Vimentin regulates mitochondrial ROS production and inflammatory responses of neutrophils.

The intermediate filament vimentin is present in immune cells and is implicated in proinflammatory immune responses. Whether and how it supports antimicrobial activities of neutrophils is not well established. Here, we developed an immortalized neutrophil model to examine the requirement of vimentin. We demonstrate that vimentin restricts the production of proinflammatory cytokines and reactive oxygen species (ROS), but enhances phagocytosis and swarming. We observe that vimentin is dispensable for neutrophil extracellular trap (NET) formation, degranulation, and inflammasome activation. Moreover, gene expression analysis demonstrated that the presence of vimentin was associated with changes in expression of multiple genes required for mitochondrial function and ROS overproduction. Treatment of wild-type cells with rotenone, an inhibitor for complex I of the electron transport chain, increases the ROS levels. Likewise, treatment with mitoTEMPO, a SOD mimetic, rescues the ROS production in cells lacking vimentin. Together, these data show vimentin regulates neutrophil antimicrobial functions and alters ROS levels through regulation of mitochondrial activity.

Pushing boundaries: mechanisms enabling bacterial pathogens to spread between cells.

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

Synaptopodin is necessary for Shigella flexneri intercellular spread.

For many intracellular pathogens, their virulence depends on an ability to spread between cells of an epithelial layer. For intercellular spread to occur, these pathogens deform the plasma membrane into a protrusion structure that is engulfed by the neighboring cell. Although the polymerization of actin is essential for spread, how these pathogens manipulate the actin cytoskeleton in a manner that enables protrusion formation is still incompletely understood. Here, we identify the mammalian actin binding protein synaptopodin as required for efficient intercellular spread. Using a model cytosolic pathogen, Shigella flexneri , we show that synaptopodin contributes to organization of actin around bacteria and increases the length of the actin tail at the posterior pole of the bacteria. We show that synaptopodin presence enables protrusions to form and to resolve at a greater rate, indicating that greater stability of the actin tail enables the bacteria to push against the membrane with greater force. We demonstrate that synaptopodin recruitment around bacteria requires the bacterial protein IcsA, and we show that this recruitment is further enhanced in a type 3 secretion system dependent manner. These data establish synaptopodin as required for intracellular bacteria to reprogram the actin cytoskeleton in a manner that enables efficient protrusion formation and enhance our understanding of the cellular function of synaptopodin. Authors Summary: Intercellular spread is essential for many cytosolic dwelling pathogens during their infectious life cycle. Despite knowing the steps required for intercellular spread, relatively little is known about the host-pathogen interactions that enable these steps to occur. Here, we identify a requirement for the actin binding protein synaptopodin during intercellular spread by cytosolic bacteria. We show synaptopodin is necessary for the stability and recruitment of polymerized actin around bacteria. We also demonstrate synaptopodin is necessary to form plasma membrane structures known as protrusions that are necessary for the movement of these bacteria between cells. Thus, these findings implicate synaptopodin as an important actin-binding protein for the virulence of intracellular pathogens that require the actin cytoskeleton for their spread between cells.

Evaluation of Escherichia coli Inactivation at High Altitudes Using Solar Water Disinfection.

INTRODUCTION: Solar disinfection (SODIS) is an effective method for microbiologic inactivation of contaminated water using ultraviolet rays at low elevations. The aim of this study was to determine the effectiveness of SODIS at higher elevations. METHODS: The ability of SODIS to inactivate Escherichia coli bacteria was evaluated at an altitude of ≥1600 m using Nalgene bottles, disposable plastic water bottles, and Ziploc plastic bags. Bacterial viability was determined through measurement of colony forming units (CFUs). Decreases in CFUs were determined at each time point relative to those at the baseline, and a multivariable regression analysis was used to assess significant changes in CFUs. RESULTS: Bacterial CFUs in exposed containers decreased by >5 log after 6 h of exposure to sunlight. In contrast, the CFUs remained nearly unchanged in unexposed containers, showing a mean decrease of 0.3 log. By 2 h, bacterial inactivation at high altitudes was 1.7-fold greater than that at lower altitudes (P<0.05). By 6 h, nearly all bacteria were inactivated at high or low altitudes. At 6 h, no statistical difference was observed in the efficiency of inactivation between elevations. Compared with Nalgene bottles, plastic bottles had a 1.4-fold greater decrease in CFUs (P<0.05). No statistical difference in bacterial inactivation was found between plastic bottles and plastic bags. CONCLUSIONS: At high altitudes, SODIS is an effective method for inactivating E coli. Further research investigating other microorganisms is warranted to determine whether SODIS is suitable for disinfecting contaminated water at high altitudes.

Longitudinal characterization of circulating neutrophils uncovers phenotypes associated with severity in hospitalized COVID-19 patients.

Mechanisms of neutrophil involvement in severe coronavirus disease 2019 (COVID-19) remain incompletely understood. Here, we collect longitudinal blood samples from 306 hospitalized COVID-19+ patients and 86 controls and perform bulk RNA sequencing of enriched neutrophils, plasma proteomics, and high-throughput antibody profiling to investigate relationships between neutrophil states and disease severity. We identify dynamic switches between six distinct neutrophil subtypes. At days 3 and 7 post-hospitalization, patients with severe disease display a granulocytic myeloid-derived suppressor cell-like gene expression signature, while patients with resolving disease show a neutrophil progenitor-like signature. Humoral responses are identified as potential drivers of neutrophil effector functions, with elevated severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-specific immunoglobulin G1 (IgG1)-to-IgA1 ratios in plasma of severe patients who survived. In vitro experiments confirm that while patient-derived IgG antibodies induce phagocytosis in healthy donor neutrophils, IgA antibodies predominantly induce neutrophil cell death. Overall, our study demonstrates a dysregulated myelopoietic response in severe COVID-19 and a potential role for IgA-dominant responses contributing to mortality.

Alveolar, Endothelial, and Organ Injury Marker Dynamics in Severe COVID-19.

Rationale: Alveolar and endothelial injury may be differentially associated with coronavirus disease (COVID-19) severity over time. Objectives: To describe alveolar and endothelial injury dynamics and associations with COVID-19 severity, cardiorenovascular injury, and outcomes. Methods: This single-center observational study enrolled patients with COVID-19 requiring respiratory support at emergency department presentation. More than 40 markers of alveolar (including receptor for advanced glycation endproducts [RAGE]), endothelial (including angiopoietin-2), and cardiorenovascular injury (including renin, kidney injury molecule-1, and troponin-I) were serially compared between invasively and spontaneously ventilated patients using mixed-effects repeated-measures models. Ventilatory ratios were calculated for intubated patients. Associations of biomarkers with modified World Health Organization scale at Day 28 were determined with multivariable proportional-odds regression. Measurements and Main Results: Of 225 patients, 74 (33%) received invasive ventilation at Day 0. RAGE was 1.80-fold higher in invasive ventilation patients at Day 0 (95% confidence interval [CI], 1.50-2.17) versus spontaneous ventilation, but decreased over time in all patients. Changes in alveolar markers did not correlate with changes in endothelial, cardiac, or renal injury markers. In contrast, endothelial markers were similar to lower at Day 0 for invasive ventilation versus spontaneous ventilation, but then increased over time only among intubated patients. In intubated patients, angiopoietin-2 was similar (fold difference, 1.02; 95% CI, 0.89-1.17) to nonintubated patients at Day 0 but 1.80-fold higher (95% CI, 1.56-2.06) at Day 3; cardiorenovascular injury markers showed similar patterns. Endothelial markers were not consistently associated with ventilatory ratios. Endothelial markers were more often significantly associated with 28-day outcomes than alveolar markers. Conclusions: Alveolar injury markers increase early. Endothelial injury markers increase later and are associated with cardiorenovascular injury and 28-day outcome. Alveolar and endothelial injury likely contribute at different times to disease progression in severe COVID-19.

Topology and Contribution to the Pore Channel Lining of Plasma Membrane-Embedded Shigella flexneri Type 3 Secretion Translocase IpaB.

Shigella spp. are human bacterial pathogens that cause bacillary dysentery. Virulence depends on a type 3 secretion system (T3SS), a highly conserved structure present in multiple important human and plant pathogens. Upon host cell contact, the T3SS translocon is delivered to the host membrane, facilitates bacterial docking to the membrane, and enables delivery of effector proteins into the host cytosol. The Shigella translocon is composed of two proteins, IpaB and IpaC, which together form this multimeric structure within host plasma membranes. Upon interaction of IpaC with host intermediate filaments, the translocon undergoes a conformational change that allows for bacterial docking onto the translocon and, together with host actin polymerization, enables subsequent effector translocation through the translocon pore. To generate additional insights into the translocon, we mapped the topology of IpaB in plasma membrane-embedded pores using cysteine substitution mutagenesis coupled with site-directed labeling and proximity-enabled cross-linking by membrane-permeant sulfhydryl reactants. We demonstrate that IpaB function is dependent on posttranslational modification by a plasmid-encoded acyl carrier protein. We show that the first transmembrane domain of IpaB lines the interior of the translocon pore channel such that the IpaB portion of the channel forms a funnel-like shape leading into the host cytosol. In addition, we identify regions of IpaB within its cytosolic domain that protrude into and are closely associated with the pore channel. Taken together, these results provide a framework for how IpaB is arranged within translocons natively delivered by Shigella during infection. IMPORTANCE Type 3 secretion systems are nanomachines employed by many bacteria, including Shigella, which deliver into human cells bacterial virulence proteins that alter cellular function in ways that promote infection. Delivery of Shigella virulence proteins occurs through a pore formed in human cell membranes by the IpaB and IpaC proteins. Here, we define how IpaB contributes to the formation of pores natively delivered into human cell membranes by Shigella flexneri. We show that a specific domain of IpaB (transmembrane domain 1) lines much of the pore channel and that portions of IpaB that lie in the inside of the human cell loop back into and/or are closely associated with the pore channel. These findings provide new insights into the organization and function of the pore in serving as the conduit for delivery of virulence proteins into human cells during Shigella infection.

Longitudinal characterization of circulating neutrophils uncovers distinct phenotypes associated with disease severity in hospitalized COVID-19 patients.

Multiple studies have identified an association between neutrophils and COVID-19 disease severity; however, the mechanistic basis of this association remains incompletely understood. Here we collected 781 longitudinal blood samples from 306 hospitalized COVID-19 + patients, 78 COVID-19 âˆ' acute respiratory distress syndrome patients, and 8 healthy controls, and performed bulk RNA-sequencing of enriched neutrophils, plasma proteomics, cfDNA measurements and high throughput antibody profiling assays to investigate the relationship between neutrophil states and disease severity or death. We identified dynamic switches between six distinct neutrophil subtypes using non-negative matrix factorization (NMF) clustering. At days 3 and 7 post-hospitalization, patients with severe disease had an enrichment of a granulocytic myeloid derived suppressor cell-like state gene expression signature, while non-severe patients with resolved disease were enriched for a progenitor-like immature neutrophil state signature. Severe disease was associated with gene sets related to neutrophil degranulation, neutrophil extracellular trap (NET) signatures, distinct metabolic signatures, and enhanced neutrophil activation and generation of reactive oxygen species (ROS). We found that the majority of patients had a transient interferon-stimulated gene signature upon presentation to the emergency department (ED) defined here as Day 0, regardless of disease severity, which persisted only in patients who subsequently died. Humoral responses were identified as potential drivers of neutrophil effector functions, as enhanced antibody-dependent neutrophil phagocytosis and reduced NETosis was associated with elevated SARS-CoV-2-specific IgG1-to-IgA1 ratios in plasma of severe patients who survived. In vitro experiments confirmed that while patient-derived IgG antibodies mostly drove neutrophil phagocytosis and ROS production in healthy donor neutrophils, patient-derived IgA antibodies induced a predominant NETosis response. Overall, our study demonstrates neutrophil dysregulation in severe COVID-19 and a potential role for IgA-dominant responses in driving neutrophil effector functions in severe disease and mortality.

Early cross-coronavirus reactive signatures of humoral immunity against COVID-19.

The introduction of vaccines has inspired hope in the battle against SARS-CoV-2. However, the emergence of viral variants, in the absence of potent antivirals, has left the world struggling with the uncertain nature of this disease. Antibodies currently represent the strongest correlate of immunity against SARS-CoV-2, thus we profiled the earliest humoral signatures in a large cohort of acutely ill (survivors and nonsurvivors) and mild or asymptomatic individuals with COVID-19. Although a SARS-CoV-2­specific immune response evolved rapidly in survivors of COVID-19, nonsurvivors exhibited blunted and delayed humoral immune evolution, particularly with respect to S2-specific antibodies. Given the conservation of S2 across ß-coronaviruses, we found that the early development of SARS-CoV-2­specific immunity occurred in tandem with preexisting common ß-coronavirus OC43 humoral immunity in survivors, which was also selectively expanded in individuals that develop a paucisymptomatic infection. These data point to the importance of cross-coronavirus immunity as a correlate of protection against COVID-19.

The type 3 secretion system requires actin polymerization to open translocon pores.

Many bacterial pathogens require a type 3 secretion system (T3SS) to establish a niche. Host contact activates bacterial T3SS assembly of a translocon pore in the host plasma membrane. Following pore formation, the T3SS docks onto the translocon pore. Docking establishes a continuous passage that enables the translocation of virulence proteins, effectors, into the host cytosol. Here we investigate the contribution of actin polymerization to T3SS-mediated translocation. Using the T3SS model organism Shigella flexneri, we show that actin polymerization is required for assembling the translocon pore in an open conformation, thereby enabling effector translocation. Opening of the pore channel is associated with a conformational change to the pore, which is dependent upon actin polymerization and a coiled-coil domain in the pore protein IpaC. Analysis of an IpaC mutant that is defective in ruffle formation shows that actin polymerization-dependent pore opening is distinct from the previously described actin polymerization-dependent ruffles that are required for bacterial internalization. Moreover, actin polymerization is not required for other pore functions, including docking or pore protein insertion into the plasma membrane. Thus, activation of the T3SS is a multilayered process in which host signals are sensed by the translocon pore leading to the activation of effector translocation.

SARS-CoV-2 viremia is associated with distinct proteomic pathways and predicts COVID-19 outcomes.

BACKGROUNDSARS-CoV-2 plasma viremia has been associated with severe disease and death in COVID-19 in small-scale cohort studies. The mechanisms behind this association remain elusive.METHODSWe evaluated the relationship between SARS-CoV-2 viremia, disease outcome, and inflammatory and proteomic profiles in a cohort of COVID-19 emergency department participants. SARS-CoV-2 viral load was measured using a quantitative reverse transcription PCR-based platform. Proteomic data were generated with Proximity Extension Assay using the Olink platform.RESULTSThis study included 300 participants with nucleic acid test-confirmed COVID-19. Plasma SARS-CoV-2 viremia levels at the time of presentation predicted adverse disease outcomes, with an adjusted OR of 10.6 (95% CI 4.4-25.5, P < 0.001) for severe disease (mechanical ventilation and/or 28-day mortality) and 3.9 (95% CI 1.5-10.1, P = 0.006) for 28-day mortality. Proteomic analyses revealed prominent proteomic pathways associated with SARS-CoV-2 viremia, including upregulation of SARS-CoV-2 entry factors (ACE2, CTSL, FURIN), heightened markers of tissue damage to the lungs, gastrointestinal tract, and endothelium/vasculature, and alterations in coagulation pathways.CONCLUSIONThese results highlight the cascade of vascular and tissue damage associated with SARS-CoV-2 plasma viremia that underlies its ability to predict COVID-19 disease outcomes.FUNDINGMark and Lisa Schwartz; the National Institutes of Health (U19AI082630); the American Lung Association; the Executive Committee on Research at Massachusetts General Hospital; the Chan Zuckerberg Initiative; Arthur, Sandra, and Sarah Irving for the David P. Ryan, MD, Endowed Chair in Cancer Research; an EMBO Long-Term Fellowship (ALTF 486-2018); a Cancer Research Institute/Bristol Myers Squibb Fellowship (CRI2993); the Harvard Catalyst/Harvard Clinical and Translational Science Center (National Center for Advancing Translational Sciences, NIH awards UL1TR001102 and UL1TR002541-01); and by the Harvard University Center for AIDS Research (National Institute of Allergy and Infectious Diseases, 5P30AI060354).

Longitudinal proteomic analysis of severe COVID-19 reveals survival-associated signatures, tissue-specific cell death, and cell-cell interactions.

Mechanisms underlying severe coronavirus disease 2019 (COVID-19) disease remain poorly understood. We analyze several thousand plasma proteins longitudinally in 306 COVID-19 patients and 78 symptomatic controls, uncovering immune and non-immune proteins linked to COVID-19. Deconvolution of our plasma proteome data using published scRNA-seq datasets reveals contributions from circulating immune and tissue cells. Sixteen percent of patients display reduced inflammation yet comparably poor outcomes. Comparison of patients who died to severely ill survivors identifies dynamic immune-cell-derived and tissue-associated proteins associated with survival, including exocrine pancreatic proteases. Using derived tissue-specific and cell-type-specific intracellular death signatures, cellular angiotensin-converting enzyme 2 (ACE2) expression, and our data, we infer whether organ damage resulted from direct or indirect effects of infection. We propose a model in which interactions among myeloid, epithelial, and T cells drive tissue damage. These datasets provide important insights and a rich resource for analysis of mechanisms of severe COVID-19 disease.

SARS-CoV-2 Viremia is Associated with Distinct Proteomic Pathways and Predicts COVID-19 Outcomes.

BACKGROUND: Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) plasma viremia has been associated with severe disease and death in coronavirus disease 2019 (COVID-19) in small-scale cohort studies. The mechanisms behind this association remain elusive. METHODS: We evaluated the relationship between SARS-CoV-2 viremia, disease outcome, inflammatory and proteomic profiles in a cohort of COVID-19 emergency department participants. SARS-CoV-2 viral load was measured using qRT-PCR based platform. Proteomic data were generated with Proximity Extension Assay (PEA) using the Olink platform. RESULTS: Three hundred participants with nucleic acid test-confirmed COVID-19 were included in this study. Levels of plasma SARS-CoV-2 viremia at the time of presentation predicted adverse disease outcomes, with an adjusted odds ratio (aOR) of 10.6 (95% confidence interval [CI] 4.4, 25.5, P<0.001) for severe disease (mechanical ventilation and/or 28-day mortality) and aOR of 3.9 (95%CI 1.5, 10.1, P=0.006) for 28-day mortality. Proteomic analyses revealed prominent proteomic pathways associated with SARS-CoV-2 viremia, including upregulation of SARS-CoV-2 entry factors (ACE2, CTSL, FURIN), heightened markers of tissue damage to the lungs, gastrointestinal tract, endothelium/vasculature and alterations in coagulation pathways. CONCLUSIONS: These results highlight the cascade of vascular and tissue damage associated with SARS-CoV-2 plasma viremia that underlies its ability to predict COVID-19 disease outcomes.

Shigella flexneri Disruption of Cellular Tension Promotes Intercellular Spread.

During infection, some bacterial pathogens invade the eukaryotic cytosol and spread between cells of an epithelial monolayer. Intercellular spread occurs when these pathogens push against the plasma membrane, forming protrusions that are engulfed by adjacent cells. Here, we show that IpaC, a Shigella flexneri type 3 secretion system protein, binds the host cell-adhesion protein ß-catenin and facilitates efficient protrusion formation. S. flexneri producing a point mutant of IpaC that cannot interact with ß-catenin is defective in protrusion formation and spread. Spread is restored by chemical reduction of intercellular tension or genetic depletion of ß-catenin, and the magnitude of the protrusion defect correlates with membrane tension, indicating that IpaC reduces membrane tension, which facilitates protrusion formation. IpaC stabilizes adherens junctions and does not alter ß-catenin localization at the membrane. Thus, Shigella, like other bacterial pathogens, reduces intercellular tension to efficiently spread between cells.

Plasma proteomics reveals tissue-specific cell death and mediators of cell-cell interactions in severe COVID-19 patients.

COVID-19 has caused over 1 million deaths globally, yet the cellular mechanisms underlying severe disease remain poorly understood. By analyzing several thousand plasma proteins in 306 COVID-19 patients and 78 symptomatic controls over serial timepoints using two complementary approaches, we uncover COVID-19 host immune and non-immune proteins not previously linked to this disease. Integration of plasma proteomics with nine published scRNAseq datasets shows that SARS-CoV-2 infection upregulates monocyte/macrophage, plasmablast, and T cell effector proteins. By comparing patients who died to severely ill patients who survived, we identify dynamic immunomodulatory and tissue-associated proteins associated with survival, providing insights into which host responses are beneficial and which are detrimental to survival. We identify intracellular death signatures from specific tissues and cell types, and by associating these with angiotensin converting enzyme 2 (ACE2) expression, we map tissue damage associated with severe disease and propose which damage results from direct viral infection rather than from indirect effects of illness. We find that disease severity in lung tissue is driven by myeloid cell phenotypes and cell-cell interactions with lung epithelial cells and T cells. Based on these results, we propose a model of immune and epithelial cell interactions that drive cell-type specific and tissue-specific damage in severe COVID-19.

Activation of Shigella flexneri type 3 secretion requires a host-induced conformational change to the translocon pore.

Type 3 secretion systems (T3SSs) are conserved bacterial nanomachines that inject virulence proteins (effectors) into eukaryotic cells during infection. Due to their ability to inject heterologous proteins into human cells, these systems are being developed as therapeutic delivery devices. The T3SS assembles a translocon pore in the plasma membrane and then docks onto the pore. Docking activates effector secretion through the pore and into the host cytosol. Here, using Shigella flexneri, a model pathogen for the study of type 3 secretion, we determined the molecular mechanisms by which host intermediate filaments trigger docking and enable effector secretion. We show that the interaction of intermediate filaments with the translocon pore protein IpaC changed the pore's conformation in a manner that was required for docking. Intermediate filaments repositioned residues of the Shigella pore protein IpaC that are located on the surface of the pore and in the pore channel. Restricting these conformational changes blocked docking in an intermediate filament-dependent manner. These data demonstrate that a host-induced conformational change to the pore enables T3SS docking and effector secretion, providing new mechanistic insight into the regulation of type 3 secretion.

A predicted Francisella tularensis DXD-motif glycosyltransferase blocks immune activation.

Pathogens enhance their survival during infections by manipulating host defenses. Francisella tularensis evades innate immune responses, which we have found to be dependent on an understudied gene ybeX (FTL_0883/FTT_0615c). To understand the function of YbeX, we sought protein interactors in F. tularensis subsp. holarctica live vaccine strain (LVS). An unstudied Francisella protein co-immunoprecipitated with recombinant YbeX, which is a predicted glycosyltransferase with a DXD-motif. There are up to four genomic copies of this gene with identical sequence in strains of F. tularensis pathogenic to humans, despite ongoing genome decay. Disruption mutations were generated by intron insertion into all three copies of this glycosyltransferase domain containing gene in LVS, gdcA1-3. The resulting strains stimulated more cytokines from macrophages in vitro than wild-type LVS and were attenuated in two in vivo infection models. GdcA was released from LVS during culture and was sufficient to block NF-κB activation when expressed in eukaryotic cells. When co-expressed in zebrafish, GdcA and YbeX were synergistically lethal to embryo development. Glycosyltransferases with DXD-motifs are found in a variety of pathogens including NleB, an Escherichia coli type-III secretion system effector that inhibits NF-κB by antagonizing death receptor signaling. To our knowledge, GdcA is the first DXD-motif glycosyltransferase that inhibits NF-κB in immune cells. Together, these findings suggest DXD-motif glycosyltransferases may be a conserved virulence mechanism used by pathogenic bacteria to remodel host defenses.

Topological Analysis of the Type 3 Secretion System Translocon Pore Protein IpaC following Its Native Delivery to the Plasma Membrane during Infection.

Many Gram-negative bacterial pathogens require a type 3 secretion system (T3SS) to deliver effector proteins into eukaryotic cells. Contact of the tip complex of the T3SS with a target eukaryotic cell initiates secretion of the two bacterial proteins that assemble into the translocon pore in the plasma membrane. The translocon pore functions to regulate effector protein secretion and is the conduit for effector protein translocation across the plasma membrane. To generate insights into how the translocon pore regulates effector protein secretion, we defined the topology of the Shigella translocon pore protein IpaC in the plasma membrane following its native delivery by the T3SS. Using single cysteine substitution mutagenesis and site-directed labeling with a membrane-impermeant chemical probe, we mapped residues accessible from the extracellular surface of the cell. Our data support a model in which the N terminus of IpaC is extracellular and the C terminus of IpaC is intracellular. These findings resolve previously conflicting data on IpaC topology that were based on nonnative delivery of IpaC to membranes. Salmonella enterica serovar Typhimurium also requires the T3SS for effector protein delivery into eukaryotic cells. Although the sequence of IpaC is closely related to the Salmonella translocon pore protein SipC, the two proteins have unique functional attributes during infection. We showed a similar overall topology for SipC and IpaC and identified subtle topological differences between their transmembrane α-helixes and C-terminal regions. Together, our data suggest that topological differences among distinct translocon pore proteins may dictate organism-specific functional differences of the T3SSs during infection.IMPORTANCE The type 3 secretion system (T3SS) is a nanomachine required for virulence of many bacterial pathogens that infect humans. The system delivers bacterial virulence proteins into the cytosol of human cells, where the virulence proteins promote bacterial infection. The T3SS forms a translocon pore in the membranes of target cells. This pore is the portal through which bacterial virulence proteins are delivered by the T3SS into the eukaryotic cytosol. The pore also regulates secretion of these virulence proteins. Our work defines the topology of translocon pore proteins in their native context during infection, resolves previously conflicting reports about the topology of the Shigella translocon pore protein IpaC, and provides new insights into how interactions of the pore with the T3SS likely produce signals that activate secretion of virulence proteins.

Intermediate filaments enable pathogen docking to trigger type 3 effector translocation.

Type 3 secretion systems (T3SSs) of bacterial pathogens translocate bacterial effector proteins that mediate disease into the eukaryotic cytosol. Effectors traverse the plasma membrane through a translocon pore formed by T3SS proteins. In a genome-wide selection, we identified the intermediate filament vimentin as required for infection by the T3SS-dependent pathogen S. flexneri. We found that vimentin is required for efficient T3SS translocation of effectors by S. flexneri and other pathogens that use T3SS, Salmonella enterica serovar Typhimurium and Yersinia pseudotuberculosis. Vimentin and the intestinal epithelial intermediate filament keratin 18 interact with the C-terminus of the Shigella translocon pore protein IpaC. Vimentin and its interaction with IpaC are dispensable for pore formation, but are required for stable docking of S. flexneri to cells; moreover, stable docking triggers effector secretion. These findings establish that stable docking of the bacterium specifically requires intermediate filaments, is a process distinct from pore formation, and is a prerequisite for effector secretion.

Shigella flexneri regulation of ARF6 activation during bacterial entry via an IpgD-mediated positive feedback loop.

UNLABELLED: Entry into cells is critical for virulence of the human bacterial pathogens Shigella spp. Shigella spp. induce membrane ruffle formation and macropinocytic uptake, but the events instigating this process are incompletely understood. The host small GTPase ADP-ribosylation factor 6 (ARF6) functions in membrane trafficking at the plasma membrane and activates membrane ruffle formation. We demonstrate that ARF6 is required for efficient Shigella flexneri entry, is activated by S. flexneri dependent on the phosphatase activity of the type III secreted effector IpgD, and depends on cytohesin guanine nucleotide exchange factors (GEFs) for recruitment to entry sites. The cytohesin GEF ARF nucleotide binding site opener (ARNO) is recruited to these sites, also dependent on IpgD phosphatase activity. ARNO recruitment is independent of ARF6, indicating that, in addition to the described recruitment of ARNO by ARF6, ARNO is recruited upstream of ARF6. Our data provide evidence that ARF6, IpgD, phosphoinositide species, and ARNO constitute a previously undescribed positive feedback loop that amplifies ARF6 activation at bacterial entry sites, thereby promoting efficient S. flexneri uptake. IMPORTANCE: Shigella spp. cause diarrhea and dysentery by infection of epithelial cells in the human colon. Critical to disease is the ability of Shigella to enter into cells, yet the mechanisms involved in entry are incompletely understood. We demonstrate that the small GTPase ADP-ribosylation factor 6 (ARF6) is required for efficient cellular entry of Shigella flexneri and that activation of ARF6 depends on the phosphatase activity of the Shigella protein IpgD, which is introduced into cells via the bacterial type III secretion system. We further show that IpgD phosphatase activity is required for recruitment of the ARF6 guanine nucleotide exchange factor (GEF) ARF nucleotide binding site opener (ARNO) to bacterial entry sites and that ARNO lies upstream of ARF6 activation. These relationships define a positive feedback loop that contributes to activation of ARF6 at S. flexneri entry sites and leads to local amplification of signals that promote bacterial entry.