Salmonella-directed recruitment of new membrane to invasion foci via the host exocyst complex

The Phase-Specific Dynamics in Gene Expression of Salmonella Typhimurium During Acanthamoeba castellanii Infection

Being facultative intracellular pathogens, the bacterium often is found in the environment. In natural habitats, Salmonella are able to survive and multiply inside free-living protists that support preservation and distribution of the pathogen, its virulence, and resistance to antimicrobial agents. At the same time, the expression profile of Salmonella genes in the eukaryotic cells has been shown not to be stable, but changes dramatically according to the sequential stages of infection. Previously, we had described the gene expression profile of S. enterica serovar Typhimurium 14028S at the early stage of interaction with Acanthamoeba castellanii. In this study, we have revealed the phase-specific dynamics in expression of several clusters and functional groups of S. Typhimurium 14028S genes. The early stage of invasion characterized by a maximum response to oxidative stress, and it was accompanied by activation of SPI-1 genes, which can contribute to the successful internalization into the host cell. At the second stage (8 h) increase in expression of SPI-2 and SPI-3 genes was accompanied with a maximum expression of iron uptake genes and lysozyme inhibitors. At the late stage of the infection (15 h), downregulation of carbon metabolism and oxidative stress response genes, as well as a decrease in the expression of all other genes, was revealed that may be an evidence of adaptation of Salmonella to intracellular conditions. The obtained results might be useful for further search of factors reducing persistence of pathogens like Salmonella in the environment.

The interplay between Salmonella and host: Mechanisms and strategies for bacterial survival

Salmonella, an intracellular pathogen, infects both humans and animals, causing diverse diseases such as gastroenteritis and enteric fever. The Salmonella type III secretion system (T3SS), encoded within its pathogenicity islands (SPIs), is critical for bacterial virulence by directly delivering multiple effectors into eukaryotic host cells. Salmonella utilizes these effectors to facilitate its survival and replication within the host through modulating cytoskeletal dynamics, inflammatory responses, the biogenesis of Salmonella-containing vacuole (SCV), and host cell survival. Moreover, these effectors also interfere with immune responses via inhibiting innate immunity or antigen presentation. In this review, we summarize the current progress in the survival strategies employed by Salmonella and the molecular mechanisms underlying its interactions with the host. Understanding the interplay between Salmonella and host can enhance our knowledge of the bacterium's pathogenic processes and provide new insights into how it manipulates host cellular physiological activities to ensure its survival.

Influenza A virus RNA localisation and the interceding trafficking pathways of the host cell

Viruses have evolved to efficiently navigate host cells to deliver, express, and replicate their genetic material. Understanding the mechanisms underlying viral RNA localisation is paramount to designing new antivirals. In this review, we discuss Influenza A Virus (IAV) as a model system to highlight some of the ways in which RNA viruses can hijack the endomembrane systems, as well as nuclear transporters, to achieve the correct localisation of their transcripts. IAV exemplifies a nuclear-replicating RNA virus with a complex and highly regulated RNA localisation and trafficking system within host cells. The virus subverts various vesicular transport systems and nuclear transporters, altering normal cellular functions. IAV RNA trafficking begins during entry; after clathrin-mediated endocytosis, the viral genome (vRNPs) is released into the cytosol after fusion with the endosomal membrane, and it is subsequently imported into the nucleus via the importin system. There, vRNPs engage with most major subnuclear structures and exploit host chromatin, the transcription machinery and splicing apparatus to achieve efficient viral mRNA synthesis and export. Subsequently, newly synthesised vRNPs are rapidly exported from the nucleus and contact the host's recycling endosome network for transport to the plasma membrane. We discuss the critical viral remodelling of the entire endomembrane system, particularly the Rab11 recycling endosome and the endoplasmic reticulum. Lastly, replicated genomes come together into bundles to be inserted in budding virions, and we discuss the current models being proposed and the evidence behind them. Despite advances in understanding these processes, several knowledge gaps remain, particularly regarding the specific export of unspliced IAV transcripts, the remodelling of the endomembrane system, and segment bundling.

Salmonella exploits LRRK2-dependent plasma membrane dynamics to invade host cells

Salmonella utilizes type 3 secreted effector proteins to induce plasma membrane (PM) perturbations during invasion of host cells1. The effectors drive mobilization of host membranes to generate cell surface ruffles, followed by invagination and scission of the PM to generate Salmonella-containing vacuoles (SCVs)2. Here, we show that LRRK2 kinase generates membrane reservoirs exploited by Salmonella during invasion. The reservoirs are tubular compartments associated with the PM under basal conditions and are formed through the phosphorylation of RAB10 GTPase by LRRK2. Mobilization of membrane reservoirs to generate invasion ruffles mediates delivery of phosphorylated RAB10 to invasion sites. Subsequently, RAB10 dephosphorylation is required for its inactivation by a bacterial GTPase activating protein and subsequent scission of the PM. RAB10 dephosphorylation is mediated by a TLR4/PIEZO1/TMEM16F-dependent pathway and is inhibited by hyperactive variants of LRRK2. Our findings reveal how Salmonella exploits LRRK2-dependent PM dynamics during invasion and provide new insight into how LRRK2 variants can protect against bacterial infection3,4.

The exocyst in context

The exocyst is a hetero-octameric complex involved in the exocytosis arm of cellular trafficking. Specifically, it tethers secretory vesicles to the plasma membrane, but it is also a main convergence point for many players of exocytosis: regulatory proteins, motor proteins, lipids and Soluble N-ethylmaleimide-sensitive factor Attachment Protein Receptor (SNARE) proteins are all connected physically by the exocyst. Despite extensive knowledge about its structure and interactions, the exocyst remains an enigma precisely because of its increasingly broad and flexible role across the exocytosis process. To solve the molecular mechanism of such a multi-tasking complex, dynamical structures with self, other proteins, and environment should be described. And to do this, interrogation within contexts increasingly close to native conditions is needed. Here we provide a perspective on how different experimental contexts have been used to study the exocyst, and those that could be used in the future. This review describes the structural breakthroughs on the isolated in vitro exocyst, followed by the use of membrane reconstitution assays for revealing in vitro exocyst functionality. Next, it moves to in situ cell contexts, reviewing imaging techniques that have been, and that ideally could be, used to look for near-native structure and organization dynamics. Finally, it looks at the exocyst structure in situ within evolutionary contexts, and the potential of structure prediction therein. From in vitro, to in situ, cross-context investigation of exocyst structure has begun, and will be critical for functional mechanism elucidation.

Pushing boundaries: mechanisms enabling bacterial pathogens to spread between cells

For multiple intracellular bacterial pathogens, the ability to spread directly into adjacent epithelial cells is an essential step for disease in humans. For pathogens such as Shigella, Listeria, Rickettsia, and Burkholderia, this intercellular movement frequently requires the pathogens to manipulate the host actin cytoskeleton and deform the plasma membrane into structures known as protrusions, which extend into neighboring cells. The protrusion is then typically resolved into a double-membrane vacuole (DMV) from which the pathogen quickly escapes into the cytosol, where additional rounds of intercellular spread occur. Significant progress over the last few years has begun to define the mechanisms by which intracellular bacterial pathogens spread. This review highlights the interactions of bacterial and host factors that drive mechanisms required for intercellular spread with a focus on how protrusion structures form and resolve.

Salmonella exploits membrane reservoirs for invasion of host cells

Salmonella utilizes a type 3 secretion system to translocate virulence proteins (effectors) into host cells during infection1. The effectors modulate host cell machinery to drive uptake of the bacteria into vacuoles, where they can establish an intracellular replicative niche. A remarkable feature of Salmonella invasion is the formation of actin-rich protuberances (ruffles) on the host cell surface that contribute to bacterial uptake. However, the membrane source for ruffle formation and how these bacteria regulate membrane mobilization within host cells remains unclear. Here, we show that Salmonella exploits membrane reservoirs for the generation of invasion ruffles. The reservoirs are pre-existing tubular compartments associated with the plasma membrane (PM) and are formed through the activity of RAB10 GTPase. Under normal growth conditions, membrane reservoirs contribute to PM homeostasis and are preloaded with the exocyst subunit EXOC2. During Salmonella invasion, the bacterial effectors SipC, SopE2, and SopB recruit exocyst subunits from membrane reservoirs and other cellular compartments, thereby allowing exocyst complex assembly and membrane delivery required for bacterial uptake. Our findings reveal an important role for RAB10 in the establishment of membrane reservoirs and the mechanisms by which Salmonella can exploit these compartments during host cell invasion.

Structural basis for subversion of host cell actin cytoskeleton during Salmonella infection

Secreted bacterial type III secretion system (T3SS) proteins are essential for successful infection by many human pathogens. Both T3SS translocator SipC and effector SipA are critical for Salmonella infection by subversion of the host cell cytoskeleton, but the precise molecular interplay between them remains unknown. Here, using cryo-electron microscopy, we show that SipA binds along the F-actin grooves with a unique binding pattern. SipA stabilizes F-actin through charged interface residues and appears to prevent inorganic phosphate release through closure of the "back door" of adenosine 5'-triphosphate pocket. We also show that SipC enhances the binding of SipA to F-actin, thus demonstrating that a sequential presence of T3SS proteins in host cells is associated with a sequence of infection events-starting with actin nucleation, filament growth, and stabilization. Together, our data explain the coordinated interplay of a precisely tuned and highly effective mechanism during Salmonella infection and provide a blueprint for interfering with Salmonella effectors acting on actin.

From Eberthella typhi to Salmonella Typhi: The Fascinating Journey of the Virulence and Pathogenicity of Salmonella Typhi

Salmonella Typhi (S. Typhi), the invasive typhoidal serovar of Salmonella enterica that causes typhoid fever in humans, is a severe threat to global health. It is one of the major causes of high morbidity and mortality in developing countries. According to recent WHO estimates, approximately 11-21 million typhoid fever illnesses occur annually worldwide, accounting for 0.12-0.16 million deaths. Salmonella infection can spread to healthy individuals by the consumption of contaminated food and water. Typhoid fever in humans sometimes is accompanied by several other critical extraintestinal complications related to the central nervous system, cardiovascular system, pulmonary system, and hepatobiliary system. Salmonella Pathogenicity Island-1 and Salmonella Pathogenicity Island-2 are the two genomic segments containing genes encoding virulent factors that regulate its invasion and systemic pathogenesis. This Review aims to shed light on a comparative analysis of the virulence and pathogenesis of the typhoidal and nontyphoidal serovars of S. enterica.

Photo-ANA enables profiling of host-bacteria protein interactions during infection

Bacterial pathogens rapidly change and adapt their proteome to cope with the environment in host cells and secrete effector proteins to hijack host targets and ensure their survival and proliferation during infection. Excessive host proteins make it difficult to profile pathogens' proteome dynamics by conventional proteomics. It is even more challenging to map pathogen-host protein-protein interactions in real time, given the low abundance of bacterial effectors and weak and transient interactions in which they may be involved. Here we report a method for selectively labeling bacterial proteomes using a bifunctional amino acid, photo-ANA, equipped with a bio-orthogonal handle and a photoreactive warhead, which enables simultaneous analysis of bacterial proteome reprogramming and pathogen-host protein interactions of Salmonella enterica serovar Typhimurium (S. Typhimurium) during infection. Using photo-ANA, we identified FLOT1/2 as host interactors of S. Typhimurium effector PipB2 in late-stage infection and globally profiled the extensive interactions between host proteins and pathogens during infection.

Determination of the Salmonella intracellular lifestyle by the diversified interaction of Type III secretion system effectors and host GTPases

Intracellular bacteria have developed sophisticated strategies to subvert the host endomembrane system to establish a stable replication niche. Small GTPases are critical players in regulating each step of membrane trafficking events, such as vesicle biogenesis, cargo transport, tethering, and fusion events. Salmonella is a widely studied facultative intracellular bacteria. Salmonella delivers several virulence proteins, termed effectors, to regulate GTPase dynamics and subvert host trafficking for their benefit. In this review, we summarize an updated and systematic understanding of the interactions between bacterial effectors and host GTPases in determining the intracellular lifestyle of Salmonella. This article is categorized under: Infectious Diseases > Molecular and Cellular Physiology.

Salmonella Typhimurium induces genome-wide expression and phosphorylation changes that modulate immune response, intracellular survival and vesicle transport in infected neutrophils

Salmonella Typhimurium is a food-borne pathogen that causes salmonellosis. When in contact with the host, neutrophils are rapidly recruited to act as first line of defense. To better understand the pathogenesis of this infection, we used an in vitro model of neutrophil infection to perform dual RNA-sequencing (both host and pathogen). In addition, and given that many pathogens interfere with kinase-mediated phosphorylation in host signaling, we performed a phosphoproteomic analysis. The immune response was overall diminished in infected neutrophils, mainly JAK/STAT and toll-like receptor signaling pathways. We found decreased expression of proinflammatory cytokine receptor genes and predicted downregulation of the mitogen-activated protein (MAPK) signaling pathway. Also, Salmonella infection inhibited interferons I and II signaling pathways by upregulation of SOCS3 and subsequent downregulation of STAT1 and STAT2. Additionally, phosphorylation of PSMC2 and PSMC4, proteasome regulatory proteins, was decreased in infected neutrophils. Cell viability and survival was increased by p53 signaling, cell cycle arrest and NFkB-proteasome pathways activation. Combined analysis of RNA-seq and phosphoproteomics also revealed inhibited vesicle transport mechanisms mediated by dynein/dynactin and exocyst complexes, involved in ER-to-Golgi transport and centripetal movement of lysosomes and endosomes. Among the overexpressed virulence genes from Salmonella we found potential effectors responsible of these dysregulations, such as spiC, sopD2, sifA or pipB2, all of them involved in intracellular replication. Our results suggest that Salmonella induces (through overexpression of virulence factors) transcriptional and phosphorylation changes that increases neutrophil survival and shuts down immune response to minimize host response, and impairing intracellular vesicle transport likely to keep nutrients for replication and Salmonella-containing vacuole formation and maintenance.

Manipulation of host cell plasma membranes by intracellular bacterial pathogens

Manipulation of the host cell plasma membrane is critical during infection by intracellular bacterial pathogens, particularly during bacterial entry into and exit from host cells. To manipulate host cells, bacteria deploy secreted proteins that modulate or modify host cell components. Here, we review recent advances that suggest common themes by which bacteria manipulate the host cell plasma membrane. One theme is that bacteria use diverse strategies to target or influence host cell plasma membrane composition and shape. A second theme is that bacteria take advantage of host cell plasma membrane-associated pathways such as signal transduction, endocytosis, and exocytosis. Future investigation into how bacterial and host factors contribute to plasma membrane manipulation by bacterial pathogens will reveal new insights into pathogenesis and fundamental principles of plasma membrane biology.

Exploitation of the host exocyst complex by bacterial pathogens

Intracellular bacterial pathogens remodel the plasma membrane of eukaryotic cells in order to establish infection. A common and well-studied mechanism of plasma membrane remodelling involves bacterial stimulation of polymerization of the host actin cytoskeleton. Here, we discuss recent results showing that several bacterial pathogens also exploit the host vesicular trafficking pathway of 'polarized exocytosis' to expand and reshape specific regions in the plasma membrane during infection. Polarized exocytosis is mediated by an evolutionarily conserved octameric protein complex termed the exocyst. We describe examples in which the bacteria Listeria monocytogenes, Salmonella enterica serovar Typhimurium, and Shigella flexneri co-opt the exocyst to promote internalization into human cells or intercellular spread within host tissues. We also discuss results showing that Legionella pneumophila or S. flexneri manipulate exocyst components to modify membrane vacuoles to favour intracellular replication or motility of bacteria. Finally, we propose potential ways that pathogens manipulate exocyst function, discuss how polarized exocytosis might promote infection and highlight the importance of future studies to determine how actin polymerization and polarized exocytosis are coordinated to achieve optimal bacterial infection.

Listeria monocytogenes Co-Opts the Host Exocyst Complex To Promote Internalin A-Mediated Entry

The bacterial pathogen Listeria monocytogenes induces its internalization (entry) into intestinal epithelial cells through interaction of its surface protein, internalin A (InlA), with the human cell-cell adhesion molecule, E-cadherin. While InlA-mediated entry requires bacterial stimulation of actin polymerization, it remains unknown whether additional host processes are manipulated to promote internalization. Here, we show that interaction of InlA with E-cadherin induces the host membrane-trafficking process of polarized exocytosis, which augments uptake of Listeria. Imaging studies revealed that exocytosis is stimulated at sites of InlA-dependent internalization. Experiments inhibiting human N-ethylmaleimide-sensitive factor (NSF) demonstrated that exocytosis is needed for efficient InlA-mediated entry. Polarized exocytosis is mediated by the exocyst complex, which comprises eight proteins, including Sec6, Exo70, and Exo84. We found that Exo70 was recruited to sites of InlA-mediated entry. In addition, depletion of Exo70, Exo84, or Sec6 by RNA interference impaired entry without affecting surface levels of E-cadherin. Similar to binding of InlA to E-cadherin, homophilic interaction of E-cadherin molecules mobilized the exocyst and stimulated exocytosis. Collectively, these results demonstrate that ligation of E-cadherin induces exocytosis that promotes Listeria entry, and they raise the possibility that the exocyst might also control the normal function of E-cadherin in cell-cell adhesion.

In through the Out Exit: the Role of the Exocyst in Listeria monocytogenes Cell Entry

The intracellular pathogen Listeria monocytogenes is one of the leading causes of death from foodborne illness in the United States. Internalin A is the key surface protein that drives Listeria uptake by epithelial cells expressing E-cadherin. G. C. Gyanwali, T. U. B. Herath, A. Gianfelice, and K. Ireton (Infect Immun 90:e00326-22, 2022, https://doi.org/10.1128/iai.00326-22) unravel the close relationship between internalin A and the exocyst, adding another layer of complexity to the bacterial internalization process.

A genetic screen identifies a protective type III interferon response to Cryptosporidium that requires TLR3 dependent recognition

Cryptosporidium is a leading cause of severe diarrhea and diarrheal-related death in children worldwide. As an obligate intracellular parasite, Cryptosporidium relies on intestinal epithelial cells to provide a niche for its growth and survival, but little is known about the contributions that the infected cell makes to this relationship. Here we conducted a genome wide CRISPR/Cas9 knockout screen to discover host genes that influence Cryptosporidium parvum infection and/or host cell survival. Gene enrichment analysis indicated that the host interferon response, glycosaminoglycan (GAG) and glycosylphosphatidylinositol (GPI) anchor biosynthesis are important determinants of susceptibility to C. parvum infection and impact on the viability of host cells in the context of parasite infection. Several of these pathways are linked to parasite attachment and invasion and C-type lectins on the surface of the parasite. Evaluation of transcript and protein induction of innate interferons revealed a pronounced type III interferon response to Cryptosporidium in human cells as well as in mice. Treatment of mice with IFNλ reduced infection burden and protected immunocompromised mice from severe outcomes including death, with effects that required STAT1 signaling in the enterocyte. Initiation of this type III interferon response was dependent on sustained intracellular growth and mediated by the pattern recognition receptor TLR3. We conclude that host cell intrinsic recognition of Cryptosporidium results in IFNλ production critical to early protection against this infection.

Gb3/cd77 Is a Predictive Marker and Promising Therapeutic Target for Head and Neck Cancer

Head and neck squamous cell carcinoma is the sixth leading cancer in the world. This cancer is difficult to treat and is characterized by recurrences that are often fatal. This cancer is generally removed surgically, but it often regrows from the edges of the lesion from where most recurrences reappear. In this study, we have investigated if the expression of GB3 in human cell lines, tissues from patient biopsies, and a murine animal model could be used as an early and determinant marker of HNC. We found that in all the investigated systems, this marker appears in neoplastic cells from the very early stages of their malignant transformation. Our conclusions support the hypothesis that GB3 is a reliable and independent target for HNC identification and selective delivery of treatments. Furthermore, we show that the level of expression of this marker correlates with the degree of malignancy of the tumor. These studies suggest that GB3 may provide the basis for the early identification and new targeted therapies for head and neck cancer.

Mitochondrial respiration restricts Listeria monocytogenes infection by slowing down host cell receptor recycling

Mutations in mitochondrial genes impairing energy production cause mitochondrial diseases (MDs), and clinical studies have shown that MD patients are prone to bacterial infections. However, the relationship between mitochondrial (dys)function and infection remains largely unexplored, especially in epithelial cells, the first barrier to many pathogens. Here, we generate an epithelial cell model for one of the most common mitochondrial diseases, Leigh syndrome, by deleting surfeit locus protein 1 (SURF1), an assembly factor for respiratory chain complex IV. We use this genetic model and a complementary, nutrient-based approach to modulate mitochondrial respiration rates and show that impaired mitochondrial respiration favors entry of the human pathogen Listeria monocytogenes, a well-established bacterial infection model. Reversely, enhanced mitochondrial energy metabolism decreases infection efficiency. We further demonstrate that endocytic recycling is reduced in mitochondrial respiration-dependent cells, dampening L. monocytogenes infection by slowing the recycling of its host cell receptor c-Met, highlighting a previously undescribed role of mitochondrial respiration during infection.

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Enhanced mitochondrial respiration decreases L. monocytogenes infection

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Bacterial entry is affected by the host cell metabolism

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Mitochondrial respiration restricts host cell receptor recycling and thus infection

Shigella flexneri subverts host polarized exocytosis to enhance cell-to-cell spread

Shigella flexneri is a gram-negative bacterial pathogen that causes dysentery. Critical for disease is the ability of Shigella to use an actin-based motility (ABM) process to spread between cells of the colonic epithelium. ABM transports bacteria to the periphery of host cells, allowing the formation of plasma membrane protrusions that mediate spread to adjacent cells. Here we demonstrate that efficient protrusion formation and cell-to-cell spread of Shigella involves bacterial stimulation of host polarized exocytosis. Using an exocytic probe, we found that exocytosis is locally upregulated in bacterial protrusions in a manner that depends on the Shigella type III secretion system. Experiments involving RNA interference (RNAi) indicate that efficient bacterial protrusion formation and spread require the exocyst, a mammalian multi-protein complex known to mediate polarized exocytosis. In addition, the exocyst component Exo70 and the exocyst regulator RalA were recruited to Shigella protrusions, suggesting that bacteria manipulate exocyst function. Importantly, RNAi-mediated depletion of exocyst proteins or RalA reduced the frequency of protrusion formation and also the lengths of protrusions, demonstrating that the exocyst controls both the initiation and elongation of protrusions. Collectively, our results reveal that Shigella co-opts the exocyst complex to disseminate efficiently in host cell monolayers.

Targeting Nanomaterials to Head and Neck Cancer Cells Using a Fragment of the Shiga Toxin as a Potent Natural Ligand

Head and Neck Cancer (HNC) is the seventh most common cancer worldwide with a 5-year survival from diagnosis of 50%. Currently, HNC is diagnosed by a physical examination followed by an histological biopsy, with surgery being the primary treatment. Here, we propose the use of targeted nanotechnology in support of existing diagnostic and therapeutic tools to prevent recurrences of tumors with poorly defined or surgically inaccessible margins. We have designed an innocuous ligand-protein, based on the receptor-binding domain of the Shiga toxin (ShTxB), that specifically drives nanoparticles to HNC cells bearing the globotriaosylceramide receptor on their surfaces. Microscopy images show how, upon binding to the receptor, the ShTxB-coated nanoparticles cause the clustering of the globotriaosylceramide receptors, the protrusion of filopodia, and rippling of the membrane, ultimately allowing the penetration of the ShTxB nanoparticles directly into the cell cytoplasm, thus triggering a biomimetic cellular response indistinguishable from that triggered by the full-length Shiga toxin. This functionalization strategy is a clear example of how some toxin fragments can be used as natural biosensors for the detection of some localized cancers and to target nanomedicines to HNC lesions.

Type 3 secretion system 1 of Salmonella typhimurium and its inhibitors: a novel strategy to combat salmonellosis

Unsuccessful vaccination against Salmonella due to a large number of serovars, and antibiotic resistance, necessitates the development of novel therapeutics to treat salmonellosis. The development of anti-virulence agents against multi-drug-resistant bacteria is a novel strategy because of its non-bacterial feature. Hence, a thorough study of the type three secretion system (T3SS) of Salmonella would help us better understand its role in bacterial pathogenesis and development of anti-virulence agents. However, T3SS can be inhibited by different chemicals at different stages of infection and sequenced delivery of effectors can be blocked to restrict the progression of disease. This review highlights the role of T3SS-1 in the internalization, survival, and replication of Salmonella within the intestinal epithelium and T3SS inhibitors. We concluded that the better we understand the structures and functions of T3SS, the more we have chances to develop anti-virulence agents. Furthermore, greater insights into the T3SS inhibitors of Salmonella would help in the mitigation of the antibiotic resistance problem and would lead us to the era of new therapeutics against salmonellosis.

New methods to decrypt emerging macropinosome functions during the host-pathogen crosstalk

Large volumes of liquid and other materials from the extracellular environment are internalised by eukaryotic cells via an endocytic process called macropinocytosis. It is now recognised that this fundamental and evolutionarily conserved pathway is hijacked by numerous intracellular pathogens as an entry portal to the host cell interior. Yet, an increasing number of additional cellular functions of macropinosomes in pathologic processes have been reported beyond this role for fluid internalisation. It emerges that the identity of macropinosomes can vary hugely and change rapidly during their lifetime. A deeper understanding of this important multi-faceted compartment is based on novel methods for their investigation. These methods are either imaging-based for the tracking of macropinosome dynamics, or they provide the means to extract macropinosomes at high purity for comprehensive proteomic analyses. Here, we portray these new approaches for the investigation of macropinosomes. We document how these method developments have provided insights for a new understanding of the intracellular lifestyle of the bacterial pathogens Shigella and Salmonella. We suggest that a systematic complete characterisation of macropinosome subversion with these approaches during other infection processes and pathologies will be highly beneficial for our understanding of the underlying cellular and molecular processes.

Myosins, an Underestimated Player in the Infectious Cycle of Pathogenic Bacteria

Myosins play a key role in many cellular processes such as cell migration, adhesion, intracellular trafficking and internalization processes, making them ideal targets for bacteria. Through selected examples, such as enteropathogenic E. coli (EPEC), Neisseria, Salmonella, Shigella, Listeria or Chlamydia, this review aims to illustrate how bacteria target and hijack host cell myosins in order to adhere to the cell, to enter the cell by triggering their internalization, to evade from the cytosolic autonomous cell defense, to promote the biogenesis of intracellular replicative niche, to disseminate in tissues by cell-to-cell spreading, to exit out the host cell, and also to evade from macrophage phagocytosis. It highlights the diversity and sophistication of the strategy evolved by bacteria to manipulate one of their privileged targets, the actin cytoskeleton.

Using proteomics to identify host cell interaction partners for VgrG and IglJ

Francisella tularensis is a highly virulent intracellular bacterium and the causative agent of tularemia. The disease is characterized by the suboptimal innate immune response and consequently by the impaired adaptive immunity. The virulence of this pathogen depends on proteins encoded by a genomic island termed the Francisella Pathogenicity Island (FPI). However, the precise biological roles of most of the FPI-encoded proteins remain to be clarified. In this study, we employed stable isotope labeling by amino acids in cell culture (SILAC) in combination with affinity protein purification coupled with liquid chromatography–mass spectrometry to identify potential protein-effector binding pairs for two FPI virulence effectors IglJ and VgrG. Our results may indicate that while the IglJ protein interactions primarily affect mitochondria, the VgrG interactions affect phagosome and/or autophagosome biogenesis via targeting components of the host’s exocyst complex.

Shigella hijacks the exocyst to cluster macropinosomes for efficient vacuolar escape

Shigella flexneri invades host cells by entering within a bacteria-containing vacuole (BCV). In order to establish its niche in the host cytosol, the bacterium ruptures its BCV. Contacts between S. flexneri BCV and infection-associated macropinosomes (IAMs) formed in situ have been reported to enhance BCV disintegration. The mechanism underlying S. flexneri vacuolar escape remains however obscure. To decipher the molecular mechanism priming the communication between the IAMs and S. flexneri BCV, we performed mass spectrometry-based analysis of the magnetically purified IAMs from S. flexneri-infected cells. While proteins involved in host recycling and exocytic pathways were significantly enriched at the IAMs, we demonstrate more precisely that the S. flexneri type III effector protein IpgD mediates the recruitment of the exocyst to the IAMs through the Rab8/Rab11 pathway. This recruitment results in IAM clustering around S. flexneri BCV. More importantly, we reveal that IAM clustering subsequently facilitates an IAM-mediated unwrapping of the ruptured vacuole membranes from S. flexneri, enabling the naked bacterium to be ready for intercellular spread via actin-based motility. Taken together, our work untangles the molecular cascade of S. flexneri-driven host trafficking subversion at IAMs to develop its cytosolic lifestyle, a crucial step en route for infection progression at cellular and tissue level.

Listeria monocytogenes exploits host exocytosis to promote cell-to-cell spread

The facultative intracellular pathogen Listeria monocytogenes uses an actin-based motility process to spread within human tissues. Filamentous actin from the human cell forms a tail behind bacteria, propelling microbes through the cytoplasm. Motile bacteria remodel the host plasma membrane into protrusions that are internalized by neighboring cells. A critical unresolved question is whether generation of protrusions by Listeria involves stimulation of host processes apart from actin polymerization. Here we demonstrate that efficient protrusion formation in polarized epithelial cells involves bacterial subversion of host exocytosis. Confocal microscopy imaging indicated that exocytosis is up-regulated in protrusions of Listeria in a manner that depends on the host exocyst complex. Depletion of components of the exocyst complex by RNA interference inhibited the formation of Listeria protrusions and subsequent cell-to-cell spread of bacteria. Additional genetic studies indicated important roles for the exocyst regulators Rab8 and Rab11 in bacterial protrusion formation and spread. The secreted Listeria virulence factor InlC associated with the exocyst component Exo70 and mediated the recruitment of Exo70 to bacterial protrusions. Depletion of exocyst proteins reduced the length of Listeria protrusions, suggesting that the exocyst complex promotes protrusion elongation. Collectively, these results demonstrate that Listeria exploits host exocytosis to stimulate intercellular spread of bacteria.

Screening and identification of SipC-interacting proteins in Salmonella enteritidis using Gal4 yeast two-hybrid system in duck

The zoonotic pathogen Salmonella not only reduces the production performance in ducks, but also poses a serious threat to human health through eggs and pollutes water bodies through feces. SipC, an effector protein of type III secretion systems (T3SS) in Salmonella, mediates translocation of effectors into the eukaryotic host. However, the precise role of SipC effectors remains unknown in ducks. In this study, the SipC from duck granulosa cells (dGCs) was selected as bait, and the SipC-interacting proteins in Salmonella enteritidis (SE) were screened using Gal4 yeast two-hybrid system in duck. Twelve SipC-interacting proteins were identified. Among those, the p53-effector related to PMP-22 (PERP) and TGF-β activated kinase 1-binding protein 2 (TAB2) were selected to further confirm the function by GST pull-down in vitro. Over-expression of PERP resulted in not only increasing SE adhesion and invasion but also triggering the production of IL-1β and IFN-α in SE infected dGCs, while knock-down PERP showed the opposite tendency (P < 0.01). In addition, TAB2 significantly induced the production of IL-6, IL-1β, IFN-α, and INF-γ in SE infected dGCs (P < 0.05), but did not cause obvious changes in SE adhesion and invasion. When the sipC in SE was deleted, the activities of duck PERP and TAB2 were abolished because they could not bind to SipC. Taken together, although the protein of PERP and TAB2 can interact with SipC, their mechanisms were different in duck challenged by SE. Therefore, PERP was involved in SE invasion and inflammatory response of dGC ovaries, and TAB2 only contributed to dGCs inflammatory response, which provided critical insights about the mechanism in host- bacterium protein interactions during Salmonella invasion in duck.

Visualization of translocons in Yersinia type III protein secretion machines during host cell infection

Type III secretion systems (T3SSs) are essential virulence factors of numerous bacterial pathogens. Upon host cell contact the T3SS machinery—also named injectisome—assembles a pore complex/translocon within host cell membranes that serves as an entry gate for the bacterial effectors. Whether and how translocons are physically connected to injectisome needles, whether their phenotype is related to the level of effector translocation and which target cell factors trigger their formation have remained unclear. We employed the superresolution fluorescence microscopy techniques Stimulated Emission Depletion (STED) and Structured Illumination Microscopy (SIM) as well as immunogold electron microscopy to visualize Y. enterocolitica translocons during infection of different target cell types. Thereby we were able to resolve translocon and needle complex proteins within the same injectisomes and demonstrate that these fully assembled injectisomes are generated in a prevacuole, a PI(4,5)P2 enriched host cell compartment inaccessible to large extracellular proteins like antibodies. Furthermore, the operable translocons were produced by the yersiniae to a much larger degree in macrophages (up to 25% of bacteria) than in HeLa cells (2% of bacteria). However, when the Rho GTPase Rac1 was activated in the HeLa cells, uptake of the yersiniae into the prevacuole, translocon formation and effector translocation were strongly enhanced reaching the same levels as in macrophages. Our findings indicate that operable T3SS translocons can be visualized as part of fully assembled injectisomes with superresolution fluorescence microscopy techniques. By using this technology, we provide novel information about the spatiotemporal organization of T3SS translocons and their regulation by host cell factors.

Many human, animal and plant pathogenic bacteria employ a molecular machine termed injectisome to inject their toxins into host cells. Because injectisomes are crucial for these bacteria’s infectious potential they have been considered as targets for antiinfective drugs. Injectisomes are highly similar between the different bacterial pathogens and most of their overall structure is well established at the molecular level. However, only little information is available for a central part of the injectisome named the translocon. This pore-like assembly integrates into host cell membranes and thereby serves as an entry gate for the bacterial toxins. We used state of the art fluorescence microscopy to watch translocons of the diarrheagenic pathogen Yersinia enterocolitica during infection of human host cells. Thereby we could for the first time—with fluorescence microscopy—visualize translocons connected to other parts of the injectisome. Furthermore, because translocons mark functional injectisomes we could obtain evidence that injectisomes only become active for secretion of translocators when the bacteria are almost completely enclosed by host cells. These findings provide a novel view on the organization and regulation of bacterial translocons and may thus open up new strategies to block the function of infectious bacteria.

The Host Scaffolding Protein Filamin A and the Exocyst Complex Control Exocytosis during InlB-Mediated Entry of Listeria monocytogenes

Listeria monocytogenes is a foodborne bacterium that causes gastroenteritis, meningitis, or abortion. Listeria induces its internalization (entry) into some human cells through interaction of the bacterial surface protein InlB with its host receptor, the Met tyrosine kinase. InlB and Met promote entry, in part, through stimulation of localized exocytosis. How exocytosis is upregulated during entry is not understood. Here, we show that the human signaling proteins mTOR, protein kinase C-α (PKC-α), and RalA promote exocytosis during entry by controlling the scaffolding protein Filamin A (FlnA). InlB-mediated uptake was accompanied by PKC-α-dependent phosphorylation of serine 2152 in FlnA. Depletion of FlnA by RNA interference (RNAi) or expression of a mutated FlnA protein defective in phosphorylation impaired InlB-dependent internalization. These findings indicate that phosphorylation of FlnA by PKC-α contributes to entry. mTOR and RalA were found to mediate the recruitment of FlnA to sites of InlB-mediated entry. Depletion of PKC-α, mTOR, or FlnA each reduced exocytosis during InlB-mediated uptake. Because the exocyst complex is known to mediate polarized exocytosis, we examined if PKC-α, mTOR, RalA, or FlnA affects this complex. Depletion of PKC-α, mTOR, RalA, or FlnA impaired recruitment of the exocyst component Exo70 to sites of InlB-mediated entry. Experiments involving knockdown of Exo70 or other exocyst proteins demonstrated an important role for the exocyst complex in uptake of Listeria Collectively, our results indicate that PKC-α, mTOR, RalA, and FlnA comprise a signaling pathway that mobilizes the exocyst complex to promote infection by Listeria.

Identification of Novel Serodiagnostic Signatures of Typhoid Fever Using a Salmonella Proteome Array

Current diagnostic tests for typhoid fever, the disease caused by Salmonella Typhi, are poor. We aimed to identify serodiagnostic signatures of typhoid fever by assessing microarray signals to 4,445 S. Typhi antigens in sera from 41 participants challenged with oral S. Typhi. We found broad, heterogeneous antibody responses with increasing IgM/IgA signals at diagnosis. In down-selected 250-antigen arrays we validated responses in a second challenge cohort (n = 30), and selected diagnostic signatures using machine learning and multivariable modeling. In four models containing responses to antigens including flagellin, OmpA, HlyE, sipC, and LPS, multi-antigen signatures discriminated typhoid (n = 100) from other febrile bacteremia (n = 52) in Nepal. These models contained combinatorial IgM, IgA, and IgG responses to 5 antigens (ROC AUC, 0.67 and 0.71) or 3 antigens (0.87), although IgA responses to LPS also performed well (0.88). Using a novel systematic approach we have identified and validated optimal serological diagnostic signatures of typhoid fever.

Swiss Army Pathogen: The Salmonella Entry Toolkit

Salmonella causes disease in humans and animals ranging from mild self-limiting gastroenteritis to potentially life-threatening typhoid fever. Salmonellosis remains a considerable cause of morbidity and mortality globally, and hence imposes a huge socio-economic burden worldwide. A key property of all pathogenic Salmonella strains is the ability to invade non-phagocytic host cells. The major determinant of this invasiveness is a Type 3 Secretion System (T3SS), a molecular syringe that injects virulence effector proteins directly into target host cells. These effectors cooperatively manipulate multiple host cell signaling pathways to drive pathogen internalization. Salmonella does not only rely on these injected effectors, but also uses several other T3SS-independent mechanisms to gain entry into host cells. This review summarizes our current understanding of the methods used by Salmonella for cell invasion, with a focus on the host signaling networks that must be coordinately exploited for the pathogen to achieve its goal.

Functional relatedness in the Inv/Mxi-Spa type III secretion system family

Type III Secretion Systems (T3SSs) are structurally conserved nanomachines that span the inner and outer bacterial membranes, and via a protruding needle complex contact host cell membranes and deliver type III effector proteins. T3SS are phylogenetically divided into several families based on structural basal body components. Here we have studied the evolutionary and functional conservation of four T3SS proteins from the Inv/Mxi-Spa family: a cytosolic chaperone, two hydrophobic translocators that form a plasma membrane-integral pore, and the hydrophilic 'tip complex' translocator that connects the T3SS needle to the translocon pore. Salmonella enterica serovar Typhimurium (S. Typhimurium), a common cause of food-borne gastroenteritis, possesses two T3SSs, one belonging to the Inv/Mxi-Spa family. We used invasion-deficient S. Typhimurium mutants as surrogates for expression of translocator orthologs identified from an extensive phylogenetic analysis, and type III effector translocation and host cell invasion as a readout for complementation efficiency, and identified several Inv/Mxi-Spa orthologs that can functionally substitute for the S. Typhimurium chaperone and translocator proteins. Functional complementation correlates with amino acid sequence identity between orthologs, but varies considerably between the four proteins. This is the first in-depth survey of the functional interchangeability of Inv/Mxi-Spa T3SS proteins acting directly at the host-pathogen interface.

Probing Functional Changes in Exocyst Configuration with Monoclonal Antibodies

Spatial regulation of exocytosis relies on the exocyst, a hetero-octameric protein complex that tethers vesicles to fusion sites at the plasma membrane. Nevertheless, our understanding of mechanisms regulating exocyst assembly/disassembly, localization, and function are incomplete. Here, we have exploited a panel of anti-Sec6 monoclonal antibodies (mAbs) to probe possible configurational changes accompanying transitions in exocyst function in epithelial MDCK cells. Sec6 is quantitatively associated with Sec8 in high molecular weight complexes, as shown by gel filtration and co-immunoprecipitation studies. We mapped epitopes recognized by more than 20 distinct mAbs to one of six Sec6 segments. Surprisingly, mAbs that bound epitopes in each segment labeled distinct subcellular structures. In general, antibodies to epitopes in N-terminal domains labeled Sec6 in either cytosolic or nuclear pools, whereas those that bound epitopes in C-terminal domains labeled membrane-associated Sec6. In this latter group, we identified antibodies that labeled distinct Sec6 populations at the apical junctional complex, desmosomes, endoplasmic reticulum and vimentin-type intermediate filaments. That each antibody was specific was verified by both Sec6 RNAi and competition with fusion proteins containing each domain. Comparison of non-polarized and polarized cells revealed that many Sec6 epitopes either redistribute or become concealed during epithelial polarization. Transitions in exocyst configurations may be regulated in part by the actions of Ral GTPases, because the exposure of Sec6 C-terminal domain epitopes at the plasma membrane is significantly reduced upon RalA RNAi. To determine whether spatio-temporal changes in epitope accessibility was correlated with differential stability of interactions between Sec6 and other exocyst subunits, we quantified relative amounts of each subunit that co-immunoprecipitated with Sec6 when antibodies to N-terminal or C-terminal epitopes were used. Antibodies to Sec6NT co-precipitated substantially more Sec5, -10, -15, Exo70 and -84 than did those to Sec6CT. In contrast, antibodies to Sec6CT co-precipitated more Sec3 and Sec8 than did those to Sec6NT. These results are consistent with a model in which exocyst activation during periods of rapid membrane expansion is accompanied by molecular rearrangements within the holocomplex or association with accessory proteins, which expose the Sec6 C-terminal domain when the complex is membrane-bound and conceal it when the complex is cytoplasmic.

The type three secreted effector SipC regulates the trafficking of PERP during Salmonella infection

Salmonella enterica Typhimurium employs type III secreted effectors to induce cellular invasion and pathogenesis. We previously reported the secreted effector SipA is in part responsible for inducing the apical accumulation of the host membrane protein PERP, a host factor we have shown is key to the inflammatory response induced by Salmonella. We now report that the S. Typhimurium type III secreted effector SipC significantly contributes to PERP redistribution to the apical membrane surface. To our knowledge, this is the first report demonstrating a role for SipC in directing the trafficking of a host membrane protein to the cell surface. In sum, facilitation of PERP trafficking appears to be a result of type III secreted effector-mediated recruitment of vesicles to the apical surface. Our study therefore reveals a new role for SipC, and builds upon previous reports suggesting recruitment of vesicles to the cell surface is important for Salmonella invasion.

The type III secretion system apparatus determines the intracellular niche of bacterial pathogens

Upon entry into host cells, intracellular bacterial pathogens establish a variety of replicative niches. Although some remodel phagosomes, others rapidly escape into the cytosol of infected cells. Little is currently known regarding how professional intracytoplasmic pathogens, including Shigella, mediate phagosomal escape. Shigella, like many other Gram-negative bacterial pathogens, uses a type III secretion system to deliver multiple proteins, referred to as effectors, into host cells. Here, using an innovative reductionist-based approach, we demonstrate that the introduction of a functional Shigella type III secretion system, but none of its effectors, into a laboratory strain of Escherichia coli is sufficient to promote the efficient vacuole lysis and escape of the modified bacteria into the cytosol of epithelial cells. This establishes for the first time, to our knowledge, a direct physiologic role for the Shigella type III secretion apparatus (T3SA) in mediating phagosomal escape. Furthermore, although protein components of the T3SA share a moderate degree of structural and functional conservation across bacterial species, we show that vacuole lysis is not a common feature of T3SA, as an effectorless strain of Yersinia remains confined to phagosomes. Additionally, by exploiting the functional interchangeability of the translocator components of the T3SA of Shigella, Salmonella, and Chromobacterium, we demonstrate that a single protein component of the T3SA translocon-Shigella IpaC, Salmonella SipC, or Chromobacterium CipC-determines the fate of intracellular pathogens within both epithelial cells and macrophages. Thus, these findings have identified a likely paradigm by which the replicative niche of many intracellular bacterial pathogens is established.

The Exocyst Complex in Health and Disease

Exocytosis involves the fusion of intracellular secretory vesicles with the plasma membrane, thereby delivering integral membrane proteins to the cell surface and releasing material into the extracellular space. Importantly, exocytosis also provides a source of lipid moieties for membrane extension. The tethering of the secretory vesicle before docking and fusion with the plasma membrane is mediated by the exocyst complex, an evolutionary conserved octameric complex of proteins. Recent findings indicate that the exocyst complex also takes part in other intra-cellular processes besides secretion. These various functions seem to converge toward defining a direction of membrane growth in a range of systems from fungi to plants and from neurons to cilia. In this review we summarize the current knowledge of exocyst function in cell polarity, signaling and cell-cell communication and discuss implications for plant and animal health and disease.

The Exocyst Subunit Sec6 Interacts with Assembled Exocytic SNARE Complexes

In eukaryotic cells, membrane-bound vesicles carry cargo between intracellular compartments, to and from the cell surface, and into the extracellular environment. Many conserved families of proteins are required for properly localized vesicle fusion, including the multisubunit tethering complexes and the SNARE complexes. These protein complexes work together to promote proper vesicle fusion in intracellular trafficking pathways. However, the mechanism by which the exocyst, the exocytosis-specific multisubunit tethering complex, interacts with the exocytic SNAREs to mediate vesicle targeting and fusion is currently unknown. We have demonstrated previously that the Saccharomyces cerevisiae exocyst subunit Sec6 directly bound the plasma membrane SNARE protein Sec9 in vitro and that Sec6 inhibited the assembly of the binary Sso1-Sec9 SNARE complex. Therefore, we hypothesized that the interaction between Sec6 and Sec9 prevented the assembly of premature SNARE complexes at sites of exocytosis. To map the determinants of this interaction, we used cross-linking and mass spectrometry analyses to identify residues required for binding. Mutation of residues identified by this approach resulted in a growth defect when introduced into yeast. Contrary to our previous hypothesis, we discovered that Sec6 does not change the rate of SNARE assembly but, rather, binds both the binary Sec9-Sso1 and ternary Sec9-Sso1-Snc2 SNARE complexes. Together, these results suggest a new model in which Sec6 promotes SNARE complex assembly, similar to the role proposed for other tether subunit-SNARE interactions.

Bacterial nucleators: actin' on actin

The actin cytoskeleton is a key target of numerous microbial pathogens, including protozoa, fungi, bacteria and viruses. In particular, bacterial pathogens produce and deliver virulence effector proteins that hijack actin dynamics to enable bacterial invasion of host cells, allow movement within the host cytosol, facilitate intercellular spread or block phagocytosis. Many of these effector proteins directly or indirectly target the major eukaryotic actin nucleator, the Arp2/3 complex, by either mimicking nucleation promoting factors or activating upstream small GTPases. In contrast, this review is focused on a recently identified class of effector proteins from Gram-negative bacteria that function as direct actin nucleators. These effector proteins mimic functional activities of formins, WH2-nucleators and Ena/VASP assembly promoting factors demonstrating that bacteria have coopted the complete set of eukaryotic actin assembly pathways. Structural and functional analyses of these nucleators have revealed several motifs and/or mechanistic activities that are shared with eukaryotic actin nucleators. However, functional effects of these proteins during infection extend beyond plain actin polymerization leading to interference with other host cell functions such as vesicle trafficking, cell cycle progression and cell death. Therefore, their use as model systems could not only help in the understanding of the mechanistic details of actin polymerization but also provide novel insights into the connection between actin dynamics and other cellular pathways.

Urothelial Defects from Targeted Inactivation of Exocyst Sec10 in Mice Cause Ureteropelvic Junction Obstructions

Most cases of congenital obstructive nephropathy are the result of ureteropelvic junction obstructions, and despite their high prevalence, we have a poor understanding of their etiology and scarcity of genetic models. The eight-protein exocyst complex regulates polarized exocytosis of intracellular vesicles in a large variety of cell types. Here we report generation of a conditional knockout mouse for Sec10, a central component of the exocyst, which is the first conditional allele for any exocyst gene. Inactivation of Sec10 in ureteric bud-derived cells using Ksp1.3-Cre mice resulted in severe bilateral hydronephrosis and complete anuria in newborns, with death occurring 6-14 hours after birth. Sec10 FL/FL;Ksp-Cre embryos developed ureteropelvic junction obstructions between E17.5 and E18.5 as a result of degeneration of the urothelium and subsequent overgrowth by surrounding mesenchymal cells. The urothelial cell layer that lines the urinary tract must maintain a hydrophobic luminal barrier again urine while remaining highly stretchable. This barrier is largely established by production of uroplakin proteins that are transported to the apical surface to establish large plaques. By E16.5, Sec10 FL/FL;Ksp-Cre ureter and pelvic urothelium showed decreased uroplakin-3 protein at the luminal surface, and complete absence of uroplakin-3 by E17.5. Affected urothelium at the UPJ showed irregular barriers that exposed the smooth muscle layer to urine, suggesting this may trigger the surrounding mesenchymal cells to overgrow the lumen. Findings from this novel mouse model show Sec10 is critical for the development of the urothelium in ureters, and provides experimental evidence that failure of this urothelial barrier may contribute to human congenital urinary tract obstructions.

PERP, a host tetraspanning membrane protein, is required for Salmonella-induced inflammation

Salmonella enterica  Typhimurium induces intestinal inflammation through the activity of type III secreted effector (T3SE) proteins. Our prior results indicate that the secretion of the T3SE SipA and the ability of SipA to induce epithelial cell responses that lead to induction of polymorphonuclear transepithelial migration are not coupled to its direct delivery into epithelial cells from Salmonella. We therefore tested the hypothesis that SipA interacts with a membrane protein located at the apical surface of intestinal epithelial cells. Employing a split ubiquitin yeast‐two‐hybrid screen, we identified the tetraspanning membrane protein, p53 effector related to PMP‐22 (PERP), as a SipA binding partner. SipA and PERP appear to have intersecting activities as we found PERP to be involved in proinflammatory pathways shown to be regulated by SipA. In sum, our studies reveal a critical role for PERP in the pathogenesis of S. Typhimurium, and for the first time demonstrate that SipA, a T3SE protein, can engage a host protein at the epithelial surface.

Proteomes of host cell membranes modified by intracellular activities of Salmonella enterica

Intracellular pathogens need to establish a growth-stimulating host niche for survival and replication. A unique feature of the gastrointestinal pathogen Salmonella enterica serovar Typhimurium is the creation of extensive membrane networks within its host. An understanding of the origin and function of these membranes is crucial for the development of new treatment strategies. However, the characterization of this compartment is very challenging, and only fragmentary knowledge of its composition and biogenesis exists. Here, we describe a new proteome-based approach to enrich and characterize Salmonella-modified membranes. Using a Salmonella mutant strain that does not form this unique membrane network as a reference, we identified a high-confidence set of host proteins associated with Salmonella-modified membranes. This comprehensive analysis allowed us to reconstruct the interactions of Salmonella with host membranes. For example, we noted that Salmonella redirects endoplasmic reticulum (ER) membrane trafficking to its intracellular niche, a finding that has not been described for Salmonella previously. Our system-wide approach therefore has the potential to rapidly close gaps in our knowledge of the infection process of intracellular pathogens and demonstrates a hitherto unrecognized complexity in the formation of Salmonella host niches.

RAB11-mediated trafficking in host-pathogen interactions

Many bacterial and viral pathogens block or subvert host cellular processes to promote successful infection. One host protein that is targeted by invading pathogens is the small GTPase RAB11, which functions in vesicular trafficking. RAB11 functions in conjunction with a protein complex known as the exocyst to mediate terminal steps in cargo transport via the recycling endosome to cell-cell junctions, phagosomes and cellular protrusions. These processes contribute to host innate immunity by promoting epithelial and endothelial barrier integrity, sensing and immobilizing pathogens and repairing pathogen-induced cellular damage. In this Review, we discuss the various mechanisms that pathogens have evolved to disrupt or subvert RAB11-dependent pathways as part of their infection strategy.

A role for exocyst in maturation and bactericidal function of staphylococci-containing endothelial cell phagosomes

Bacteria that invade human endothelial cells can be efficiently eliminated in phagolysosomes. We investigated the role of vesicle tethering exocyst complex in maturation and function of endothelial cell phagosomes harbouring staphylococci or latex beads. Exocyst complex proteins (Sec5, -8, -10, Exo70) together with recycling endosome marker Rab11 were detected in vesicles that dynamically interacted and seemingly fused with endothelial cell phagosomes. Knockdown of exocyst proteins Sec8 and Exo70 inhibited the accumulation of Rab11-positive vesicles at the phagosomes. Furthermore, knockdown of exocyst proteins and Rab11 greatly reduced acidification of phagosomes and significantly diminished the elimination of invaded staphylococci in endothelial cells. The inhibitory effect of Exo70 knockdown on bacterial elimination could be rescued by constitutively active Rab11-Q70L. Our data suggest that exocyst complex controls the interaction of recycling endocytic vesicles with phagosomes and this process is involved in maturation and functioning of the phagosomes in endothelial cells.

The exocyst at the interface between cytoskeleton and membranes in eukaryotic cells

Delivery and final fusion of the secretory vesicles with the relevant target membrane are hierarchically organized and reciprocally interconnected multi-step processes involving not only specific protein-protein interactions, but also specific protein-phospholipid interactions. The exocyst was discovered as a tethering complex mediating initial encounter of arriving exocytic vesicles with the plasma membrane. The exocyst complex is regulated by Rab and Rho small GTPases, resulting in docking of exocytic vesicles to the plasma membrane (PM) and finally their fusion mediated by specific SNARE complexes. In model Opisthokont cells, the exocyst was shown to directly interact with both microtubule and microfilament cytoskeleton and related motor proteins as well as with the PM via phosphatidylinositol 4, 5-bisphosphate specific binding, which directly affects cortical cytoskeleton and PM dynamics. Here we summarize the current knowledge on exocyst-cytoskeleton-PM interactions in order to open a perspective for future research in this area in plant cells.