Expression of the eukaryotic Trypanosoma cruzi CRA gene in Yersinia enterocolitica and induction of an immune response against CRA in mice

Recruitment of heterologous substrates by bacterial secretion systems for transkingdom translocation

Bacterial secretion systems mediate the selective exchange of macromolecules between bacteria and their environment, playing a pivotal role in processes such as horizontal gene transfer or virulence. Among the different families of secretion systems, Type III, IV and VI (T3SS, T4SS and T6SS) share the ability to inject their substrates into human cells, opening up the possibility of using them as customized injectors. For this to happen, it is necessary to understand how substrates are recruited and to be able to engineer secretion signals, so that the transmembrane machineries can recognize and translocate the desired substrates in place of their own. Other factors, such as recruiting proteins, chaperones, and the degree of unfolding required to cross through the secretion channel, may also affect transport. Advances in the knowledge of the secretion mechanism have allowed heterologous substrate engineering to accomplish translocation by T3SS, and to a lesser extent, T4SS and T6SS into human cells. In the case of T4SS, transport of nucleoprotein complexes adds a bonus to its biotechnological potential. Here, we review the current knowledge on substrate recognition by these secretion systems, the many examples of heterologous substrate translocation by engineering of secretion signals, and the current and future biotechnological and biomedical applications derived from this approach.

A bacterial type III secretion-based protein delivery tool for broad applications in cell biology

Methods enabling the delivery of proteins into eukaryotic cells are essential to address protein functions. Here we propose broad applications to cell biology for a protein delivery tool based on bacterial type III secretion (T3S). We show that bacterial, viral, and human proteins, fused to the N-terminal fragment of the Yersinia enterocolitica T3S substrate YopE, are effectively delivered into target cells in a fast and controllable manner via the injectisome of extracellular bacteria. This method enables functional interaction studies by the simultaneous injection of multiple proteins and allows the targeting of proteins to different subcellular locations by use of nanobody-fusion proteins. After delivery, proteins can be freed from the YopE fragment by a T3S-translocated viral protease or fusion to ubiquitin and cleavage by endogenous ubiquitin proteases. Finally, we show that this delivery tool is suitable to inject proteins in living animals and combine it with phosphoproteomics to characterize the systems-level impact of proapoptotic human truncated BID on the cellular network.

Yersinia as oral live carrier vaccine: influence of Yersinia outer proteins (Yops) on the T-cell response

Attenuated enteropathogenic Yersinia strains are attractive candidates for the development of oral live carrier vaccines. Yersiniae colonize the small intestine and invade lymphoid tissue of the terminal ileum where they replicate extracellularly. Yersiniae can be engineered to secrete or translocate heterologous antigens into the cytosol of antigen-presenting cells by their type 3 secretion system (T3SS). This results in the induction of both cellular and humoral immune responses to heterologous antigens of viral, bacterial and parasitic origin. In this review, we summarize the progress in developing Yersinia-based vaccine carrier strains by mutating the T3SS effector proteins of Yersinia called Yops (Yersinia outer proteins) to both attenuate the strains and to modulate the T-cell response.

Attenuated Yersinia pseudotuberculosis carrier vaccine for simultaneous antigen-specific CD4 and CD8 T-cell induction

Yersinia pseudotuberculosis employs a type III secretion system for targeting of several virulence factors directly to the cytosol of eukaryotic cells. This protein translocation mechanism mediates the ability of Yersinia to resist phagocytosis and is required for sustained extracellular bacterial replication. In the present study, the Yersinia outer protein E (YopE) was used as a carrier molecule for type III-dependent secretion and translocation of listeriolysin O (LLO) from Listeria monocytogenes. In comparison to wild-type Yersinia, an attenuated Y. pseudotuberculosis yopK-null mutant strain hypertranslocates chimeric YopE/LLO into the cytosol of macrophages, resulting in enhanced major histocompatibility complex (MHC) class I-restricted antigen presentation of an LLO-derived CD8 T-cell epitope. Remarkably, T-cell activation assays also revealed a superior ability of translocated over secreted LLO to induce MHC class II-restricted antigen presentation. These in vitro observations were confirmed after immunization of mice with a single dose of the yopK-null mutant strain. Animals orally inoculated with recombinant Yersinia expressing translocated chimeric YopE/LLO revealed high numbers of gamma interferon-producing LLO-specific CD4 and CD8 T cells. For the first time, it is shown that cytosolic antigen display mediated by an extracellular bacterial carrier vaccine results in simultaneous CD4 and CD8 T-cell priming, conferring protection against an intracellular pathogen.

Type iii protein secretion in yersinia species

The type III mechanism of protein secretion is a pathogenic strategy shared by a number of gram-negative pathogens of plants and animals that has evolved in order to inject virulence proteins into the cytosol of target eukaryotic cells. The pathogens of the Yersinia genus represent a model system where much progress has been made in understanding this secretion pathway. Herein, we review what has been recently learned in yersiniae about the various environmental signals that induce type III secretion, how the synthesis of secretion substrates is regulated, and how such a diverse group of proteins is recognized as a substrate for secretion.

Use of live bacterial vaccine vectors for antigen delivery: potential and limitations

Most infectious agents are restricted to the mucosal membranes or their transit through the mucosa constitutes a critical step in the infection process. Therefore, the elicitation of an efficient immune response, not only at systemic, but also at mucosal level, after vaccination is highly desirable, representing a significant advantage in order to prevent infection. This goal can be only achieved, when the vaccine formulation is administered by the mucosal route. However, soluble antigens given by this route are usually poorly immunogenic. Among the available approaches to stimulate efficient mucosal responses, the use of bacterial carriers to deliver vaccine antigens, probably, constitutes one of the most successful strategies. The potential and limitations of the most extensively studied bacterial carrier systems will be discussed.

Rational live oral carrier vaccine design by mutating virulence-associated genes of Yersinia enterocolitica

Three different Yersinia enterocolitica serotype O8 strains harboring mutations in virulence-associated genes coding for Yersinia adhesin A (YadA), Mn-cofactored superoxide dismutase (SodA), and high-molecular-weight protein 1 were analyzed for their ability to colonize and persist in tissues after orogastric immunization of C57BL/6 mice. We demonstrated that all three Yersinia mutant strains were markedly impaired in their ability to disseminate into the spleens and livers of immunized mice but were able to colonize the Peyer's patches for at least 12 days, resulting in the induction of significant antibody titers against Yersinia outer proteins (Yops) and in the priming of Yersinia antigen-specific CD4+ Th1 cells isolated from spleens. The high level of attenuation did not diminish the immunogenic properties of the mutant strains. In fact, mice immunized with a single oral dose of any of the mutant strains were protected against a lethal oral-challenge infection with wild-type Y. enterocolitica. Moreover, adoptive transfer of Yersinia-specific antibodies from sera of mice immunized with the mutant WAP-314 sodA revealed that this protection could be mediated by Yersinia-specific immunoglobulins.

The virulence plasmid of Yersinia, an antihost genome

The 70-kb virulence plasmid enables Yersinia spp. (Yersinia pestis, Y. pseudotuberculosis, and Y. enterocolitica) to survive and multiply in the lymphoid tissues of their host. It encodes the Yop virulon, an integrated system allowing extracellular bacteria to disarm the cells involved in the immune response, to disrupt their communications, or even to induce their apoptosis by the injection of bacterial effector proteins. This system consists of the Yop proteins and their dedicated type III secretion apparatus, called Ysc. The Ysc apparatus is composed of some 25 proteins including a secretin. Most of the Yops fall into two groups. Some of them are the intracellular effectors (YopE, YopH, YpkA/YopO, YopP/YopJ, YopM, and YopT), while the others (YopB, YopD, and LcrV) form the translocation apparatus that is deployed at the bacterial surface to deliver the effectors into the eukaryotic cells, across their plasma membrane. Yop secretion is triggered by contact with eukaryotic cells and controlled by proteins of the virulon including YopN, TyeA, and LcrG, which are thought to form a plug complex closing the bacterial secretion channel. The proper operation of the system also requires small individual chaperones, called the Syc proteins, in the bacterial cytosol. Transcription of the genes is controlled both by temperature and by the activity of the secretion apparatus. The virulence plasmid of Y. enterocolitica and Y. pseudotuberculosis also encodes the adhesin YadA. The virulence plasmid contains some evolutionary remnants including, in Y. enterocolitica, an operon encoding resistance to arsenic compounds.

YopT, a new Yersinia Yop effector protein, affects the cytoskeleton of host cells

Extracellular Yersinia disarm the immune system of their host by injecting effector Yop proteins into the cytosol of target cells. Five effectors have been described: YopE, YopH, YpkA/YopO, YopP and YopM. Delivery of these effectors by Yersinia adhering at the cell surface requires other Yops (translocators) including YopB. Effector and translocator Yops are secreted by the type III Ysc secretion apparatus, and some Yops also need a specific cytosolic chaperone, called Syc. In this paper, we describe a new Yop, which we have called YopT (35.5kDa). Its secretion required an intact Ysc apparatus and SycT (15.0kDa, pl4.4), a new chaperone resembling SycE. Infection of macrophages with a Yersinia, producing a hybrid YopT-adenylate cyclase, led to the accumulation of intracellular cAMP, indicating that YopT is delivered into the cytosol of eukaryotic cells. Infection of HeLa cells with a mutant strain devoid of the five known Yop effectors (deltaHOPEM strain) but producing YopT resulted in the alteration of the cell cytoskeleton and the disruption of the actin filament structure. This cytotoxic effect was caused by YopT and dependent on YopB. YopT is thus a new effector Yop and a new bacterial toxin affecting the cytoskeleton of eukaryotic cells.

LcrG is required for efficient translocation of Yersinia Yop effector proteins into eukaryotic cells

Extracellular Yersinia disables the immune system of its host by injecting effector Yop proteins into host cells. We show that a Yersinia enterocolitica nonpolar lcrG mutant is severely impaired in the translocation of YopE, YopH, YopM, YpkA/YopO, and YopP into eukaryotic cells. LcrG is thus required for efficient internalization of all the known Yop effectors.

Targeting of Yersinia Yop proteins into the cytosol of HeLa cells: one-step translocation of YopE across bacterial and eukaryotic membranes is dependent on SycE chaperone

Pathogenic Yersiniae adhere to and kill macrophages by targeting some of their Yop proteins into the eukaryotic cytosol. There is debate about whether YopE targeting proceeds as a direct translocation of polypeptide between cells or in two distinct steps, each requiring specific signals for YopE secretion across the bacterial envelope and for translocation into the eukaryotic cytosol. Here, we used the selective solubilization of the eukaryotic plasma membrane with digitonin to measure Yop targeting during Yersinia infections of HeLa cells. YopE, YopH, YopM and YopN were found in the eukaryotic cytosol but not in the extracellular medium. When bound to SycE chaperone in the Yersinia cytoplasm, YopE residues 1-100 are necessary and sufficient for the targeting of hybrid neomycin phosphotransferase. Electron microscopic analysis failed to detect an extracellular intermediate of YopE targeting, suggesting a one-step translocation mechanism.

TyeA, a protein involved in control of Yop release and in translocation of Yersinia Yop effectors

Extracellular Yersinia spp. disarm the immune system by injecting the effector Yersinia outer proteins (Yops) into the target cell. Yop secretion is triggered by contact with eukaryotic cells or by Ca2+ chelation. Two proteins, YopN and LcrG, are known to be involved in Yop-secretion control. Here we describe TyeA, a third protein involved in the control of Yop release. Like YopN, TyeA is localized at the bacterial surface. A tyeA knock-out mutant secreted Yops in the presence of Ca2+ and in the absence of eukaryotic cells. Unlike a yopN null mutant, the tyeA mutant was defective for translocation of YopE and YopH, but not YopM, YopO and YopP, into eukaryotic cells. This is the first observation suggesting that Yop effectors can be divided into two sets for delivery into eukaryotic cells. TyeA was found to interact with the translocator YopD and with residues 242-293 of YopN. In contrast with a yopN null mutant, a yopNDelta248-272 mutant was also unable to translocate YopE and YopH. Our results suggest that TyeA forms part of the translocation-control apparatus together with YopD and YopN, and that the interaction of these proteins is required for selective translocation of Yops inside eukaryotic cells.

Status of YopM and YopN in the Yersinia Yop virulon: YopM of Y.enterocolitica is internalized inside the cytosol of PU5-1.8 macrophages by the YopB, D, N delivery apparatus

The Yersinia Yop virulon is an anti-host system made up of four elements: (i) a type III secretion system called Ysc; (ii) a system designed to deliver bacterial proteins into eukaryotic target cells (YopB, YopD); (iii) a control element (YopN); and (iv) a set of intracellularly delivered proteins designed to disarm these cells or disrupt their communications (YopE, YopH and possibly others). YopM, another Yop protein, binds thrombin and is thus presumed to act as an extracellular effector. Here, we analyzed YopM from Y.enterocolitica and we wondered whether it could also be delivered inside eukaryotic cells. To answer this question we applied the Yop-Cya reporter strategy. Hybrids made of 141 or 100 N-terminal residues of YopM fused to Cya were delivered inside PU5-1.8 macrophages by recombinant Y.enterocolitica strains. YopB and YopD were required as translocators. Leakage of the reporters into the macrophage culture supernatant during the bacterial infection increased strongly when YopN was missing, showing that YopN is involved in the control of delivery of YopM inside eukaryotic cells. YopN itself was not delivered into the macrophages. In conclusion, YopM is translocated inside the eukaryotic cells and its physiopathological role should be revised or completed.

The cytosolic SycE and SycH chaperones of Yersinia protect the region of YopE and YopH involved in translocation across eukaryotic cell membranes

Yersinia adhering at the surface of eukaryotic cells secrete a set of proteins called Yops. This secretion which occurs via a type III secretion pathway is immediately followed by the injection of some Yops into the cytosol of eukaryotic cells. Translocation of YopE and YopH across the eukaryotic cell membranes requires the presence of the translocators YopB and YopD. YopE and YopH are modular proteins composed of an N-terminal secretion signal, an internalization domain, and an effector domain. Secretion of YopE and YopH requires the presence of the specific cytosolic chaperones SycE and SycH, respectively. In this work, we have mapped the regions of YopE and YopH that are involved in binding of their cognate chaperone. There is only one Syc-binding domain in YopE (residues 15-50) and YopH (residues 20-70). This domain is localized immediately after the secretion signal and it corresponds to the internalization domain. Removal of this bifunctional domain did not affect secretion of YopE and YopH and even suppressed the need for the chaperone in the secretion process. Thus SycE and SycH are not secretion pilots. Instead, we propose that they prevent intrabacterial interaction of YopE and YopH with proteins involved in translocation of these Yops across eukaryotic cell membranes.

Identification of the YopE and YopH domains required for secretion and internalization into the cytosol of macrophages, using the cyaA gene fusion approach

Pathogenic yersiniae secrete a set of antihost proteins, called Yops, by a type III secretion mechanism. Upon infection of cultured epithelial cells, extracellular Yersinia pseudotuberculosis and Yersinia enterocolitica translocate cytotoxin YopE across the host cell plasma membrane. Several lines of evidence suggest that tyrosine phosphatase YopH follows the same pathway. We analyzed internalization of YopE and YopH into murine PU5-1.8 macrophages by using recombinant Y. enterocolitica producing truncated YopE and YopH proteins fused to a calmodulin-dependent adenylate cyclase. The YopE-cyclase and YopH-cyclase hybrids were readily secreted by Y. enterocolitica. The N-terminal domain required for secretion was not longer than 15 residues of YopE and 17 residues of YopH. Internalization into eukaryotic cells, revealed by cAMP production, only required the N-terminal 50 amino acid residues of YopE and the N-terminal 71 amino acid residues of YopH. YopE and YopH are thus modular proteins composed of a secretion domain, a translocation domain, and an effector domain. Translocation of YopE and YopH across host cell's membranes was also dependent on the secretion of YopB and YopD by the same bacterium. The cyclase fusion approach could be readily extended to study the fate of other proteins secreted by invasive bacterial pathogens.

Translocation of a hybrid YopE-adenylate cyclase from Yersinia enterocolitica into HeLa cells

Pathogenic bacteria of the genus Yersinia release in vitro a set of antihost proteins called Yops. Upon infection of cultured epithelial cells, extracellular Yersinia pseudotuberculosis transfers YopE across the host cell plasma membrane. To facilitate the study of this translocation process, we constructed a recombinant Yersinia enterocolitica strain producing YopE fused to a reporter enzyme. As a reporter, we selected the calmodulin-dependent adenylate cyclase of Bordetella pertussis and we monitored the accumulation of cyclic AMP (cAMP). Since bacteria do not produce calmodulin, cyclase activity marks the presence of hybrid enzyme in the cytoplasmic compartment of the eukaryotic cell. Infection of a monolayer of HeLa cells by the recombinant Y. enterocolitica strain led to a significant increase of cAMP. This phenomenon was dependent not only on the integrity of the Yop secretion pathway but also on the presence of YopB and/or YopD. It also required the presence of the adhesin YadA at the bacterial surface. In contrast, the phenomenon was not affected by cytochalasin D, indicating that internalization of the bacteria themselves was not required for the translocation process. Our results demonstrate that Y. enterocolitica is able to transfer hybrid proteins into eukaryotic cells. This system can be used not only to study the mechanism of YopE translocation but also the fate of the other Yops or even of proteins secreted by other bacterial pathogens.

Individual chaperones required for Yop secretion by Yersinia

Pathogenic yersiniae secrete anti-host proteins called Yops, by a recently discovered Sec-independent pathway. The Yops do not have a classical signal peptide at their N terminus and they are not processed during membrane translocation. The secretion domain is nevertheless contained in their N-terminal part but these domains do not resemble each other in the different Yops. We have previously shown that YopE secretion requires SycE, a 15-kDa acidic protein acting as a specific cytosolic chaperone. Here we show that the gene downstream from yopH encodes a 16-kDa acidic protein that binds to hybrid proteins made of the N-terminal part of YopH and either the bacterial alkaline phosphatase or the cholera toxin B subunit. Loss of this protein by mutagenesis led to accumulation of YopH in the cytoplasm and to a severe and selective reduction of YopH secretion. This protein thus behaves like the counterpart of SycE and we called it SycH. We also engineered a mutation in lcrH, the gene upstream from yopB and yopD, known to encode a 19-kDa acidic protein. Although this mutation was nonpolar, the mutant no longer secreted YopB and YopD. The product of lcrH could be immunoprecipitated together with cytoplasmic YopD. lcrH therefore seems to encode a YopD-specific chaperone, which we called SycD. Determination of the dependence of YopB on SycD requires further investigation. SycE, SycH, and SycD appear to be members of a new family of cytosolic chaperones required for Yop secretion.

YscU, a Yersinia enterocolitica inner membrane protein involved in Yop secretion

Pathogenic yersiniae secrete antihost Yop proteins by a recently discovered secretion pathway which is also encountered in several animal and plant pathogens. The components of the export machinery are encoded by the virA (lcrA), virB (lcrB), and virC (lcrC) loci of the 70-kb pYV plasmid. In the present paper we describe yscU, the last gene of the virB locus. We determined the DNA sequence and mutated the gene on the pYV plasmid. After inactivation of yscU, the mutant strain was unable to secrete Yop proteins. The topology of YscU was investigated by the analysis of YscU-PhoA translational fusions generated by TnphoA transposition. This showed that the 40.3-kDa yscU product contains four transmembrane segments anchoring a large cytoplasmic carboxyl-terminal domain to the inner membrane. YscU is related to Spa40 from Shigella flexneri, to SpaS from Salmonella typhimurium, to FlhB from Bacillus subtilis, and to HrpN from Pseudomonas solanacearum.