Role for cis-acting RNA sequences in the temperature-dependent expression of the multiadhesive lig proteins in Leptospira interrogans

RNA thermometers and other regulatory elements: Diversity and importance in bacterial pathogenesis

Survival of microorganisms depends to a large extent on environmental conditions and the occupied host. By adopting specific strategies, microorganisms can thrive in the surrounding environment and, at the same time, preserve their viability. Evading the host defenses requires several mechanisms compatible with the host survival which include the production of RNA thermometers to regulate the expression of genes responsible for heat or cold shock as well as of those involved in virulence. Microorganisms have developed a variety of molecules in response to the environmental changes in temperature and even more specifically to the host they invade. Among all, RNA-based regulatory mechanisms are the most common ones, highlighting the importance of such molecules in gene expression control and novel drug development by suitable structure-based alterations. This article is categorized under: RNA Structure and Dynamics > RNA Structure, Dynamics and Chemistry RNA in Disease and Development > RNA in Disease RNA Structure and Dynamics > Influence of RNA Structure in Biological Systems.

The FUR-like regulators PerRA and PerRB integrate a complex regulatory network that promotes mammalian host-adaptation and virulence of Leptospira interrogans

Leptospira interrogans, the causative agent of most cases of human leptospirosis, must respond to myriad environmental signals during its free-living and pathogenic lifestyles. Previously, we compared L. interrogans cultivated in vitro and in vivo using a dialysis membrane chamber (DMC) peritoneal implant model. From these studies emerged the importance of genes encoding the Peroxide responsive regulators PerRA and PerRB. First described in in Bacillus subtilis, PerRs are widespread in Gram-negative and -positive bacteria, where regulate the expression of gene products involved in detoxification of reactive oxygen species and virulence. Using perRA and perRB single and double mutants, we establish that L. interrogans requires at least one functional PerR for infectivity and renal colonization in a reservoir host. Our finding that the perRA/B double mutant survives at wild-type levels in DMCs is noteworthy as it demonstrates that the loss of virulence is not due to a metabolic lesion (i.e., metal starvation) but instead reflects dysregulation of virulence-related gene products. Comparative RNA-Seq analyses of perRA, perRB and perRA/B mutants cultivated within DMCs identified 106 genes that are dysregulated in the double mutant, including ligA, ligB and lvrA/B sensory histidine kinases. Decreased expression of LigA and LigB in the perRA/B mutant was not due to loss of LvrAB signaling. The majority of genes in the perRA and perRB single and double mutant DMC regulons were differentially expressed only in vivo, highlighting the importance of host signals for regulating gene expression in L. interrogans. Importantly, the PerRA, PerRB and PerRA/B DMC regulons each contain multiple genes related to environmental sensing and/or transcriptional regulation. Collectively, our data suggest that PerRA and PerRB are part of a complex regulatory network that promotes host adaptation by L. interrogans within mammals.

Distinct transcriptional profiles of Leptospira borgpetersenii serovar Hardjo strains JB197 and HB203 cultured at different temperatures

Background

Leptospirosis is a zoonotic, bacterial disease, posing significant health risks to humans, livestock, and companion animals around the world. Symptoms range from asymptomatic to multi-organ failure in severe cases. Complex species-specific interactions exist between animal hosts and the infecting species, serovar, and strain of pathogen. Leptospira borgpetersenii serovar Hardjo strains HB203 and JB197 have a high level of genetic homology but cause different clinical presentation in the hamster model of infection; HB203 colonizes the kidney and presents with chronic shedding while JB197 causes severe organ failure and mortality. This study examines the transcriptome of L. borgpetersenii and characterizes differential gene expression profiles of strains HB203 and JB197 cultured at temperatures during routine laboratory conditions (29°C) and encountered during host infection (37°C).

Methodology/Principal findings

L. borgpetersenii serovar Hardjo strains JB197 and HB203 were isolated from the kidneys of experimentally infected hamsters and maintained at 29°C and 37°C. RNAseq revealed distinct gene expression profiles; 440 genes were differentially expressed (DE) between JB197 and HB203 at 29°C, and 179 genes were DE between strains at 37°C. Comparison of JB197 cultured at 29°C and 37°C identified 135 DE genes while 41 genes were DE in HB203 with those same culture conditions. The consistent differential expression of ligB, which encodes the outer membrane virulence factor LigB, was validated by immunoblotting and 2D-DIGE. Differential expression of lipopolysaccharide was also observed between JB197 and HB203.

Conclusions/Significance

Investigation of the L. borgpetersenii JB197 and HB203 transcriptome provides unique insight into the mechanistic differences between acute and chronic disease. Characterizing the nuances of strain to strain differences and investigating the environmental sensitivity of Leptospira to temperature is critical to the development and progress of leptospirosis prevention and treatment technologies, and is an important consideration when serovars are selected and propagated for use as bacterin vaccines as well as for the identification of novel therapeutic targets.

Leptospirosis is a global zoonotic, neglected tropical disease. Interestingly, a high level of species specificity (both bacteria and host) plays a major role in the severity of disease presentation which can vary from asymptomatic to multi-organ failure. Pathogenic Leptospira colonize the kidneys of infected individuals and are shed in urine into the environment where they can survive until they are contracted by another host. This study looks at two strains of L. borgpetersenii, HB203 and JB197 which are genetically very similar, and identical by serotyping as serovar Hardjo, yet HB203 causes a chronic infection in the hamster while JB197 causes organ failure and mortality. To better characterize bacterial factors causing different disease outcomes, we examined the gene expression profile of these strains in the context of temperatures that would reflect natural Leptospira life cycles (environmentally similar 29°C and 37°C which is more indicative of host environment). We found vast differences in gene expression both between the strains and within strains between temperatures. Characterization of the transcriptome of L. borgpetersenii serovar Hardjo strains JB197 and HB203 provides insights into factors that can determine acute versus chronic disease in the hamster model of infection. Additionally, these studies highlight strain to strain variability within the same species, and serovar, at different growth temperatures, which needs to be considered when serovars are selected and propagated for use as bacterin vaccines used to immunize domestic animal species.

A Salmonella Typhi RNA thermosensor regulates virulence factors and innate immune evasion in response to host temperature

Sensing and responding to environmental signals is critical for bacterial pathogens to successfully infect and persist within hosts. Many bacterial pathogens sense temperature as an indication they have entered a new host and must alter their virulence factor expression to evade immune detection. Using secondary structure prediction, we identified an RNA thermosensor (RNAT) in the 5' untranslated region (UTR) of tviA encoded by the typhoid fever-causing bacterium Salmonella enterica serovar Typhi (S. Typhi). Importantly, tviA is a transcriptional regulator of the critical virulence factors Vi capsule, flagellin, and type III secretion system-1 expression. By introducing point mutations to alter the mRNA secondary structure, we demonstrate that the 5' UTR of tviA contains a functional RNAT using in vitro expression, structure probing, and ribosome binding methods. Mutational inhibition of the RNAT in S. Typhi causes aberrant virulence factor expression, leading to enhanced innate immune responses during infection. In conclusion, we show that S. Typhi regulates virulence factor expression through an RNAT in the 5' UTR of tviA. Our findings demonstrate that limiting inflammation through RNAT-dependent regulation in response to host body temperature is important for S. Typhi's "stealthy" pathogenesis.

Leptospiral Immunoglobulin-Like Domain Proteins: Roles in Virulence and Immunity

The virulence mechanisms required for infection and evasion of immunity by pathogenic Leptospira species remain poorly understood. A number of L. interrogans surface proteins have been discovered, lying at the interface between the pathogen and host. Among these proteins, the functional properties of the Lig (leptospiral immunoglobulin-like domain) proteins have been examined most thoroughly. LigA, LigB, and LigC contain a series of, 13, 12, and 12 closely related domains, respectively, each containing a bacterial immunoglobulin (Big) -like fold. The multidomain region forms a mostly elongated structure that exposes a large surface area. Leptospires wield the Lig proteins to promote interactions with a range of specific host proteins, including those that aid evasion of innate immune mechanisms. These diverse binding events mediate adhesion of L. interrogans to the extracellular matrix, inhibit hemostasis, and inactivate key complement proteins. These interactions may help L. interrogans overcome the physical, hematological, and immunological barriers that would otherwise prevent the spirochete from establishing a systemic infection. Despite significant differences in the affinities of the LigA and LigB proteins for host targets, their functions overlap during lethal infection of hamsters; virulence is lost only when both ligA and ligB transcription is knocked down simultaneously. Lig proteins have been shown to be promising vaccine antigens through evaluation of a variety of different adjuvant strategies. This review serves to summarize current knowledge of Lig protein roles in virulence and immunity and to identify directions needed to better understand the precise functions of the Lig proteins during infection.

Feeling the heat at the millennium: Thermosensors playing with fire

An outstanding question regards the ability of organisms to sense their environments and respond in a suitable way. Pathogenic bacteria in particular exploit host-temperature sensing as a cue for triggering virulence gene expression. This micro-review does not attempt to fully cover the field of bacterial thermosensors and in detail describe each identified case. Instead, the review focus on the time-period at the end of the 1990's and beginning of the 2000's when several key discoveries were made, identifying protein, DNA and RNA as potential thermosensors controlling gene expression in several different bacterial pathogens in general and on the prfA thermosensor of Listeria monocytogenes in particular.

RNA-based thermoregulation of a Campylobacter jejuni zinc resistance determinant

RNA thermometers (RNATs) trigger bacterial virulence factor expression in response to the temperature shift on entering a warm-blooded host. At lower temperatures these secondary structures sequester ribosome-binding sites (RBSs) to prevent translation initiation, whereas at elevated temperatures they "melt" allowing translation. Campylobacter jejuni is the leading bacterial cause of human gastroenteritis worldwide yet little is known about how it interacts with the host including host induced gene regulation. Here we demonstrate that an RNAT regulates a C. jejuni gene, Cj1163c or czcD, encoding a member of the Cation Diffusion Facilitator family. The czcD upstream untranslated region contains a predicted stem loop within the mRNA that sequesters the RBS to inhibit translation at temperatures below 37°C. Mutations that disrupt or enhance predicted secondary structure have significant and predictable effects on temperature regulation. We also show that in an RNAT independent manner, CzcD expression is induced by Zn(II). Mutants lacking czcD are hypersensitive to Zn(II) and also over-accumulate Zn(II) relative to wild-type, all consistent with CzcD functioning as a Zn(II) exporter. Importantly, we demonstrate that C. jejuni Zn(II)-tolerance at 32°C, a temperature at which the RNAT limits CzcD production, is increased by RNAT disruption. Finally we show that czcD inactivation attenuates larval killing in a Galleria infection model and that at 32°C disrupting RNAT secondary structure to allow CzcD production can enhance killing. We hypothesise that CzcD regulation by metals and temperature provides a mechanism for C. jejuni to overcome innate immune system-mediated Zn(II) toxicity in warm-blooded animal hosts.

Isolation and propagation of leptospires at 37 °C directly from the mammalian host

The causative agent of leptospirosis includes multiple serovars and species of pathogenic leptospires that are excreted via urine from reservoir hosts of infection. Primary isolation takes weeks to months, and is limited to semi-solid media at 28-30 °C. Here we present an alternative media formulation, HAN, compared to commercially available EMJH and the more specialized T80/40/LH media formulations, in semi-solid and liquid compositions, for the primary isolation of two diverse species and serovars of pathogenic leptospires directly from host kidney tissue. All three media types supported the isolation and propagation of L. interrogans serovar Copenhageni strain IC:20:001 in semi-solid media at 29 °C. However, only HAN and T80/40/LH supported the growth of L. borgpetersenii serovar Hardjo strain HB15B203 at 29 °C. In addition, HAN supported primary isolation at 37 °C. Both T80/40/LH and HAN supported primary isolation of strain IC:20:001 in liquid media at 29 °C but only HAN supported growth of strain HB15B203 in liquid media, at both 29 and 37 °C. HAN media supports the primary isolation of fastidious pathogenic leptospires directly from infected host tissue at either 29 or 37 °C: this formulation represents a more defined media for the continued optimization of growth factors required to support the primary isolation of the large and diverse range of species and serovars within the genus Leptospira circulating within domestic and wild animal populations.

An RNA thermometer dictates production of a secreted bacterial toxin

Frequent transitions of bacterial pathogens between their warm-blooded host and external reservoirs are accompanied by abrupt temperature shifts. A temperature of 37°C serves as reliable signal for ingestion by a mammalian host, which induces a major reprogramming of bacterial gene expression and metabolism. Enteric Yersiniae are Gram-negative pathogens accountable for self-limiting gastrointestinal infections. Among the temperature-regulated virulence genes of Yersinia pseudotuberculosis is cnfY coding for the cytotoxic necrotizing factor (CNF_Y), a multifunctional secreted toxin that modulates the host’s innate immune system and contributes to the decision between acute infection and persistence. We report that the major determinant of temperature-regulated cnfY expression is a thermo-labile RNA structure in the 5’-untranslated region (5’-UTR). Various translational gene fusions demonstrated that this region faithfully regulates translation initiation regardless of the transcription start site, promoter or reporter strain. RNA structure probing revealed a labile stem-loop structure, in which the ribosome binding site is partially occluded at 25°C but liberated at 37°C. Consistent with translational control in bacteria, toeprinting (primer extension inhibition) experiments in vitro showed increased ribosome binding at elevated temperature. Point mutations locking the 5’-UTR in its 25°C structure impaired opening of the stem loop, ribosome access and translation initiation at 37°C. To assess the in vivo relevance of temperature control, we used a mouse infection model. Y. pseudotuberculosis strains carrying stabilized RNA thermometer variants upstream of cnfY were avirulent and attenuated in their ability to disseminate into mesenteric lymph nodes and spleen. We conclude with a model, in which the RNA thermometer acts as translational roadblock in a two-layered regulatory cascade that tightly controls provision of the CNF_Y toxin during acute infection. Similar RNA structures upstream of various cnfY homologs suggest that RNA thermosensors dictate the production of secreted toxins in a wide range of pathogens.

Bacterial pathogens closely survey the ambient conditions and induce virulence genes only at appropriate conditions. Upon host contact, many pathogens secrete toxins in order to subvert host defense systems. We find that such a secreted toxin in enteropathogenic Yersinia pseudotuberculosis is produced only at host body temperature. This regulation depends on a temperature-responsive RNA structure, an RNA thermometer, in the 5’-untranslated region of the toxin mRNA, which prevents translation at low temperatures when the bacterium is outside the host. Preventing melting of the RNA structure at 37°C by nucleotide substitutions that stabilize base pairing resulted in avirulent Yersinia strains unable to infect mice. Given that similar RNA thermometer-like structures exist upstream of related toxin genes in various bacterial pathogens, we propose that RNA thermometer-mediated toxin production is an evolutionary conserved mechanism. Interfering with opening of such regulatory structures might thus be a promising strategy targeting a broad spectrum of bacterial pathogens.

RNA Thermometers in Bacterial Pathogens

Temperature variation is one of the multiple parameters a microbial pathogen encounters when it invades a warm-blooded host. To survive and thrive at host body temperature, human pathogens have developed various strategies to sense and respond to their ambient temperature. An instantaneous response is mounted by RNA thermometers (RNATs), which are integral sensory structures in mRNAs that modulate translation efficiency. At low temperatures outside the host, the folded RNA blocks access of the ribosome to the translation initiation region. The temperature shift upon entering the host destabilizes the RNA structure and thus permits ribosome binding. This reversible zipper-like mechanism of RNATs is ideally suited to fine-tune virulence gene expression when the pathogen enters or exits the body of its host. This review summarizes our present knowledge on virulence-related RNATs and discusses recent developments in the field.

cis-Acting Determinant Limiting Expression of Sphingomyelinase Gene sph2 in Leptospira interrogans, Identified with a gfp Reporter Plasmid

Many strains of the spirochete Leptospira interrogans serovar Pomona express the osmotically inducible sphingomyelinase gene sph2 at much higher levels than strains from other serovars. We developed a new green fluorescent protein (GFP) reporter plasmid to examine sph2 gene expression determinants. The vector enables the fusion of the test promoter to the ribosome-binding site and coding region of gfp We fused the sph2 promoters from the L. interrogans serovar Lai strain 56601 and from the L. interrogans serovar Pomona strain LC82-25 to gfp to examine the molecular determinants of differential sph2 expression between the two strains. Similar to what was observed with the native sph2 genes, the introduction of the plasmids into the Lai 56601 strain resulted in near background levels of gfp expression from the Lai sph2 promoter, while the expression from the Pomona sph2 promoter was high. The expression of both fusions increased at physiologic levels of osmolarity achieved by adding sodium chloride to the culture medium. We examined the role of a 17-bp upstream element found in all L. interrogans strains expressing low basal levels of sph2 and missing from Pomona strains that express sph2 at high levels. When the 17-bp sequence present upstream of the Lai sph2 promoter was deleted or scrambled, the fusion expression increased substantially. Conversely, the insertion of the 17-bp sequence upstream of the Pomona sph2 promoter diminished fusion expression. In contrast, the removal of an insertion sequence-like element that is found only in the Pomona sph2 upstream sequence had no effect on the expression from the Pomona sph2 fusion in the Lai strain. These findings demonstrate the utility of the gfp reporter plasmid in analyzing gene expression in L. interrogans IMPORTANCE Genetic tools are needed to examine gene expression in the pathogen Leptospira interrogans We developed a reporter plasmid that replicates in L. interrogans with green fluorescent protein (GFP) as the readout of promoter activity. We demonstrated an application of the new reporter plasmid by identifying an upstream element responsible for the poor basal expression of the sph2 sphingomyelinase gene in an L. interrogans serovar Lai strain. This new tool is useful for the discovery of the molecular determinants of L. interrogans gene expression.

Pathogenic Leptospires Modulate Protein Expression and Post-translational Modifications in Response to Mammalian Host Signals

Pathogenic species of Leptospira cause leptospirosis, a bacterial zoonotic disease with a global distribution affecting over one million people annually. Reservoir hosts of leptospirosis, including rodents, dogs, and cattle, exhibit little to no signs of disease but shed large numbers of organisms in their urine. Transmission occurs when mucosal surfaces or abraded skin come into contact with infected urine or urine-contaminated water or soil. Whilst little is known about how Leptospira adapt to and persist within a reservoir host, in vitro studies suggest that leptospires alter their transcriptomic and proteomic profiles in response to environmental signals encountered during mammalian infection. We applied the dialysis membrane chamber (DMC) peritoneal implant model to compare the whole cell proteome of in vivo derived leptospires with that of leptospires cultivated in vitro at 30°C and 37°C by 2-dimensional difference in-gel electrophoresis (2-D DIGE). Of 1,735 protein spots aligned across 9 2-D DIGE gels, 202 protein spots were differentially expressed (p < 0.05, fold change >1.25 or < −1.25) across all three conditions. Differentially expressed proteins were excised for identification by mass spectrometry. Data are available via ProteomeXchange with identifier PXD006995. The greatest differences were detected when DMC-cultivated leptospires were compared with IV30- or IV37-cultivated leptospires, including the increased expression of multiple isoforms of Loa22, a known virulence factor. Unexpectedly, 20 protein isoforms of LipL32 and 7 isoforms of LipL41 were uniformly identified by DIGE as differentially expressed, suggesting that unique post-translational modifications (PTMs) are operative in response to mammalian host conditions. To test this hypothesis, a rat model of persistent renal colonization was used to isolate leptospires directly from the urine of experimentally infected rats. Comparison of urinary derived leptospires to IV30 leptospires by 2-D immunoblotting confirmed that modification of proteins with trimethyllysine and acetyllysine occurs to a different degree in response to mammalian host signals encountered during persistent renal colonization. These results provide novel insights into differential protein and PTMs present in response to mammalian host signals which can be used to further define the unique equilibrium that exists between pathogenic leptospires and their reservoir host of infection.

RNA-mediated signal perception in pathogenic bacteria

Bacterial pathogens encounter several different environments during an infection, many of them possibly being detrimental. In order to sense its surroundings and adjust the gene expression accordingly, different regulatory schemes are undertaken. With these, the bacterium appropriately can differentiate between various environmental cues to express the correct virulence factor at the appropriate time and place. An attractive regulator device is RNA, which has an outstanding ability to alter its structure in response to external stimuli, such as metabolite concentration or alterations in temperature, to control its downstream gene expression. This review will describe the function of riboswitches and thermometers, with a particular emphasis on regulatory RNAs being important for bacterial pathogenicity. WIREs RNA 2017, 8:e1429. doi: 10.1002/wrna.1429 For further resources related to this article, please visit the WIREs website.

Genome-Wide Transcriptional Start Site Mapping and sRNA Identification in the Pathogen Leptospira interrogans

Leptospira are emerging zoonotic pathogens transmitted from animals to humans typically through contaminated environmental sources of water and soil. Regulatory pathways of pathogenic Leptospira spp. underlying the adaptive response to different hosts and environmental conditions remains elusive. In this study, we provide the first global Transcriptional Start Site (TSS) map of a Leptospira species. RNA was obtained from the pathogen Leptospira interrogans grown at 30°C (optimal in vitro temperature) and 37°C (host temperature) and selectively enriched for 5′ ends of native transcripts. A total of 2865 and 2866 primary TSS (pTSS) were predicted in the genome of L. interrogans at 30 and 37°C, respectively. The majority of the pTSSs were located between 0 and 10 nucleotides from the translational start site, suggesting that leaderless transcripts are a common feature of the leptospiral translational landscape. Comparative differential RNA-sequencing (dRNA-seq) analysis revealed conservation of most pTSS at 30 and 37°C. Promoter prediction algorithms allow the identification of the binding sites of the alternative sigma factor sigma 54. However, other motifs were not identified indicating that Leptospira consensus promoter sequences are inherently different from the Escherichia coli model. RNA sequencing also identified 277 and 226 putative small regulatory RNAs (sRNAs) at 30 and 37°C, respectively, including eight validated sRNAs by Northern blots. These results provide the first global view of TSS and the repertoire of sRNAs in L. interrogans. These data will establish a foundation for future experimental work on gene regulation under various environmental conditions including those in the host.

Spirochetal Lipoproteins in Pathogenesis and Immunity

Lipoproteins are lipid-modified proteins that dominate the spirochetal proteome. While found in all bacteria, spirochetal lipoproteins have unique features and play critical roles in spirochete biology. For this reason, considerable effort has been devoted to determining how the lipoproteome is generated. Essential features of the structural elements of lipoproteins are now understood with greater clarity, enabling greater confidence in identification of lipoproteins from genomic sequences. The journey from the ribosome to the outer membrane, and in some cases, to the cellular surface has been defined, including secretion, lipidation, sorting, and export across the outer membrane. Given their abundance and importance, it is not surprising that spirochetes have developed a number of strategies for regulating the spatiotemporal expression of lipoproteins. In some cases, lipoprotein expression is tied to various environmental cues, while in other cases, it is linked to growth rate. This regulation enables spirochetes to express certain lipoproteins at high levels in one phase of the spirochete lifecycle, while dramatically downregulating the same lipoproteins in other phases. The mammalian host has developed specialized mechanisms for recognizing lipoproteins and triggering an immune response. Evasion of that immune response is essential for spirochete persistence. For this reason, spirochetes have developed mechanisms for altering lipoproteins. Lipoproteins recognized by antibodies formed during infection are key serodiagnostic antigens. In addition, lipoprotein vaccines have been developed for generating an immune response to control or prevent a spirochete infection. This chapter summarizes our current understanding of lipoproteins in interactions of spirochetes with their hosts.

Temperature-responsive in vitro RNA structurome of Yersinia pseudotuberculosis

RNA structures are fundamentally important for RNA function. Dynamic, condition-dependent structural changes are able to modulate gene expression as shown for riboswitches and RNA thermometers. By parallel analysis of RNA structures, we mapped the RNA structurome of Yersinia pseudotuberculosis at three different temperatures. This human pathogen is exquisitely responsive to host body temperature (37 °C), which induces a major metabolic transition. Our analysis profiles the structure of more than 1,750 RNAs at 25 °C, 37 °C, and 42 °C. Average mRNAs tend to be unstructured around the ribosome binding site. We searched for 5'-UTRs that are folded at low temperature and identified novel thermoresponsive RNA structures from diverse gene categories. The regulatory potential of 16 candidates was validated. In summary, we present a dynamic bacterial RNA structurome and find that the expression of virulence-relevant functions in Y. pseudotuberculosis and reprogramming of its metabolism in response to temperature is associated with a restructuring of numerous mRNAs.

Control of Gene Expression in Leptospira spp. by Transcription Activator-Like Effectors Demonstrates a Potential Role for LigA and LigB in Leptospira interrogans Virulence

Leptospirosis is a zoonotic disease that affects ∼1 million people annually, with a mortality rate of >10%. Currently, there is an absence of effective genetic manipulation tools for targeted mutagenesis in pathogenic leptospires. Transcription activator-like effectors (TALEs) are a recently described group of repressors that modify transcriptional activity in prokaryotic and eukaryotic cells by directly binding to a targeted sequence within the host genome. To determine the applicability of TALEs within Leptospira spp., two TALE constructs were designed. First, a constitutively expressed TALE gene specific for the lacO-like region upstream of bgaL was trans inserted in the saprophyte Leptospira biflexa (the TALEβgal strain). Reverse transcriptase PCR (RT-PCR) analysis and enzymatic assays demonstrated that BgaL was not expressed in the TALEβgal strain. Second, to study the role of LigA and LigB in pathogenesis, a constitutively expressed TALE gene with specificity for the homologous promoter regions of ligA and ligB was cis inserted into the pathogen Leptospira interrogans (TALElig). LigA and LigB expression was studied by using three independent clones: TALElig1, TALElig2, and TALElig3. Immunoblot analysis of osmotically induced TALElig clones demonstrated 2- to 9-fold reductions in the expression levels of LigA and LigB, with the highest reductions being noted for TALElig1 and TALElig2, which were avirulent in vivo and nonrecoverable from animal tissues. This study reconfirms galactosidase activity in the saprophyte and suggests a role for LigA and LigB in pathogenesis. Collectively, this study demonstrates that TALEs are effective at reducing the expression of targeted genes within saprophytic and pathogenic strains of Leptospira spp., providing an additional genetic manipulation tool for this genus.

Thermal control of virulence factors in bacteria: a hot topic

Pathogenic bacteria sense environmental cues, including the local temperature, to control the production of key virulence factors. Thermal regulation can be achieved at the level of DNA, RNA or protein and although many virulence factors are subject to thermal regulation, the exact mechanisms of control are yet to be elucidated in many instances. Understanding how virulence factors are regulated by temperature presents a significant challenge, as gene expression and protein production are often influenced by complex regulatory networks involving multiple transcription factors in bacteria. Here we highlight some recent insights into thermal regulation of virulence in pathogenic bacteria. We focus on bacteria which cause disease in mammalian hosts, which are at a significantly higher temperature than the outside environment. We outline the mechanisms of thermal regulation and how understanding this fundamental aspect of the biology of bacteria has implications for pathogenesis and human health.

RNA structures are involved in the thermoregulation of bacterial virulence-associated traits

Pathogenic bacteria are exposed to temperature changes during colonization of the human body and during exposure to environmental conditions. Virulence-associated traits are mainly expressed by pathogenic bacteria at 37°C. We review different cases of post-transcriptional regulation of virulence-associated proteins through RNA structures (called RNA thermometers or RNATs) that modulate the translation of mRNAs. The analysis of RNATs in pathogenic bacteria has started to produce a comprehensive picture of the structures involved, and of the genes regulated by this mechanism. However, we are still not able to predict the functionality of putative RNATs predicted by bioinformatics methods, and there is not a global approach to measure the effect of these RNA structures in gene regulation during bacterial infections.

The leptospiral outer membrane

The outer membrane (OM) is the front line of leptospiral interactions with their environment and the mammalian host. Unlike most invasive spirochetes, pathogenic leptospires must be able to survive in both free-living and host-adapted states. As organisms move from one set of environmental conditions to another, the OM must cope with a series of conflicting challenges. For example, the OM must be porous enough to allow nutrient uptake, yet robust enough to defend the cell against noxious substances. In the host, the OM presents a surface decorated with adhesins and receptors for attaching to, and acquiring, desirable host molecules such as the complement regulator, Factor H.Factor H. On the other hand, the OM must enable leptospires to evade detection by the host's immune system on their way from sites of invasion through the bloodstream to the protected niche of the proximal tubule. The picture that is emerging of the leptospiral OM is that, while it shares many of the characteristics of the OMs of spirochetes and Gram-negative bacteria, it is also unique and different in ways that make it of general interest to microbiologists. For example, unlike most other pathogenic spirochetes, the leptospiral OM is rich in lipopolysaccharide (LPS). Leptospiral LPS is similar to that of Gram-negative bacteria but has a number of unique structural features that may explain why it is not recognized by the LPS-specific Toll-like receptor 4 of humans. As in other spirochetes, lipoproteins are major components of the leptospiral OM, though their roles are poorly understood. The functions of transmembrane outer membrane proteins (OMPs) in many cases are better understood, thanks to homologies with their Gram-negative counterparts and the emergence of improved genetic techniques. This chapter will review recent discoveries involving the leptospiral OM and its role in leptospiral physiology and pathogenesis.

Genomics, proteomics, and genetics of leptospira

Recent advances in molecular genetics, such as the ability to construct defined mutants, have allowed the study of virulence factors and more generally the biology in Leptospira. However, pathogenic leptospires remain much less easily transformable than the saprophyte L. biflexa and further development and improvement of genetic tools are required. Here, we review tools that have been used to genetically manipulate Leptospira. We also describe the major advances achieved in both genomics and postgenomics technologies, including transcriptomics and proteomics.

How to find RNA thermometers

Temperature is one of the decisive signals that a mammalian pathogen has entered its warm-blooded host. Among the many ways to register temperature changes, bacteria often use temperature-modulated structures in the untranslated region of mRNAs. In this article, we describe how such RNA thermometers (RNATs) have been discovered one by one upstream of heat shock and virulence genes in the past, and how next-generation sequencing approaches are able to reveal novel temperature-responsive RNA structures on a global scale.

The Cross-Talk between Spirochetal Lipoproteins and Immunity

Spirochetal diseases such as syphilis, Lyme disease, and leptospirosis are major threats to public health. However, the immunopathogenesis of these diseases has not been fully elucidated. Spirochetes interact with the host through various structural components such as lipopolysaccharides (LPS), surface lipoproteins, and glycolipids. Although spirochetal antigens such as LPS and glycolipids may contribute to the inflammatory response during spirochetal infections, spirochetes such as Treponema pallidum and Borrelia burgdorferi lack LPS. Lipoproteins are most abundant proteins that are expressed in all spirochetes and often determine how spirochetes interact with their environment. Lipoproteins are pro-inflammatory, may regulate responses from both innate and adaptive immunity and enable the spirochetes to adhere to the host or the tick midgut or to evade the immune system. However, most of the spirochetal lipoproteins have unknown function. Herein, the immunomodulatory effects of spirochetal lipoproteins are reviewed and are grouped into two main categories: effects related to immune evasion and effects related to immune activation. Understanding lipoprotein-induced immunomodulation will aid in elucidating innate immunopathogenesis processes and subsequent adaptive mechanisms potentially relevant to spirochetal disease vaccine development and to inflammatory events associated with spirochetal diseases.

Temperature-driven differential gene expression by RNA thermosensors

Many prokaryotic genes are organized in operons. Genes organized in such transcription units are co-transcribed into a polycistronic mRNA. Despite being clustered in a single mRNA, individual genes can be subjected to differential regulation, which is mainly achieved at the level of translation depending on initiation and elongation. Efficiency of translation initiation is primarily determined by the structural accessibility of the ribosome binding site (RBS). Structured cis-regulatory elements like RNA thermometers (RNATs) can contribute to differential regulation of individual genes within a polycistronic mRNA. RNATs are riboregulators that mediate temperature-responsive regulation of a downstream gene by modulating the accessibility of its RBS. At low temperature, the RBS is trapped by intra-molecular base pairing prohibiting translation initiation. The secondary structure melts with increasing temperature thus liberating the RBS. Here, we present an overview of different RNAT types and specifically highlight recently discovered RNATs. The main focus of this review is on RNAT-based differential control of polycistronic operons. Finally, we discuss the influence of temperature on other riboregulators and the potential of RNATs in synthetic RNA biology. This article is part of a Special Issue entitled: Riboswitches.

Pre-erythrocytic antibody profiles induced by controlled human malaria infections in healthy volunteers under chloroquine prophylaxis

Complete sterile protection to Plasmodium falciparum (Pf) infection mediated by pre-erythrocytic immunity can be experimentally induced under chloroquine prophylaxis, through immunization with sporozoites from infected mosquitoes' bites (CPS protocol). To characterize the profile of CPS induced antibody (Ab) responses, we developed a proteome microarray containing 809 Pf antigens showing a distinct Ab profile with recognition of antigens expressed in pre-erythrocytic life-cycle stages. In contrast, plasma from naturally exposed semi-immune individuals from Kenya was skewed toward antibody reactivity against asexual blood stage antigens. CPS-immunized and semi-immune individuals generated antibodies against 192 and 202 Pf antigens, respectively, but only 60 antigens overlapped between the two groups. Although the number of reactive antigens varied between the CPS-immunized individuals, all volunteers reacted strongly against the pre-erythrocytic antigens circumsporozoite protein (CSP) and liver stage antigen 1 (LSA1). Well classified merozoite and erythrocytic antigens were strongly reactive in semi-immune individuals but lacking in the CPS immunized group. These data show that the antibody profile of CPS-immunized and semi-immune groups have quite distinct profiles reflecting their protective immunity; antibodies from CPS immunized individuals react strongly against pre-erythrocytic while semi-immune individuals mainly react against erythrocytic antigens.

Oral immunization with Escherichia coli expressing a lipidated form of LigA protects hamsters against challenge with Leptospira interrogans serovar Copenhageni

Leptospirosis is a potentially fatal zoonosis transmitted by reservoir host animals that harbor leptospires in their renal tubules and shed the bacteria in their urine. Leptospira interrogans serovar Copenhageni transmitted from Rattus norvegicus to humans is the most prevalent cause of urban leptospirosis. We examined L. interrogans LigA, domains 7 to 13 (LigA7-13), as an oral vaccine delivered by Escherichia coli as a lipidated, membrane-associated protein. The efficacy of the vaccine was evaluated in a susceptible hamster model in terms of the humoral immune response and survival from leptospiral challenge. Four weeks of oral administration of live E. coli expressing LigA7-13 improved survival from intraperitoneal (i.p.) and intradermal (i.d.) challenge by L. interrogans serovar Copenhageni strain Fiocruz L1-130 in Golden Syrian hamsters. Immunization with E. coli expressing LigA7-13 resulted in a systemic antibody response, and a significant LigA7-13 IgG level after the first 2 weeks of immunization was completely predictive of survival 28 days after challenge. As in previous LigA vaccine studies, all immunized hamsters that survived infection had renal leptospiral colonization and histopathological changes. In summary, an oral LigA-based vaccine improved survival from leptospiral challenge by either the i.p. or i.d. route.