The cell biology of infection by intracellular bacterial pathogens

Prevotella melaninogenica disrupted oral epithelial barrier function via myosin light chain kinase

OBJECTIVE

Our previous studies have found that the composition ratio of Prevotella melaninogenica (Pm) on buccal mucosa surface of oral lichen planus (OLP) patients increased significantly compared with control. Furthermore, Pm could invade the epithelium of OLP patients. This study aimed to further explore the impact of Pm on oral keratinocytes.

MATERIALS AND METHODS

The Pm-human oral keratinocyte (HOK) co-culture model was established to detect monolayer permeability, zona occludens-1 (ZO-1) expression, and intracellular survival of Pm. We performed RNA-seq followed by identification of differentially expressed genes (DEGs) and Gene Ontology (GO) analysis, with a particular focus on myosin light chain kinase (MLCK). An MLCK inhibitor ML-7 was utilized in Pm-HOK co-culture model to assess its effects on monolayer permeability and ZO-1 expression.

RESULTS

HOK monolayer permeability was increased, and ZO-1 expression was decreased after co-culture (p < 0.05). Pm could survive in HOK cells. RNA-seq revealed MLCK was an upregulated common DEG. The expression of MLCK in the Pm-HOK co-culture model was upregulated. Inhibition of MLCK rescued the increased epithelial permeability, and ZO-1 expression was upregulated (p < 0.05).

CONCLUSION

MLCK may be involved in disrupting epithelial barrier function by Pm.

Cytoskeleton as a roadmap navigating rhizobia to establish symbiotic root nodulation in legumes

Legumes enter into symbiotic associations with soil nitrogen-fixing rhizobia, culminating in the creation of new organs, root nodules. This complex process relies on chemical and physical interaction between legumes and rhizobia, including early signalling events informing the host legume plant of a potentially beneficial microbe and triggering the nodulation program. The great significance of this plant-microbe interaction rests upon conversion of atmospheric dinitrogen not accessible to plants into a biologically active form of ammonia available to plants. The plant cytoskeleton consists in a highly dynamic network and undergoes rapid remodelling upon sensing various developmental and environmental cues, including response to attachment, internalization, and accommodation of rhizobia in plant root and nodule cells. This dynamic nature is governed by cytoskeleton-associated proteins that modulate cytoskeletal behaviour depending on signal perception and transduction. Precisely localized cytoskeletal rearrangements are therefore essential for the uptake of rhizobia, their targeted delivery, and establishing beneficial root nodule symbiosis. This review summarizes current knowledge about rhizobia-dependent rearrangements and functions of the cytoskeleton in legume roots and nodules. General patterns and nodule type-, nodule stage-, and species-specific aspects of actin filaments and microtubules remodelling are discussed. Moreover, emerging evidence is provided about fine-tuning the root nodulation process through cytoskeleton-associated proteins. We also consider future perspectives on dynamic localization studies of the cytoskeleton during early symbiosis utilizing state of the art molecular and advanced microscopy approaches. Based on acquired detailed knowledge of the mutualistic interactions with microbes, these approaches could contribute to broader biotechnological crop improvement.

Applications of bacteriophages against intracellular bacteria

Infectious diseases pose a significant threat to both human and animal populations. Intracellular bacteria are a group of pathogens that invade and survive within the interior of eukaryotic cells, which in turn protect them from antibacterial drugs and the host immune system. Limited penetration of antibacterials into host cells results in insufficient bacterial clearance and treatment failure. Bacteriophages have, over the decades, been proved to play an important role in combating bacterial infections (phage therapy), making them an important alternative to classical antibiotic strategies today. Phages have been found to be effective at killing various species of extracellular bacteria, but little is still known about how phages control intracellular infections. With advances in phage genomics and mechanisms of delivery and cell uptake, the development of phage-based antibacterial strategies to address the treatment of intracellular bacteria has general potential. In this review, we present the current state of knowledge regarding the application of bacteriophages against intracellular bacteria. We cover phage deployment against the most common intracellular pathogens with special attention to therapeutic and preventive strategies.

Smuggle tau through a secret(ory) pathway

Secretion of misfolded tau, a microtubule-binding protein enriched in nerve cells, is linked to the progression of tau pathology. However, the molecular mechanisms underlying tau secretion are poorly understood. Recent work by Lee et al. [Biochemical J. (2021) 478: 1471-1484] demonstrated that the transmembrane domains of syntaxin6 and syntaxin8 could be exploited for tau release, setting a stage for testing a novel hypothesis that has profound implications in tauopathies (e.g. Alzheimer's disease, FTDP-17, and CBD/PSP) and other related neurodegenerative diseases. The present commentary highlights the importance and limitations of the study, and discusses opportunities and directions for future investigations.

Human stem cell-based models for studying host-pathogen interactions

The use of human cell lines and primary cells as in vitro models represents a valuable approach to study cellular responses to infection. However, with the advent of new molecular technologies and tools available, there is a growing need to develop more physiologically relevant systems to overcome cell line model limitations and better mimic human disease. Since the discovery of human stem cells, its use has revolutionised the development of in vitro models. This is because after differentiation, these cells have the potential to reflect in vivo cell phenotypes and allow for probing questions in numerous fields of the biological sciences. Moreover, the possibility to combine the advantages of stem cell-derived cell types with genome editing technologies and engineered 3D microenvironments, provides enormous potential for producing in vitro systems to investigate cellular responses to infection that are both relevant and predictive. Here, we discuss recent advances in the use of human stem cells to model host-pathogen interactions, highlighting emerging technologies in the field of stem cell biology that can be exploited to investigate the fundamental biology of infection. TAKE AWAYS: hPSC overcome current limitations to study host-pathogen interactions in vitro. Genome editing can be used in hPSC to study cellular responses to infection. hPSC, 3D models and genome editing can recreate physiological in vitro systems.

Macrophage-microbe interaction: lessons learned from the pathogen Mycobacterium tuberculosis

Macrophages, being the cornerstone of the immune system, have adapted the ancient nutrient acquisition mechanism of phagocytosis to engulf various infectious organisms thereby helping to orchestrate an appropriate host response. Phagocytosis refers to the process of internalization and degradation of particulate material, damaged and senescent cells and microorganisms by specialized cells, after which the vesicle containing the ingested particle, the phagosome, matures into acidic phagolysosomes upon fusion with hydrolytic enzyme-containing lysosomes. The destructive power of the macrophage is further exacerbated through the induction of macrophage activation upon a variety of inflammatory stimuli. Despite being the end-point for many phagocytosed microbes, the macrophage can also serve as an intracellular survival niche for a number of intracellular microorganisms. One microbe that is particularly successful at surviving within macrophages is the pathogen Mycobacterium tuberculosis, which can efficiently manipulate the macrophage at several levels, including modulation of the phagocytic pathway as well as interfering with a number of immune activation pathways that normally would lead to eradication of the internalized bacilli. M. tuberculosis excels at circumventing destruction within macrophages, thus establishing itself successfully for prolonged times within the macrophage. In this contribution, we describe a number of general features of macrophages in the context of their function to clear an infection, and highlight the strategies employed by M. tuberculosis to counter macrophage attack. Interestingly, research on the evasion tactics employed by M. tuberculosis within macrophages not only helps to design strategies to curb tuberculosis, but also allows a better understanding of host cell biology.

The State of the Union Is Strong: a Review of ASM's 6th Conference on Cell-Cell Communication in Bacteria

The 6th American Society for Microbiology Conference on Cell-Cell Communication in Bacteria convened from 16 to 19 October 2017 in Athens, GA. In this minireview, we highlight some of the research presented at that meeting that addresses central questions emerging in the field, including the following questions. How are cell-cell communication circuits designed to generate responses? Where are bacteria communicating? Finally, why are bacteria engaging in such behaviors?

Shifts in the fluorescence lifetime of EGFP during bacterial phagocytosis measured by phase-sensitive flow cytometry

Phase-sensitive flow cytometry (PSFC) is a technique in which fluorescence excited state decay times are measured as fluorescently labeled cells rapidly transit a finely focused, frequency-modulated laser beam. With PSFC the fluorescence lifetime is taken as a cytometric parameter to differentiate intracellular events that are challenging to distinguish with standard flow cytometry. For example PSFC can report changes in protein conformation, expression, interactions, and movement, as well as differences in intracellular microenvironments. This contribution focuses on the latter case by taking PSFC measurements of macrophage cells when inoculated with enhanced green fluorescent protein (EGFP)-expressing E. coli. During progressive internalization of EGFP-E. coli, fluorescence lifetimes were acquired and compared to control groups. It was hypothesized that fluorescence lifetimes would correlate well with phagocytosis because phagosomes become acidified and the average fluorescence lifetime of EGFP is known to be affected by pH. We confirmed that average EGFP lifetimes consistently decreased (3 to 2 ns) with inoculation time. The broad significance of this work is the demonstration of how high-throughput fluorescence lifetime measurements correlate well to changes that are not easily tracked by intensity-only cytometry, which is affected by heterogeneous protein expression, cell-to-cell differences in phagosome formation, and number of bacterium engulfed.

Strategies of Intracellular Pathogens for Obtaining Iron from the Environment

Most microorganisms are destroyed by the host tissues through processes that usually involve phagocytosis and lysosomal disruption. However, some organisms, called intracellular pathogens, are capable of avoiding destruction by growing inside macrophages or other cells. During infection with intracellular pathogenic microorganisms, the element iron is required by both the host cell and the pathogen that inhabits the host cell. This minireview focuses on how intracellular pathogens use multiple strategies to obtain nutritional iron from the intracellular environment in order to use this element for replication. Additionally, the implications of these mechanisms for iron acquisition in the pathogen-host relationship are discussed.

In vitro invasion and survival of Porphyromonas gingivalis in gingival fibroblasts; role of the capsule

Porphyromonas gingivalis is a Gram-negative, anaerobic bacterium involved in periodontitis and peri-implantitis that can invade and survive inside host cells in vitro. P. gingivalis can invade human gingival fibroblasts (GF), but no data are available about the role of P. gingivalis' capsule in GF invasion. In the current study, we aimed to determine the ability of three strains of P. gingivalis (encapsulated wild type W83, non-encapsulated HG91 and the non-encapsulated insertional isogenic knockout mutant of W83, ΔEpsC) to invade GF and the ability of internalized P. gingivalis to survive in vitro antibiotic treatment. The ability of P. gingivalis strains to invade GF was tested using an antibiotic protection assay at multiplicity of infection (MOI) 100 and 1000. The survival of internalized P. gingivalis cells was further analyzed by subsequent in vitro treatment with either metronidazole or amoxicillin alone or a combination of metronidazole and amoxicillin and anaerobic culture viability counts. All strains of P. gingivalis used in this study were able to invade GFs. The non-encapsulated mutant of W83 (ΔEpsC mutant) was significantly more invasive than the wild type W83 at MOI 100 (p value 0.025) and MOI 1000 (p value 0.038). Furthermore, internalized P. gingivalis was able to resist in vitro antibiotic treatment. As demonstrated by the differences in invasion efficiencies of P. gingivalis strain W83 and its isogenic mutant ΔEpsC, the capsule of P. gingivalis makes it less efficient in invading gingival fibroblasts. Moreover, internalized P. gingivalis can survive antibiotic treatment in vitro.

Nanomedicine as an emerging approach against intracellular pathogens

Diseases such as tuberculosis, hepatitis, and HIV/AIDS are caused by intracellular pathogens and are a major burden to the global medical community. Conventional treatments for these diseases typically consist of long-term therapy with a combination of drugs, which may lead to side effects and contribute to low patient compliance. The pathogens reside within intracellular compartments of the cell, which provide additional barriers to effective treatment. Therefore, there is a need for improved and more effective therapies for such intracellular diseases. This review will summarize, for the first time, the intracellular compartments in which pathogens can reside and discuss how nanomedicine has the potential to improve intracellular disease therapy by offering properties such as targeting, sustained drug release, and drug delivery to the pathogen’s intracellular location. The characteristics of nanomedicine may prove advantageous in developing improved or alternative therapies for intracellular diseases.

A novel pseudopodial component of the dendritic cell anti-fungal response: the fungipod

Fungal pathologies are seen in immunocompromised and healthy humans. C-type lectins expressed on immature dendritic cells (DC) recognize fungi. We report a novel dorsal pseudopodial protrusion, the “fungipod”, formed by DC after contact with yeast cell walls. These structures have a convoluted cell-proximal end and a smooth distal end. They persist for hours, exhibit noticeable growth and total 13.7±5.6 µm long and 1.8±0.67 µm wide at the contact. Fungipods contain clathrin and an actin core surrounded by a sheath of cortactin. The actin cytoskeleton, but not microtubules, is required for fungipod integrity and growth. An apparent rearward flow (225±55 nm/second) exists from the zymosan contact site into the distal fungipod. The phagocytic receptor Dectin-1 is not required for fungipod formation, but CD206 (Mannose Receptor) is the generative receptor for these protrusions. The human pathogen Candida parapsilosis induces DC fungipod formation strongly, but the response is species specific since the related fungal pathogens Candida tropicalis and Candida albicans induce very few and no fungipods, respectively. Our findings show that fungipods are dynamic actin-driven cellular structures involved in fungal recognition by DC. They may promote yeast particle phagocytosis by DC and are a specific response to large (i.e., 5 µm) particulate ligands. Our work also highlights the importance of this novel protrusive structure to innate immune recognition of medically significant Candida yeasts in a species specific fashion.

Yeasts are normal microbial commensals of humans and a significant source of opportunistic infections, especially in immunocompromised individuals. We report a novel cellular protrusive structure, the fungipod, which participates in the host-microbe interaction between human immature dendritic cells (DC) and yeasts. The fungipod's structure is based on and propelled by a robust process of local actin cytoskeleton growth at the DC-yeast contact site, and this cytoskeletal remodeling results in a durable tubular structure over 10 µm long connecting the dorsal DC membrane and yeast. The fungal cell wall polysaccharides mannan and chitin trigger fungipod formation by stimulating the carbohydrate pattern recognition receptor CD206. Fungipods are part of a specific response to large particulate objects (i.e., yeast), and they may promote the human immature DC's relatively poor phagocytosis of yeast. The human fungal pathogen, Candida parapsilosis, induces a strong fungipod response from DC, and this response is highly species specific since the related pathogens Candida albicans and Candida tropicalis induce fungipods rarely. Our work highlights a novel cell biological element of fungal recognition by the innate immune system.

Myosin motor function: the ins and outs of actin-based membrane protrusions

Cells build plasma membrane protrusions supported by parallel bundles of F-actin to enable a wide variety of biological functions, ranging from motility to host defense. Filopodia, microvilli and stereocilia are three such protrusions that have been the focus of intense biological and biophysical investigation in recent years. While it is evident that actin dynamics play a significant role in the formation of these organelles, members of the myosin superfamily have also been implicated as key players in the maintenance of protrusion architecture and function. Based on a simple analysis of the physical forces that control protrusion formation and morphology, as well as our review of available data, we propose that myosins play two general roles within these structures: (1) as cargo transporters to move critical regulatory components toward distal tips and (2) as mediators of membrane-cytoskeleton adhesion.

Association of small Rho GTPases and actin ring formation in epithelial cells during the invasion by Candida albicans

Invasion of epithelial cells is a major virulence determinant of Candida albicans; however, the molecular events that occur during invasion are not discerned. This study is aimed to elucidate the role of the host's actin remodeling and involvement of small GTPases during invasion. Actin filaments formed a rigid ring-like structure in the rabbit corneal epithelial cell line SIRC after C. albicans invasion. During invasion, an increase in the mRNA content of Cdc42, Rac1 and RhoA GTPase was observed in SIRC cells. Immunochemical staining and expression of chimeric green fluorescent protein (GFP)-GTPases showed that all three GTPases colocalize at invasion and actin polymerization sites. This colocalization was not seen in SIRC cells expressing a GFP-tagged dominant-negative mutant of GTPases. Inhibition of invasion was observed in SIRC cells expressing dominant-negative mutants of Rac1 and RhoA GTPases. Involvement of zonula occludens-1 (ZO-1) was observed in the process of actin-mediated endocytosis of C. albicans. Actin, GTPases and ZO-1 were colocalized in epithelial cells during uptake of polymethylmethacrylate beads coated with spent medium from a C. albicans culture. The results indicate that host actin remodeling and recruitment of small GTPases occur during invasion and molecules that are shed or secreted by C. albicans are probably responsible for cytoskeletal reorganization.

New opto-plasmonic tweezers for manipulation and rotation of biological cells--design and fabrication

Opto-Plasmonic Tweezers are proposed as a new optical manipulator and rotator for biological cells. The approach utilizes polarized light to excite localized surface plasmon resonance (LSPR) on an array of Au nanostructure. Large dielectrophoretic trapping force is expected to be induced by the highly non-uniform scattering field from the resonant oscillating dipoles. Fine orientation control of the cells can be realized by tuning the polarization state of the input light.

Mechanisms of host cell exit by the intracellular bacterium Chlamydia

The mechanisms that mediate the release of intracellular bacteria from cells are poorly understood, particularly for those that live within a cellular vacuole. The release pathway of the obligate intracellular bacterium Chlamydia from cells is unknown. Using a GFP-based approach to visualize chlamydial inclusions within cells by live fluorescence videomicroscopy, we identified that Chlamydia release occurred by two mutually exclusive pathways. The first, lysis, consisted of an ordered sequence of membrane permeabilizations: inclusion, nucleus and plasma membrane rupture. Treatment with protease inhibitors abolished inclusion lysis. Intracellular calcium signaling was shown to be important for plasma membrane breakdown. The second release pathway was a packaged release mechanism, called extrusion. This slow process resulted in a pinching of the inclusion, protrusion out of the cell within a cell membrane compartment, and ultimately detachment from the cell. Treatment of Chlamydia-infected cells with specific pharmacological inhibitors of cellular factors demonstrated that extrusion required actin polymerization, neuronal Wiskott-Aldrich syndrome protein, myosin II and Rho GTPase. The participation of Rho was unique in that it functioned late in extrusion. The dual nature of release characterized for Chlamydia has not been observed as a strategy for intracellular bacteria.

Role of Protein Phosphorylation on Serine/Threonine and Tyrosine in the Virulence of Bacterial Pathogens

Bacterial pathogens have developed a diversity of strategies to interact with host cells, manipulate their behaviors, and thus to survive and propagate. During the process of pathogenesis, phosphorylation of proteins on hydroxyl amino acids (serine, threonine, tyrosine) occurs at different stages, including cell-cell interaction and adherence, translocation of bacterial effectors into host cells, and changes in host cellular structure and function induced by infection. The phosphorylation reactions are catalyzed in a reversible fashion by specific protein kinases and phosphatases that belong to either the invading bacterial cells or the infected eukaryotic host cells. Among the various virulence factors involved in bacterial pathogenesis, special attention has been paid recently to the cell wall components, exopolysaccharides. A major breakthrough has been made by showing the existence of a biological link between the activity of certain protein-tyrosine kinases/phosphatases and the production and/or transport of surface polysaccharides. In addition, genetic studies have revealed a key role played by some serine/threonine kinases in pathogenesis. Considering the structural organization and membrane topology of these different kinases, it can be envisaged that they operate as one-component systems in signal transduction pathways, in the form of single proteins containing input and output domains on the same polypeptide chain. From a general standpoint, the demonstration of a direct relationship between protein phosphorylation on serine/threonine/tyrosine and bacterial virulence represents a novel concept of great importance in deciphering the molecular and cellular mechanisms that underlie pathogenesis.

Production of IFN-gamma by CD4 T cells is essential for resolving ehrlichia infection

To address the role of cellular immunity during ehrlichia infection, we have used a newly described model of monocytic ehrlichiosis that results from infection of mice by an ehrlichia that was isolated from an Ixodes ovatus tick (Ixodes ovatus ehrlichia, IOE). Immunocompetent C57BL/6 and BALB/c mice exhibited a dose-dependent susceptibility to IOE infection. Mice infected with a high dose inoculum ( approximately 1000 organisms) exhibited pronounced thrombocytopenia, lymphopenia, anemia, and morbidity within 12 days postinfection. Infection was associated with bacterial colonization of a number of tissues. In contrast, mice infected with a low dose inoculum ( approximately 100 organisms) exhibited only transient disease and were able to resolve the infection. SCID mice were highly susceptible to low-dose infection, indicating that adaptive immunity was required. Resistance to sublethal challenge in both C57BL/6 and BALB/c mice was CD4-, but not CD8-, dependent and required IL-12p40-dependent cytokines, IFN-gamma, and TNF-alpha, but not IL-4. CD4 T cells purified from infected mice proliferated in vitro in response to IOE Ags. T cell proliferation was associated with production of IFN-gamma, and the production of this cytokine by CD4 T cells rescued IFN-gamma-deficient mice from fatal infection. Exogenous IFN-gamma was capable of inducing microbiocidal activity in infected macrophages. The data suggest that classical immune mechanisms involving CD4 cells and type 1 cytokines are responsible for macrophage activation and for elimination of this intracellular bacterial pathogen.

Kinome analysis of host response to mycobacterial infection: a novel technique in proteomics

An array of mammalian phospho-specific antibodies was used to screen for a host response upon mycobacterial infection, reflected as changes in host protein phosphorylation. Changes in the phosphorylation state of 31 known signaling molecules were tracked after infection with live or heat killed Mycobacterium bovis BCG or after incubation with the mycobacterial cell wall component lipoarabinomannan (LAM). Mycobacterial infection triggers a signaling cascade leading to activation of stress-activated protein kinase and its subsequent downstream target, c-Jun. Mycobacteria were also shown to inhibit the activation of protein kinase C epsilon and to induce phosphorylation of proteins not yet known to be involved in mycobacterial infection, such as the cytoskeletal protein alpha-adducin, glycogen synthase kinase 3beta, and a receptor subunit involved in regulation of intracellular Ca(2+) levels. The mycobacterial cell wall component LAM has been identified as a trigger for some of these modulation events.

Expression of Th1-mediated immunity in mouse lungs induces a Mycobacterium tuberculosis transcription pattern characteristic of nonreplicating persistence

The lung is the primary target of infection with Mycobacterium tuberculosis. It is well established that, in mouse lung, expression of adaptive, Th1-mediated host immunity inhibits further multiplication of M. tuberculosis. Here, real-time RT-PCR was used to define the pattern of expression against time of lung infection of key genes involved in Th1-mediated immunity and of selected genes of M. tuberculosis. Inhibition of bacterial multiplication was preceded by increased mRNA synthesis for IFN-gamma and inducible NO synthase (NOS2) and by NOS2 protein synthesis in infected macrophages. Concurrently, the pattern of transcription of bacterial genes underwent dramatic changes. mRNA synthesis increased for alpha-crystallin (acr), rv2626c, and rv2623 and decreased for superoxide dismutase C (sodC), sodA, and fibronectin-binding protein B (fbpB). This pattern of M. tuberculosis transcription is characteristic of the nonreplicating persistence [Wayne, L. G. & Sohaskey, C. D. (2001) Annu. Rev. Microbiol. 55, 139-163] associated with adaptation of tubercle bacilli to hypoxia in vitro. Based on this similarity, we infer that host immunity induces bacterial growth arrest. In IFN-gamma gene-deleted mice, bacterial growth was not controlled; NOS2 protein was not detected in macrophages; sodC, sodA, and fbpB transcription showed no decrease; and acr, rv2626c, and rv2623 transcription increased only at the terminal stages of lung pathology. These findings define the transcription signature of M. tuberculosis as it transitions from growth to persistence in the mouse lung. The bacterial transcription changes measured at onset of Th1-mediated immunity are likely induced, directly or indirectly, by nitric oxide generated by infected macrophages.

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.

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.

Molecular cloning, sequence and characterization of cjsT, a putative protease from Rickettsia rickettsii

The cloning and sequencing of a gene from Rickettsia rickettsii which confers haemolytic activity on Escherichia coli strain TB1 is described. The open reading frame of the haemolysis-promoting gene, cjsT, is 1041 bp and encodes a putative protein with a molecular mass of 33 825 Da. CjsT has high sequence similarity to several bacterial proteases, particularly type IV signal peptidases. Cell lysates from an E. coli clone containing cjsT in pUC19 (pJON1) exhibited greater protease activity in functional assays than found in E. coli containing pUC19 alone. Disruption of the cjsT gene by insertional inactivation with a kanamycin cassette reduced both the protease and haemolytic activities conferred by cjsT. The protease inhibitors antipain and diisopropylfluorophosphate (DFP) both reduced the proteolytic activity of pJON1. The mechanism by which the R. rickettsii cjsT promotes haemolysis in E. coli remains unclear.

How the assembly dynamics of the nematode major sperm protein generate amoeboid cell motility

Nematode sperm are amoeboid cells that use a major sperm protein (MSP) cytoskeleton in place of a conventional actin cytoskeleton to power their amoeboid motility. In these simple, specialized cells cytoskeletal dynamics is tightly coupled to locomotion. Studies have capitalized on this feature to explore the key structural properties of MSP and to reconstitute motility both in vivo and in vitro. This review discusses how the mechanistic properties shared by the MSP machinery and actin-based motility systems lead to a "push-pull" mechanism for amoeboid cell motility in which cytoskeletal assembly and disassembly at opposite ends of the lamellipodium are associated with independent forces for protrusion of the leading edge and retraction of the cell body.

Relationships between IFNgamma, IL-6, corticosterone, and Listeria monocytogenes pathogenesis in BALB/c mice

The relationships between Listeria monocytogenes (LM) pathogenesis, based on bacterial load, and serum levels of IL-6, IFNgamma, and corticosterone (CORT) were quantified. Serum IFNgamma levels increased along with the LM burden; however, with LM burdens > or =3 x 10(6) CFU per spleen, the serum IFNgamma level decreased along with a decrease in splenic weight. Serum IL-6 levels exponentially increased with increases of LM, and the CORT level positively correlated with the increase in IL-6 and LM. The serum level of IFNgamma appeared to be a good biomarker of the host's ability to combat the infection only when the LM burden did not exceed a critical level (>3 x 10(6) CFU per spleen). Interestingly, the LM load at which the IFNgamma level began to decline was near the dose at which the IL-6 concentration exponentially increased, suggesting a transition point shift from stress (assessed as CORT level) being immunoenhancing to becoming immunosuppressive. The IL-6:IFNgamma ratio may be a good indicator of disease severity and/or the ability to cope with an infection.

Secrets of actin-based motility revealed by a bacterial pathogen

Actin-based cell motility is a complex process involving a dynamic, self-organizing cellular system. Experimental problems initially limited our understanding of this type of motility, but the use of a model system derived from a bacterial pathogen has led to a breakthrough. Now, all the molecular components necessary for dynamic actin self-organization and motility have been identified, setting the stage for future mechanistic studies.

The actin-driven movement and formation of acetylcholine receptor clusters

A new method was devised to visualize actin polymerization induced by postsynaptic differentiation signals in cultured muscle cells. This entails masking myofibrillar filamentous (F)-actin with jasplakinolide, a cell-permeant F-actin-binding toxin, before synaptogenic stimulation, and then probing new actin assembly with fluorescent phalloidin. With this procedure, actin polymerization associated with newly induced acetylcholine receptor (AChR) clustering by heparin-binding growth-associated molecule-coated beads and by agrin was observed. The beads induced local F-actin assembly that colocalized with AChR clusters at bead-muscle contacts, whereas both the actin cytoskeleton and AChR clusters induced by bath agrin application were diffuse. By expressing a green fluorescent protein-coupled version of cortactin, a protein that binds to active F-actin, the dynamic nature of the actin cytoskeleton associated with new AChR clusters was revealed. In fact, the motive force generated by actin polymerization propelled the entire bead-induced AChR cluster with its attached bead to move in the plane of the membrane. In addition, actin polymerization is also necessary for the formation of both bead and agrin-induced AChR clusters as well as phosphotyrosine accumulation, as shown by their blockage by latrunculin A, a toxin that sequesters globular (G)-actin and prevents F-actin assembly. These results show that actin polymerization induced by synaptogenic signals is necessary for the movement and formation of AChR clusters and implicate a role of F-actin as a postsynaptic scaffold for the assembly of structural and signaling molecules in neuromuscular junction formation.

Expression patterns of E-cadherin, involucrin, and connexin gap junction proteins in the lining epithelia of inflamed gingiva

The structural integrity and functional differentiation of the lining epithelium were studied in relation to inflammatory changes associated with destructive periodontitis. In the different regions of lining epithelia from clinically healthy gingiva and periodontitis, comparisons were made of the expression patterns of E-cadherin, which is critical in intercellular adhesion; of proteins associated with gap junction communication channels; and of involucrin, which is a key marker of differentiation in stratified epithelia. Filamentous actin (F-actin), which is important in cell structural integrity, attachment, and migration, was also examined. Semiquantitative immunohistochemical analysis revealed that in both clinically healthy gingiva and lesions of advanced periodontitis, expression patterns of E-cadherin, involucrin, and connexins 26 and 43 were similar, with a statistically significant reduction in staining intensity from the external oral epithelium, through the gingival sulcus, to the junctional epithelium or pocket epithelium, respectively. Furthermore, there was a striking reduction in staining for E-cadherin, involucrin, and both connexins in the pathological lining epithelium of the periodontal pocket. These changes were associated with marked alterations of filamentous actin expression, collectively indicating profound perturbation of the epithelial structure. The data reported support the concept that the ability of the pathological lining epithelium to function as an effective barrier against the ingress of microbial products into the tissues is severely compromised.

Burkholderia pseudomallei induces cell fusion and actin-associated membrane protrusion: a possible mechanism for cell-to-cell spreading

Burkholderia pseudomallei, a facultative intracellular bacterium, is the causative agent of a broad spectrum of diseases collectively known as melioidosis. Its ability to survive inside phagocytic and nonphagocytic cells and to induce multinucleated giant cell (MNGC) formation has been demonstrated. This study was designed to assess a possible mechanism(s) leading to this cellular change, using virulent and nonvirulent strains of B. pseudomallei to infect both phagocytic and nonphagocytic cell lines. We demonstrated that when the cells were labeled with two different cell markers (CMFDA or CMTMR), mixed, and then infected with B. pseudomallei, direct cell-to-cell fusion could be observed, leading to MNGC formation. Staining of the infected cells with rhodamine-conjugated phalloidin indicated that immediately after the infection, actin rearrangement into a comet tail appearance occurred, similar to that described earlier for other bacteria. The latter rearrangement led to the formation of bacterium-containing, actin-associated membrane protrusions which could lead to a direct cell-to-cell spreading of B. pseudomallei in the infected hosts. Results from 4', 6'-diamidine-2-phenylindole dihydrochloride (DAPI) nuclear staining, poly-ADP ribose polymerase cleavage, staining of infected cells for phosphatidylserine exposure with annexin V, and electrophoresis of the DNA extracted from these infected cells showed that B. pseudomallei could kill the host cells by inducing apoptosis in both phagocytic and nonphagocytic cells.

Reconstitution of Listeria motility: implications for the mechanism of force transduction

Listeria monocytogenes and some other infectious bacteria polymerize their host cell's actin into tails that propel the bacteria through the cytoplasm. Here we show that reconstitution of this behavior in simpler media resolves two aspects of the mechanism of force transduction. First, since dilute reconstitution media have no cytoskeleton, we consider what keeps the tail from being pushed backward rather than the bacterium being propelled forward. The dependence of the partitioning of motion on the friction coefficient of the tail is derived. Consistent with experiments, we find that the resistance of the tail to motion is sensitive to its length. That even small tails are stationary in intact cells is attributed to anchoring to the cytoskeleton. Second, the comparatively low viscosity of some reconstitution media magnifies the effects of diffusion, such that a large gap will develop between the bacterium and its tail if they are unattached. At the viscosities of diluted platelet extracts, steady-state gaps of several bacterium lengths are predicted. Since such gaps are not observed, we conclude that Listeria must be attached to their tails. We consider what purposes such attachments might serve under physiological conditions. The implications for related pathogens and amoeboid locomotion are also discussed.

Actin machinery: pushing the envelope

The reconstitution of microbial rocketing motility in vitro with purified proteins has recently established definitively that no myosin motor is required for protrusion. Instead, actin polymerization, in conjunction with a small number of proteins, is sufficient. A dendritic pattern of nucleation controlled by the Arp2/3 complex provides an efficient pushing force for lamellipodial motility.

The polymerization motor

Polymerization and depolymerization of actin filaments and microtubules are thought to generate force for movement in various kinds of cell motility, ranging from lamellipodial protrusion to chromosome segregation. This article reviews the thermodynamic and physical theories of how a nonequilibrium polymerization reaction can be used to transduce chemical energy into mechanical energy, and summarizes the evidence suggesting that actin polymerization produces motile force in several biological systems.

Proteins of the ADF/cofilin family: essential regulators of actin dynamics

Ubiquitous among eukaryotes, the ADF/cofilins are essential proteins responsible for the high turnover rates of actin filaments in vivo. In vertebrates, ADF and cofilin are products of different genes. Both bind to F-actin cooperatively and induce a twist in the actin filament that results in the loss of the phalloidin-binding site. This conformational change may be responsible for the enhancement of the off rate of subunits at the minus end of ADF/cofilin-decorated filaments and for the weak filament-severing activity. Binding of ADF/cofilin is competitive with tropomyosin. Other regulatory mechanisms in animal cells include binding of phosphoinositides, phosphorylation by LIM kinases on a single serine, and changes in pH. Although vertebrate ADF/cofilins contain a nuclear localization sequence, they are usually concentrated in regions containing dynamic actin pools, such as the leading edge of migrating cells and neuronal growth cones. ADF/cofilins are essential for cytokinesis, phagocytosis, fluid phase endocytosis, and other cellular processes dependent upon actin dynamics.

An analysis of actin delivery in the acrosomal process of thyone

The acrosomal process of the sea cucumber Thyone briareus can extend 90 microm in 10 s, but an epithelial goldfish keratocyte can only glide a few microns in the same time. Both speeds reflect the rate of extension of an actin network. The difference is in the delivery of actin monomers to the polymerization region. Diffusion supplies monomers fast enough to support the observed speed of goldfish keratocytes, but previous models have indicated that the acrosomal process of Thyone extends too rapidly for diffusion to keep up. Here we reexamine the assumptions made in earlier models and present a new model, the Actin Reconcentration Model, that includes more biological detail. Salt and water fluxes during the acrosomal reaction and the nonideality of the cytoplasm are particularly significant for actin delivery. We find that the variability of the acrosomal growth curve can be explained by the salt and water fluxes, and that nonideality magnifies the effect of actin concentration changes. We calculate the speed of process growth using biologically relevant parameters from the literature and find that the predictions of the model fall among the experimental data.

Role of actin-filament disassembly in lamellipodium protrusion in motile cells revealed using the drug jasplakinolide

BACKGROUND

In motile cells, protrusion of the lamellipodium (a type of cell margin) requires assembly of actin monomers into actin filaments at the tip of the lamellipodium. The importance of actin-filament disassembly in this process is less well understood, and is assessed here using the actin drug jasplakinolide, which has two known activities - inhibition of filament disassembly and induction of an increase in actin polymer.

RESULTS

In cells the two activities of jasplakinolide were found to be separable; 1 microM jasplakinolide could permeate cells, bind cellular filamentous actin (F-actin) and inhibit filament disassembly within 3.5 minutes, but significant increase in actin polymer was not detected until 60 minutes of treatment. In live, permeabilised cells, jasplakinolide did not inhibit filament assembly from supplied, purified actin monomers. In migrating chick fibroblasts, lamellipodium protrusion was blocked within 1-5 minutes of treatment with 1 microM jasplakinolide, without any perturbation of actin organisation. In non-migrating chick fibroblasts, there was a delay in the onset of jasplakinolide-induced inhibition of lamellipodium protrusion, during which lamellipodium length increased linearly with no increase in protrusion rate. Motility of the bacterium Listeria in infected PtK2 cells was reduced 2.3-fold within 3 minutes of treatment with 1 microM jasplakinolide.

CONCLUSIONS

Actin-filament disassembly is tightly coupled to lamellipodium protrusion in migrating chick fibroblasts and motility of Listeria in PtK2 cells. One simple interpretation of these data is a situation whereby ongoing actin-filament assembly uses free actin monomer derived from filament disassembly, in preference to stored monomer.

Localized depolymerization of the major sperm protein cytoskeleton correlates with the forward movement of the cell body in the amoeboid movement of nematode sperm

The major sperm protein (MSP)-based amoeboid motility of Ascaris suum sperm requires coordinated lamellipodial protrusion and cell body retraction. In these cells, protrusion and retraction are tightly coupled to the assembly and disassembly of the cytoskeleton at opposite ends of the lamellipodium. Although polymerization along the leading edge appears to drive protrusion, the behavior of sperm tethered to the substrate showed that an additional force is required to pull the cell body forward. To examine the mechanism of cell body movement, we used pH to uncouple cytoskeletal polymerization and depolymerization. In sperm treated with pH 6.75 buffer, protrusion of the leading edge slowed dramatically while both cytoskeletal disassembly at the base of the lamellipodium and cell body retraction continued. At pH 6.35, the cytoskeleton pulled away from the leading edge and receded through the lamellipodium as its disassembly at the cell body continued. The cytoskeleton disassembled rapidly and completely in cells treated at pH 5.5, but reformed when the cells were washed with physiological buffer. Cytoskeletal reassembly occurred at the lamellipodial margin and caused membrane protrusion, but the cell body did not move until the cytoskeleton was rebuilt and depolymerization resumed. These results indicate that cell body retraction is mediated by tension in the cytoskeleton, correlated with MSP depolymerization at the base of the lamellipodium.

Dynamics of actin-based movement by Rickettsia rickettsii in vero cells

Actin-based motility (ABM) is a virulence mechanism exploited by invasive bacterial pathogens in the genera Listeria, Shigella, and Rickettsia. Due to experimental constraints imposed by the lack of genetic tools and their obligate intracellular nature, little is known about rickettsial ABM relative to Listeria and Shigella ABM systems. In this study, we directly compared the dynamics and behavior of ABM of Rickettsia rickettsii and Listeria monocytogenes. A time-lapse video of moving intracellular bacteria was obtained by laser-scanning confocal microscopy of infected Vero cells synthesizing beta-actin coupled to green fluorescent protein (GFP). Analysis of time-lapse images demonstrated that R. rickettsii organisms move through the cell cytoplasm at an average rate of 4.8 +/- 0.6 micrometer/min (mean +/- standard deviation). This speed was 2.5 times slower than that of L. monocytogenes, which moved at an average rate of 12.0 +/- 3.1 micrometers/min. Although rickettsiae moved more slowly, the actin filaments comprising the actin comet tail were significantly more stable, with an average half-life approximately three times that of L. monocytogenes (100.6 +/- 19.2 s versus 33.0 +/- 7.6 s, respectively). The actin tail associated with intracytoplasmic rickettsiae remained stationary in the cytoplasm as the organism moved forward. In contrast, actin tails of rickettsiae trapped within the nucleus displayed dramatic movements. The observed phenotypic differences between the ABM of Listeria and Rickettsia may indicate fundamental differences in the mechanisms of actin recruitment and polymerization.

Genomic clues for defining bacterial pathogenicity

Genomic sequences are becoming available from both pathogenic and nonpathogenic bacteria. Here we analyze an increasing body of information available on the molecular mechanisms Salmonella typhimurium uses to cause disease, in order to divine clues for identifying sequences that play a role in pathogenesis in other bacterial pathogens.

Actinobacillus actinomycetemcomitans may utilize either actin-dependent or actin-independent mechanisms of invasion

Actinobacillus actinomycetemcomitans is an important pathogen implicated in juvenile and adult periodontal diseases. An important virulence factor of A. actinomycetemcomitans is the ability to invade human oral epithelial cells. A clinical isolate, A. actinomycetemcomitans SUNY 465, has previously been shown to enter epithelial cells by an actin-dependent mechanism. The internalized bacteria are surrounded by an actin halo upon entry. These data are consistent with the mode of entry associated with many enteric pathogens. We tested the effects of cytochalasin D, an inhibitor of the actin microfilament network, on bacterial entry to determine whether this mode of entry was common to other A. actinomycetemcomitans clinical isolates. Cytochalasin D was added prior to infection. A. actinomycetemcomitans SUNY 523 and A. actinomycetemcomitans 4065 exhibited enhanced ability to enter epithelial cells in the presence of cytochalasin D. Immunofluorescent labeling of bacteria and host cell actin confirmed that actin was not being mobilized by the entry of A. actinomycetemcomitans SUNY 523. Inhibitors of receptor-mediated endocytosis inhibited invasion of A. actinomycetemcomitans SUNY 523 and A. actinomycetemcomitans 4065. Microtubule effectors did not inhibit invasion of A. actinomycetemcomitans. A. actinomycetemcomitans SUNY 523, but not A. actinomycetemcomitans 4065, was deficient in exit from epithelial cells as determined by the absence of organisms in the assay medium. These data suggest that A. actinomycetemcomitans strains utilize at least two distinct mechanisms for entry into epithelial cells, and that A. actinomycetemcomitans SUNY 523 may be defective in exit and cell-to-cell spread.

A role for host phosphoinositide 3-kinase and cytoskeletal remodeling during Cryptosporidium parvum infection

Cryptosporidium parvum preferentially infects epithelial cells lining the intestinal mucosa of mammalian hosts. Parasite development and propagation occurs within a unique intracellular but extracytoplasmic parasitophorous vacuole at the apical surface of infected cells. Parasite-induced host cell signaling events and subsequent cytoskeletal remodeling were investigated by using cultured bovine fallopian tube epithelial (BFTE) cells inoculated with C. parvum sporozoites. Indirect-immunofluorescence microscopy detected host tyrosine phosphorylation within 30 s of inoculation. At >30 min postinoculation, actin aggregates were detected at the site of parasite attachment by fluorescein isothiocyanate-conjugated phalloidin staining as well as by indirect immunolabeling with monoclonal anti-actin. The actin-binding protein villin was also detected in focal aggregates at the site of attachment. Host cytoskeletal rearrangement persisted for the duration of the parasitophorous vacuole and contributed to the formation of long, branched microvilli clustered around the cryptosporidial vacuole. The phosphoinositide 3-kinase inhibitor wortmannin significantly inhibited (P < 0.05) C. parvum infection when BFTE cells were pretreated for 60 min at 37 degreesC prior to inoculation. Similarly, treatment of BFTE cells with the protein kinase inhibitors genistein and staurosporine and the cytoskeletally acting compounds 1-(5-iodonaphthalene-1-sulfonyl)-1H-hexahydro-1,4-diazapine, cytochalasin D, and 2,3-butanedione monoxime significantly inhibited (P < 0.05) in vitro infection at 24 h postinoculation. These findings demonstrate a prominent role for phosphoinositide 3-kinase activity during the early C. parvum infection process and suggest that manipulation of host signaling pathways results in actin rearrangement at the site of sporozoite attachment.

Intracellular pathogens and the actin cytoskeleton

Many pathogens actively exploit the actin cytoskeleton during infection. This exploitation may take place during entry into mammalian cells after engagement of a receptor and/or as series of signaling events culminating in the engulfment of the microorganism. Although actin rearrangements are a common feature of most internalization events (e.g. entry of Listeria, Salmonella, Shigella, Yersinia, Neisseria, and Bartonella), bacterial and other cellular factors involved in entry are specific to each bacterium. Another step during which pathogens harness the actin cytoskeleton takes place in the cytosol, within which some bacteria (Listeria, Shigella, Rickettsia) or viruses (vaccinia virus) are able to move. Movement is coupled to a polarized actin polymerization process, with the formation of characteristic actin tails. Increasing attention has focused on this phenomenon due to its striking similarity to cellular events occurring at the leading edge of locomoting cells. Thus pathogens are convenient systems in which to study actin cytoskeleton rearrangements in response to stimuli at the plasma membrane or inside cells.

Solution structure of the motile major sperm protein (MSP) of Ascaris suum - evidence for two manganese binding sites and the possible role of divalent cations in filament formation

The major sperm protein (MSP) of Ascaris suum mediates amoeboid motility by forming an extensive intermeshed system of cytoskeletal filaments analogous to that formed by actin in many other amoeboid cells. MSP is a dimeric molecule that polymerizes to form non-polar filaments constructed from two helical subfilaments that wind round one another. Moreover, MSP filaments can interact with one another to form higher-order assemblies without requiring the range of accessory proteins usually employed in actin-based systems. A knowledge of how MSP polymerizes and forms the hierarchical series of helical MSP macromolecular assemblies is fundamental to understanding locomotion in these cells. Here we describe the solution structure of MSP dimers determined by NMR spectroscopy under conditions where MSP does not polymerize to form filaments. The solution structure is indistinguishable from that observed in putative MSP subfilament helices by X-ray crystallography, indicating that MSP polymerization is not accompanied by a major conformational change. We also show that the rate of MSP polymerization associated with movement of vesicles in an in vitro motility assay is enhanced by the presence of magnesium and manganese ions and use NMR to show that the primary residues that bind these ions are 24-25 and 83-86. These residues are distant from the interface formed between MSP dimers in subfilament helices, and so are probably not involved in this level of polymerization. Instead the manganese and magnesium ion binding appears to be associated with the assembly of subfilaments into filaments and their subsequent aggregation into bundles.