Septin 11 restricts InlB-mediated invasion by Listeria

Blebs promote cell survival by assembling oncogenic signalling hubs

Most human cells require anchorage for survival. Cell-substrate adhesion activates diverse signalling pathways, without which cells undergo anoikis-a form of programmed cell death1. Acquisition of anoikis resistance is a pivotal step in cancer disease progression, as metastasizing cells often lose firm attachment to surrounding tissue2,3. In these poorly attached states, cells adopt rounded morphologies and form small hemispherical plasma membrane protrusions called blebs4-11. Bleb function has been thoroughly investigated in the context of amoeboid migration, but it has been examined far less in other scenarios12. Here we show by three-dimensional imaging and manipulation of cell morphological states that blebbing triggers the formation of plasma membrane-proximal signalling hubs that confer anoikis resistance. Specifically, in melanoma cells, blebbing generates plasma membrane contours that recruit curvature-sensing septin proteins as scaffolds for constitutively active mutant NRAS and effectors. These signalling hubs activate ERK and PI3K-well-established promoters of pro-survival pathways. Inhibition of blebs or septins has little effect on the survival of well-adhered cells, but in detached cells it causes NRAS mislocalization, reduced MAPK and PI3K activity, and ultimately, death. This unveils a morphological requirement for mutant NRAS to operate as an effective oncoprotein. Furthermore, whereas some BRAF-mutated melanoma cells do not rely on this survival pathway in a basal state, inhibition of BRAF and MEK strongly sensitizes them to both bleb and septin inhibition. Moreover, fibroblasts engineered to sustain blebbing acquire the same anoikis resistance as cancer cells even without harbouring oncogenic mutations. Thus, blebs are potent signalling organelles capable of integrating myriad cellular information flows into concerted cellular responses, in this case granting robust anoikis resistance.

Cellular functions of actin- and microtubule-associated septins

Septins are an integral component of the cytoskeleton, assembling into higher-order oligomers and filamentous polymers that associate with actin filaments, microtubules and membranes. Here, we review septin interactions with actin and microtubules, and septin-mediated regulation of the organization and dynamics of these cytoskeletal networks, which is critical for cellular morphogenesis. We discuss how actomyosin-associated septins function in cytokinesis, cell migration and host defense against pathogens. We highlight newly emerged roles of septins at the interface of microtubules and membranes with molecular motors, which point to a 'septin code' for the regulation of membrane traffic. Additionally, we revisit the functions of microtubule-associated septins in mitosis and meiosis. In sum, septins comprise a unique module of cytoskeletal regulators that are spatially and functionally specialized and have properties of bona fide actin-binding and microtubule-associated proteins. With many questions still outstanding, the study of septins will continue to provide new insights into fundamental problems of cytoskeletal organization and function.

The history of septin biology and bacterial infection

Investigation of cytoskeleton during bacterial infection has significantly contributed to both cell and infection biology. Bacterial pathogens Listeria monocytogenes and Shigella flexneri are widely recognised as paradigms for investigation of the cytoskeleton during bacterial entry, actin-based motility, and cell-autonomous immunity. At the turn of the century, septins were a poorly understood component of the cytoskeleton mostly studied in the context of yeast cell division and human cancer. In 2002, a screen performed in the laboratory of Pascale Cossart identified septin family member MSF (MLL septin-like fusion, now called SEPT9) associated with L. monocytogenes entry into human epithelial cells. These findings inspired the investigation of septins during L. monocytogenes and S. flexneri infection at the Institut Pasteur, illuminating important roles for septins in host-microbe interactions. In this review, we revisit the history of septin biology and bacterial infection, and discuss how the comparative study of L. monocytogenes and S. flexneri has been instrumental to understand septin roles in cellular homeostasis and host defence.

Host Lipid Rafts as the Gates for Listeria monocytogenes Infection: A Mini-Review

Listeria monocytogenes is a Gram-positive foodborne bacterial pathogen capable of interacting and crossing the intestinal barrier, blood–brain barrier, and placental barrier to cause deadly infection with high mortality. L. monocytogenes is an intracellular pathogen characterized by its ability to enter non-phagocytic cells. Expression of the cytolysin listeriolysin O has been shown to be the main virulence determinant in vitro and in vivo in mouse models. L. monocytogenes can also perform cell-to-cell spreading using actin-rich membrane protrusions to infect neighboring cells, which also constitutes an important strategy for infection. These events including entry into host cells, interaction between listeriolysin O and host plasma membrane, and bacterial cell-to-cell spreading have been demonstrated to implicate the cholesterol-rich lipid rafts or molecules in these microdomains in the host plasma membrane in vitro with tissue culture models. Here we review the contribution of lipid rafts on plasma membrane to L. monocytogenes infection.

A role for Taok2 in Listeria monocytogenes vacuolar escape

The bacterial pathogen Listeria monocytogenes invades host cells, ruptures the internalization vacuole, and reaches the cytosol for replication. A high-content small interfering RNA (siRNA) microscopy screen allowed us to identify epithelial cell factors involved in L. monocytogenes vacuolar rupture, including the serine/threonine kinase Taok2. Kinase activity inhibition using a specific drug validated a role for Taok2 in favoring L. monocytogenes cytoplasmic access. Furthermore, we showed that Taok2 recruitment to L. monocytogenes vacuoles requires the presence of pore-forming toxin listeriolysin O. Overall, our study identified the first set of host factors modulating L. monocytogenes vacuolar rupture and cytoplasmic access in epithelial cells.

Role of septins in microbial infection

Septins are widely recognized as a component of the cytoskeleton that is essential for cell division, and new work has shown that septins can recognise cell shape by assembling into filaments on membrane regions that display micrometer-scale curvature (e.g. at the cytokinetic furrow). Moreover, infection biology studies have illuminated important roles for septins in mediating the outcome of host-microbe interactions. In this Review, we discuss a selection of mechanistic insights recently gained from studying three infection paradigms: the rice blast fungus Magnaporthe oryzae, the poxvirus family member vaccinia virus and the Gram-negative bacterium Shigella flexneri These studies have respectively discovered that higher-order septin assemblies enable fungal invasion into plant cells, entrap viral particles at the plasma membrane and recognize dividing bacterial cells for delivery to lysosomes. Collectively, these insights illustrate how studying septin biology during microbial infection can provide fundamental advances in both cell and infection biology, and suggest new concepts underlying infection control.

Septin Polymerization Slows Synaptic Vesicle Recycling in Motor Nerve Endings

Septins are GTP-binding proteins recognized as a component of the cytoskeleton. Despite the fact that septins are highly expressed by neurons and can interact with the proteins that participate in synaptic vesicle exocytosis and endocytosis, the role of septins in synaptic transmission and the synaptic vesicle recycling mechanisms is poorly understood. In this study, neurotransmitter release and synaptic vesicle exocytosis and endocytosis were investigated by microelectrode intracellular recording of end-plate potentials and fluorescent confocal microscopy in mouse diaphragm motor nerve endings during septin polymerization induced by forchlorfenuron application. It was shown that forchlorfenuron application reduces neurotransmission during prolonged high-frequency (20 and 50 pulses/s) stimulation. Application of pairs of short high-frequency stimulation trains showed that forchlorfenuron slows the replenishment of the readily releasable pool. Forchlorfenuron enhanced FM 1-43 fluorescent dye loading by synaptic vesicle endocytosis but decreased dye unloading from the preliminarily stained nerve endings by synaptic vesicle exocytosis. It was concluded that the septin polymerization induced by forchlorfenuron application slows the rate of synaptic vesicle recycling in motor nerve endings due to the impairment of synaptic vesicle transport.

Listeria monocytogenes: cell biology of invasion and intracellular growth

The Gram-positive pathogen Listeria monocytogenes is able to promote its entry into a diverse range of mammalian host cells by triggering plasma membrane remodeling, leading to bacterial engulfment. Upon cell invasion, L. monocytogenes disrupts its internalization vacuole and translocates to the cytoplasm, where bacterial replication takes place. Subsequently, L. monocytogenes uses an actin-based motility system that allows bacterial cytoplasmic movement and cell-to-cell spread. L. monocytogenes therefore subverts host cell receptors, organelles and the cytoskeleton at different infection steps, manipulating diverse cellular functions that include ion transport, membrane trafficking, post-translational modifications, phosphoinositide production, innate immune responses as well as gene expression and DNA stability.

Reorganization of Septins Modulates Synaptic Transmission at Neuromuscular Junctions

Septins (Sept) are highly conserved Guanosine-5'-triphosphate (GTP)-binding cytoskeletal proteins involved in neuronal signaling in the central nervous system but their involvement in signal transmission in peripheral synapses remains unclear. Sept5 and Sept9 proteins were detected in mouse peripheral neuromuscular junctions by immunofluorescence with a greater degree of co-localization with presynaptic than postsynaptic membranes. Preincubation of neuromuscular junction preparations with the inhibitor of Sept dynamics, forchlorfenuron (FCF), decreased co-localization of Sept with presynaptic membranes. FCF introduced ex vivo or in vivo had no effect on the amplitude of the spontaneous endplate currents (EPCs), indicating the absence of postsynaptic effects of FCF. However, FCF decreased acetylcholine (ACh) quantal release in response to nerve stimulation, reduced the amplitude of evoked quantal currents and decreased the number of quanta with long synaptic delays, demonstrating the presynaptic action of FCF. Nevertheless, FCF had no effect on the amplitude of calcium transient in nerve terminals, as detected by calcium-sensitive dye, and slightly decreased the ratio of the second response amplitude to the first one in paired-pulse experiments. These results suggest that FCF-induced decrease in ACh quantal secretion is not due to a decrease in Ca²⁺ influx but is likely related to the impairment of later stages occurring after Ca²⁺ entry, such as trafficking, docking or membrane fusion of synaptic vesicles. Therefore, Sept9 and Sept5 are abundantly expressed in presynaptic membranes, and disruption of Sept dynamics suppresses the evoked synchronous and delayed asynchronous quantal release of ACh, strongly suggesting an important role of Sept in the regulation of neurotransmission in peripheral synapses.

A requirement for septins and the autophagy receptor p62 in the proliferation of intracellular Shigella

Shigella flexneri, a Gram-negative enteroinvasive pathogen, causes inflammatory destruction of the human intestinal epithelium. During infection of epithelial cells, Shigella escape from the phagosome to the cytosol, where they reroute host cell glycolysis to obtain nutrients for proliferation. Septins, a poorly understood component of the cytoskeleton, can entrap cytosolic Shigella targeted to autophagy in cage-like structures to restrict bacterial proliferation. Although bacterial entrapment by septin caging has been the subject of intense investigation, the role of septins and the autophagy machinery in the proliferation of noncaged Shigella is mostly unknown. Here, we found that intracellular Shigella fail to efficiently proliferate in SEPT2-, SEPT7-, or p62/SQSTM1-depleted cells. Consistent with a failure to proliferate, single cell analysis of bacteria not entrapped in septin cages showed that the number of metabolically active Shigella in septin- or p62-depleted cells is reduced. Targeted metabolomic analysis revealed that host cell glycolysis is dysregulated in septin-depleted cells, suggesting a key role for septins in modulation of glycolysis. Together, these results suggest that septins and the autophagy machinery may regulate metabolic pathways that promote the proliferation of intracellular Shigella not entrapped in septin cages.

Use of zebrafish to study Shigella infection

Shigella is a leading cause of dysentery worldwide, responsible for up to 165 million cases of shigellosis each year. Shigella is also recognised as an exceptional model pathogen to study key issues in cell biology and innate immunity. Several infection models have been useful to explore Shigella biology; however, we still lack information regarding the events taking place during the Shigella infection process in vivo Here, we discuss a selection of mechanistic insights recently gained from studying Shigella infection of zebrafish (Danio rerio), with a focus on cytoskeleton rearrangements and cellular immunity. We also discuss how infection of zebrafish can be used to investigate new concepts underlying infection control, including emergency granulopoiesis and the use of predatory bacteria to combat antimicrobial resistance. Collectively, these insights illustrate how Shigella infection of zebrafish can provide fundamental advances in our understanding of bacterial pathogenesis and vertebrate host defence. This information should also provide vital clues for the discovery of new therapeutic strategies against infectious disease in humans.

CodY-Mediated c-di-GMP-Dependent Inhibition of Mammalian Cell Invasion in Listeria monocytogenes

Elevated levels of the second messenger c-di-GMP suppress virulence in diverse pathogenic bacteria, yet mechanisms are poorly characterized. In the foodborne pathogen Listeria monocytogenes, high c-di-GMP levels inhibit mammalian cell invasion. Here, we show that invasion is impaired because of the decreased expression levels of internalin genes whose products are involved in invasion. We further show that at high c-di-GMP levels, the expression of the entire virulence regulon is suppressed, and so is the expression of the prfA gene encoding the master activator of the virulence regulon. Analysis of mechanisms controlling prfA expression pointed to the transcription factor CodY as a c-di-GMP-sensitive component. In high-c-di-GMP strains, codY gene expression is decreased, apparently due to the lower activity of CodY, which functions as an activator of codY transcription. We found that listerial CodY does not bind c-di-GMP in vitro and therefore investigated whether c-di-GMP levels affect two known cofactors of listerial CodY, branched-chain amino acids and GTP. Our manipulation of branched-chain amino acid levels did not perturb the c-di-GMP effect; however, our replacement of listerial CodY with the streptococcal CodY homolog, whose activity is GTP independent, abolished the c-di-GMP effect. The results of this study suggest that elevated c-di-GMP levels decrease the activity of the coordinator of metabolism and virulence, CodY, possibly via lower GTP levels, and that decreased CodY activity suppresses L. monocytogenes virulence by the decreased expression of the PrfA virulence regulon.IMPORTANCEListeria monocytogenes is a pathogen causing listeriosis, a disease responsible for the highest mortality rate among foodborne diseases. Understanding how the virulence of this pathogen is regulated is important for developing treatments to decrease the frequency of listerial infections in susceptible populations. In this study, we describe the mechanism through which elevated levels of the second messenger c-di-GMP inhibit listerial invasion in mammalian cells. Inhibition is caused by the decreased activity of the transcription factor CodY that coordinates metabolism and virulence.

The role of septins in infections with vacuole-dwelling intracellular bacteria

Septins are a relatively little understood group of GTPases that form large assemblies in cells from all eukaryotes other than plants. Septins were first identified in cell division but have also been implicated in microbial infections. Septins often associate with cytoskeletal proteins - most often described for filamentous (F-) actin - and are considered cytoskeletal components themselves. Septins have increasingly been found to partake in processes that are linked to intracellular membranes, from mitochondria to phagosomes, and evidence is accumulating that septins specifically bind to membranes. Since a number of microorganisms have specialized to live and grow inside membranous vacuoles in the cytosol of mammalian cells, this membrane-association of septins suggests that septins may also be involved in the membranous, vacuolar structures that develop around these microbes. However, data are limited on this issue: septins have been identified by proteome analysis on some microbe-bearing vacuoles, but more extensive experimental data are only available for infections with the obligate intracellular bacterium Chlamydia trachomatis. In this review article I will discuss the available data and speculate about the mechanisms of recruitment and potential functions of septins for vacuole-dwelling microorganisms, which may be peculiar to Chlamydia or may pertain more generally to this class of microbes.

How the study of Listeria monocytogenes has led to new concepts in biology

The opportunistic intracellular bacterial pathogen Listeria monocytogenes has in 30 years emerged as an exceptional bacterial model system in infection biology. Research on this bacterium has provided considerable insight into how pathogenic bacteria adapt to mammalian hosts, invade eukaryotic cells, move intracellularly, interfere with host cell functions and disseminate within tissues. It also contributed to unveil features of normal host cell pathways and unsuspected functions of previously known cellular proteins. This review provides an updated overview of our knowledge on this pathogen. In many examples, findings on L. monocytogenes provided the basis for new concepts in bacterial regulation, cell biology and infection processes.

Septins and Bacterial Infection

Septins, a unique cytoskeletal component associated with cellular membranes, are increasingly recognized as having important roles in host defense against bacterial infection. A role for septins during invasion of Listeria monocytogenes into host cells was first proposed in 2002. Since then, work has shown that septins assemble in response to a wide variety of invasive bacterial pathogens, and septin assemblies can have different roles during the bacterial infection process. Here we review the interplay between septins and bacterial pathogens, highlighting septins as a structural determinant of host defense. We also discuss how investigation of septin assembly in response to bacterial infection can yield insight into basic cellular processes including phagocytosis, autophagy, and mitochondrial dynamics.

Manipulation of host membranes by the bacterial pathogens Listeria, Francisella, Shigella and Yersinia

Bacterial pathogens display an impressive arsenal of molecular mechanisms that allow survival in diverse host niches. Subversion of plasma membrane and cytoskeletal functions are common themes associated to infection by both extracellular and intracellular pathogens. Moreover, intracellular pathogens modify the structure/stability of their membrane-bound compartments and escape degradation from phagocytic or autophagic pathways. Here, we review the manipulation of host membranes by Listeria monocytogenes, Francisella tularensis, Shigella flexneri and Yersinia spp. These four bacterial model pathogens exemplify generalized strategies as well as specific features observed during bacterial infection processes.

Investigation of septins using infection by bacterial pathogens

Investigation of the host cytoskeleton during infection by bacterial pathogens has significantly contributed to our understanding of cell biology and host defense. Work has shown that septins are recruited to the phagocytic cup as collarlike structures and enable bacterial entry into host cells. In the cytosol, septins can entrap actin-polymerizing bacteria in cage-like structures for targeting to autophagy, a highly conserved intracellular degradation process. In this chapter, we describe methods to investigate septin assembly and function during infection by bacterial pathogens. Use of these methods can lead to in-depth understanding of septin biology and suggest therapeutic approaches to combat infectious disease.

Septin oligomerization regulates persistent expression of ErbB2/HER2 in gastric cancer cells

Septins are a family of cytoskeletal GTP-binding proteins that assemble into membrane-associated hetero-oligomers and organize scaffolds for recruitment of cytosolic proteins or stabilization of membrane proteins. Septins have been implicated in a diverse range of cancers, including gastric cancer, but the underlying mechanisms remain unclear. The hypothesis tested here is that septins contribute to cancer by stabilizing the receptor tyrosine kinase ErbB2, an important target for cancer treatment. Septins and ErbB2 were highly over-expressed in gastric cancer cells. Immunoprecipitation followed by MS analysis identified ErbB2 as a septin-interacting protein. Knockdown of septin-2 or cell exposure to forchlorfenuron (FCF), a well-established inhibitor of septin oligomerization, decreased surface and total levels of ErbB2. These treatments had no effect on epidermal growth factor receptor (EGFR), emphasizing the specificity and functionality of the septin-ErbB2 interaction. The level of ubiquitylated ErbB2 at the plasma membrane was elevated in cells treated with FCF, which was accompanied by a decrease in co-localization of ErbB2 with septins at the membrane. Cathepsin B inhibitor, but not bafilomycin or lactacystin, prevented FCF-induced decrease in total ErbB2 by increasing accumulation of ubiquitylated ErbB2 in lysosomes. Therefore, septins protect ErbB2 from ubiquitylation, endocytosis and lysosomal degradation. The FCF-induced degradation pathway is distinct from and additive with the degradation induced by inhibiting ErbB2 chaperone Hsp90. These results identify septins as novel regulators of ErbB2 expression that contribute to the remarkable stabilization of the receptor at the plasma membrane of cancer cells and may provide a basis for the development of new ErbB2-targeting anti-cancer therapies.

A Dual Microscopy-Based Assay To Assess Listeria monocytogenes Cellular Entry and Vacuolar Escape

Listeria monocytogenes is a Gram-positive bacterium and a facultative intracellular pathogen that invades mammalian cells, disrupts its internalization vacuole, and proliferates in the host cell cytoplasm. Here, we describe a novel image-based microscopy assay that allows discrimination between cellular entry and vacuolar escape, enabling high-content screening to identify factors specifically involved in these two steps. We first generated L. monocytogenes and Listeria innocua strains expressing a β-lactamase covalently attached to the bacterial cell wall. These strains were then incubated with HeLa cells containing the Förster resonance energy transfer (FRET) probe CCF4 in their cytoplasm. The CCF4 probe was cleaved by the bacterial surface β-lactamase only in cells inoculated with L. monocytogenes but not those inoculated with L. innocua, thereby demonstrating bacterial access to the host cytoplasm. Subsequently, we performed differential immunofluorescence staining to distinguish extracellular versus total bacterial populations in samples that were also analyzed by the FRET-based assay. With this two-step analysis, bacterial entry can be distinguished from vacuolar rupture in a single experiment. Our novel approach represents a powerful tool for identifying factors that determine the intracellular niche of L. monocytogenes.

The cytoskeleton in cell-autonomous immunity: structural determinants of host defence

Host cells use antimicrobial proteins, pathogen-restrictive compartmentalization and cell death in their defence against intracellular pathogens. Recent work has revealed that four components of the cytoskeleton--actin, microtubules, intermediate filaments and septins, which are well known for their roles in cell division, shape and movement--have important functions in innate immunity and cellular self-defence. Investigations using cellular and animal models have shown that these cytoskeletal proteins are crucial for sensing bacteria and for mobilizing effector mechanisms to eliminate them. In this Review, we highlight the emerging roles of the cytoskeleton as a structural determinant of cell-autonomous host defence.

How bacterial pathogens colonize their hosts and invade deeper tissues

Bacterial pathogens have evolved a wide range of strategies to colonize and invade human organs, despite the presence of multiple host defense mechanisms. In this review, we will describe how pathogenic bacteria can adhere and multiply at the surface of host cells, how some bacteria can enter and proliferate inside these cells, and finally how pathogens may cross epithelial or endothelial host barriers and get access to internal tissues, leading to severe diseases in humans.

Septin dynamics are essential for exocytosis

Septins are a family of 14 cytoskeletal proteins that dynamically form hetero-oligomers and organize membrane microdomains for protein complexes. The previously reported interactions with SNARE proteins suggested the involvement of septins in exocytosis. However, the contradictory results of up- or down-regulation of septin-5 in various cells and mouse models or septin-4 in mice suggested either an inhibitory or a stimulatory role for these septins in exocytosis. The involvement of the ubiquitously expressed septin-2 or general septin polymerization in exocytosis has not been explored to date. Here, by nano-LC with tandem MS and immunoblot analyses of the septin-2 interactome in mouse brain, we identified not only SNARE proteins but also Munc-18-1 (stabilizes assembled SNARE complexes), N-ethylmaleimide-sensitive factor (NSF) (disassembles SNARE complexes after each membrane fusion event), and the chaperones Hsc70 and synucleins (maintain functional conformation of SNARE proteins after complex disassembly). Importantly, α-soluble NSF attachment protein (SNAP), the adaptor protein that mediates NSF binding to the SNARE complex, did not interact with septin-2, indicating that septins undergo reorganization during each exocytosis cycle. Partial depletion of septin-2 by siRNA or impairment of septin dynamics by forchlorfenuron inhibited constitutive and stimulated exocytosis of secreted and transmembrane proteins in various cell types. Forchlorfenuron impaired the interaction between SNAP-25 and its chaperone Hsc70, decreasing SNAP-25 levels in cultured neuroendocrine cells, and inhibited both spontaneous and stimulated acetylcholine secretion in mouse motor neurons. The results demonstrate a stimulatory role of septin-2 and the dynamic reorganization of septin oligomers in exocytosis.

Use of Shigella flexneri to study autophagy-cytoskeleton interactions

Shigella flexneri is an intracellular pathogen that can escape from phagosomes to reach the cytosol, and polymerize the host actin cytoskeleton to promote its motility and dissemination. New work has shown that proteins involved in actin-based motility are also linked to autophagy, an intracellular degradation process crucial for cell autonomous immunity. Strikingly, host cells may prevent actin-based motility of S. flexneri by compartmentalizing bacteria inside ‘septin cages’ and targeting them to autophagy. These observations indicate that a more complete understanding of septins, a family of filamentous GTP-binding proteins, will provide new insights into the process of autophagy. This report describes protocols to monitor autophagy-cytoskeleton interactions caused by S. flexneri in vitro using tissue culture cells and in vivo using zebrafish larvae. These protocols enable investigation of intracellular mechanisms that control bacterial dissemination at the molecular, cellular, and whole organism level.

Septin functions in organ system physiology and pathology

Human septins comprise a family of 13 genes that encode for >30 protein isoforms with ubiquitous and tissue-specific expressions. Septins are GTP-binding proteins that assemble into higher-order oligomers and filamentous polymers, which associate with cell membranes and the cytoskeleton. In the last decade, much progress has been made in understanding the biochemical properties and cell biological functions of septins. In parallel, a growing number of studies show that septins play important roles for the development and physiology of specific tissues and organs. Here, we review the expression and function of septins in the cardiovascular, immune, nervous, urinary, digestive, respiratory, endocrine, reproductive, and integumentary organ systems. Furthermore, we discuss how the tissue-specific functions of septins relate to the pathology of human diseases that arise from aberrations in septin expression.

Cyclic di-GMP-dependent signaling pathways in the pathogenic Firmicute Listeria monocytogenes

We characterized key components and major targets of the c-di-GMP signaling pathways in the foodborne pathogen Listeria monocytogenes, identified a new c-di-GMP-inducible exopolysaccharide responsible for motility inhibition, cell aggregation, and enhanced tolerance to disinfectants and desiccation, and provided first insights into the role of c-di-GMP signaling in listerial virulence. Genome-wide genetic and biochemical analyses of c-di-GMP signaling pathways revealed that L. monocytogenes has three GGDEF domain proteins, DgcA (Lmo1911), DgcB (Lmo1912) and DgcC (Lmo2174), that possess diguanylate cyclase activity, and three EAL domain proteins, PdeB (Lmo0131), PdeC (Lmo1914) and PdeD (Lmo0111), that possess c-di-GMP phosphodiesterase activity. Deletion of all phosphodiesterase genes (ΔpdeB/C/D) or expression of a heterologous diguanylate cyclase stimulated production of a previously unknown exopolysaccharide. The synthesis of this exopolysaccharide was attributed to the pssA-E (lmo0527-0531) gene cluster. The last gene of the cluster encodes the fourth listerial GGDEF domain protein, PssE, that functions as an I-site c-di-GMP receptor essential for exopolysaccharide synthesis. The c-di-GMP-inducible exopolysaccharide causes cell aggregation in minimal medium and impairs bacterial migration in semi-solid agar, however, it does not promote biofilm formation on abiotic surfaces. The exopolysaccharide also greatly enhances bacterial tolerance to commonly used disinfectants as well as desiccation, which may contribute to survival of L. monocytogenes on contaminated food products and in food-processing facilities. The exopolysaccharide and another, as yet unknown c-di-GMP-dependent target, drastically decrease listerial invasiveness in enterocytes in vitro, and lower pathogen load in the liver and gallbladder of mice infected via an oral route, which suggests that elevated c-di-GMP levels play an overall negative role in listerial virulence.

Listeria monocytogenes is ubiquitously present in the environment, highly adaptable and tolerant to various stresses. L. monocytogenes is also a foodborne pathogen associated with the largest foodborne outbreaks in recent US history. Signaling pathways involving the second messenger c-di-GMP play important roles in increased stress survival of proteobacteria and mycobacteria, yet roles of c-di-GMP signaling pathways in L. monocytogenes have remained unexplored. Here, we identified and systematically characterized functions of the proteins involved in c-di-GMP synthesis, degradation and sensing. We show that elevated c-di-GMP levels in L. monocytogenes result in synthesis of a previously unknown exopolysaccharide that promotes cell aggregation, inhibits motility in semi-solid media, and importantly, enhances bacterial tolerance to commonly used disinfectants as well as desiccation. These properties of the exopolysaccharide may increase listerial survival in food processing plants as well as on produce during transportation and storage. Elevated c-di-GMP levels also grossly diminish listerial invasiveness in enterocytes in vitro, and impair bacterial accumulation in selected mouse organs during oral infection.

Role of host GTPases in infection by Listeria monocytogenes

The bacterial pathogen Listeria monocytogenes induces internalization into mammalian cells and uses actin-based motility to spread within tissues. Listeria accomplishes this intracellular life cycle by exploiting or antagonizing several host GTPases. Internalization into human cells is mediated by the bacterial surface proteins InlA or InlB. These two modes of uptake each require a host actin polymerization pathway comprised of the GTPase Rac1, nucleation promotion factors, and the Arp2/3 complex. In addition to Rac1, InlB-mediated internalization involves inhibition of the GTPase Arf6 and participation of Dynamin and septin family GTPases. After uptake, Listeria is encased in host phagosomes. The bacterial protein GAPDH inactivates the human GTPase Rab5, thereby delaying phagosomal acquisition of antimicrobial properties. After bacterial-induced destruction of the phagosome, cytosolic Listeria uses the surface protein ActA to stimulate actin-based motility. The GTPase Dynamin 2 reduces the density of microtubules that would otherwise limit bacterial movement. Cell-to-cell spread results when motile Listeria remodel the host plasma membrane into protrusions that are engulfed by neighbouring cells. The human GTPase Cdc42, its activator Tuba, and its effector N-WASP form a complex with the potential to restrict Listeria protrusions. Bacteria overcome this restriction through two microbial factors that inhibit Cdc42-GTP or Tuba/N-WASP interaction.

A trip in the "New Microbiology" with the bacterial pathogen Listeria monocytogenes

Listeria monocytogenes is a food-borne pathogen causing an opportunistic disease called listeriosis. This bacterium invades and replicates in most cell types, due to its multiple strategies to exploit host molecular mechanisms. Research aiming at unravelling Listeria invasion and intracellular lifestyle has led to a number of key discoveries in infection biology, cell biology and also microbiology. In this review, we report on our most recent advances in understanding the intimate crosstalk between the bacterium and its host, resulting from in-depth studies performed over the past five years. We specifically highlight new concepts in RNA-based regulation in bacteria and discuss important findings in cell biology, including a new role for clathrin and an atypical mitochondrial fragmentation mechanism. We also illustrate the notion that bacterial infection regulates host gene expression at the chromatin level, contributing to an emerging field called patho-epigenetics. This review corresponds to the lecture given by one of us (P.C.) on the occasion of the 2014 FEBS|EMBO Woman in Science Award.

Fungal pathogens are platforms for discovering novel and conserved septin properties

Septins are filament-forming GTP-binding proteins that act as scaffolds in diverse cell functions including division, polarity and membrane remodeling. In a variety of fungal pathogens, it has been observed that septins are required for virulence because cells are unable to survive or are misshapen when septins are mutated. Cell morphology is interconnected with pathogenesis and thus septin mutants displaying aberrant cell morphologies are commonly deficient in host tissue invasion. The degree to which septins orchestrate versus maintain changes in fungal cell morphology during pathogenesis remains to be determined. Aside from the importance of septins in the process of pathogenesis, animal and plant fungal pathogens display complexity in septin form, dynamics, and function not seen in Saccharomyces cerevisiae making these organisms important models for uncovering diversity in septin behavior. Additionally, host septins have recently been implicated in the process of Candida albicans invasion, motivating the need to examine host septins in fungal pathogenesis. Understanding the role of septins in the host-pathogen interaction not only illuminates pathogenesis mechanisms but importantly also expands our understanding of septin biology in general.

Role of endothelial cell septin 7 in the endocytosis of Candida albicans

Candida albicans invades endothelial cells by binding to N-cadherin and other cell surface receptors. This binding induces rearrangement of endothelial cell actin microfilaments, which results in the formation of pseudopods that surround the organism and pull it into the endothelial cell. Here, we investigated the role of endothelial cell septin 7 (SEPT7) in the endocytosis of C. albicans hyphae. Using confocal microscopy, we determined that SEPT7 accumulated with N-cadherin and actin microfilaments around C. albicans as it was endocytosed by endothelial cells. Affinity purification studies indicated that a complex containing N-cadherin and SEPT7 was recruited by C. albicans and that formation of this complex around C. albicans was mediated by the fungal Als3 and Ssa1 invasins. Knockdown of N-cadherin by small interfering RNA (siRNA) reduced recruitment of SEPT7 to C. albicans, suggesting that N-cadherin functions as a link between SEPT7 and the fungus. Also, depolymerization of actin microfilaments with cytochalasin D decreased the association between SEPT7 and N-cadherin and inhibited recruitment of both SEPT7 and N-cadherin to C. albicans, indicating the necessity of an intact cytoskeleton in the functional interaction between SEPT7 and N-cadherin. Importantly, knockdown of SEPT7 decreased accumulation of N-cadherin around C. albicans in intact endothelial cells and reduced binding of N-cadherin to this organism, as revealed by the affinity purification assay. Furthermore, SEPT7 knockdown significantly inhibited the endocytosis of C. albicans. Therefore, in response to C. albicans infection, SEPT7 forms a complex with endothelial cell N-cadherin, is required for normal accumulation of N-cadherin around C. albicans hyphae, and is necessary for maximal endocytosis of the organism.

During hematogenously disseminated infection, Candida albicans invades the endothelial cell lining of the blood vessels to invade the deep tissues. C. albicans can invade endothelial cells by inducing its own endocytosis, which is triggered when the C. albicans Als3 and Ssa1 invasins bind to N-cadherin on the endothelial cell surface. How this binding induces endocytosis is incompletely understood. Septins are intracellular GTP-binding proteins that influence the function and localization of cell surface proteins. We found that C. albicans Als3 and Ssa1 bind to a complex containing N-cadherin and septin 7, which in turn interacts with endothelial cell microfilaments, thereby inducing endocytosis of the organism. The key role of septin 7 in governing receptor-mediated endocytosis is likely relevant to host cell invasion by other microbial pathogens, in addition to C. albicans.

Entry of Listeria monocytogenes in mammalian epithelial cells: an updated view

Listeria monocytogenes is a bacterial pathogen that promotes its internalization into host epithelial cells. Interaction between the bacterial surface molecules InlA and InlB and their cellular receptors E-cadherin and Met, respectively, triggers the recruitment of endocytic effectors, the subversion of the phosphoinositide metabolism, and the remodeling of the actin cytoskeleton that lead to bacterial engulfment. Additional bacterial surface and secreted virulence factors also contribute to entry, albeit to a lesser extent. Here we review the increasing number of signaling effectors that are reported as being subverted by L. monocytogenes during invasion of cultured cell lines. We also update the current knowledge of the early steps of in vivo cellular infection, which, as shown recently, challenges previous concepts generated from in vitro data.

Actin recruitment to the Chlamydia inclusion is spatiotemporally regulated by a mechanism that requires host and bacterial factors

The ability to exit host cells at the end of their developmental growth is a critical step for the intracellular bacterium Chlamydia. One exit strategy, extrusion, is mediated by host signaling pathways involved with actin polymerization. Here, we show that actin is recruited to the chlamydial inclusion as a late event, occurring after 20 hours post-infection (hpi) and only within a subpopulation of cells. This event increases significantly in prevalence and extent from 20 to 68 hpi, and actin coats strongly correlated with extrusions. In contrast to what has been reported for other intracellular pathogens, actin nucleation on Chlamydia inclusions did not 'flash', but rather exhibited moderate depolymerization dynamics. By using small molecule agents to selectively disrupt host signaling pathways involved with actin nucleation, modulate actin polymerization dynamics and also to disable the synthesis and secretion of chlamydial proteins, we further show that host and bacterial proteins are required for actin coat formation. Transient disruption of either host or bacterial signaling pathways resulted in rapid loss of coats in all infected cells and a reduction in extrusion formation. Inhibition of Chlamydia type III secretion also resulted in rapid loss of actin association on inclusions, thus implicating chlamydial effector proteins(s) as being central factors for engaging with host actin nucleating factors, such as formins. In conclusion, our data illuminate the host and bacterial driven process by which a dense actin matrix is dynamically nucleated and maintained on the Chlamydia inclusion. This late stage event is not ubiquitous for all infected cells in a population, and escalates in prevalence and extent throughout the developmental cycle of Chlamydia, culminating with their exit from the host cell by extrusion. The initiation of actin recruitment by Chlamydia appears to be novel, and may serve as an upstream determinant of the extrusion mechanism.

Septins: the fourth component of the cytoskeleton

Septins belong to a family of proteins that is highly conserved in eukaryotes and is increasingly recognized as a novel component of the cytoskeleton. All septins are GTP-binding proteins that form hetero-oligomeric complexes and higher-order structures, including filaments and rings. Recent studies have provided structural information about the different levels of septin organization; however, the crucial structural determinants and factors responsible for septin assembly remain unclear. Investigations on the molecular functions of septins have highlighted their roles as scaffolds for protein recruitment and as diffusion barriers for subcellular compartmentalization in numerous biological processes, including cell division and host-microorganism interactions.

Virulence factors that modulate the cell biology of Listeria infection and the host response

The Gram-positive bacterial pathogen Listeria monocytogenes has become one of the best studied models in infection biology. This review will update our knowledge of Listeria virulence factors and highlight their role during the Listeria infection process.

Mammalian septins: dynamic heteromers with roles in cellular morphogenesis and compartmentalization

The septins are a family of GTP-binding proteins, evolutionarily conserved from yeast through to mammals, with roles in multiple core cellular functions. Here we provide an overview of our current knowledge of septin structure and function and focus mainly on mammalian septins, but gain much insight by drawing on knowledge of septins in other organisms. We describe their genomic and transcriptional complexity: a complexity manifest also in the diversity of scaffold structures that septins can form. Septin complexes can act to localize interacting proteins at specific intracellular locales and can also define membrane compartments by defining diffusion barriers. By such activities, septins can contribute to the definition of spatial asymmetry and cell polarity and we suggest a potential role in stem cell biology. Finally, we review the evidence that septins contribute to various disease states and argue that it is a breakdown in the tight regulation of their expression (particularly of individual isoforms), and also their inherent ability to oligomerize, which is pathogenic. Study of the perturbation of septin complex formation in disease will provide valuable insights into septin biology and will be a fertile ground for study.

Illuminating the landscape of host-pathogen interactions with the bacterium Listeria monocytogenes

Listeria monocytogenes has, in 25 y, become a model in infection biology. Through the analysis of both its saprophytic life and infectious process, new concepts in microbiology, cell biology, and pathogenesis have been discovered. This review will update our knowledge on this intracellular pathogen and highlight the most recent breakthroughs. Promising areas of investigation such as the increasingly recognized relevance for the infectious process, of RNA-mediated regulations in the bacterium, and the role of bacterially controlled posttranslational and epigenetic modifications in the host will also be discussed.

A glance at Listeria and Salmonella cell invasion: different strategies to promote host actin polymerization

The facultative intracellular bacterial pathogens Listeria monocytogenes and Salmonella enterica have evolved multiple strategies to invade a large panel of mammalian cells. These pathogens use the host cell actin system for invasion and became a paradigm for the study of host-pathogen interactions and bacterial adaptation to mammalian hosts. The key signaling component that these pathogens use to orchestrate actin remodeling is the Arp2/3 complex, which is related to polymerization of actin filaments. These bacterial pathogens are able to trigger distinct invasion mechanisms. On the one hand, L. monocytogenes invade a host cell in a way dependent on the specific interactions between bacterial and host cell proteins, which in turn activate the host cell actin polymerizing machinery that culminates with bacterial internalization. Also, Listeria escapes from the newly formed parasitophorous vacuole and moves among adjacent cells by triggering actin polymerization. On the other hand, Salmonella invades a host cell by delivering into the cytoplasm virulence factors which directly interact with host regulators of actin polymerization which leads to bacterial uptake. Moreover, Salmonella avoids vacuole lyses and modulates the early and late endosomal markers presented in the vacuole membrane. This mini-review focuses on the different pathways that L. monocytogenes and S. enterica activate to modulate the actin cytoskeleton in order to invade, to form the parasitophorous vacuole, and to migrate inside host cells.

Septins as key regulators of actin based processes in bacterial infection

Many pathogens have evolved a variety of mechanisms to exploit the host-cell actin cytoskeleton during infection, either to enter into cells or to move within cells. These events have been investigated and documented in detail. Yet, a complete picture of the molecules and mechanisms regulating entry and intracellular movement remains to be established. Here we present a series of studies revealing that in addition to actin rearrangements the host cell also employs septins, a relatively newly characterized component of the cell cyto-skeleton, to regulate bacterial entry and restrict the dissemination of cytosolic bacteria. The challenge now is to decipher the precise role of septins during actin rearrangements and how these different cytoskeleton components orchestrate infection processes.

p62 and NDP52 proteins target intracytosolic Shigella and Listeria to different autophagy pathways

Autophagy is an important mechanism of innate immune defense. We have recently shown that autophagy components are recruited with septins, a new and increasingly characterized cytoskeleton component, to intracytosolic Shigella that have started to polymerize actin. On the other hand, intracytosolic Listeria avoids autophagy recognition by expressing ActA, a bacterial effector required for actin polymerization. Here, we exploit Shigella and Listeria as intracytosolic tools to characterize different pathways of selective autophagy. We show that the ubiquitin-binding adaptor proteins p62 and NDP52 target Shigella to an autophagy pathway dependent upon septin and actin. In contrast, p62 or NDP52 targets the Listeria ActA mutant to an autophagy pathway independent of septin or actin. TNF-α, a host cytokine produced upon bacterial infection, stimulates p62-mediated autophagic activity and restricts the survival of Shigella and the Listeria ActA mutant. These data provide a new molecular framework to understand the emerging complexity of autophagy and its ability to achieve specific clearance of intracytosolic bacteria.

Tannerella forsythia invasion in oral epithelial cells requires phosphoinositide 3-kinase activation and clathrin-mediated endocytosis

Tannerella forsythia, a Gram-negative anaerobe implicated in periodontitis, has been detected within human buccal epithelial cells and shown to invade oral epithelial cells in vitro. We have previously shown that this bacterium triggers host tyrosine kinase-dependent phosphorylation and actin-dependent cytoskeleton reorganization for invasion. On the bacterial side, the leucine-rich repeat cell-surface BspA protein is important for entry. The present study was undertaken to identify host signalling molecules during T. forsythia entry into human oral and cervical epithelial cells. Specifically, the roles of phosphatidylinositol 3-kinase (PI3K), Rho-family GTPases, cholesterol-rich membrane microdomains and the endocytic protein clathrin were investigated. For this purpose, cell lines were pretreated with chemical inhibitors or small interfering RNAs (siRNAs) that target PI3Ks, Rho GTPases, clathrin and cholesterol (a critical component of 'lipid rafts'), and the resulting effects on T. forsythia uptake were determined. Our studies revealed that T. forsythia entry is dependent on host PI3K signalling, and that purified BspA protein causes activation of this lipid kinase. Bacterial entry also requires the cooperation of host Rac1 GTPase. Finally, our findings indicate an important role for clathrin and cholesterol-rich lipid microdomains in the internalization process.

A role for septins in the interaction between the Listeria monocytogenes INVASION PROTEIN InlB and the Met receptor

Septins are conserved GTPases that form filaments and are required for cell division. During interphase, septin filaments associate with cellular membrane and cytoskeleton networks, yet the functional significance of these associations have, to our knowledge, remained unknown. We recently discovered that different septins, SEPT2 and SEPT11, regulate the InlB-mediated entry of Listeria monocytogenes into host cells. Here we address the role of SEPT2 and SEPT11 in the InlB-Met interactions underlying Listeria invasion to explore how septins modulate surface receptor function. We observed that differences in InlB-mediated Listeria entry correlated with differences in Met surface expression caused by septin depletion. Using atomic force microscopy on living cells, we show that septin depletion significantly reduced the unbinding force of InlB-Met interaction and the viscosity of membrane tethers at locations where the InlB-Met interaction occurs. Strikingly, the same order of difference was observed for cells in which the actin cytoskeleton was disrupted. Consistent with a proposed role of septins in association with the actin cytoskeleton, we show that cell elasticity is decreased upon septin or actin inactivation. Septins are therefore likely to participate in anchorage of the Met receptor to the actin cytoskeleton, and represent a critical determinant in surface receptor function.

Entrapment of intracytosolic bacteria by septin cage-like structures

Actin-based motility is used by various pathogens for dissemination within and between cells. Yet host factors restricting this process have not been identified. Septins are GTP-binding proteins that assemble as filaments and are essential for cell division. However, their role during interphase has remained elusive. Here, we report that septin assemblies are recruited to different bacteria that polymerize actin. We observed that intracytosolic Shigella either become compartmentalized in septin cage-like structures or form actin tails. Inactivation of septin caging increases the number of Shigella with actin tails and enhances cell-to-cell spread. TNF-α, a host cytokine produced upon Shigella infection, stimulates septin caging and restricts actin tail formation and cell-to-cell spread. Finally, we show that septin cages entrap bacteria targeted to autophagy. Together, these results reveal an unsuspected mechanism of host defense that restricts dissemination of invasive pathogens.

Analysis of differential proteomes of induced pluripotent stem cells by protein-based reprogramming of fibroblasts

The recent generation of induced pluripotent stem (iPS) cells represents a novel opportunity to complement embryonic stem (ES) cell-based approaches. iPS cells can be generated by viral transduction of specific transcription factors, but there is a potential risk of tumorigenicity by random retroviral integration. We have generated novel iPS (sFB-protein-iPS) cells from murine dermal fibroblasts (FVB-sFB) that have ES cell characteristics, using ES cell-derived cell extracts instead of performing viral transduction. Notably, only cell extracts from an ES cell line (C57-mES) on the C57/BL6 background generated iPS cells in our protocol-not an ES cell line (E14-mES) on the 129 background. Hypothesizing that determining the differences in these 2 mES cell lines will provide vital insight into the reprogramming machinery, we performed proteomic and global gene expression analysis by iTRAQ and mRNA microarray, respectively. We observed that pluripotent ES cells and ES cell extract-derived iPS cells had differential proteomes and global gene expression patterns. Notably, reprogramming-competent C57-mES cells highly expressed proteins that regulate protein synthesis and metabolism, compared with reprogramming-incompetent 129-mES cells, suggesting that there is a threshold that protein synthetic machinery must exceed to initiate reprogramming.

Conquering the complex world of human septins: implications for health and disease

Septins are highly conserved filamentous proteins first characterized in budding yeast and subsequently identified in must eukaryotes. Septins can bind and hydrolyze GTP, which is intrinsically related to their formation of septin hexamers and functional protein interactions. The human septin family is composed of 14 loci, SEPT1-SEPT14, which encode dozens of different septin proteins. Their central GTPase and polybasic domain regions are highly conserved but they diverge in their N-terminus and/or C-terminus. The mechanism by which the different isoforms are generated is not yet well understood, but one can hypothesize that the use of different promoters and/or alternative splicing could give rise to these variants. Septins perform diverse cellular functions according to tissue expression and their interacting partners. Functions identified to date include cell division, chromosome segregation, protein scaffolding, cellular polarity, motility, membrane dynamics, vesicle trafficking, exocytosis, apoptosis, and DNA damage response. Their expression is tightly regulated to maintain proper filament assembly and normal cellular functions. Alterations of these proteins, by mutation or expression changes, have been associated with a variety of cancers and neurological diseases. The association of septins with cancer results from alterations of expression in solid tumors or translocations in leukemias [mixed lineage leukemia (MLL)]. Expression changes in septins have also been associated with neurological conditions such as Alzheimer's and Parkinson's disease, as well as retinopathies, hepatitis C, spermatogenesis and Listeria infection. Pathogenic mutations of SEPT9 were identified in the autosomal dominant neurological disorder hereditary neuralgic amyotrophy (HNA). Human septin research over the past decade has established their importance in cell biology and human disease. Further functional characterization of septins is crucial to our understanding of their possible diagnostic, prognostic, and therapeutic applications.

Cytoskeleton rearrangements during Listeria infection: clathrin and septins as new players in the game

The study of an infection process can reveal how microbes exploit the host, and can illuminate unknown host cellular functions. Invasive pathogens have evolved efficient strategies to promote their internalization within normally non-phagocytic host cells. The so-called "zippering" bacteria present to host cell receptors molecules that mimic endogenous ligands, thereby inducing specific intracellular signaling cascades ultimately resulting in actin polymerization and uptake. Here we review how the bacterial pathogen Listeria monocytogenes enters into cells, and present a series of studies revealing that in addition to actin rearrangements this bacterium exploits the clathrin-mediated endocytosis machinery together with septins, a novel cytoskeleton element. The challenge is now to decipher how all of these components orchestrate themselves to permit entry into normally non-phagocytic cells.

From pathogenesis to cell biology and back

The study of how microbes cause disease has initiated unique research avenues in both infection biology and cell biology. As our knowledge of the infected cell improves and innovative technologies become available, new domains of investigation hold promise for better understanding pathogenesis and fundamental cellular processes.