Ubiquitin and its relatives as wizards of the endolysosomal system.

The endolysosomal system comprises a dynamic constellation of vesicles working together to sense and interpret environmental cues and facilitate homeostasis. Integrating extracellular information with the internal affairs of the cell requires endosomes and lysosomes to be proficient in decision-making: fusion or fission; recycling or degradation; fast transport or contacts with other organelles. To effectively discriminate between these options, the endolysosomal system employs complex regulatory strategies that crucially rely on reversible post-translational modifications (PTMs) with ubiquitin (Ub) and ubiquitin-like (Ubl) proteins. The cycle of conjugation, recognition and removal of different Ub- and Ubl-modified states informs cellular protein stability and behavior at spatial and temporal resolution and is thus well suited to finetune macromolecular complex assembly and function on endolysosomal membranes. Here, we discuss how ubiquitylation (also known as ubiquitination) and its biochemical relatives orchestrate endocytic traffic and designate cargo fate, influence membrane identity transitions and support formation of membrane contact sites (MCSs). Finally, we explore the opportunistic hijacking of Ub and Ubl modification cascades by intracellular bacteria that remodel host trafficking pathways to invade and prosper inside cells.

Shigella hijacks the exocyst to cluster macropinosomes for efficient vacuolar escape.

Shigella flexneri invades host cells by entering within a bacteria-containing vacuole (BCV). In order to establish its niche in the host cytosol, the bacterium ruptures its BCV. Contacts between S. flexneri BCV and infection-associated macropinosomes (IAMs) formed in situ have been reported to enhance BCV disintegration. The mechanism underlying S. flexneri vacuolar escape remains however obscure. To decipher the molecular mechanism priming the communication between the IAMs and S. flexneri BCV, we performed mass spectrometry-based analysis of the magnetically purified IAMs from S. flexneri-infected cells. While proteins involved in host recycling and exocytic pathways were significantly enriched at the IAMs, we demonstrate more precisely that the S. flexneri type III effector protein IpgD mediates the recruitment of the exocyst to the IAMs through the Rab8/Rab11 pathway. This recruitment results in IAM clustering around S. flexneri BCV. More importantly, we reveal that IAM clustering subsequently facilitates an IAM-mediated unwrapping of the ruptured vacuole membranes from S. flexneri, enabling the naked bacterium to be ready for intercellular spread via actin-based motility. Taken together, our work untangles the molecular cascade of S. flexneri-driven host trafficking subversion at IAMs to develop its cytosolic lifestyle, a crucial step en route for infection progression at cellular and tissue level.

New methods to decrypt emerging macropinosome functions during the host-pathogen crosstalk.

Large volumes of liquid and other materials from the extracellular environment are internalised by eukaryotic cells via an endocytic process called macropinocytosis. It is now recognised that this fundamental and evolutionarily conserved pathway is hijacked by numerous intracellular pathogens as an entry portal to the host cell interior. Yet, an increasing number of additional cellular functions of macropinosomes in pathologic processes have been reported beyond this role for fluid internalisation. It emerges that the identity of macropinosomes can vary hugely and change rapidly during their lifetime. A deeper understanding of this important multi-faceted compartment is based on novel methods for their investigation. These methods are either imaging-based for the tracking of macropinosome dynamics, or they provide the means to extract macropinosomes at high purity for comprehensive proteomic analyses. Here, we portray these new approaches for the investigation of macropinosomes. We document how these method developments have provided insights for a new understanding of the intracellular lifestyle of the bacterial pathogens Shigella and Salmonella. We suggest that a systematic complete characterisation of macropinosome subversion with these approaches during other infection processes and pathologies will be highly beneficial for our understanding of the underlying cellular and molecular processes.

The entry of Salmonella in a distinct tight compartment revealed at high temporal and ultrastructural resolution.

Salmonella enterica induces membrane ruffling and genesis of macropinosomes during its interactions with epithelial cells. This is achieved through the type three secretion system-1, which first mediates bacterial attachment to host cells and then injects bacterial effector proteins to alter host behaviour. Next, Salmonella enters into the targeted cell within an early membrane-bound compartment that matures into a slow growing, replicative niche called the Salmonella Containing Vacuole (SCV). Alternatively, the pathogen disrupts the membrane of the early compartment and replicate at high rate in the cytosol. Here, we show that the in situ formed macropinosomes, which have been previously postulated to be relevant for the step of Salmonella entry, are key contributors for the formation of the mature intracellular niche of Salmonella. We first clarify the primary mode of type three secretion system-1 induced Salmonella entry into epithelial cells by combining classical fluorescent microscopy with cutting edge large volume electron microscopy. We observed that Salmonella, similarly to Shigella, enters epithelial cells inside tight vacuoles rather than in large macropinosomes. We next apply this technology to visualise rupturing Salmonella containing compartments, and we use extended time-lapse microscopy to establish early markers that define which Salmonella will eventually hyper replicate. We show that at later infection stages, SCVs harbouring replicating Salmonella have previously fused with the in situ formed macropinosomes. In contrast, such fusion events could not be observed for hyper-replicating Salmonella, suggesting that fusion of the Salmonella entry compartment with macropinosomes is the first committed step of SCV formation.

Identification of Parameters of Host Cell Vulnerability during Salmonella Infection by Quantitative Image Analysis and Modeling.

Salmonella targets and enters epithelial cells at permissive entry sites: some cells are more likely to be infected than others. However, the parameters that lead to host cell heterogeneity are not known. Here, we quantitatively characterized host cell vulnerability to Salmonella infection based on imaged parameters. We performed successive infections of the same host cell population followed by automated high-throughput microscopy and observed that infected cells have a higher probability of being reinfected. Establishing a predictive model, we identified two combined origins of host cell vulnerability: pathogen-induced cellular vulnerability emerging from Salmonella uptake and persisting at later stages of the infection and host cell-inherent vulnerability. We linked the host cell-inherent vulnerability with its morphological attributes, such as local cell crowding, and with host cell cholesterol content. This showed that the probability of Salmonella infection success can be forecast from morphological or molecular host cell parameters.

Soft agar colony formation assay to quantify mouse embryonic fibroblast transformation after Salmonella infection.

Links between bacterial infections and cancer are actively investigated. Cost-effective assays to quantify bacterial oncogenic potential can shed new light on these links. Here, we present a soft agar colony formation assay to quantify mouse embryonic fibroblast transformation after Salmonella Typhimurium infection. We describe how to infect and seed cells in soft agar for anchorage-independent growth, a hallmark of cell transformation. We further detail automated cell colony enumeration. This protocol is adaptable to other bacteria or host cells. For complete details on the use and execution of this protocol, please refer to Van Elsland et al.1.

Control of host PTMs by intracellular bacteria: An opportunity toward novel anti-infective agents.

Intracellular bacteria have developed a multitude of mechanisms to influence the post-translational modifications (PTMs) of host proteins to pathogen advantages. The recent explosion of insights into the diversity and sophistication of host PTMs and their manipulation by infectious agents challenges us to formulate a comprehensive vision of this complex and dynamic facet of the host-pathogen interaction landscape. As new discoveries continue to shed light on the central roles of PTMs in infectious diseases, technological advances foster our capacity to detect old and new PTMs and investigate their control and impact during pathogenesis, opening new possibilities for chemical intervention and infection treatment. Here, we present a comprehensive overview of these pathogenic mechanisms and offer perspectives on how these insights may contribute to the development of a new class of therapeutics that are urgently needed to face rising antibiotic resistances.

Repetitive non-typhoidal Salmonella exposure is an environmental risk factor for colon cancer and tumor growth.

During infection, Salmonella hijacks essential host signaling pathways. These molecular manipulations disrupt cellular integrity and may induce oncogenic transformation. Systemic S. Typhi infections are linked to gallbladder cancer, whereas severe non-typhoidal Salmonella (NTS) infections are associated with colon cancer (CC). These diagnosed infections, however, represent only a small fraction of all NTS infections as many infections are mild and go unnoticed. To assess the overall impact of NTS infections, we performed a retrospective serological study on NTS exposure in patients with CC. The magnitude of exposure to NTS, as measured by serum antibody titer, is significantly positively associated with CC. Repetitively infecting mice with low NTS exposure showed similar accelerated tumor growth to that observed after high NTS exposure. At the cellular level, NTS preferably infects (pre-)transformed cells, and each infection round exponentially increases the rate of transformed cells. Thus, repetitive exposure to NTS associates with CC risk and accelerates tumor growth.

Purification of infection-associated macropinosomes by magnetic isolation for proteomic characterization.

Macropinocytosis refers to the nonselective uptake of extracellular molecules into many different types of eukaryotic cells within large fluid-filled vesicles named macropinosomes. Macropinosomes are relevant for a wide variety of cellular processes, such as antigen sampling in immune cells, homeostasis in the kidney, cell migration or pathogen uptake. Understanding the molecular composition of the different macropinosomes formed during these processes has helped to differentiate their regulations from other endocytic events. Here, we present a magnetic purification protocol that segregates scarce macropinosomes from other endocytic vesicles at a high purity and in a low-cost and unbiased manner. Our protocol takes advantage of moderate-sized magnetic beads of 100 nm in diameter coupled to mass-spectrometry-based proteomic analysis. Passing the cell lysate through a table-top magnet allows the quick retention of the bead-containing macropinosomes. Unlike other cell-fractionation-based methodologies, our protocol minimizes sample loss and production cost without prerequisite knowledge of the macropinosomes and with minimal laboratory experience. We describe a detailed procedure for the isolation of infection-associated macropinosomes during bacterial invasion and the optimization steps to readily adapt it to various studies. The protocol can be performed in 3 d to provide highly purified and enriched macropinosomes for qualitative proteomic composition analysis.

The phosphoinositide coincidence detector Phafin2 promotes macropinocytosis by coordinating actin organisation at forming macropinosomes.

Uptake of large volumes of extracellular fluid by actin-dependent macropinocytosis has an important role in infection, immunity and cancer development. A key question is how actin assembly and disassembly are coordinated around macropinosomes to allow them to form and subsequently pass through the dense actin network underlying the plasma membrane to move towards the cell center for maturation. Here we show that the PH and FYVE domain protein Phafin2 is recruited transiently to newly-formed macropinosomes by a mechanism that involves coincidence detection of PtdIns3P and PtdIns4P. Phafin2 also interacts with actin via its PH domain, and recruitment of Phafin2 coincides with actin reorganization around nascent macropinosomes. Moreover, forced relocalization of Phafin2 to the plasma membrane causes rearrangement of the subcortical actin cytoskeleton. Depletion of Phafin2 inhibits macropinosome internalization and maturation and prevents KRAS-transformed cancer cells from utilizing extracellular protein as an amino acid source. We conclude that Phafin2 promotes macropinocytosis by controlling timely delamination of actin from nascent macropinosomes for their navigation through the dense subcortical actin network.

Cellular Imaging of Intracellular Bacterial Pathogens.

The spatial dimensions of host cells and bacterial microbes are perfectly suited to being studied by microscopy techniques. Therefore, cellular imaging has been instrumental in uncovering many paradigms of the intracellular lifestyle of microbes. Initially, microscopy was used as a qualitative, descriptive tool. However, with the onset of specific markers and the power of computer-assisted image analysis, imaging can now be used to gather quantitative data on biological processes. This makes imaging a driving force for the study of cellular phenomena. One particular imaging modality stands out, which is based on the physical principles of fluorescence. Fluorescence is highly specific and therefore can be exploited to label biomolecules of choice. It is also very sensitive, making it possible to follow individual molecules with this approach. Also, microscopy hardware has played an important role in putting microscopy in the spotlight for host-pathogen investigations. For example, microscopes have been automated for microscopy-based screenings. A new generation of microscopes and molecular probes are being used to image events below the resolution limit of light. Finally, workflows are being developed to link light microscopy with electron microscopy methods via correlative light electron microscopy. We are witnessing a golden age of cellular imaging in cellular microbiology.

Dynamic Growth and Shrinkage of the Salmonella-Containing Vacuole Determines the Intracellular Pathogen Niche.

Salmonella is a human and animal pathogen that causes gastro-enteric diseases. The key to Salmonella infection is its entry into intestinal epithelial cells, where the bacterium resides within a Salmonella-containing vacuole (SCV). Salmonella entry also induces the formation of empty macropinosomes, distinct from the SCV, in the vicinity of the entering bacteria. A few minutes after its formation, the SCV increases in size through fusions with the surrounding macropinosomes. Salmonella also induces membrane tubules that emanate from the SCV and lead to SCV shrinkage. Here, we show that these antipodal events are utilized by Salmonella to either establish a vacuolar niche or to be released into the cytosol by SCV rupture. We identify the molecular machinery underlying dynamic SCV growth and shrinkage. In particular, the SNARE proteins SNAP25 and STX4 participate in SCV inflation by fusion with macropinosomes. Thus, host compartment size control emerges as a pathogen strategy for intracellular niche regulation.

High-throughput Microscopic Analysis of Salmonella Invasion of Host Cells.

Salmonella is a Gram-negative bacterium causing a gastro-enteric disease called salmonellosis. During the first phase of infection, Salmonella uses its flagella to swim near the surface of the epithelial cells and to target specific site of infection. In order to study the selection criteria that determine which host cells are targeted by the pathogen, and to analyze the relation between infecting Salmonella (i.e., cooperation or competition), we have established a high-throughput microscopic assay of HeLa cells sequentially infected with fluorescent bacteria. Using an automated pipeline of image analysis, we quantitatively characterized a multitude of parameters of infected and non-infected cells. Based on this, we established a predictive model that allowed us to identify those parameters involved in host cell vulnerability towards infection. We revealed that host cell vulnerability has two origins: a pathogen-induced cellular vulnerability emerging from Salmonella uptake and persisting at later stages of the infection process; and a host cell-inherent vulnerability linked with cell inherent attributes, such as local cell crowding, and cholesterol content. Our method forecasts the probability of Salmonella infection within monolayers of epithelial cells based on morphological or molecular host cell parameters. Here, we provide a detailed description of the workflow including the computer-based analysis pipeline. Our method has the potential to be applied to study other combinations of host-pathogen interactions.

FHOD1 is needed for directed forces and adhesion maturation during cell spreading and migration.

Matrix adhesions provide critical signals for cell growth or differentiation. They form through a number of distinct steps that follow integrin binding to matrix ligands. In an early step, integrins form clusters that support actin polymerization by an unknown mechanism. This raises the question of how actin polymerization occurs at the integrin clusters. We report here that a major formin in mouse fibroblasts, FHOD1, is recruited to integrin clusters, resulting in actin assembly. Using cell-spreading assays on lipid bilayers, solid substrates, and high-resolution force-sensing pillar arrays, we find that knockdown of FHOD1 impairs spreading, coordinated application of adhesive force, and adhesion maturation. Finally, we show that targeting of FHOD1 to the integrin sites depends on the direct interaction with Src family kinases and is upstream of the activation by Rho kinase. Thus, our findings provide insights into the mechanisms of cell migration with implications for development and disease.