our understanding, notably of the glycolipid–lectin (GL-Lect) endocytosis of bacterial Shiga and cholera toxins. The methods discussed here are also applicable to the design of membrane-penetrating nanoparticles and the study of the phenomenon of protein phase separation at the surface of the membrane. Finally, we discuss other likely routes for future development.

The Protein Toxins Ricin and Shiga Toxin as Tools to Explore Cellular Mechanisms of Internalization and Intracellular Transport

Protein toxins secreted by bacteria and found in plants can be threats to human health. However, their extreme toxicity can also be exploited in different ways, e.g., to produce hybrid toxins directed against cancer cells and to study transport mechanisms in cells. Investigations during the last decades have shown how powerful these molecules are as tools in cell biological research. Here, we first present a partly historical overview, with emphasis on Shiga toxin and ricin, of how such toxins have been used to characterize processes and proteins of importance for their trafficking. In the second half of the article, we describe how one can now use toxins to investigate the role of lipid classes for intracellular transport. In recent years, it has become possible to quantify hundreds of lipid species using mass spectrometry analysis. Thus, it is also now possible to explore the importance of lipid species in intracellular transport. The detailed analyses of changes in lipids seen under conditions of inhibited toxin transport reveal previously unknown connections between syntheses of lipid classes and demonstrate the ability of cells to compensate under given conditions.

Complexity of the Nano-Bio Interface and the Tortuous Path of Metal Oxides in Biological Systems

Metal oxide nanoparticles (NPs) have received a great deal of attention as potential theranostic agents. Despite extensive work on a wide variety of metal oxide NPs, few chemically active metal oxide NPs have received Food and Drug Administration (FDA) clearance. The clinical translation of metal oxide NP activity, which often looks so promising in preclinical studies, has not progressed as rapidly as one might expect. The lack of FDA approval for metal oxide NPs appears to be a consequence of the complex transformation of NP chemistry as any given NP passes through multiple extra- and intracellular environments and interacts with a variety of proteins and transport processes that may degrade or transform the chemical properties of the metal oxide NP. Moreover, the translational models frequently used to study these materials do not represent the final therapeutic environment well, and studies in reduced preparations have, all too frequently, predicted fundamentally different physico-chemical properties from the biological activity observed in intact organisms. Understanding the evolving pharmacology of metal oxide NPs as they interact with biological systems is critical to establish translational test systems that effectively predict future theranostic activity.

Structural Domains of the Herpes Simplex Type 1 gD Protein that Restrict HIV-1 Particle Infectivity

Previously, we showed that the presence of the herpes simplex virus type 1 (HSV-1) gD glycoprotein but not gB potently restricted HIV-1 particle infectivity. This restriction was characterized by incorporation of HSV-1 gD and the exclusion of the HIV-1 gp120/gp41 from budding virus particles. To determine the structural domains involved in gD restriction of HIV-1, a series of deletion mutants and chimeric proteins between gD and the non-restrictive gB were generated. Our results show that deletion of the cytoplasmic tail domain (CTD) of gD or that replacement of the transmembrane domain (TMD) with the TMD from gB slightly reduced restriction activity. However, replacement of the gD CTD with that of gB resulted in lower cell surface expression, significantly less incorporation into HIV-1 particles, and inefficient restriction of the release of infectious HIV-1. Analysis of gB/gD chimeric proteins revealed that removal of the gB CTD or replacement with gD CTD resulted in enhanced surface expression and an increase in restriction activity. Finally, we show that expression of gD without other HSV-1 proteins resulted in gD fractionation into detergent resistant membranes (DRM) and that gD co-localized with the raft marker GM1, which may partially explain its incorporation into budding virus particles. Taken together, our results suggest that expression of gD at the cell surface is likely a major factor but that other intrinsic properties are also involved in the gD-mediated restriction of HIV-1 particle infectivity.IMPORTANCE Previously, we showed that unlike the HSV-1, the presence of the gD glycoprotein in virus producer cells but not gB potently restricted HIV-1 particle infectivity. To better understand the relationship between cell surface expression, virus incorporation and restriction of HIV-1, we analyzed a series of deletion mutants and chimeric proteins in which domains of gD and gB were swapped. Our results indicate that: a) gD/gB chimeras having the cytoplasmic domain (CTD) of gB significantly reduced cell surface expression, release from cells, incorporation into virus, and reduced HIV-1 restriction; b) removal of the gB CTD or replacement with the gD CTD resulted in better surface expression, incorporation into HIV-1, and enhanced restriction; and c) the transmembrane domain of gB can influence transport and ultimately effect incorporation of gB into HIV-1. Overall, these data support a role for gD surface expression as crucial to restriction of infectious HIV-1 release.

Endocytic Internalization of Herpes Simplex Virus 1 in Human Keratinocytes at Low Temperature

Herpes simplex virus 1 (HSV-1) can adopt a variety of pathways to accomplish cellular internalization. In human keratinocytes representing the natural target cell of HSV-1, both direct plasma membrane fusion and endocytic uptake have been found. The impact of either pathway in successful infection, however, remains to be fully understood. To address the role of each internalization mode, we performed infection studies at low temperature as a tool to interfere with endocytic pathways. Interestingly, successful HSV-1 entry in primary human keratinocytes and HaCaT cells was observed even at 7°C, although delayed compared to infection at 37°C. Moreover, ex vivo infection of murine epidermis demonstrated that virus entry at 7°C is not only accomplished in cultured cells but also in tissue. Control experiments with cholera toxin B confirmed a block of endocytic uptake at 7°C. In addition, uptake of dextran by macropinosomes and phagocytic uptake of latex beads was also inhibited at 7°C. Infection of nectin-1-deficient murine keratinocytes affirmed that the entry at 7°C was receptor-dependent. Strikingly, the lysosomotropic agent, ammonium chloride, strongly inhibited HSV-1 entry suggesting a role for endosomal acidification. Ultrastructural analyses in turn revealed free capsids in the cytoplasm as well as virus particles in vesicles after infection at 7°C supporting both plasma membrane fusion and endocytic internalization as already observed at 37°C. Overall, entry of HSV-1 at 7°C suggests that the virus can efficiently adopt nectin-1-dependent unconventional vesicle uptake mechanisms in keratinocytes strengthening the role of endocytic internalization for successful infection.IMPORTANCE The human pathogen herpes simplex virus 1 (HSV-1) relies on multiple internalization pathways to initiate infection. Our focus is on the entry in human keratinocytes, the major in vivo target during primary and recurrent infection. While antivirals reduce the severity of clinical cases, there is no cure or vaccine against HSV. To develop strategies that interfere with virus penetration, we need to understand the various parameters and conditions that determine virus entry. Here, we addressed the impact of virus internalization via vesicles by blocking endocytic processes at low temperature. Intriguingly, we detected entry of HSV-1 even at 7°C which led to infection of primary keratinocytes and epidermal tissue. Moreover, electron microscopy of human keratinocytes at 7°C support that internalization is based on fusion of the viral envelope with the plasma membrane as well as vesicle membranes. These results provide novel insights into conditions that still allow endocytic internalization of HSV-1.

The role of lipid species in membranes and cancer-related changes

Several studies have demonstrated interactions between the two leaflets in membrane bilayers and the importance of specific lipid species for such interaction and membrane function. We here discuss these investigations with a focus on the sphingolipid and cholesterol-rich lipid membrane domains called lipid rafts, including the small flask-shaped invaginations called caveolae, and the importance of such membrane structures in cell biology and cancer. We discuss the possible interactions between the very long-chain sphingolipids in the outer leaflet of the plasma membrane and the phosphatidylserine species PS 18:0/18:1 in the inner leaflet and the importance of cholesterol for such interactions. We challenge the view that lipid rafts contain a large fraction of lipids with two saturated fatty acyl groups and argue that it is important in future studies of membrane models to use asymmetric membrane bilayers with lipid species commonly found in cellular membranes. We also discuss the need for more quantitative lipidomic studies in order to understand membrane function and structure in general, and the importance of lipid rafts in biological systems. Finally, we discuss cancer-related changes in lipid rafts and lipid composition, with a special focus on changes in glycosphingolipids and the possibility of using lipid therapy for cancer treatment.

Clathrin-independent endocytosis: an increasing degree of complexity

This article aims at providing an update on the complexity of clathrin-independent endocytosis. It is now almost 30 years since we first wrote a review about its existence; at that time many people believed that with the exception of macropinocytosis, which will only be briefly mentioned in this review, all uptake could be accounted for by clathrin-dependent endocytosis. Now it is generally accepted that there are different clathrin-independent mechanisms, some of them regulated by ligands and membrane lipid composition. They can be both dynamin-dependent and -independent, meaning that the uptake cannot be accounted for by caveolae and other dynamin-dependent processes such as tubular structures that can be induced by toxins, e.g. Shiga toxin, or the fast endophilin mediated endocytosis recently described. Caveolae seem to be mostly quite stable structures with other functions than endocytosis, but evidence suggests that they may have cell-type dependent functions. Although several groups have been working on endocytic mechanisms for years, and new advanced methods have improved our ability to study mechanistic details, there are still a number of important questions we need to address, such as: How many endocytic mechanisms does a cell have? How quantitatively important are they? What about the complexity in polarized cells where clathrin-independent endocytosis is differentially regulated on the apical and basolateral poles? These questions are not easy to answer since one and the same molecule may contribute to more than one process, and manipulating one mechanism can affect another. Also, several inhibitors of endocytic processes commonly used turn out to be less specific than originally thought. We will here describe the current view of clathrin-independent endocytic processes and the challenges in studying them.

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.

Clathrin-Dependent Uptake of Paraquat into SH-SY5Y Cells and Its Internalization into Different Subcellular Compartments

The herbicide paraquat (PQ) is an exogenous toxin that allows the selective activation of dopaminergic neurons in the mesencephalon to induce injury and also causes its apoptosis in vitro. However, uptake mechanisms between PQ and neurons remain elusive. To address this issue, we undertook a study of PQ endocytosis in a dopaminergic SH-SY5Y cell line as well as explored the subsequent subcellular location and potential functional analysis of PQ. The PQ was found to bind the SH-SY5Y cell membrane and then became internalized via a clathrin-dependent pathway. PQ was internalized by many subcellular organelles in a time- and dose-dependent manner. Interestingly, the taken up PQ and secretogranin III (SCG3), which became dysregulated with PQ treatment that induced SH-SY5Y apoptosis in our previous study, colocalized in cytoplasmic vesicles. Taken together, our findings indicate that PQ is endocytosed by SH-SY5Y cells and that its multiple, subcellular localizations indicate PQ may potentially be involved in subcellular-level functions. More importantly, PQ distributing preferentially into SCG3-positive vesicles demonstrates its selective targeting which may affect SCG3 and cargoes carried by SCG3-positive vesicles. Therefore, it is reasonable to infer that PQ toxic insults may potentially interfere with neurotransmitter storage and transport associated with secretory granules.

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.

Isolation and Analysis of Detergent-Resistant Membrane Fractions

The hypothesis that the Golgi apparatus is capable of sorting proteins and sending them to the plasma membrane through "lipid rafts," membrane lipid domains highly enriched in glycosphingolipids, sphingomyelin, ceramide, and cholesterol, was formulated by van Meer and Simons in 1988 and came to a turning point when it was suggested that lipid rafts could be isolated thanks to their resistance to solubilization by some detergents, namely Triton X-100. An incredible number of papers have described the composition and properties of detergent-resistant membrane fractions. However, the use of this method has also raised the fiercest criticisms. In this chapter, we would like to discuss the most relevant methodological aspects related to the preparation of detergent-resistant membrane fractions, and to discuss the importance of discriminating between what is present on a cell membrane and what we can prepare from cell membranes in a laboratory tube.

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.

Determination of cell uptake pathways for tumor inhibitor lysyl oxidase propeptide

The lysyl oxidase propeptide (LOX-PP) is derived from pro-lysyl oxidase (Pro-LOX) by extracellular biosynthetic proteolysis. LOX-PP inhibits breast and prostate cancer xenograft tumor growth and has tumor suppressor activity. Although, several intracellular targets and molecular mechanisms of action of LOX-PP have been identified, LOX-PP uptake pathways have not been reported. Here we demonstrate that the major uptake pathway for recombinant LOX-PP (rLOX-PP) is PI3K-dependent macropinocytosis in PWR-1E, PC3, SCC9, MDA-MB-231 cell lines. A secondary pathway appears to be dynamin- and caveola dependent. The ionic properties of highly basic rLOX-PP provide buffering capacity at both high and low pHs. We suggest that the buffering capacity of rLOX-PP, which serves to limit endosomal acidification, sustains PI3K-dependent macropinocytosis in endosomes which in turn is likely to facilitate LOX-PP endosomal escape into the cytoplasm and its observed interactions with cytoplasmic targets and nuclear uptake.

Loss of Cell Surface CD47 Clustering Formation and Binding Avidity to SIRPα Facilitate Apoptotic Cell Clearance by Macrophages

CD47, a self recognition marker expressed on tissue cells, interacts with immunoreceptor SIRPα expressed on the surface of macrophages to initiate inhibitory signaling that prevents macrophage phagocytosis of healthy host cells. Previous studies suggested that cells may lose surface CD47 during aging or apoptosis to enable phagocytic clearance. In the current study, we demonstrate that the level of cell surface CD47 is not decreased, but the distribution pattern of CD47 is altered, during apoptosis. On nonapoptotic cells, CD47 molecules are clustered in lipid rafts forming punctates on the surface, whereas on apoptotic cells, CD47 molecules are diffused on the cell surface following the disassembly of lipid rafts. We show that clustering of CD47 in lipid rafts provides a high binding avidity for cell surface CD47 to ligate macrophage SIRPα, which also presents as clusters, and elicits SIRPα-mediated inhibitory signaling that prevents phagocytosis. In contrast, dispersed CD47 on the apoptotic cell surface is associated with a significant reduction in the binding avidity to SIRPα and a failure to trigger SIRPα signal transduction. Disruption of plasma membrane lipid rafts with methyl-β-cyclodextrin diffuses CD47 clusters, leading to a decrease in the cell binding avidity to SIRPα and a concomitant increase in cells being engulfed by macrophages. Taken together, our study reveals that CD47 normally is clustered in lipid rafts on nonapoptotic cells but is diffused in the plasma membrane when apoptosis occurs; this transformation of CD47 greatly reduces the strength of CD47-SIRPα engagement, resulting in the phagocytosis of apoptotic cells.

Tracking cholesterol/sphingomyelin-rich membrane domains with the ostreolysin A-mCherry protein

Ostreolysin A (OlyA) is an ∼15-kDa protein that has been shown to bind selectively to membranes rich in cholesterol and sphingomyelin. In this study, we investigated whether OlyA fluorescently tagged at the C-terminal with mCherry (OlyA-mCherry) labels cholesterol/sphingomyelin domains in artificial membrane systems and in membranes of Madin-Darby canine kidney (MDCK) epithelial cells. OlyA-mCherry showed similar lipid binding characteristics to non-tagged OlyA. OlyA-mCherry also stained cholesterol/sphingomyelin domains in the plasma membranes of both fixed and living MDCK cells, and in the living cells, this staining was abolished by pretreatment with either methyl-β-cyclodextrin or sphingomyelinase. Double labelling of MDCK cells with OlyA-mCherry and the sphingomyelin-specific markers equinatoxin II-Alexa488 and GST-lysenin, the cholera toxin B subunit as a probe that binds to the ganglioside GM1, or the cholesterol-specific D4 domain of perfringolysin O fused with EGFP, showed different patterns of binding and distribution of OlyA-mCherry in comparison with these other proteins. Furthermore, we show that OlyA-mCherry is internalised in living MDCK cells, and within 90 min it reaches the juxtanuclear region via caveolin-1-positive structures. No binding to membranes could be seen when OlyA-mCherry was expressed in MDCK cells. Altogether, these data clearly indicate that OlyA-mCherry is a promising tool for labelling a distinct pool of cholesterol/sphingomyelin membrane domains in living and fixed cells, and for following these domains when they are apparently internalised by the cell.

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.

Retrograde transport of protein toxins through the Golgi apparatus

A number of protein toxins from plants and bacteria take advantage of transport through the Golgi apparatus to gain entry into the cytosol where they exert their action. These toxins include the plant toxin ricin, the bacterial Shiga toxins, and cholera toxin. Such toxins bind to lipids or proteins at the cell surface, and they are endocytosed both by clathrin-dependent and clathrin-independent mechanisms. Sorting to the Golgi and retrograde transport to the endoplasmic reticulum (ER) are common to these toxins, but the exact mechanisms turn out to be toxin and cell-type dependent. In the ER, the enzymatically active part is released and then transported into the cytosol, exploiting components of the ER-associated degradation system. In this review, we will discuss transport of different protein toxins, but we will focus on factors involved in entry and sorting of ricin and Shiga toxin into and through the Golgi apparatus.

Implication of the proprotein convertases in iron homeostasis: proprotein convertase 7 sheds human transferrin receptor 1 and furin activates hepcidin

The first seven members of the proprotein convertase (PC) family activate protein precursors by cleavage after basic residues. While PC7 has no known specific substrates, it shows redundancy with other PCs. A genome-wide association study suggested that circulating levels of shed human transferrin receptor 1 (hTfR1) are regulated by PC7. We thus examined whether hTfR1 constitutes a specific substrate for PC7. Coexpression of hTfR1 with PCs in several cell lines indicated that PC7 is the only convertase that sheds this receptor into the medium. Site-directed mutagenesis showed that cleavage occurs at the unusual site KTECER100 ↓LA, in which the P1 Arg100 and P6 Lys95 are critical. Pharmacological treatments revealed that shedding of hTfR1 by PC7 requires endocytosis into acidic clathrin-coated vesicles. A PC7 chimera, in which the transmembrane domain and the cytosolic tail of PC7 were replaced by that of the convertase furin, lost its ability to cleave the receptor, demonstrating the importance of these domains in the regulation of PC7 function. Analysis of primary hepatocytes from mice lacking furin, PC5, PACE4, or PC7 revealed that hepcidin, which limits iron availability in the circulation, is specifically generated by furin and not by PC7. Finally, depletion of iron in the medium of hepatoma cell lines incubated with the iron chelator desferrioxamine resulted in PC7 down-regulation.

CONCLUSION

Among the PC family members, only furin activates hepcidin in hepatocytes, and uniquely the full-length membrane-bound PC7 can directly shed hTfR1 by cleavage at Arg100 ↓. Our results support the notion that, when iron is limiting, hTfR1 levels increase at least in part by way of the down-regulation of PC7 expression. (HEPATOLOGY 2013;).

Proteome analysis of Cry4Ba toxin-interacting Aedes aegypti lipid rafts using geLC-MS/MS

Lipid rafts are microdomains in the plasma membrane of eukaryotic cells. Among their many functions, lipid rafts are involved in cell toxicity caused by pore forming bacterial toxins including Bacillus thuringiensis (Bt) Cry toxins. We isolated lipid rafts from brush border membrane vesicles (BBMV) of Aedes aegypti larvae as a detergent resistant membrane (DRM) fraction on density gradients. Cholesterol, aminopeptidase (APN), alkaline phosphatase (ALP) and the raft marker flotillin were preferentially partitioned into the lipid raft fraction. When mosquitocidal Cry4Ba toxin was preincubated with BBMV, Cry4Ba localized to lipid rafts. A proteomic approach based on one-dimensional gel electrophoresis, in-gel trypsin digestion, followed by liquid chromatography-mass spectrometry (geLC-MS/MS) identified a total of 386 proteins. Of which many are typical lipid raft marker proteins including flotillins and glycosylphosphatidylinositol (GPI)-anchored proteins. Identified raft proteins were annotated in silico for functional and physicochemical characteristics. Parameters such as distribution of isoelectric point, molecular mass, and predicted post-translational modifications relevant to lipid raft proteins (GPI anchorage and myristoylation or palmitoylation) were analyzed for identified proteins in the DRM fraction. From a functional point of view, this study identified proteins implicated in Cry toxin interactions as well as membrane-associated proteins expressed in the mosquito midgut that have potential relevance to mosquito biology and vector management.

Axonal transport deficits and degeneration can evolve independently in mouse models of amyotrophic lateral sclerosis

Axonal transport deficits have been reported in many neurodegenerative conditions and are widely assumed to be an immediate causative step of axon and synapse loss. By imaging changes in axonal morphology and organelle transport over time in several animal models of amyotrophic lateral sclerosis (ALS), we now find that deficits in axonal transport of organelles (mitochondria, endosomes) and axon degeneration can evolve independently. This conclusion rests on the following results: (i) Axons can survive despite long-lasting transport deficits: In the SOD(G93A) model of ALS, transport deficits are detected soon after birth, months before the onset of axon degeneration. (ii) Transport deficits are not necessary for axon degeneration: In the SOD(G85R) model of ALS, motor axons degenerate, but transport is unaffected. (iii) Axon transport deficits are not sufficient to cause immediate degeneration: In mice that overexpress wild-type superoxide dismutase-1 (SOD(WT)), axons show chronic transport deficits, but survive. These data suggest that disturbances of organelle transport are not a necessary step in the emergence of motor neuron degeneration.

A human IgM signals axon outgrowth: coupling lipid raft to microtubules

Mouse and human IgMs support neurite extension from primary cerebellar granule neurons. In this study using primary hippocampal and cortical neurons, we demonstrate that a recombinant human IgM, rHIgM12, promotes axon outgrowth by coupling membrane domains (lipid rafts) to microtubules. rHIgM12 binds to the surface of neuron and induces clustering of cholesterol and ganglioside GM1. After cell binding and membrane fractionation, rHIgM12 gets segregated into two pools, one associated with lipid raft fractions and the other with the detergent-insoluble cytoskeleton-containing pellet. Membrane-bound rHIgM12 co-localized with microtubules and co-immuno precipitated with β3-tubulin. rHIgM12-membrane interaction also enhanced the tyrosination of α-tubulin indicating a stabilization of new neurites. When presented as a substrate, rHIgM12 induced axon outgrowth from primary neurons. We now demonstrate that a recombinant human mAb can induce signals in neurons that regulate membrane lipids and microtubule dynamics required for axon extension. We propose that the pentameric structure of the IgM is critical to cross-link membrane lipids and proteins resulting in signaling cascades.

Internalization of Sambucus nigra agglutinins I and II in insect midgut CF-203 cells

In this project, the uptake mechanisms and localization of two lectins from Sambucus nigra, further referred to as S. nigra agglutinin (SNA)-I and SNA-II, into insect midgut CF-203 cells were studied. SNA-I is a chimeric lectin belonging to the class of ribosome-inactivating proteins, whereas SNA-II is a hololectin devoid of enzymatic activity. Internalization of the fluorescein isothiocyanate-labeled lectin was investigated using confocal microscopy. Both lectins were internalized into the cytoplasm of CF-203 cells at similar rates. Preexposure of the insect midgut cells to specific inhibitors of clathrin- and caveolae-mediated endocytosis resulted in an inhibition of lectin uptake in CF-203 cells and caspase-induced cytotoxicity caused by SNA-I and SNA-II, confirming the involvement of both endocytosis pathways. Further studies demonstrated that the uptake mechanism(s) for both lectins required phosphoinositide 3-kinases, but did not depend on the actin cytoskeleton. Since the hololectin SNA-II apparently uses a similar endocytosis pathway as the chimerolectin SNA-I, it can be concluded that the endocytosis process mainly relies on the carbohydrate-binding activity of the lectins under investigation. © 2011 Wiley Periodicals, Inc.

Endocytosis unplugged: multiple ways to enter the cell

Endocytosis occurs at the cell surface and involves internalization of the plasma membrane (PM) along with its constituent membrane proteins and lipids. Endocytosis is involved in sampling of the extracellular milieu and also serves to regulate various processes initiated at the cell surface. These include nutrient uptake, signaling from cell-surface receptors, and many other processes essential for cell and tissue functioning in metazoans. It is also central to the maintenance of PM lipid and protein homeostasis. There are multiple means of internalization that operate concurrently, at the cell surface. With advancement in high-resolution visualization techniques, it is now possible to track multiple endocytic cargo at the same time, revealing a remarkable diversity of endocytic processes in a single cell. A combination of live cell imaging and efficient genetic manipulations has also aided in understanding the functional hierarchy of molecular players in these mechanisms of internalization. Here we provide an account of various endocytic routes, their mechanisms of operation and occurrence across phyla.

Imaging lipid membrane domains with lipid-specific probes

Imaging membrane lipid domains to characterize their organization and function has been hindered by the lack of reliable lipid-specific probes. In this paper, we provide detailed methods to investigate, mainly by confocal microscopy, the distribution and dynamics of two components of the "lipid rafts," sphingomyelin (SM) and cholesterol, using two specific lipid probes that have been extensively studied in the laboratory: lysenin, a SM-binding toxin and the fluorescent esters of poly(ethylene glycol) cholesteryl ether (PEG-Chol) that label cholesterol-rich domains. The production of nontoxic forms of lysenin as well as its specific binding behavior have allowed monitoring the distribution and the dynamics of SM-rich domains in living cell membranes. Because of its water-solubility and low toxicity, the fluorescent PEG-Chol can be used to follow the reorganization of cell surface cholesterol-rich domains as well as intracellular cholesterol dynamics in living cells. These probes can thus provide important informations on lipid distribution and traffic in living cell membranes.

Gangliosides as regulators of cell membrane organization and functions

Gangliosides, characteristic complex lipids present in the external layer of plasma membranes, deeply influence the organization of the membrane as a whole and the function of specific membrane associated proteins due to lipid-lipid and lipid-protein lateral interaction. Here we discuss the basis for the membrane-organizing potential of gangliosides, examples of ganglioside-regulated membrane protein complexes and the mechanisms for the regulation of ganglioside membrane composition.

Alternate raft pathways cooperate to mediate slow diffusion and efficient uptake of a sphingolipid tracer to degradative and recycling compartments

Several cholesterol-dependent cellular uptake pathways involving microdomain-resident sphingolipids have been characterized, but little is known about what controls the further intracellular trafficking routes of those domains. Here, we present evidence that the uptake and intracellular trafficking of a recently described sphingolipid-binding probe, the sphingolipid binding domain (SBD) peptide, is mediated by two parallel cooperating mechanisms requiring flotillin, dynamin and cdc42, which act in concert to direct a distinct surface behavior and trafficking itinerary. Diffusion measurements of SBD at the cell surface by fluorescence correlation spectroscopy suggest that cdc42- and flotillin-associated uptake sites both correspond to domains of intermediate mobility, but that they can cooperate to form low-mobility, efficiently internalized domains. Interestingly, we find that the choice of uptake mechanism affects subsequent trafficking of SBD, as does cholesterol content. Interference with one or other uptake pathway acts as a toggle switch for the trafficking of SBD to recycling endosomes or endolysosomes, whereas both of these pathways are bypassed if cholesterol is reduced. The data are in accordance with a scenario in which SBD mirrors the trafficking response of raft-borne lipids towards a degradative or recycling target. In summary, we suggest that both the surface behavior of a cargo and its subsequent trafficking are determined by a combination of endocytic accessory proteins and the cholesterol content of different membrane compartments.

Cholera toxin: A paradigm for multi-functional engagement of cellular mechanisms (Review)

Cholera toxin (Ctx) from Vibrio cholerae and its closely related homologue, heat-labile enterotoxin (Etx) from Escherichia coli have become superb tools for illuminating pathways of cellular trafficking and immune cell function. These bacterial protein toxins should be viewed as conglomerates of highly evolved, multi-functional elements equipped to engage the trafficking and signalling machineries of cells. Ctx and Etx are members of a larger family of A-B toxins of bacterial (and plant) origin that are comprised of structurally and functionally distinct enzymatically active A and receptor-binding B sub-units or domains. Intoxication of mammalian cells by Ctx and Etx involves B pentamer-mediated receptor binding and entry into a vesicular pathway, followed by translocation of the enzymatic A1 domain of the A sub-unit into the target cell cytosol, where covalent modification of intracellular targets leads to activation of adenylate cyclase and a sequence of events culminating in life-threatening diarrhoeal disease. Importantly, Ctx and Etx also have the capacity to induce a wide spectrum of remarkable immunological processes. With respect to the latter, it has been found that these toxins activate signalling pathways that modulate the immune system. This review explores the complexities of the cellular interactions that are engaged by these bacterial protein toxins, and highlights some of the new insights to have recently emerged.

Lipid rafts and clathrin cooperate in the internalization of PrP in epithelial FRT cells

BACKGROUND

The cellular prion protein (PrP(C)) plays a key role in the pathogenesis of Transmissible Spongiform Encephalopathies in which the protein undergoes post-translational conversion to the infectious form (PrP(Sc)). Although endocytosis appears to be required for this conversion, the mechanism of PrP(C) internalization is still debated, as caveolae/raft- and clathrin-dependent processes have all been reported to be involved.

METHODOLOGY/PRINCIPAL FINDINGS

We have investigated the mechanism of PrP(C) endocytosis in Fischer Rat Thyroid (FRT) cells, which lack caveolin-1 (cav-1) and caveolae, and in FRT/cav-1 cells which form functional caveolae. We show that PrP(C) internalization requires activated Cdc-42 and is sensitive to cholesterol depletion but not to cav-1 expression suggesting a role for rafts but not for caveolae in PrP(C) endocytosis. PrP(C) internalization is also affected by knock down of clathrin and by the expression of dominant negative Eps15 and Dynamin 2 mutants, indicating the involvement of a clathrin-dependent pathway. Notably, PrP(C) co-immunoprecipitates with clathrin and remains associated with detergent-insoluble microdomains during internalization thus indicating that PrP(C) can enter the cell via multiple pathways and that rafts and clathrin cooperate in its internalization.

CONCLUSIONS/SIGNIFICANCE

These findings are of particular interest if we consider that the internalization route/s undertaken by PrP(C) can be crucial for the ability of different prion strains to infect and to replicate in different cell lines.

Polymeric linear Peptide chimeric vaccine-induced antimalaria immunity is associated with enhanced in vitro antigen loading

Immunization of mice with Plasmodium berghei or Plasmodium yoelii synthetic linear peptide chimeras (LPCs) based on the circumsporozoite protein protects against experimental challenge with viable sporozoites. The immunogenicity of LPCs is significantly enhanced by spontaneous polymerization. To better understand the antigenic properties of polymeric antimalarial peptides, we studied the immune responses elicited in mice immunized with a polymer or a monomer of a linear peptide construct specific for P. yoelii and compared the responses of antigen-presenting cells following incubation with both peptide species. Efficient uptake of the polymeric peptide in vitro resulted in higher expression of the coactivation markers CD80, CD40, and CD70 on dendritic cells and higher proinflammatory cytokine production than with the monomeric peptide. Macropinocytosis seems to be the main route used by polymeric peptides internalized by antigen-presenting cells. Spontaneous polymerization of synthetic antimalarial-peptide constructs to target professional antigen-presenting cells shows promise for simple delivery of subunit malaria vaccines.

Mechanism of the metal-mediated endocytosis of the prion protein

The cellular form of the prion protein, PrPc, is critically required for the establishment of prion diseases, such as Creutzfeldt–Jakob disease. Within the N-terminal half of PrPc are four octapeptide repeats that bind Cu2+. Exposure of neuronal cells expressing PrPc to Cu2+ results in the rapid endocytosis of the protein. First, PrPc translocates laterally out of detergent-resistant lipid rafts into detergent-soluble regions of the plasma membrane, then it is internalized through clathrin-coated pits. The extreme N-terminal region of PrPc is critically required for its endocytosis, as is the transmembrane LRP1 (low-density lipoprotein receptor-related protein-1). Incubation of cells with a competitive inhibitor of LRP1 ligands, receptor-associated protein, or down-regulation of LRP1 with siRNA (short interfering RNA) reduces the endocytosis of PrPc. Zn2+ also promotes the endocytosis of PrPc, a phenomenon that is also dependent on the octapeptide repeats and requires LRP1.

Mechanisms of infectious diarrhea

Infectious diarrhea is an important public health problem worldwide. Research has provided new insights into the mechanisms of diarrhea caused by various pathogens that are classified as noninflammatory, inflammatory or invasive. These three groups of organisms cause two diarrheal syndromes--noninflammatory diarrhea and inflammatory diarrhea. The noninflammatory diarrheas are caused by enterotoxin-producing organisms such as Vibrio cholerae and enterotoxigenic Escherichia coli, or by viruses that adhere to the mucosa and disrupt the absorptive and/or secretory processes of the enterocyte without causing acute inflammation or mucosal destruction. Inflammatory diarrhea is caused by two groups of organisms--cytotoxin-producing, noninvasive bacteria (e.g. enteroaggregative Escherichia coli, enterohemorrhagic Escherichia coli and Clostridium difficile), or by invasive organisms (e.g. Salmonella spp., Shigella spp., Campylobacter spp., Entamoeba histolytica). The cytotoxin-producing organisms adhere to the mucosa, activate cytokines and stimulate the intestinal mucosa to release inflammatory mediators. Invasive organisms, which can also produce cytotoxins, invade the intestinal mucosa to induce an acute inflammatory reaction, involving the activation of cytokines and inflammatory mediators. Regardless of the underlying mechanism they use, these various types of pathogen have all successfully evolved to evade and modulate the host defense systems. The mechanisms by which the different pathogens invade the host and cause infectious diarrhea are the topic of this Review.

Lipid content of brain, brain membrane lipid domains, and neurons from acid sphingomyelinase deficient mice

The cholesterol, sphingolipid, and glycerophospholipid content of total brain, of detergent-resistant membranes prepared from the total brain, and of cerebellar granule cells differentiated in culture from wild type (WT) and acid sphingomyelinase knockout (ASMKO) were studied. Brains derived from 7-month-old ASMKO animals showed a fivefold higher level of sphingomyelin and a significant increase in ganglioside content, mainly because of monosialogangliosides GM3 and GM2 accumulation, while the cholesterol and glycerophospholipid content was unchanged with respect to WT animals. An increase in sphingomyelin, but not in gangliosides, was also detected in cultured cerebellar granule neurons from ASMKO mice, indicating that ganglioside accumulation is not a direct consequence of the enzyme defect. When a detergent-resistant membrane fraction was prepared from ASMKO brains, we observed that a higher detergent-to-protein ratio was needed than in WT animals. This likely reflects a reduced fluidity in restricted membrane areas because of a higher enrichment in sphingolipids in the case of ASMKO brain.

Differential intracellular transport and binding of verotoxin 1 and verotoxin 2 to globotriaosylceramide-containing lipid assemblies

Although verotoxin-1 (VT1) and verotoxin-2 (VT2) share a common receptor, globotriaosyl ceramide (Gb(3)), VT2 induces distinct animal pathology and is preferentially associated with human disease. Moreover VT2 cytotoxicity in vitro is less than VT1. We therefore investigated whether these toxins similarly traffic within cells via similar Gb(3) assemblies. At 4 degrees C, fluorescent-VT1 and VT2 bound both coincident and distinct punctate surface Gb(3) microdomains. After 10 min at 37 degrees C, similar distinct/coincident micropunctate intracellular localization was observed. Most internalized VT2, but not VT1, colocalized with transferrin. After 1 h, VT1 and VT2 coalesced during retrograde transport to the Golgi. During prolonged incubation (3-6 h), VT1, and VT2 (more slowly), exited the Golgi to reach the ER/nuclear envelope. At this time, VT2 induced a previously unreported, retrograde transport-dependent vacuolation. Cell surface and intracellular VT1 showed greater detergent resistance than VT2, suggesting differential 'raft' association. >90% (125)I-VT1 cell surface bound, or added to detergent-resistant cell membrane extracts (DRM), was in the Gb(3)-containing sucrose gradient 'insoluble' fraction, whereas only 30% (125)I-VT2 was similarly DRM-associated. VT1 bound more efficiently to Gb(3)/cholesterol DRMs generated in vitro. Only VT1 binding was inhibited by high cholesterol/Gb(3) ratios. VT2 competed less effectively for (125)I-VT1/Gb(3) DRM-binding but only VT2-Gb(3)/cholesterol DRM-binding was augmented by sphingomyelin. Differential VT1/VT2 Gb(3) raft-binding may mediate differential cell binding/intracellular trafficking and cytopathology.

A fluorescent glycolipid-binding peptide probe traces cholesterol dependent microdomain-derived trafficking pathways

BACKGROUND

The uptake and intracellular trafficking of sphingolipids, which self-associate into plasma membrane microdomains, is associated with many pathological conditions, including viral and toxin infection, lipid storage disease, and neurodegenerative disease. However, the means available to label the trafficking pathways of sphingolipids in live cells are extremely limited. In order to address this problem, we have developed an exogenous, non-toxic probe consisting of a 25-amino acid sphingolipid binding domain, the SBD, derived from the amyloid peptide Abeta, and conjugated by a neutral linker with an organic fluorophore. The current work presents the characterization of the sphingolipid binding and live cell trafficking of this novel probe, the SBD peptide. SBD was the name given to a motif originally recognized by Fantini et al in a number of glycolipid-associated proteins, and was proposed to interact with sphingolipids in membrane microdomains.

METHODOLOGY/PRINCIPAL FINDINGS

In accordance with Fantini's model, optimal SBD binding to membranes depends on the presence of sphingolipids and cholesterol. In synthetic membrane binding assays, SBD interacts preferentially with raft-like lipid mixtures containing sphingomyelin, cholesterol, and complex gangliosides in a pH-dependent manner, but is less glycolipid-specific than Cholera toxin B (CtxB). Using quantitative time-course colocalization in live cells, we show that the uptake and intracellular trafficking route of SBD is unlike that of either the non-raft marker Transferrin or the raft markers CtxB and Flotillin2-GFP. However, SBD traverses an endolysosomal route that partially intersects with raft-associated pathways, with a major portion being diverted at a late time point to rab11-positive recycling endosomes. Trafficking of SBD to acidified compartments is strongly disrupted by cholesterol perturbations, consistent with the regulation of sphingolipid trafficking by cholesterol.

CONCLUSIONS/SIGNIFICANCE

The current work presents the characterization and trafficking behavior of a novel sphingolipid-binding fluorescent probe, the SBD peptide. We show that SBD binding to membranes is dependent on the presence of cholesterol, sphingomyelin, and complex glycolipids. In addition, SBD targeting through the endolysosomal pathway in neurons is highly sensitive to cholesterol perturbations, making it a potentially useful tool for the analysis of sphingolipid trafficking in disease models that involve changes in cholesterol metabolism and storage.

Hydrocortisone modulates cholera toxin endocytosis by regulating immature enterocyte plasma membrane phospholipids

BACKGROUND & AIMS

Diarrheal disease is a major cause of morbidity and mortality in infants and children worldwide. Evidence has indicated immature human enterocytes and their interaction with bacteria and enterotoxins may account for the noted increased susceptibility of neonates to diarrhea. Our aim was to characterize the developmental difference in cholera toxin (CT)-GM1-mediated endocytosis.

METHODS

We used H4 cells (a fetal human small intestinal epithelial cell line), T84 cells, primary cultured mature human small intestinal epithelial cells, and human fetal small intestine xenografts. In addition, hydrocortisone was used as a potent intestinal trophic factor to induce maturation of the human enterocytes.

RESULTS

Here we show an increase in CT-caveolae and a decrease in CT-clathrin colocalization in H4/hydrocortisone compared with H4 cells by electron microscopy. In T84 and freshly isolated human small intestinal epithelial cells, a significant amount of GM1 was partitioned into the lipid rafts. In contrast, there was little CT-GM1/lipid raft association in H4 cells. However, hydrocortisone significantly increased GM1/lipid raft association in H4 cells. Furthermore, we noted an increase in the level of phosphatidylcholine, sphingomyelin, and the ratio of phosphatidylcholine/phosphatidylinositol in mature compared with immature enterocytes and that hydrocortisone can accelerate this maturational process. Disruption of phosphatidylinositol transfer protein alpha using small interference RNA showed an increase in GM1/lipid raft association in H4 cells and resulted in a decreased CT response.

CONCLUSIONS

Our studies suggest that the developmental change in CT endocytosis is partially caused by an increased GM1-lipid raft association through a maturational change of phospholipid composition on the cell surface of immature enterocytes.

Regulation of AMPA receptor localization in lipid rafts

Lipid rafts are special microdomains enriched in cholesterol, sphingolipids and certain proteins, and play important roles in a variety of cellular functions including signal transduction and protein trafficking. We report that in cultured cortical and hippocampal neurons the distribution of lipid rafts is development-dependent. Lipid rafts in mature neurons exist on the entire cell-surface and display a high degree of mobility. AMPA receptors co-localize and associate with lipid rafts in the plasma membrane. The association of AMPARs with rafts is under regulation; through the NOS-NO pathway, NMDA receptor activity increases AMPAR localization in rafts. During membrane targeting, AMPARs insert into or at close proximity of the surface raft domains. Perturbation of lipid rafts dramatically suppresses AMPA receptor exocytosis, resulting in significant reduction in AMPAR cell-surface expression.

Clathrin-independent endocytosis: from nonexisting to an extreme degree of complexity

Today it is generally accepted that there are several endocytic mechanisms, both the clathrin-dependent one and mechanisms which operate without clathrin and with different requirements when it comes to dynamin, small GTP-binding proteins of the Rho family and specific lipids. It should be noted that clathrin-independent endocytosis can occur even when the cholesterol level in the membrane has been reduced to so low levels that caveolae are gone and clathrin-coated membrane areas are flat. Although new investigators in the field take it for granted that there is a multitude of entry mechanisms, it has taken a long time for this to become accepted. However, more work needs to be done, because one can still ask the question: How many endocytic mechanisms does a cell have, what are their function, and how are they regulated? This article describes some of the history of endocytosis research and attempts to give an overview of the complexity of the mechanisms and their regulation.

Conversion of apical plasma membrane sphingomyelin to ceramide attenuates the intoxication of host cells by cholera toxin

Cholera toxin (CT) enters host cells by binding to ganglioside GM1 in the apical plasma membrane (PM). GM1 carries CT retrograde from the PM to the endoplasmic reticulum (ER), where a portion of the toxin, the A1-chain, retro-translocates to the cytosol, causing disease. Trafficking in this pathway appears to depend on the association of CT-GM1 complexes with sphingomyelin (SM)- and cholesterol-rich membrane microdomains termed lipid rafts. Here, we find that in polarized intestinal epithelia, the conversion of apical membrane SM to ceramide by bacterial sphingomyelinase attenuates CT toxicity, consistent with the lipid raft hypothesis. The effect is reversible, specific to toxin entry via the apical membrane, and recapitulated by the addition of exogenous long-chain ceramides. Conversion of apical membrane SM to ceramide inhibits the efficiency of toxin endocytosis, but retrograde trafficking from the apical PM to the Golgi and ER is not affected. This result suggests that the cause for toxin resistance occurs at steps required for retro-translocation of the CT A1-chain to the cytosol.

Intoxication of epithelial cells by plasmid-encoded toxin requires clathrin-mediated endocytosis

It has been shown that the autotransporter plasmid-encoded toxin (Pet) of enteroaggregative Escherichia coli (EAEC) produces cytotoxic and enterotoxic effects. Both effects can be explained by the proteolytic activity of Pet on its intracellular target alpha-fodrin (alphaII spectrin). In addition, Pet cytotoxicity and enterotoxicity depend on Pet serine protease activity, and on its internalization into epithelial cells. However, the mechanisms of Pet uptake by epithelial cells are unknown. Here, we show that Pet interacts with the plasma membrane of epithelial cells, and afterwards is detected inside the cells. Furthermore, Pet was internalized via clathrin-mediated endocytosis, since its internalization was inhibited by monodansylcadaverine and sucrose, but not by filipin or methyl-beta-cyclodextrin, which are drugs that interfere with protein entry via a clathrin-independent pathway. Additionally, Pet was immunoprecipitated by anti-clathrin antibodies, but not by anti-caveolin antibodies. Moreover, small interfering RNA (siRNA), designed to knock out clathrin gene expression in HEp-2 cells, prevented Pet internalization, and thereby the Pet-induced cytotoxic effect. However, the use of siRNA to knock out caveolin expression had no effect on Pet internalization, and the cytotoxic effect was clearly observed. Together, these data indicate that Pet secreted by EAEC binds to the cell surface via an unknown receptor, to be taken up by clathrin-mediated endocytosis, and exert its toxic effect in the cytoplasm.

Endosomal transport of neurotrophins: roles in signaling and neurodegenerative diseases

The internalization and retrograde axonal transport of neurotrophin receptors is important for their retrograde signal transduction supporting neuronal differentiation, plasticity, and survival. To influence transcription, neurotrophin signals initiated at synapses have to be conveyed retrogradely to the cell body. Signaling endosomes containing neurotrophin receptor signaling complexes mediate retrograde neurotrophin signaling from synapses to the nucleus. Interestingly, many neurodegenerative diseases, including Alzheimer's disease, Niemann Pick disease Type C, and Charcot-Marie-Tooth neuropathies, show alterations of vesicular transport, suggesting that traffic jams within neuronal processes may cause neurodegeneration. Although most of these diseases are complex and may be modulated by diverse pathways contributing to neuronal death, altered neurotrophin transport is emerging as a strong candidate influence on neurodegeneration. In this article, we review the mechanisms of internalization and endocytic trafficking of neurotrophin receptors, and discuss the potential roles of perturbations in neurotrophin trafficking in a number of neurodegenerative diseases.