Intracellular pathogens and the actin cytoskeleton

Variation in the expression of a Plasmodium falciparum protein family implicated in erythrocyte invasion

The PfRH protein family of Plasmodium falciparum is implicated in erythrocyte invasion. Here we report variations in the sequence, transcription, and protein expression of four different members of this family in three parasite lines, 3D7, T996, and FCB1. There are sequence polymorphisms in PfRH1, PfRH2a, PfRH2b, and PfRH3, ranging from variations across repeat regions to a 585-bp deletion in the 3' end of PfRH2b in T996. Not all the genes are transcribed: although all members of the family are transcribed in 3D7 and T996, PfRH2a and PfRH2b are not transcribed in FCB1. The PfRH1, PfRH2a, and PfRH2b proteins are expressed in late schizonts and merozoites and are located in apical organelles and on the apical surface. However, the PfRH1 protein does not appear to be correctly targeted to the apex in 3D7 and T996. In contrast, the PfRH1 protein is present at the apical end of FCB1 merozoites, but the PfRH2a and PfRH2b proteins are undetectable. The apparent redundancy in the PfRH family of proteins at the level of gene number and sequence and the variations in transcription and protein expression may allow the parasite to use alternative invasion pathways.

Activation of Rho and Rab GTPases dissociates Brucella abortus internalization from intracellular trafficking

Brucella abortus is an intracellular pathogen that relies on unconventional virulence factors to infect hosts. In non-professional phagocytes, Rho GTPases-activation by the Escherichia coli cytotoxic necrotizing factor (CNF) promoted massive Brucella entrance by membrane ruffling, a mechanism that differs from the common mode of entrance used by this bacterium in non-treated cells. Cytotoxic necrotizing factor treatment, however, did not alter the intracellular route followed by the wild type or non-virulent defined mutants. In contrast, expression of a constitutively active Rab5Q79L GTPase did not alter cell-invasion by Brucella but hampered its ability to reach the endoplasmic reticulum. The CNF-induced Brucella super-infection did not reduce the ability of host cells to synthesize DNA and progress through the cell cycle. Furthermore, CNF-treatment increased the isolation of Brucella-containing compartments by a factor of 15. These results demonstrate that in non-professional phagocytic cells, Brucella manipulates two different sets of GTPases during its biogenesis, being internalization and intracellular trafficking two consecutive but independent processes. Besides, CNF-induced super-infection demonstrates that Brucella does not interfere with crucial cellular processes and has shown its potential as tool to characterize the intracellular compartments occupied by this bacterium.

InlB, a surface protein ofListeria monocytogenesthat behaves as an invasin and a growth factor

Molecules from some pathogenic bacteria mimic natural host cell ligands and trigger engulfment of the bacterium after specifically interacting with cell-surface receptors. The leucine-rich repeat (LRR)-containing protein InlB of Listeria monocytogenes is one such molecule. It triggers bacterial entry by interacting with the hepatocyte growth factor receptor (HGF-R or Met)and two other cellular components: gC1q-R and proteoglycans. Recent studies point to significant similarities between the molecular mechanisms underlying InlB-mediated entry into cells and classic phagocytosis. In addition, InlB, in common with HGF, activates signaling cascades that are not involved in bacterial entry. Therefore, studies of InlB may help us to analyze the previously noticed similarities between growth factor receptor activation and phagocytosis.

Chlamydia trachomatis induces remodeling of the actin cytoskeleton during attachment and entry into HeLa cells

To elucidate the host cell machinery utilized by Chlamydia trachomatis to invade epithelial cells, we examined the role of the actin cytoskeleton in the internalization of chlamydial elementary bodies (EBs). Treatment of HeLa cells with cytochalasin D markedly inhibited the internalization of C. trachomatis serovar L2 and D EBs. Association of EBs with HeLa cells induced localized actin polymerization at the site of attachment, as visualized by either phalloidin staining of fixed cells or the active recruitment of GFP-actin in viable infected cells. The recruitment of actin to the specific site of attachment was accompanied by dramatic changes in the morphology of cell surface microvilli. Ultrastructural studies revealed a transient microvillar hypertrophy that was dependent upon C. trachomatis attachment, mediated by structural components on the EBs, and cytochalasin D sensitive. In addition, a mutant CHO cell line that does not support entry of C. trachomatis serovar L2 did not display such microvillar hypertrophy following exposure to L2 EBs, which is in contrast to infection with serovar D, to which it is susceptible. We propose that C. trachomatis entry is facilitated by an active actin remodeling process that is induced by the attachment of this pathogen, resulting in distinct microvillar reorganization throughout the cell surface and the formation of a pedestal-like structure at the immediate site of attachment and entry.

Local actin polymerization and dynamin recruitment in SV40-induced internalization of caveolae

Simian virus 40 (SV40) utilizes endocytosis through caveolae for infectious entry into host cells. We found that after binding to caveolae, virus particles induced transient breakdown of actin stress fibers. Actin was then recruited to virus-loaded caveolae as actin patches that served as sites for actin "tail" formation. Dynamin II was also transiently recruited. These events depended on the presence of cholesterol and on the activation of tyrosine kinases that phosphorylated proteins in caveolae. They were necessary for formation of caveolae-derived endocytic vesicles and for infection of the cell. Thus, caveolar endocytosis is ligand-triggered and involves extensive rearrangement of the actin cytoskeleton.

Regulation of actin filament network formation through ARP2/3 complex: activation by a diverse array of proteins

Actin filament assembly and turnover drive many forms of cellular motility, particularly extension of the leading edge of locomoting cells and rocketing of pathogenic microorganisms through host cell cytoplasm. De novo nucleation of actin filaments appears to be required for these movements. A complex of seven proteins called Arp2/3 complex is the best characterized cellular initiator of actin filament nucleation. Arp2/3 complex is intrinsically inactive, relying on nucleation promoting factors for activation. WASp/Scar family proteins are prominent cellular nucleation promoting factors. They bring together an actin monomer and Arp2/3 complex in solution or on the side of an existing actin filament to initiate a new filament that grows in the barbed end direction. WASp and N-WASP are intrinsically autoinhibited, and their activity is regulated by Rho-family GTPases such as Cdc42, membrane polyphosphoinositides, WIP/verprolin, and SH3 domain proteins. These interactions provide a final common pathway for many signaling inputs to regulate actin polymerization. Microorganisms either activate Arp2/3 complex directly or usurp N-WASP to initiate actin polymerization.

Survival of Tropheryma whipplei, the agent of Whipple's disease, requires phagosome acidification

Tropheryma whipplei was established as the agent of Whipple's disease in 2000, but the mechanisms by which it survives within host cells are still unknown. We show here that T. whipplei survives within HeLa cells by controlling the biogenesis of its phagosome. Indeed, T. whipplei colocalized with lysosome-associated membrane protein 1, a membrane marker of late endosomal and lysosomal compartments, but not with cathepsin D, a lysosomal hydrolase. This defect in phagosome maturation is specific to live organisms, since heat-killed bacilli colocalized with cathepsin D. In addition, T. whipplei survived within HeLa cells by adapting to acidic pH. The vacuoles containing T. whipplei were acidic (pH 4.7 +/- 0.3) and acquired vacuolar ATPase, responsible for the acidic pH of late phagosomes. The treatment of HeLa cells with pH-neutralizing reagents, such as ammonium chloride, N-ethylmaleimide, bafilomycin A1, and chloroquine, increased the intravacuolar pH and promoted the killing of T. whipplei. The ability of T. whipplei to survive in an acidic environment and to interfere with phagosome-lysosome fusion is likely critical for its prolonged persistence in host cells during the course of Whipple's disease. Our results suggest that manipulating the intravacuolar pH may provide a new approach for the treatment of Whipple's disease.

SCAR is a primary regulator of Arp2/3-dependent morphological events in Drosophila

The Arp2/3 complex and its activators, Scar/WAVE and Wiskott-Aldrich Syndrome protein (WASp), promote actin polymerization in vitro and have been proposed to influence cell shape and motility in vivo. We demonstrate that the Drosophila Scar homologue, SCAR, localizes to actin-rich structures and is required for normal cell morphology in multiple cell types throughout development. In particular, SCAR function is essential for cytoplasmic organization in the blastoderm, axon development in the central nervous system, egg chamber structure during oogenesis, and adult eye morphology. Highly similar developmental requirements are found for subunits of the Arp2/3 complex. In the blastoderm, SCAR and Arp2/3 mutations result in a reduction in the amount of cortical filamentous actin and the disruption of dynamically regulated actin structures. Remarkably, the single Drosophila WASp homologue, Wasp, is largely dispensable for these numerous Arp2/3-dependent functions, whereas SCAR does not contribute to cell fate decisions in which Wasp and Arp2/3 play an essential role. These results identify SCAR as a major component of Arp2/3-dependent cell morphology during Drosophila development and demonstrate that the Arp2/3 complex can govern distinct cell biological events in response to SCAR and Wasp regulation.

Bacterial persistence within erythrocytes: a unique pathogenic strategy of Bartonella spp

The genus Bartonella comprises human-specific and zoonotic pathogens responsible for a wide range of clinical manifestations, including Carrion's disease, trench fever, cat scratch disease, bacillary angiomatosis and peliosis, endocarditis and bacteremia. These arthropod-borne pathogens typically parasitise erythrocytes in their mammalian reservoir host(s), resulting in a long-lasting haemotropic infection. We have studied the process of Bartonella erythrocyte parasitism by tracking green fluorescent protein-expressing bacteria in the blood of experimentally infected animals. Following intravenous infection, bacteria colonise a yet enigmatic primary niche, from where they are seeded into the blood stream in regular intervals of approximately five days. Bacteria invade mature erythrocytes, replicate temporarily and persist in this unique intracellular niche for the remaining life span of the infected erythrocytes. A triggered antibody response typically results in an abrogation of bacteremia within 3 months of infection, likely by blocking new waves of bacterial invasion into erythrocytes. The recent establishment of genetic tools for Bartonella spp. permitted us to identify several putative pathogenicity determinants. Application of differential fluorescence induction technology resulted in the isolation of bacterial genes differentially expressed during infection in vitro and in vivo, including an unknown family of autotransporter proteins as well as a novel type IV secretion system homologous to the conjugation system of E. coli plasmid R388. Mutational analysis of a previously described type IV secretion system displaying homology to the virB locus of Agrobacterium tumefaciens provided the first example of an essential pathogenicity locus in Bartonella. Though required for establishing haemotropic infection, it remains to be demonstrated if this type IV secretion system is necessary for colonisation of the primary niche or for the subsequent colonisation of erythrocytes.

Direct modulation of the host cell cytoskeleton by Salmonella actin-binding proteins

Invasive Salmonella trigger their own uptake into non-phagocytic eukaryotic cells by delivering virulence proteins that stimulate signaling pathways and remodel the actin cytoskeleton. It has recently emerged that Salmonella encodes two actin-binding proteins, SipC and SipA, which together efficiently nucleate actin polymerization and stabilize the resulting supramolecular filament architecture. Therefore, Salmonella might directly initiate actin polymerization independently of the cellular Arp2/3 complex early in the cell entry process. This is an unprecedented example of a direct intervention strategy to facilitate entry of a pathogen into a target cell. Here, we discuss the Salmonella actin-binding proteins and how they might function in combination with entry effectors that stimulate Rho GTPases. We propose that membrane-targeted bacterial effector proteins might trigger actin polymerization through diverse mechanisms during cell entry by bacterial pathogens.

Phenotypic silencing of cytoplasmic genes using sequence-specific double-stranded short interfering RNA and its application in the reverse genetics of wild type negative-strand RNA viruses

BACKGROUND

Post-transcriptional gene silencing (PTGS) by short interfering RNA has opened up new directions in the phenotypic mutation of cellular genes. However, its efficacy on non-nuclear genes and its effect on the interferon pathway remain unexplored. Since directed mutation of RNA genomes is not possible through conventional mutagenesis, we have tested sequence-specific 21-nucleotide long double-stranded RNAs (dsRNAs) for their ability to silence cytoplasmic RNA genomes.

RESULTS

Short dsRNAs were generated against specific mRNAs of respiratory syncytial virus, a nonsegmented negative-stranded RNA virus with a cytoplasmic life cycle. At nanomolar concentrations, the dsRNAs specifically abrogated expression of the corresponding viral proteins, and produced the expected mutant phenotype ex vivo. The dsRNAs did not induce an interferon response, and did not inhibit cellular gene expression. The ablation of the viral proteins correlated with the loss of the specific mRNAs. In contrast, viral genomic and antigenomic RNA, which are encapsidated, were not directly affected.

CONCLUSIONS

Synthetic inhibitory dsRNAs are effective in specific silencing of RNA genomes that are exclusively cytoplasmic and transcribed by RNA-dependent RNA polymerases. RNA-directed RNA gene silencing does not require cloning, expression, and mutagenesis of viral cDNA, and thus, will allow the generation of phenotypic null mutants of specific RNA viral genes under normal infection conditions and at any point in the infection cycle. This will, for the first time, permit functional genomic studies, attenuated infections, reverse genetic analysis, and studies of host-virus signaling pathways using a wild type RNA virus, unencumbered by any superinfecting virus.

A role for N-WASP in invasin-promoted internalisation

Phagocytosis of Yersinia pseudotuberculosis occurs through interaction of the bacterial protein invasin with beta1-integrins. Here we report that N-WASP plays a role in internalisation of an invasin-expressing, avirulent strain of Y. pseudotuberculosis. Ectopic expression of N-WASP mutants, which affect recruitment of the Arp2/3 complex to the phagosome, reduces uptake of Yersinia. In addition, expression of the Cdc42/Rac-binding (CRIB) region of N-WASP has an inhibitory effect on uptake. Using GFP-tagged Rho GTPase mutants, we provide evidence that Rac1, but not Cdc42, is important for internalisation. Furthermore, activated Rac1 rescues Toxin B, CRIB and Src family kinase inhibitor PP2-mediated impairment of uptake. Our observations indicate that invasin-mediated phagocytosis occurs via a Src and WASP family-dependent mechanism(s), involving the Arp2/3 complex and Rac, but does not require Cdc42.

Activation of ErbB2 receptor tyrosine kinase supports invasion of endothelial cells by Neisseria meningitidis

ErbB2 is a receptor tyrosine kinase belonging to the family of epidermal growth factor (EGF) receptors which is generally involved in cell differentiation, proliferation, and tumor growth, and activated by heterodimerization with the other members of the family. We show here that type IV pilus-mediated adhesion of Neisseria meningitidis onto endothelial cells induces tyrosyl phosphorylation and massive recruitment of ErbB2 underneath the bacterial colonies. However, neither the phosphorylation status nor the cellular localization of the EGF receptors, ErbB3 or ErbB4, were affected in infected cells. ErbB2 phosphorylation induced by N. meningitidis provides docking sites for the kinase src and leads to its subsequent activation. Specific inhibition of either ErbB2 and/or src activity reduces bacterial internalization into endothelial cells without affecting bacteria-induced actin cytoskeleton reorganization or ErbB2 recruitment. Moreover, inhibition of both actin polymerization and the ErbB2/src pathway totally prevents bacterial entry. Altogether, our results provide new insight into ErbB2 function by bringing evidence of a bacteria-induced ErbB2 clustering leading to src kinase phosphorylation and activation. This pathway, in cooperation with the bacteria-induced reorganization of the actin cytoskeleton, is required for the efficient internalization of N. meningitidis into endothelial cells, an essential process enabling this pathogen to cross host cell barriers.

Evidence for intraaxonal spread of Listeria monocytogenes from the periphery to the central nervous system

Rhombencephalitis due to Listeria monocytogenes is characterized by progressive cranial nerve palsies and subacute inflammation in the brain stem. In this paper, we report observations made on mice infected with L. monocytogenes. Unilateral inoculation of bacteria into facial muscle, or peripheral parts of a cranial nerve, induced clinical and histological signs of mainly ipsilateral rhombencephalitis. Similarly, unilateral inoculation of bacteria into lower leg muscle or peripheral parts of sciatic nerve was followed by lumbar myelitis. In these animals, intraaxonal bacteria were seen in the sciatic nerve and its corresponding nerve roots ipsilateral to the bacterial application site. Development of myelitis was prevented by transsection of the sciatic nerve proximally to the hindleg inoculation site. Altogether, our results support the hypothesis that Listeria rhombencephalitis is caused by intraaxonal bacterial spread from peripheral sites to the central nervous system.

N-WASP deficiency reveals distinct pathways for cell surface projections and microbial actin-based motility

The Wiskott-Aldrich syndrome protein (WASP) family of molecules integrates upstream signalling events with changes in the actin cytoskeleton. N-WASP has been implicated both in the formation of cell-surface projections (filopodia) required for cell movement and in the actin-based motility of intracellular pathogens. To examine N-WASP function we have used homologous recombination to inactivate the gene encoding murine N-WASP. Whereas N-WASP-deficient embryos survive beyond gastrulation and initiate organogenesis, they have marked developmental delay and die before embryonic day 12. N-WASP is not required for the actin-based movement of the intracellular pathogen Listeria but is absolutely required for the motility of Shigella and vaccinia virus. Despite these distinct defects in bacterial and viral motility, N-WASP-deficient fibroblasts spread by using lamellipodia and can protrude filopodia. These results imply a crucial and non-redundant role for N-WASP in murine embryogenesis and in the actin-based motility of certain pathogens but not in the general formation of actin-containing structures.

Rho proteins: linking signaling with membrane trafficking

Rho proteins are well known for their effects on the actin cytoskeleton, and are activated in response to a variety of extracellular stimuli. Several Rho family members are localized to vesicular compartments, and increasing evidence suggests that they play important roles in the trafficking of vesicles on both endocytic and exocytic pathways. In particular, RhoA, RhoB, RhoD, Rac and Cdc42 have been shown to affect various steps of membrane trafficking. The underlying molecular basis for these effects of Rho proteins are incompletely understood, but in the case of Cdc42 it appears that it can drive vesicle movement through Arp2/3 complex-mediated actin polymerization at the surface of the vesicle. This is similar to what is believed to happen when Rac and Cdc42 stimulate actin polymerization at the plasma membrane. Rho proteins may also affect membrane trafficking by altering phosphatidylinositide composition of membrane compartments, or through interactions with microtubules.

Remodelling of the actin cytoskeleton is essential for replication of intravacuolar Salmonella

Maturation and maintenance of the intracellular vacuole in which Salmonella replicates is controlled by virulence proteins including the type III secretion system encoded by Salmonella pathogenicity island 2 (SPI-2). Here, we show that, several hours after bacterial uptake into different host cell types, Salmonella induces the formation of an F-actin meshwork around bacterial vacuoles. This structure is assembled de novo from the cellular G-actin pool in close proximity to the Salmonella vacuolar membrane. We demonstrate that the phenomenon does not require the Inv/Spa type III secretion system or cognate effector proteins, which induce actin polymerization during bacterial invasion, but does require a functional SPI-2 type III secretion system, which plays an important role in intracellular replication and systemic infection in mice. Treatment with actin-depolymerizing agents significantly inhibited intramacrophage replication of wild-type Salmonella typhimurium. Furthermore, after this treatment, wild-type bacteria were released into the host cell cytoplasm, whereas SPI-2 mutant bacteria remained within vacuoles. We conclude that actin assembly plays an important role in the establishment of an intracellular niche that sustains bacterial growth.

Curved tails in polymerization-based bacterial motility

The curved actin "comet-tail" of the bacterium Listeria monocytogenes is a visually striking signature of actin polymerization-based motility. Similar actin tails are associated with Shigella flexneri, spotted-fever Rickettsiae, the Vaccinia virus, and vesicles and microspheres in related in vitro systems. We show that the torque required to produce the curvature in the tail can arise from randomly placed actin filaments pushing the bacterium or particle. We find that the curvature magnitude determines the number of actively pushing filaments, independent of viscosity and of the molecular details of force generation. The variation of the curvature with time can be used to infer the dynamics of actin filaments at the bacterial surface.

Annexin 2 has an essential role in actin-based macropinocytic rocketing

Annexin 2 is a Ca(2+) binding protein that binds to and aggregates secretory vesicles at physiological Ca(2+) levels [1] and that also associates Ca(2+) independently with early endosomes [2, 3]. These properties suggest roles in both exocytosis and endocytosis, but little is known of the dynamics of Annexin 2 distribution in live cells during these processes. We have used evanescent field microscopy to image Annexin 2-GFP in live, secreting rat basophilic leukemia cells and in cells performing pinocytosis. Although we found no evidence of Annexin 2 involvement in exocytosis, we observed an enrichment of Annexin 2-GFP in actin tails propeling macropinosomes. The association of Annexin 2-GFP with rocketing macropinosomes was specific because Annexin 2-GFP was absent from the actin tails of rocketing Listeria. This finding suggests that the association of Annexin 2 with macropinocytic rockets requires native pinosomal membrane. Annexin 2 is necessary for the formation of macropinocytic rockets since overexpression of a dominant-negative Annexin 2 construct abolished the formation of these structures. The same construct did not prevent the movement of Listeria in infected cells. These results show that recruitment of Annexin 2 to nascent macropinosome membranes 16656is an essential prerequisite for actin polymerization-dependent vesicle locomotion.

Listeria pathogenesis and molecular virulence determinants

The gram-positive bacterium Listeria monocytogenes is the causative agent of listeriosis, a highly fatal opportunistic foodborne infection. Pregnant women, neonates, the elderly, and debilitated or immunocompromised patients in general are predominantly affected, although the disease can also develop in normal individuals. Clinical manifestations of invasive listeriosis are usually severe and include abortion, sepsis, and meningoencephalitis. Listeriosis can also manifest as a febrile gastroenteritis syndrome. In addition to humans, L. monocytogenes affects many vertebrate species, including birds. Listeria ivanovii, a second pathogenic species of the genus, is specific for ruminants. Our current view of the pathophysiology of listeriosis derives largely from studies with the mouse infection model. Pathogenic listeriae enter the host primarily through the intestine. The liver is thought to be their first target organ after intestinal translocation. In the liver, listeriae actively multiply until the infection is controlled by a cell-mediated immune response. This initial, subclinical step of listeriosis is thought to be common due to the frequent presence of pathogenic L. monocytogenes in food. In normal individuals, the continual exposure to listerial antigens probably contributes to the maintenance of anti-Listeria memory T cells. However, in debilitated and immunocompromised patients, the unrestricted proliferation of listeriae in the liver may result in prolonged low-level bacteremia, leading to invasion of the preferred secondary target organs (the brain and the gravid uterus) and to overt clinical disease. L. monocytogenes and L. ivanovii are facultative intracellular parasites able to survive in macrophages and to invade a variety of normally nonphagocytic cells, such as epithelial cells, hepatocytes, and endothelial cells. In all these cell types, pathogenic listeriae go through an intracellular life cycle involving early escape from the phagocytic vacuole, rapid intracytoplasmic multiplication, bacterially induced actin-based motility, and direct spread to neighboring cells, in which they reinitiate the cycle. In this way, listeriae disseminate in host tissues sheltered from the humoral arm of the immune system. Over the last 15 years, a number of virulence factors involved in key steps of this intracellular life cycle have been identified. This review describes in detail the molecular determinants of Listeria virulence and their mechanism of action and summarizes the current knowledge on the pathophysiology of listeriosis and the cell biology and host cell responses to Listeria infection. This article provides an updated perspective of the development of our understanding of Listeria pathogenesis from the first molecular genetic analyses of virulence mechanisms reported in 1985 until the start of the genomic era of Listeria research.

Bartonella interactions with endothelial cells and erythrocytes

Bartonella species are emerging human pathogens responsible for a wide range of clinical manifestations, including Carrion's disease, trench fever, cat-scratch disease, bacillary angiomatosis-peliosis, endocarditis and bacteraemia. During infection of their human or animal reservoir host(s), these arthropod-borne pathogens typically invade and persistently colonize mature erythrocytes. However, in both reservoir and incidentally infected hosts, endothelial cells are target cells for bartonellae. Endothelial interactions involve a unique mode of cellular invasion, the activation of a proinflammatory phenotype and the formation of vasoproliferative tumours. Based on the establishment of bacterial genetics and appropriate infection models, recent work has begun to elucidate the cell and molecular biology of these unusual pathogen-host cell interactions.

Activation of protein tyrosine kinases by Coxiella burnetii: role in actin cytoskeleton reorganization and bacterial phagocytosis

Coxiella burnetii, the agent of Q fever, is an obligate intracellular microorganism that grows in monocytes/macrophages. The internalization of virulent organisms by monocytes is lower than that of avirulent variants and is associated with actin cytoskeleton reorganization. We studied the activation of protein tyrosine kinases (PTKs) by C. burnetii in THP-1 monocytes. Virulent organisms induced early PTK activation and the tyrosine phosphorylation of several endogenous substrates, including Hck and Lyn, two Src-related kinases. PTK activation reflects C. burnetii virulence since avirulent variants were unable to stimulate PTK. We also investigated the role of PTK activation in C. burnetii-stimulated F-actin reorganization. Tyrosine-phosphorylated proteins were colocalized with F-actin inside cell protrusions induced by C. burnetii, and PTK activity was increased in Triton X-100-insoluble fractions. In addition, lavendustin A, a PTK inhibitor, and PP1, a Src kinase inhibitor, prevented C. burnetii-induced cell protrusions and F-actin reorganization. We finally assessed the role of PTK activation in bacterial phagocytosis. Pretreatment of THP-1 cells with lavendustin A and PP1 upregulated the uptake of virulent C. burnetii but had no effect on the phagocytosis of avirulent organisms. Thus, it is likely that PTK activation by C. burnetii negatively regulates bacterial uptake by interfering with cytoskeleton organization.

Adenovirus-activated PKA and p38/MAPK pathways boost microtubule-mediated nuclear targeting of virus

Nuclear targeting of adenovirus is mediated by the microtubule-dependent, minus-end-directed motor complex dynein/dynactin, in competition with plus- end-directed motility. We demonstrate that adenovirus transiently activates two distinct signaling pathways to enhance nuclear targeting. The first pathway activates integrins and cAMP-dependent protein kinase A (PKA). The second pathway activates the p38/MAP kinase and the downstream MAPKAP kinase 2 (MK2), dependent on the p38/MAPK kinase MKK6, but independent of integrins and PKA. Motility measurements in PKA-inhibited, p38-inhibited or MK2-lacking (MK2(-/-)) cells indicate that PKA and p38 stimulated both the frequency and velocity of minus-end-directed viral motility without affecting the perinuclear localization of transferrin-containing endosomal vesicles. p38 also suppressed lateral viral motilities and MK2 boosted the frequency of minus-end-directed virus transport. Nuclear targeting of adenovirus was rescued in MK2(-/-) cells by overexpression of hsp27, an MK2 target that enhances actin metabolism. Our results demonstrate that complementary activities of PKA, p38 and MK2 tip the transport balance of adenovirus towards the nucleus and thus enhance infection.

The use of host cell machinery in the pathogenesis of Listeria monocytogenes

The bacterial pathogen, Listeria monocytogenes, exploits the host cell's machinery, enabling the pathogen to enter into cells and spread from cell to cell. Three bacterial surface proteins are crucial for these processes: internalin and InlB, which mediate entry into cells, and ActA, which induces actin polymerisation at one pole of the bacterium and promotes intracellular and intercellular motility. Recent studies have identified several of the cellular factors involved in the entry process and major discoveries have unravelled the mechanisms underlying the actin-based motility. Increasing evidence shows that many cellular genes are up- or down-regulated during infection and probably play a role in the establishment of infection, inflammation and induction of the host immune response.

Actin is ADP-ribosylated by the Salmonella enterica virulence-associated protein SpvB

The Salmonella enterica virulence-associated protein SpvB was recently shown to contain a carboxy-terminal mono(ADP-ribosyl)transferase domain. We demonstrate here that the catalytic domain of SpvB as well bacterial extracts containing full-length SpvB modifies a 43 kDa protein from macrophage-like J774-A.1 and epithelial MDCK cells as shown by label transfer from [32P]-nicotinamide adenine dinucleotide (NAD) to the 43 kDa protein. When analysed by two-dimensional gel electrophoresis, the same protein was modified in cells infected with S. enterica serovariant Dublin strain SH9325, whereas infection with an isogenic spvB mutant strain did not result in modification. Immunoprecipitation and immunoblotting experiments using SH9325-infected cells identified the modified protein as actin. The isolated catalytic domain of SpvB mediated transfer of 32P from [32P]-NAD to actins from various sources in vitro, whereas isolated eukaryotic control proteins or bacterial proteins were not modified. In an in vitro actin polymerization assay, the isolated catalytic SpvB domain prevented the conversion of G actin into F actin. Microscopic examination of MDCK cells infected with SH9325 revealed morphological changes and loss of filamentous actin content, whereas cells infected with the spvB mutant remained virtually unaffected. We conclude that actin is a target for an SpvB-mediated modification, most probably ADP-ribosylation, and that the modification of G actin interferes with actin polymerization.

Dendritic organization of actin comet tails

Polymerization of actin filaments is necessary for both protrusion of the leading edge of crawling cells and propulsion of certain intracellular pathogens, and it is sufficient for generating force for bacterial motility in vitro. Motile intracellular pathogens are associated with actin-rich comet tails containing many of the same molecular components present in lamellipodia, and this suggests that these two systems use a similar mechanism for motility. However, available structural evidence suggests that the organization of comet tails differs from that of lamellipodia. Actin filaments in lamellipodia form branched arrays, which are thought to arise by dendritic nucleation mediated by the Arp2/3 complex. In contrast, comet tails have been variously described as consisting of short, randomly oriented filaments, with a higher degree of alignment at the periphery, or as containing long, straight axial filaments with a small number of oblique filaments. Because the assembly of pathogen-associated comet tails has been used as a model system for lamellipodial protrusion, it is important to resolve this apparent discrepancy. Here, using a platinum replica approach, we show that actin filament arrays in comet tails in fact have a dendritic organization with the Arp2/3 complex localizing to Y-junctions as in lamellipodia. Thus, comet tails and lamellipodia appear to share a common dendritic nucleation mechanism for protrusive motility. However, comet tails differ from lamellipodia in that their actin filaments are usually twisted and appear to be under significant torsional stress.

Virus-like gene transfer into cells mediated by polyoma virus pseudocapsids

Mouse polyoma virus-like particles (or pseudocapsids) are composed solely of recombinant viral coat protein. They can interact with DNA and transport it to cells, resulting in gene expression both in tissue culture and in mice. We demonstrate that DNA transfer in vitro depends on partial packaging of DNA within the virus-like capsid. Cell surface sialic acid residues and an intact microtubule network, required for viral infectivity, are also necessary for pseudocapsid-mediated gene expression from heterologous DNA. Thus, gene delivery in this system requires pathways utilised by polyoma virions, rather than proceeding via the 'nonspecific' endosomal route typical of nonviral systems such as liposomes or calcium phosphate precipitates. Despite the fact that all cells appear to internalise pseudocapsid/DNA complexes, only a proportion show productive gene delivery. Bulk internalisation of complexes is dependent on actin fibres, but not cell surface sialic acid or microtubules, indicating that a second transport pathway exists for pseudocapsids which is nonproductive for gene transfer. The model suggested by these data demonstrates the virus-like properties of the pseudocapsid system, and provides a basis for further development to produce a highly effective gene delivery vehicle. Gene Therapy (2000) 7, 2122-2131.

The invasion protein InIB from Listeria monocytogenes activates PLC-gamma1 downstream from PI 3-kinase

Entry of the bacterial pathogen Listeria monocytogenes into non-phagocytic mammalian cells is mainly mediated by the InlB protein. Here we show that in the human epithelial cell line HEp-2, the invasion protein InlB activates sequentially a p85beta-p110 class I(A) PI 3-kinase and the phospholipase C-gamma1 (PLC-gamma1) without detectable tyrosine phosphorylation of PLC-gamma1. Purified InlB stimulates association of PLC-gamma1 with one or more tyrosine-phosphorylated proteins, followed by a transient increase in intracellular inositol 1,4,5-trisphosphate (IP3) levels and a release of intracellular Ca2+ in a PI 3-kinase-dependent manner. Infection of HEp-2 cells with wild-type L. monocytogenes bacteria also induces association of PLC-gamma1 with phosphotyrosyl proteins. This interaction is undetectable upon infection with a deltainlB mutant revealing an InlB specific signal. Interestingly, pharmacological or genetic inactivation of PLC-gamma1 does not significantly affect InlB-mediated bacterial uptake, suggesting that InlB-mediated PLC-gamma1 activation and calcium mobilization are involved in post-internalization steps.

The role of the CFTR in susceptibility to Pseudomonas aeruginosa infections in cystic fibrosis

Recent molecular and cellular studies have shed new light on the basis for the susceptibility of cystic fibrosis (CF) patients to Pseudomonas aeruginosa infection. Changes in airway liquid composition and/or viscosity, enhanced bacterial binding to mucin and epithelial cell receptors, increased innate inflammation owing to disruptions in lipid metabolism and a role for the CFTR protein in bacterial ingestion and clearance have all been postulated. The high P. aeruginosa infection rate in CF patients can potentially be explained by the specificity of the interaction between the CFTR and P. aeruginosa.

Endocytosis and nuclear trafficking of adeno-associated virus type 2 are controlled by rac1 and phosphatidylinositol-3 kinase activation

Adeno-associated virus (AAV) is a single-stranded DNA parvovirus that causes no currently known pathology in humans. Despite the fact that this virus is of increasing interest to molecular medicine as a vector for gene delivery, relatively little is known about the cellular mechanisms controlling infection. In this study, we have examined endocytic and intracellular trafficking of AAV-2 using fluorescent (Cy3)-conjugated viral particles and molecular techniques. Our results demonstrate that internalization of heparan sulfate proteoglycan-bound AAV-2 requires alphaVbeta5 integrin and activation of the small GTP-binding protein Rac1. Following endocytosis, activation of a phosphatidylinositol-3 (PI3) kinase pathway was necessary to initiate intracellular movement of AAV-2 to the nucleus via both microfilaments and microtubules. Inhibition of Rac1 using a dominant N17Rac1 mutant led to a decrease in AAV-2-mediated PI3 kinase activation, indicating that Rac1 may act proximal to PI3 kinase during AAV-2 infection. In summary, our results indicate that alphaVbeta5 integrin-mediated endocytosis of AAV-2 occurs through a Rac1 and PI3 kinase activation cascade, which directs viral movement along the cytoskeletal network to the nucleus.

Infection of human endothelial cells with Bartonella bacilliformis is dependent on Rho and results in activation of Rho

Bartonella bacilliformis was continuously internalized into human endothelial cells beginning shortly after addition of the bacteria and continuing for at least 24 h after infection in vitro, with a major increase in uptake occurring between 16 and 24 h. Preincubation of endothelial cells with C3 exoenzyme, which inactivated intracellular Rho-GTPase, blocked internalization of the bacteria. Addition of C3 exoenzyme at any time after addition of the bacteria blocked further internalization of bacteria, including the major uptake of bacteria internalized at 16 to 24 h. Rho, a key signaling protein in pathways involving actin organization, was directly shown to be activated in endothelial cells undergoing infection with B. bacilliformis, with maximal activation and translocation to the plasma membrane at 12 to 16 h. At late times of infection, most of the bacteria were found in a perinuclear location. Staining of the Golgi complex with specific markers, anti-human Golgin-97, anti-KDEL receptor, and BODIPY-TR ceramide, showed colocalization of bacteria in the Golgi complex region. Disruption of the Golgi complex with brefeldin A scattered the bacteria from this perinuclear location and resulted in inhibition of internalization of the bacteria in endothelial cells.

Dual epitope recognition by the VASP EVH1 domain modulates polyproline ligand specificity and binding affinity

The Ena-VASP family of proteins act as molecular adaptors linking the cytoskeletal system to signal transduction pathways. Their N-terminal EVH1 domains use groups of exposed aromatic residues to specifically recognize 'FPPPP' motifs found in the mammalian zyxin and vinculin proteins, and ActA protein of the intracellular bacterium Listeria monocytogenes. Here, evidence is provided that the affinities of these EVH1-peptide interactions are strongly dependent on the recognition of residues flanking the core FPPPP motifs. Determination of the VASP EVH1 domain solution structure, together with peptide library screening, measurement of individual K(d)s by fluorescence titration, and NMR chemical shift mapping, revealed a second affinity-determining epitope present in all four ActA EVH1-binding motifs. The epitope was shown to interact with a complementary hydrophobic site on the EVH1 surface and to increase strongly the affinity of ActA for EVH1 domains. We propose that this epitope, which is absent in the sequences of the native EVH1-interaction partners zyxin and vinculin, may provide the pathogen with an advantage when competing for the recruitment of the host VASP and Mena proteins in the infected cell.

Neisseria gonorrhoeae elicits membrane ruffling and cytoskeletal rearrangements upon infection of primary human endocervical and ectocervical cells

Neisseria gonorrhoeae is a strict human pathogen that is, primarily, transmitted by close sexual contact with an infected individual. Gonococcal infection of the male urogenital tract has been well studied in experimental human models and in urethral cell culture systems. Recent studies, using tissue culture cell systems, have suggested a role for the cervical epithelium in gonococcal infection of females; however, the nature of gonococcal infection of the normal uterine cervix remains controversial. To address this enigma, we have developed two primary human cervical epithelial cell systems from surgical biopsies. Gonococcal infection studies and electron microscopy show that N. gonorrhoeae is capable of infecting and invading both the endo- and the ectocervix. Invasion was found to occur primarily in an actin-dependent manner, but it does not appear to require de novo protein synthesis by either the bacterium or the host cervical cell. Membrane ruffles appear to be induced in response to gonococci. Consistent with membrane ruffling, gonococci were found residing within macropinosomes, and a concentrated accumulation of actin-associated proteins was observed to occur in response to gonococcal infection. Electron microscopy of clinically derived cervical biopsies show that lamellipodia formation and cytoskeletal changes, suggestive of membrane ruffles, also occur in the cervical epithelium of women with naturally acquired gonococcal cervicitis. These studies demonstrate the ability of N. gonorrhoeae to infect and invade both the endo- and the ectocervix of the normal uterine cervix. Gonococcal induced ruffling is a novel finding and may be unique to the cervical epithelium.

Signaling antibodies complexed with adenovirus circumvent CAR and integrin interactions and improve gene delivery

Current adenoviral (Ad) vectors cannot be targeted to specific cell types due to the widespread distribution of the Ad receptor (CAR). Moreover, CAR and/or internalization receptors (alphav integrins) are absent or present at low levels on some cell types, rendering them resistant to Ad-mediated gene delivery. To address these problems, we have developed a novel vector targeting approach that takes advantage of the common cell signaling pathways initiated by ligation of alphav integrins and growth factor receptors. Recombinant growth factor/cytokines (TNF-alpha, IGF-1, EGF) which trigger phosphatidylinositol-3-OH kinase (PI3K) activation, a signaling molecule involved in adenovirus internalization, were fused to a monoclonal antibody specific for the viral penton base. Ad vectors complexed with these bifunctional mAbs increased gene delivery 10 to 50-fold to human melanoma cells lacking alphav integrins. The bifunctional mAbs also enhanced gene delivery by fiberless adenovirus particles which cannot bind to CAR. Improved gene delivery correlated with increased virus internalization and attachment as well as PI3K activity. The use of bifunctional mAbs to trigger specific cell signaling pathways offers a widely applicable method for bypassing the normal Ad receptors in gene delivery and potentially increasing the selectivity of gene transfer.

Membrane recruitment of Rac1 triggers phagocytosis

Rac1 is a Ρ-family GTP-binding protein that controls lamellipodia formation and membrane ruffling in fibroblasts. Recently, Rac1 and Cdc42, another member of the Ρ-family, have been shown to regulate Fc receptor-mediated phagocytosis in macrophages by controlling different steps of membrane and actin dynamics leading to particle engulfment. Here, we investigated the function of Rac1 using a membrane recruitment system that mimics phagocytosis. Recruitment of an activated Rac1 protein to the cytoplasmic domain of an engineered membrane receptor by using rapamycin as a bridge induces ingestion of latex beads bound to the receptor. Rac1-mediated bead uptake depends on actin polymerisation since actin filaments accumulate at the bead/membrane binding sites and internalisation is inhibited by cytochalasin D. Internalisation is also abolished upon substitution of Phe37 to Leu in the Rac1 effector region. Our results indicate that by promoting actin polymerisation at particle attachment sites, Rac1 by acting through specific downstream effectors induces plasma membrane remodeling that allows particle internalisation in a membrane-enclosed phagosome.

A role for alpha-and beta-catenins in bacterial uptake

Interaction of internalin with E-cadherin promotes entry of Listeria monocytogenes into human epithelial cells. This process requires actin cytoskeleton rearrangements. Here we show, by using a series of stably transfected cell lines expressing E-cadherin variants, that the ectodomain of E-cadherin is sufficient for bacterial adherence and that the intracytoplasmic domain is required for entry. The critical cytoplasmic region was further mapped to the beta-catenin binding domain. Because beta-catenin is known to interact with alpha-catenin, which binds to actin, we generated a fusion molecule consisting of the ectodomain of E-cadherin and the actin binding site of alpha-catenin. Cells expressing this chimera were as permissive as E-cadherin-expressing cells. In agreement with these data, alpha- and beta-catenins as well as E-cadherin clustered and colocalized at the entry site, where F-actin then accumulated. Taken together, these results reveal that E-cadherin, via beta- and alpha-catenins, can trigger dynamic events of actin polymerization and membrane extensions culminating in bacterial uptake.

Cellular actin is affected by interaction with Candida albicans

Attachment of Candida albicans, an important opportunistic pathogen, to host tissues is an initial step in the development of the infection. The events occurring in the fungal and in the host cells after interaction are poorly understood. In this study we concentrated on the events occurring in the mammalian cells after the interaction with Candida, with emphasis on the cytoskeleton actin. Human cell line cells (HEp2) were exposed to C. albicans or C. albicans-secreted material (culture filtrate) (actin-rearranging Candida-secreted factor, arcsf). The HEp2 cells were examined for cellular changes using confocal laser microscopy (CLSM), transmission and scanning electron microscopy (TEM and SEM). The CLSM studies, using fluorescein isothiocyanate-labeled C. albicans and rhodamine phalloidin actin staining, revealed yeasts adhering to the HEp2 cells or internalized into the cells, with actin surrounding the fungi. Furthermore, actin rearrangement from filamentous network to actin aggregates was noticed. Interaction between the HEp2 cells and C. albicans could be demonstrated also by SEM and TEM after a 2-4-h exposure of the cells to the fungus. Yeasts and hyphae were found attaching to the surface and within the cells. CLSM studies revealed that exposure of HEp2 cells to arcsf was also followed by cellular actin rearrangement, reduced membrane ruffling and decreased cellular motility. The effect was dose- and time-dependent. All these data indicate that the interaction of Candida with HEp2 cells involves signaling events and affects the cellular actin.

Ultrastructure of Rickettsia rickettsii actin tails and localization of cytoskeletal proteins

Actin-based motility (ABM) is a mechanism for intercellular spread that is utilized by vaccinia virus and the invasive bacteria within the genera Rickettsia, Listeria, and Shigella. Within the Rickettsia, ABM is confined to members of the spotted fever group (SFG), such as Rickettsia rickettsii, the agent of Rocky Mountain spotted fever. Infection by each agent induces the polymerization of host cell actin to form the typical F (filamentous)-actin comet tail. Assembly of the actin tail propels the pathogen through the host cytosol and into cell membrane protrusions that can be engulfed by neighboring cells, initiating a new infectious cycle. Little is known about the structure and morphogenesis of the Rickettsia rickettsii actin tail relative to Shigella and Listeria actin tails. In this study we examined the ultrastructure of the rickettsial actin tail by confocal, scanning electron, and transmission electron microscopy. Confocal microscopy of rhodamine phalloidin-stained infected Vero cells revealed the typhus group rickettsiae, Rickettsia prowazekii and Rickettsia typhi, to have no actin tails and short (approximately 1- to 3-micrometer) straight or hooked actin tails, respectively. The SFG rickettsia, R. rickettsii, displayed long actin tails (>10 micrometer) that were frequently comprised of multiple, distinct actin bundles, wrapping around each other in a helical fashion. Transmission electron microscopy, in conjunction with myosin S1 subfragment decoration, revealed that the individual actin filaments of R. rickettsii tails are >1 micrometer long, arranged roughly parallel to one another, and oriented with the fast-growing barbed end towards the rickettsial pole. Scanning electron microscopy of intracellular rickettsiae demonstrated R. rickettsii to have polar associations of cytoskeletal material and R. prowazekii to be devoid of cytoskeletal interactions. By indirect immunofluorescence, both R. rickettsii and Listeria monocytogenes actin tails were shown to contain the cytoskeletal proteins vasodilator-stimulated phosphoprotein profilin, vinculin, and filamin. However, rickettsial tails lacked ezrin, paxillin, and tropomyosin, proteins that were associated with actin tails of cytosolic or protrusion-bound Listeria. The unique ultrastructural and compositional characteristics of the R. rickettsii actin tail suggest that rickettsial ABM is mechanistically different from previously described microbial ABM systems.

Vaccinia virus infection disrupts microtubule organization and centrosome function

We examined the role of the microtubule cytoskeleton during vaccinia virus infection. We found that newly assembled virus particles accumulate in the vicinity of the microtubule-organizing centre in a microtubule- and dynein-dynactin complex-dependent fashion. Microtubules are required for efficient intracellular mature virus (IMV) formation and are essential for intracellular enveloped virus (IEV) assembly. As infection proceeds, the microtubule cytoskeleton becomes dramatically reorganized in a fashion reminiscent of overexpression of microtubule-associated proteins (MAPs). Consistent with this, we report that the vaccinia proteins A10L and L4R have MAP-like properties and mediate direct binding of viral cores to microtubules in vitro. In addition, vaccinia infection also results in severe reduction of proteins at the centrosome and loss of centrosomal microtubule nucleation efficiency. This represents the first example of viral-induced disruption of centrosome function. Further studies with vaccinia will provide insights into the role of microtubules during viral pathogenesis and regulation of centrosome function.

Entry of the two infectious forms of vaccinia virus at the plasma membane is signaling-dependent for the IMV but not the EEV

The simpler of the two infectious forms of vaccinia virus, the intracellular mature virus (IMV) is known to infect cells less efficiently than the extracellular enveloped virus (EEV), which is surrounded by an additional, TGN-derived membrane. We show here that when the IMV binds HeLa cells, it activates a signaling cascade that is regulated by the GTPase rac1 and rhoA, ezrin, and both tyrosine and protein kinase C phosphorylation. These cascades are linked to the formation of actin and ezrin containing protrusions at the plasma membrane that seem to be essential for the entry of IMV cores. The identical cores of the EEV also appear to enter at the cell surface, but surprisingly, without the need for signaling and actin/membrane rearrangements. Thus, in addition to its known role in wrapping the IMV and the formation of intracellular actin comets, the membrane of the EEV seems to have evolved the capacity to enter cells silently, without a need for signaling.

A complex of N-WASP and WIP integrates signalling cascades that lead to actin polymerization

Wiskott-Aldrich syndrome protein (WASP) and N-WASP have emerged as key proteins connecting signalling cascades to actin polymerization. Here we show that the amino-terminal WH1 domain, and not the polyproline-rich region, of N-WASP is responsible for its recruitment to sites of actin polymerization during Cdc42-independent, actin-based motility of vaccinia virus. Recruitment of N-WASP to vaccinia is mediated by WASP-interacting protein (WIP), whereas in Shigella WIP is recruited by N-WASP. Our observations show that vaccinia and Shigella activate the Arp2/3 complex to achieve actin-based motility, by mimicking either the SH2/SH3-containing adaptor or Cdc42 signalling pathways to recruit the N-WASP-WIP complex. We propose that the N-WASP-WIP complex has a pivotal function in integrating signalling cascades that lead to actin polymerization.

Enteropathogenic E. coli translocated intimin receptor, Tir, interacts directly with alpha-actinin

Enteropathogenic Escherichia coli (EPEC) triggers a dramatic rearrangement of the host epithelial cell actin cytoskeleton to form an attaching and effacing lesion, or pedestal. The pathogen remains attached extracellularly to the host cell through the pedestal for the duration of the infection. At the tip of the pedestal is a bacterial protein, Tir, which is secreted from the bacterium into the host cell plasma membrane, where it functions as the receptor for an EPEC outer membrane protein, intimin [1]. Delivery of Tir to the host cell results in its tyrosine phosphorylation, followed by Tir-intimin binding. Tir is believed to anchor EPEC firmly to the host cell, although its direct linkage to the cytoskeleton is unknown. Here, we show that Tir directly binds the cytoskeletal protein alpha-actinin. alpha-Actinin is recruited to the pedestal in a Tir-dependent manner and colocalizes with Tir in infected host cells. Binding is mediated through the amino terminus of Tir. Recruitment of alpha-actinin occurs independently of Tir tyrosine phosphorylation. Recruitment of actin, VASP, and N-WASP, however, is abolished in the absence of this tyrosine phosphorylation. These results suggest that Tir plays at least three roles in the host cell during infection: binding intimin on EPEC; mediating a stable anchor with alpha-actinin through its amino terminus in a phosphotyrosine-independent manner; and recruiting additional cytoskeletal proteins at the carboxyl terminus in a phosphotyrosine-dependent manner. These findings demonstrate the first known direct linkage between extracellular EPEC, through the transmembrane protein Tir, to the host cell actin cytoskeleton via alpha-actinin.