Shigella MreB promotes polar IcsA positioning for actin tail formation

Louis Pasteur continues to shape the future of microbiology

Louis Pasteur made seminal discoveries in microbiology, immunology and vaccinology that transformed clinical science and saved millions of lives. Since the 19th century, our ability to study infectious disease has undergone radical changes due to newly emerging technologies and infection models. In this Editorial, I consider Pasteur's impact on our ability to understand and combat infectious disease in the context of two modern-day pandemics: coronavirus disease 2019 (COVID-19) and antimicrobial resistance (AMR). During the COVID-19 pandemic, we witnessed remarkable ambition to understand severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and to innovate effective vaccines to prevent disease. For the comparatively overlooked pandemic of AMR, we require the same level of urgency to develop alternative approaches to combat antibiotic-resistant bacterial strains that cause millions of deaths annually. Pasteur's statement "chance only favours the mind which is prepared" is a principle that captures 'l'esprit Pasteur'. This principle should continue to guide modern-day research on infectious disease, and for this we need to support the development of predictive disease models and cutting-edge mechanistic research that prepare us for discovery and therapeutic impact.

Localization of host endocytic and actin-associated proteins during Shigella flexneri intracellular motility and intercellular spreading

Shigella flexneri (S. flexneri), the causative agent of bacillary dysentery, uses an effector-mediated strategy to hijack host cells and cause disease. To propagate and spread within human tissues, S. flexneri bacteria commandeer the host actin cytoskeleton to generate slender actin-rich comet tails to move intracellularly, and later, plasma membrane actin-based protrusions to move directly between adjacent host cells. To facilitate intercellular bacterial spreading, large micron-sized endocytic-like membrane invaginations form at the periphery of neighboring host cells that come into contact with S. flexneri-containing membrane protrusions. While S. flexneri comet tails and membrane protrusions consist primarily of host actin cytoskeletal proteins, S. flexneri membrane invaginations remain poorly understood with only clathrin and the clathrin adapter epsin-1 localized to the structures. Tangentially, we recently reported that Listeria monocytogenes, another actin-hijacking pathogen, exploits an assortment of caveolar and actin-bundling proteins at their micron-sized membrane invaginations formed during their cell-to-cell movement. Thus, to further characterize the S. flexneri disease process, we set out to catalog the distribution of a variety of actin-associated and caveolar proteins during S. flexneri actin-based motility and cell-to-cell spreading. Here we show that actin-associated proteins found at L. monocytogenes comet tails and membrane protrusions mimic those present at S. flexneri comet tails with the exception of α-actinins 1 and 4, which were shed from S. flexneri membrane protrusions. We also demonstrate that all known host endocytic components found at L. monocytogenes membrane invaginations are also present at those formed during S. flexneri infections.

Mechanistic insight into bacterial entrapment by septin cage reconstitution

Septins are cytoskeletal proteins that assemble into hetero-oligomeric complexes and sense micron-scale membrane curvature. During infection with Shigella flexneri, an invasive enteropathogen, septins restrict actin tail formation by entrapping bacteria in cage-like structures. Here, we reconstitute septin cages in vitro using purified recombinant septin complexes (SEPT2-SEPT6-SEPT7), and study how these recognize bacterial cells and assemble on their surface. We show that septin complexes recognize the pole of growing Shigella cells. An amphipathic helix domain in human SEPT6 enables septins to sense positively curved membranes and entrap bacterial cells. Shigella strains lacking lipopolysaccharide components are more efficiently entrapped in septin cages. Finally, cryo-electron tomography of in vitro cages reveals how septins assemble as filaments on the bacterial cell surface.

Septins are cytoskeletal proteins that assemble into complexes and contribute to immunity by entrapping intracellular bacteria in cage-like structures. Here, Lobato-Márquez et al. reconstitute septin cages in vitro using purified recombinant complexes, and study how these recognize bacterial cells and assemble as filaments on their surface.

The history of septin biology and bacterial infection

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

Autophagy-A Story of Bacteria Interfering with the Host Cell Degradation Machinery

Autophagy is a highly conserved and fundamental cellular process to maintain cellular homeostasis through recycling of defective organelles or proteins. In a response to intracellular pathogens, autophagy further acts as an innate immune response mechanism to eliminate pathogens. This review will discuss recent findings on autophagy as a reaction to intracellular pathogens, such as Salmonella typhimurium, Listeria monocytogenes, Mycobacterium tuberculosis, Staphylococcus aureus, and pathogenic Escherichia coli. Interestingly, while some of these bacteria have developed methods to use autophagy for their own benefit within the cell, others have developed fascinating mechanisms to evade recognition, to subvert the autophagic pathway, or to escape from autophagy.

Localization of alpha-actinin-4 during infections by actin remodeling bacteria

Bacterial pathogens cause disease by subverting the structure and function of their target host cells. Several foodborne agents such as Listeria monocytogenes (L. monocytogenes), Shigella flexneri (S. flexneri), Salmonella enterica serovar Typhimurium (S. Typhimurium) and enteropathogenic Escherichia coli (EPEC) manipulate the host actin cytoskeleton to cause diarrheal (and systemic) infections. During infections, these invasive and adherent pathogens hijack the actin filaments of their host cells and rearrange them into discrete actin-rich structures that promote bacterial adhesion (via pedestals), invasion (via membrane ruffles and endocytic cups), intracellular motility (via comet/rocket tails) and/or intercellular dissemination (via membrane protrusions and invaginations). We have previously shown that actin-rich structures generated by L. monocytogenes contain the host actin cross-linker α-actinin-4. Here we set out to examine α-actinin-4 during other key steps of the L. monocytogenes infectious cycle as well as characterize the subcellular distribution of α-actinin-4 during infections with other model actin-hijacking bacterial pathogens (S. flexneri, S. Typhimurium and EPEC). Although α-actinin-4 is absent at sites of initial L. monocytogenes invasion, we show that it is a new component of the membrane invaginations formed during secondary infections of neighboring host cells. Importantly, we reveal that α-actinin-4 also localizes to the major actin-rich structures generated during cell culture infections with S. flexneri (comet/rocket tails and membrane protrusions), S. Typhimurium (membrane ruffles) and EPEC (pedestals). Taken together, these findings suggest that α-actinin-4 is a host factor that is exploited by an assortment of actin-hijacking bacterial pathogens.

Status of Targeting MreB for the Development of Antibiotics

Although many prospective antibiotic targets are known, bacterial infections and resistance to antibiotics remain a threat to public health partly because the druggable potentials of most of these targets have yet to be fully tapped for the development of a new generation of therapeutics. The prokaryotic actin homolog MreB is one of the important antibiotic targets that are yet to be significantly exploited. MreB is a bacterial cytoskeleton protein that has been widely studied and is associated with the determination of rod shape as well as important subcellular processes including cell division, chromosome segregation, cell wall morphogenesis, and cell polarity. Notwithstanding that MreB is vital and conserved in most rod-shaped bacteria, no approved antibiotics targeting it are presently available. Here, the status of targeting MreB for the development of antibiotics is concisely summarized. Expressly, the known therapeutic targets and inhibitors of MreB are presented, and the way forward in the search for a new generation of potent inhibitors of MreB briefly discussed.

Environmental and cellular factors affecting the localization of T6SS proteins in Burkholderia thailandensis

The type VI secretion system (T6SS) injects effector proteins into neighboring bacteria and host cells. Effector translocation is driven by contraction of a tubular sheath in the cytoplasm that expels an inner needle across the cell envelope. The AAA + ATPase ClpV disassembles and recycles the contracted sheath. While ClpV-1-GFP of the Burkholderia T6SS-1, which targets prokaryotic cells, assembles into randomly localized foci, ClpV-5-GFP of the virulence-associated T6SS-5 displays a polar distribution. The mechanisms underlying the localization of T6SSs to a particular site in the bacterial cell are currently unknown. We recently showed that ClpV-5-GFP retains its polar localization in the absence of all T6SS-5 components during infection of host cells. Herein, we set out to identify factors involved in the distribution of ClpV-5 and ClpV-1 in Burkholderia thailandensis. We show that focal assembly and polar localization of ClpV-5-GFP is not dependent on the intracellular host cell environment, known to contain the signal to induce T6SS-5 gene expression. In contrast to ClpV-5-GFP, localization of ClpV-1-GFP was dependent on the cognate T6SS. Foci formation of both ClpV5-GFP and ClpV-1-GFP was decreased by D cycloserine-mediated inhibition of peptidoglycan synthesis while treatment of B. thailandensis with A22 blocking the cytoskeletal protein MreB did not affect assembly of ClpV-5 and ClpV-1 into single discrete foci. Furthermore, we found that surface contact promotes but is not essential for localization of ClpV-5-GFP to the pole whereas expression of clpV-1-gfp appears to be induced by surface contact. In summary, the study provides novel insights into the localization of ClpV ATPases of T6SSs targeting prokaryotic and eukaryotic cells.

First person – Sina Krokowski

First Person is a series of interviews with the first authors of a selection of papers published in Journal of Cell Science, helping early-career researchers promote themselves alongside their papers. Sina Krokowski is first author on ‘Shigella MreB promotes polar IcsA positioning for actin tail formation’, published in JCS. Sina conducted the research described in this article while a PhD student in Serge Mostowy's lab at Imperial College London, UK. She is now a postdoc in the lab of Andres Floto on the Cambridge Biomedical Campus, Cambridge, UK investigating the cell biology underlying host–pathogen interactions, and specifically cell autonomous immunity towards bacterial pathogens.

Role of septins in microbial infection

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