The Salmonella Typhimurium effector SteC inhibits Cdc42-mediated signaling through binding to the exchange factor Cdc24 in Saccharomyces cerevisiae

Salmonella manipulates macrophage migration via SteC-mediated myosin light chain activation to penetrate the gut-vascular barrier

The intestinal pathogen Salmonella enterica rapidly enters the bloodstream after the invasion of intestinal epithelial cells, but how Salmonella breaks through the gut-vascular barrier is largely unknown. Here, we report that Salmonella enters the bloodstream through intestinal CX3CR1+ macrophages during early infection. Mechanistically, Salmonella induces the migration/invasion properties of macrophages in a manner dependent on host cell actin and on the pathogen effector SteC. SteC recruits host myosin light chain protein Myl12a and phosphorylates its Ser19 and Thr20 residues. Myl12a phosphorylation results in actin rearrangement, and enhanced migration and invasion of macrophages. SteC is able to utilize a wide range of NTPs other than ATP to phosphorylate Myl12a. We further solved the crystal structure of SteC, which suggests an atypical dimerization-mediated catalytic mechanism. Finally, in vivo data show that SteC-mediated cytoskeleton manipulation is crucial for Salmonella breaching the gut vascular barrier and spreading to target organs.

Prophage-mediated genome differentiation of the Salmonella Derby ST71 population

Although Salmonella Derby ST71 strains have been recognized as poultry-specific by previous studies, multiple swine-associated S. Derby ST71 strains were identified in this long-term, multi-site epidemic study. Here, 15 representative swine-associated S. Derby ST71 strains were sequenced and compared with 65 (one swine-associated and 64 poultry-associated) S. Derby ST71 strains available in the NCBI database at a pangenomic level through comparative genomics analysis to identify genomic features related to the differentiation of swine-associated strains and previously reported poultry-associated strains. The distribution patterns of known Salmonella pathogenicity islands (SPIs) and virulence factor (VF) encoding genes were not capable of differentiating between the two strain groups. The results demonstrated that the S. Derby ST71 population harbours an open pan-genome, and swine-associated ST71 strains contain many more genes than the poultry-associated strains, mainly attributed to the prophage sequence contents in the genomes. The numbers of prophage sequences identified in the swine-associated strains were higher than those in the poultry-associated strains. Prophages specifically harboured by the swine-associated strains were found to contain genes that facilitate niche adaptation for the bacterial hosts. Gene deletion experiments revealed that the dam gene specifically present in the prophage of the swine-associated strains is important for S. Derby to adhere onto the host cells. This study provides novel insights into the roles of prophages during the genome differentiation of Salmonella.

SteC and the intracellular Salmonella-induced F-actin meshwork

Salmonella enterica serovars infect a broad range of mammalian hosts including humans, causing both gastrointestinal and systemic diseases. Following uptake into host cells, bacteria replicate within vacuoles (Salmonella-containing vacuoles; SCVs). Clusters of SCVs are frequently associated with a meshwork of F-actin. This meshwork is dependent on the Salmonella pathogenicity island 2 encoded type III secretion system and its effector SteC. SteC contains a region with weak similarity to conserved subdomains of eukaryotic kinases and has kinase activity that is required for the formation of the F-actin meshwork. Several substrates of SteC have been identified. In this mini-review, we attempt to integrate these findings and propose a more unified model to explain SCV-associated F-actin: SteC (i) phosphorylates the actin sequestering protein Hsp27, which increases the local G-actin concentration (ii) binds to and phosphorylates formin family FMNL proteins, which enables actin polymerisation and (iii) phosphorylates MEK, resulting in activation of the MEK/ERK/MLCK/Myosin II pathway, leading to F-actin bundling. We also consider the possible physiological functions of SCV-associated F-actin and similar structures produced by other intracellular bacterial pathogens.

Salmonella secretion systems: Differential roles in pathogen-host interactions

The bacterial genus Salmonella includes a large group of food-borne pathogens that cause a variety of gastrointestinal or systemic diseases in hosts. Salmonella use several secretion devices to inject various effectors targeting eukaryotic hosts, or bacteria. In the past few years, considerable progress has been made towards understanding the structural features and molecular mechanisms of the secretion systems of Salmonella, particularly regarding their roles in host-pathogen interactions. In this review, we summarize the current advances about the main characteristics of the Salmonella secretion systems. Clarifying the roles of the secretion systems in the process of infecting various hosts will broaden our understanding of the importance of microbial interactions in maintaining human health and will provide information for developing novel therapeutic approaches.

The TIR-domain containing effectors BtpA and BtpB from Brucella abortus impact NAD metabolism

Brucella species are facultative intracellular Gram-negative bacteria relevant to animal and human health. Their ability to establish an intracellular niche and subvert host cell pathways to their advantage depends on the delivery of bacterial effector proteins through a type IV secretion system. Brucella Toll/Interleukin-1 Receptor (TIR)-domain-containing proteins BtpA (also known as TcpB) and BtpB are among such effectors. Although divergent in primary sequence, they interfere with Toll-like receptor (TLR) signaling to inhibit the innate immune responses. However, the molecular mechanisms implicated still remain unclear. To gain insight into the functions of BtpA and BtpB, we expressed them in the budding yeast Saccharomyces cerevisiae as a eukaryotic cell model. We found that both effectors were cytotoxic and that their respective TIR domains were necessary and sufficient for yeast growth inhibition. Growth arrest was concomitant with actin depolymerization, endocytic block and a general decrease in kinase activity in the cell, suggesting a failure in energetic metabolism. Indeed, levels of ATP and NAD⁺ were low in yeast cells expressing BtpA and BtpB TIR domains, consistent with the recently described enzymatic activity of some TIR domains as NAD⁺ hydrolases. In human epithelial cells, both BtpA and BtpB expression reduced intracellular total NAD levels. In infected cells, both BtpA and BtpB contributed to reduction of total NAD, indicating that their NAD⁺ hydrolase functions are active intracellularly during infection. Overall, combining the yeast model together with mammalian cells and infection studies our results show that BtpA and BtpB modulate energy metabolism in host cells through NAD⁺ hydrolysis, assigning a novel role for these TIR domain-containing effectors in Brucella pathogenesis.

Brucella is a genus of zoonotic bacteria that cause severe disease in a variety of mammals, ranging from farm animals (as bovines, swine and ovine) to marine mammals. Transmission to humans, often by ingestion of non-treated dairy products, leads to serious systemic infection. Brucella abortus invades host cells and replicates intracellularly. Such behavior relies on the injection of bacterial proteins into the host cytoplasm via specialized secretion systems. Our work focuses on the study of two of these factors, BtpA and BtpB, previously described to contain Toll/Interleukin-1 Receptor (TIR)-domains that modulate innate immunity. We use here two biological models: the yeast Saccharomyces cerevisiae and human cell lines. We found that the TIR domains of both Brucella proteins were necessary and sufficient to collapse energy metabolism in yeast by depleting ATP and NAD⁺. This result was translatable to higher cells and consistent with the recently described NADase activity of some TIR domains both in mammalian and bacterial proteins. Importantly, we demonstrate that Brucella down-regulates total NAD levels in host cells by using both BtpA and BtpB effectors. Our results show that NAD⁺ is targeted by Brucella during infection, which may constitute a novel mechanism for its pathogenicity.

Identification of putative effectors of the Type IV secretion system from the Wolbachia endosymbiont of Brugia malayi

Wolbachia is an unculturable, intracellular bacterium that persists within an extremely broad range of arthropod and parasitic nematode hosts, where it is transmitted maternally to offspring via vertical transmission. In the filarial nematode Brugia malayi, a causative agent of human lymphatic filariasis, Wolbachia is an endosymbiont, and its presence is essential for proper nematode development, survival, and pathogenesis. While the elucidation of Wolbachia:nematode interactions that promote the bacterium’s intracellular persistence is of great importance, research has been hampered due to the fact that Wolbachia cannot be cultured in the absence of host cells. The Wolbachia endosymbiont of B. malayi (wBm) has an active Type IV secretion system (T4SS). Here, we have screened 47 putative T4SS effector proteins of wBm for their ability to modulate growth or the cell biology of a typical eukaryotic cell, Saccharomyces cerevisiae. Five candidates strongly inhibited yeast growth upon expression, and 6 additional proteins showed toxicity in the presence of zinc and caffeine. Studies on the uptake of an endocytic vacuole-specific fluorescent marker, FM4-64, identified 4 proteins (wBm0076 wBm00114, wBm0447 and wBm0152) involved in vacuole membrane dynamics. The WAS(p)-family protein, wBm0076, was found to colocalize with yeast cortical actin patches and disrupted actin cytoskeleton dynamics upon expression. Deletion of the Arp2/3-activating protein, Abp1p, provided resistance to wBm0076 expression, suggesting a role for wBm0076 in regulating eukaryotic actin dynamics and cortical actin patch formation. Furthermore, wBm0152 was found to strongly disrupt endosome:vacuole cargo trafficking in yeast. This study provides molecular insight into the potential role of the T4SS in the Wolbachia endosymbiont:nematode relationship.

Yeast as a Heterologous Model System to Uncover Type III Effector Function

Type III effectors (T3E) are key virulence proteins that are injected by bacterial pathogens inside the cells of their host to subvert cellular processes and contribute to disease. The budding yeast Saccharomyces cerevisiae represents an important heterologous system for the functional characterisation of T3E proteins in a eukaryotic environment. Importantly, yeast contains eukaryotic processes with low redundancy and are devoid of immunity mechanisms that counteract T3Es and mask their function. Expression in yeast of effectors from both plant and animal pathogens that perturb conserved cellular processes often resulted in robust phenotypes that were exploited to elucidate effector functions, biochemical properties, and host targets. The genetic tractability of yeast and its amenability for high-throughput functional studies contributed to the success of this system that, in recent years, has been used to study over 100 effectors. Here, we provide a critical view on this body of work and describe advantages and limitations inherent to the use of yeast in T3E research. “Favourite” targets of T3Es in yeast are cytoskeleton components and small GTPases of the Rho family. We describe how mitogen-activated protein kinase (MAPK) signalling, vesicle trafficking, membrane structures, and programmed cell death are also often altered by T3Es in yeast and how this reflects their function in the natural host. We describe how effector structure–function studies and analysis of candidate targeted processes or pathways can be carried out in yeast. We critically analyse technologies that have been used in yeast to assign biochemical functions to T3Es, including transcriptomics and proteomics, as well as suppressor, gain-of-function, or synthetic lethality screens. We also describe how yeast can be used to select for molecules that block T3E function in search of new antibacterial drugs with medical applications. Finally, we provide our opinion on the limitations of S. cerevisiae as a model system and its most promising future applications.

Structural insight into effector proteins of Gram-negative bacterial pathogens that modulate the phosphoproteome of their host

Invading pathogens manipulate cellular process of the host cell to establish a safe replicative niche. To this end they secrete a spectrum of proteins called effectors that modify cellular environment through a variety of mechanisms. One of the most important mechanisms is the manipulation of cellular signaling through modifications of the cellular phosphoproteome. Phosphorylation/dephosphorylation plays a pivotal role in eukaryotic cell signaling, with ∼ 500 different kinases and ∼ 130 phosphatases in the human genome. Pathogens affect the phosphoproteome either directly through the action of bacterial effectors, and/or indirectly through downstream effects of host proteins modified by the effectors. Here we review the current knowledge of the structure, catalytic mechanism and function of bacterial effectors that modify directly the phosphorylation state of host proteins. These effectors belong to four enzyme classes: kinases, phosphatases, phospholyases and serine/threonine acetylases.

Structural basis of eukaryotic cell targeting by type III secretion system (T3SS) effectors

Type III secretion systems (T3SS) are macromolecular complexes that translocate a wide number of effector proteins into eukaryotic host cells. Once within the cytoplasm, many T3SS effectors mimic the structure and/or function of eukaryotic proteins in order to manipulate signaling cascades, and thus play pivotal roles in colonization, invasion, survival and virulence. Structural biology techniques have played key roles in the unraveling of bacterial strategies employed for mimicry and targeting. This review provides an overall view of our current understanding of structure and function of T3SS effectors, as well as of the different classes of eukaryotic proteins that are targeted and the consequences for the infected cell.

Global impact of Salmonella pathogenicity island 2-secreted effectors on the host phosphoproteome

During the late stages of infection, Salmonella secretes numerous effectors through a type III secretion system that is encoded within Salmonella pathogenicity island 2 (SPI2). Despite the importance of SPI2 as a major virulence factor leading to the systemic spread of the bacteria and diseases, a global view of its effects on host responses is still lacking. Here, we measured global impacts of SPI2 effectors on the host phosphorylation and protein expression levels in RAW264.7 and in HeLa cells, as macrophage and nonphagocytic models of infection. We observe that SPI2 effectors differentially modulate the host phosphoproteome and cellular processes (e.g. protein trafficking, cytoskeletal regulation, and immune signaling) in a host cell-dependent manner. Our unbiased approach reveals the involvement of many previously unrecognized proteins, including E3 ligases (HERC4, RanBP2, and RAD18), kinases (CDK, SIK3, and WNK1), and histones (H2B1F, H4, and H15), in late stages of Salmonella infection. Furthermore, from this phosphoproteome analysis and other quantitative screens, we identified HSP27 as a direct in vitro and in vivo molecular target of the only type III secreted kinase, SteC. Using biochemical and cell biological assays, we demonstrate that SteC phosphorylates multiple sites in HSP27 and induces actin rearrangement through this protein. Together, these results provide a broader landscape of host players contributing to specific processes/pathways mediated by SPI2 effectors than was previously appreciated.

Impact of Salmonella enterica Type III Secretion System Effectors on the Eukaryotic Host Cell

Type III secretion systems are molecular machines used by many Gram-negative bacterial pathogens to inject proteins, known as effectors, directly into eukaryotic host cells. These proteins manipulate host signal transduction pathways and cellular processes to the pathogen’s advantage. Salmonella enterica possesses two virulence-related type III secretion systems that deliver more than forty effectors. This paper reviews our current knowledge about the functions, biochemical activities, host targets, and impact on host cells of these effectors. First, the concerted action of effectors at the cellular level in relevant aspects of the interaction between Salmonella and its hosts is analyzed. Then, particular issues that will drive research in the field in the near future are discussed. Finally, detailed information about each individual effector is provided.