RegX3-dependent transcriptional activation of kdpDE and repression of rv0500A are linked to potassium homeostasis in Mycobacterium tuberculosis.

Ionic homeostasis is essential for the survival and replication of Mycobacterium tuberculosis within its host. Low potassium ion concentrations trigger a transition of M. tuberculosis into dormancy. Our current knowledge of the transcriptional regulation mechanisms governing genes involved in potassium homeostasis remains limited. Potassium transport is regulated by the constitutive Trk system and the inducible Kdp system in M. tuberculosis. The two-component system KdpDE (also known as KdpD/KdpE) activates expression of the kdpFABC operon, encoding the four protein subunits of the Kdp potassium uptake system (KdpFABC). We show that, under potassium deficiency, expression of the two-component system senX3/regX3 is upregulated, and bacterial survival is compromised in a regX3-inactivated mutant, ΔregX3. Electrophoretic mobility shift assays (EMSAs), promoter reporter assays and chromatin immunoprecipitation (ChIP) show that RegX3 binds to the kdpDE promoter and activates it under potassium deficiency, whereas RegX3 (K204A), a DNA binding-deficient mutant, fails to bind to the promoter. Mutation of the RegX3 binding motifs on the kdpDE promoter abrogates RegX3 binding. In addition, EMSAs and ChIP assays show that RegX3 represses Rv0500A, a repressor of kdpFABC, by binding to consensus RegX3 binding motifs on the rv0500A promoter. Our findings provide important insight into two converging pathways regulated by RegX3; one in which it activates an activator of kdpFABC, and the other in which it represses a repressor of kdpFABC, during potassium insufficiency. This culminates in increased expression of the potassium uptake system encoded by kdpFABC, enabling bacterial survival. These results further expand the growing transcriptional network in which RegX3 serves as a central node to enable bacterial survival under stress.

Nur77 influences immunometabolism to regulate the release of proinflammatory cytokines and the formation of lipid bodies during Mycobacterium tuberculosis infection of macrophages.

Infection of macrophages with Mycobacterium tuberculosis induces innate immune responses designed to clear the invading bacterium. However, bacteria often survive within the intracellular environment by exploiting these responses triggered by macrophages. Here, the role of the orphan nuclear receptor Nur77 (Nr4a1) in regulating the response of macrophages infected with M. tuberculosis (Mtb) has been delineated. Nur77 is induced early during infection, regulates metabolism by binding directly at the promoter of the TCA cycle enzyme, isocitrate dehydrogenase 2 (IDH2), to act as its repressor, and shifts the balance from a proinflammatory to an anti-inflammatory phenotype. Depletion of Nur77 increased transcription of IDH2 and, consequently, the levels of intracellular succinate, leading to enhanced levels of the proinflammatory cytokine IL-1ß. Further, Nur77 inhibited the production of antibacterial nitric oxide and IL-1ß in a succinate dehydrogenase (SDH)-dependent manner, suggesting that its induction favors bacterial survival by suppressing bactericidal responses. Indeed, depletion of Nur77 inhibited the intracellular survival of Mtb. On the other hand, depletion of Nur77 enhanced lipid body formation, suggesting that the fall in Nur77 levels as infection progresses likely favors foamy macrophage formation and long-term survival of Mtb in the host milieu.

Uncovering the role of microRNA671-5p/CDCA7L/monoamine oxidase-A signaling in Helicobacter pylori mediated apoptosis in gastric epithelial cells.

Helicobacter pylori is a gram-negative microaerophilic bacterium and is associated with gastrointestinal diseases ranging from peptic ulcer and gastritis to gastric cancer and mucosa-associated lymphoid tissue lymphoma. In our laboratory, the transcriptomes and miRnomes of AGS cells infected with H. pylori have been profiled, and an miRNA-mRNA network has been constructed. MicroRNA 671-5p is upregulated during H. pylori infection of AGS cells or of mice. In this study, the role of miR-671-5p during infection has been investigated. It has been validated that miR-671-5p targets the transcriptional repressor CDCA7L, which is downregulated during infection (in vitro and in vivo) concomitant with miR-671-5p upregulation. Further, it has been established that the expression of monoamine oxidase A (MAO-A) is repressed by CDCA7L, and that MAO-A triggers the generation of reactive oxygen species (ROS). Consequently, miR-671-5p/CDCA7L signaling is linked to the generation of ROS during H. pylori infection. Finally, it has been demonstrated that ROS-mediated caspase 3 activation and apoptosis that occurs during H. pylori infection, is dependent on the miR-671-5p/CDCA7L/MAO-A axis. Based on the above reports, it is suggested that targeting miR-671-5p could offer a means of regulating the course and consequences of H. pylori infection.

A systems approach to decipher a role of transcription factor RegX3 in the adaptation of Mycobacterium tuberculosis to hypoxic stress.

Two-component systems (TCSs) are required for the ability of Mycobacterium tuberculosis to respond to stress. The paired TCS, SenX3-RegX3 is known to respond to phosphate starvation and acid stress. The other stress conditions under which RegX3 is required for M. tuberculosis to mount an appropriate response, remain incompletely understood. Here we have employed genome-wide microarray profiling to compare gene expression in a ΔregX3 mutant with the wild-type under phosphate stress, in order to gain information on the probable RegX3 regulon. We pulled out a set of 128 hypoxia-associated genes, which could potentially be regulated by RegX3, by overlapping the gene set downregulated at least twofold in ΔregX3 with the gene set reported in the literature to be associated with the response to hypoxia. We identified potential RegX3 binding inverted repeats at the loci of 41 of these genes, in silico. We also observed that ΔregX3 was attenuated in terms of its ability to withstand hypoxia, and this was reversed upon complementation with regX3, corroborating a role of RegX3 in the response of M. tuberculosis to hypoxia. We validated the binding of RegX3 at the upstream regions of a selected set of these genes. Electrophoretic mobility shift assays (EMSAs) confirmed that RegX3 binds to the upstream regions of the hypoxia-associated genes Rv3334, whiB7, Rv0195, Rv0196 and Rv1960c. Gene expression analyses showed that the expression of these genes is regulated by RegX3 under hypoxia. We also show that the expression of whiB7, Rv3334 and Rv0195 in macrophage-grown M. tuberculosis, is dependent on RegX3. Finally, we show that attenuation of survival of ΔregX3 under hypoxia is partly reversed upon overexpression of either Rv0195 or Rv3334, suggesting that the RegX3-Rv0195 and the RegX3-Rv3334 axis are involved in the adaptation of M. tuberculosis to a hypoxic environment.

Unraveling novel roles of the Mycobacterium tuberculosis transcription factor Rv0081 in regulation of the nucleoid-associated proteins Lsr2 and EspR, cholesterol utilization, and subversion of lysosomal trafficking in macrophages.

The transcriptional network of Mycobacterium tuberculosis is designed to enable the organism to withstand host-associated stresses and to exploit the host milieu for its own survival and multiplication. Rv0081 (MT0088) is a transcriptional regulator whose interplay with other gene regulatory proteins and role in enabling M. tuberculosis to thrive within its host is incompletely understood. M. tuberculosis utilizes cholesterol within the granuloma. We show that deletion of Rv0081 compromises the ability of M. tuberculosis to utilize cholesterol as the sole carbon source, to subvert lysosomal trafficking, and to form granulomas in vitro. Rv0081 downregulates expression of the nucleoid-associated repressor Lsr2, leading to increased expression of the cholesterol catabolism-linked gene kshA and genes of the cholesterol importing operon, accounting for the requirement of Rv0081 in cholesterol utilization. Furthermore, Rv0081 activates EspR which is required for secretion of ESX-1 substrates, which in turn are involved in subversion of lysosomal trafficking of M. tuberculosis and granuloma expansion. These results provide new insight into the role of Rv0081 under conditions which resemble the environment encountered by M. tuberculosis within its host. Rv0081 emerges as a central regulator of genes linked to various pathways which are crucial for the survival of the bacterium in vivo.

The Role of microRNAs and Long Non-Coding RNAs in the Regulation of the Immune Response to Mycobacterium tuberculosis Infection.

Non-coding RNAs have emerged as critical regulators of the immune response to infection. MicroRNAs (miRNAs) are small non-coding RNAs which regulate host defense mechanisms against viruses, bacteria and fungi. They are involved in the delicate interplay between Mycobacterium tuberculosis, the causative agent of tuberculosis (TB), and its host, which dictates the course of infection. Differential expression of miRNAs upon infection with M. tuberculosis, regulates host signaling pathways linked to inflammation, autophagy, apoptosis and polarization of macrophages. Experimental evidence suggests that virulent M. tuberculosis often utilize host miRNAs to promote pathogenicity by restricting host-mediated antibacterial signaling pathways. At the same time, host- induced miRNAs augment antibacterial processes such as autophagy, to limit bacterial proliferation. Targeting miRNAs is an emerging option for host-directed therapies. Recent studies have explored the role of long non-coding RNA (lncRNAs) in the regulation of the host response to mycobacterial infection. Among other functions, lncRNAs interact with chromatin remodelers to regulate gene expression and also function as miRNA sponges. In this review we attempt to summarize recent literature on how miRNAs and lncRNAs are differentially expressed during the course of M. tuberculosis infection, and how they influence the outcome of infection. We also discuss the potential use of non-coding RNAs as biomarkers of active and latent tuberculosis. Comprehensive understanding of the role of these non-coding RNAs is the first step towards developing RNA-based therapeutics and diagnostic tools for the treatment of TB.

Applications of Transcriptomics and Proteomics for Understanding Dormancy and Resuscitation in Mycobacterium tuberculosis.

Mycobacterium tuberculosis can survive within its host for extended periods of time without any clinical symptoms of disease and reactivate when the immune system is weakened. A detailed understanding of how M. tuberculosis enters into and exits out of dormancy, is necessary in order to develop new strategies for tackling tuberculosis. Omics methodologies are unsupervised and unbiased to any hypothesis, making them useful tools for the discovery of new drug targets. This review summarizes the findings of transcriptomic and proteomic approaches toward understanding dormancy and reactivation of M. tuberculosis. Within the granuloma of latently infected individuals, the bacteria are dormant, with a marked slowdown of growth, division and metabolism. In vitro models have attempted to simulate these features by subjecting the bacterium to hypoxia, nutrient starvation, potassium depletion, growth in the presence of vitamin C, or growth in the presence of long-chain fatty acids. The striking feature of all the models is the upregulation of the DosR regulon, which includes the transcriptional regulator Rv0081, one of the central hubs of dormancy. Also upregulated are chaperone proteins, fatty acid and cholesterol degrading enzymes, the sigma factors SigE and SigB, enzymes of the glyoxylate and the methylcitrate cycle, the Clp proteases and the transcriptional regulator ClgR. Further, there is increased expression of genes involved in mycobactin synthesis, fatty acid degradation, the glyoxylate shunt and gluconeogenesis, in granulomas formed in vitro from peripheral blood mononuclear cells from latently infected individuals compared to naïve individuals. Genes linked to aerobic respiration, replication, transcription, translation and cell division, are downregulated during dormancy in vitro, but upregulated during reactivation. Resuscitation in vitro is associated with upregulation of genes linked to the synthesis of mycolic acids, phthiocerol mycocerosate (PDIM) and sulfolipids; ribosome biosynthesis, replication, transcription and translation, cell division, and genes encoding the five resuscitation promoting factors (Rpfs). The expression of proteases, transposases and insertion sequences, suggests genome reorganization during reactivation.

Perspectives About Modulating Host Immune System in Targeting SARS-CoV-2 in India.

Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), the causative agent of coronavirus induced disease-2019 (COVID-19), is a type of common cold virus responsible for a global pandemic which requires immediate measures for its containment. India has the world's largest population aged between 10 and 40 years. At the same time, India has a large number of individuals with diabetes, hypertension and kidney diseases, who are at a high risk of developing COVID-19. A vaccine against the SARS-CoV-2, may offer immediate protection from the causative agent of COVID-19, however, the protective memory may be short-lived. Even if vaccination is broadly successful in the world, India has a large and diverse population with over one-third being below the poverty line. Therefore, the success of a vaccine, even when one becomes available, is uncertain, making it necessary to focus on alternate approaches of tackling the disease. In this review, we discuss the differences in COVID-19 death/infection ratio between urban and rural India; and the probable role of the immune system, co-morbidities and associated nutritional status in dictating the death rate of COVID-19 patients in rural and urban India. Also, we focus on strategies for developing masks, vaccines, diagnostics and the role of drugs targeting host-virus protein-protein interactions in enhancing host immunity. We also discuss India's strengths including the resources of medicinal plants, good food habits and the role of information technology in combating COVID-19. We focus on the Government of India's measures and strategies for creating awareness in the containment of COVID-19 infection across the country.

RegX3 Activates whiB3 Under Acid Stress and Subverts Lysosomal Trafficking of Mycobacterium tuberculosis in a WhiB3-Dependent Manner.

Two-component systems (TCSs) are central to the ability of Mycobacterium tuberculosis to respond to stress. One such paired TCS is SenX3-RegX3, which responds to phosphate starvation. Here we show that RegX3 is required for M. tuberculosis to withstand low pH, one of the challenges encountered by the bacterium in the host environment, and that RegX3 activates the cytosolic redox sensor WhiB3 to launch an appropriate response to acid stress. We show that the whiB3 promoter of M. tuberculosis harbors a RegX3 binding motif. Electrophoretic mobility shift assays (EMSAs) show that phosphorylated RegX3 (RegX3-P) (but not its unphosphorylated counterpart) binds to this motif, whereas a DNA binding mutant, RegX3 (K204A) fails to do so. Mutation of the putative RegX3 binding motif on the whiB3 promoter, abrogates the binding of RegX3-P. The significance of this binding is established by demonstrating that the expression of whiB3 is significantly attenuated under phosphate starvation or under acid stress in the regX3-inactivated mutant, ΔregX3. Green fluorescent protein (GFP)-based reporter assays further confirm the requirement of RegX3 for the activation of the whiB3 promoter. The compromised survival of ΔregX3 under acid stress and its increased trafficking to the lysosomal compartment are reversed upon complementation with either regX3 or whiB3, suggesting that RegX3 exerts its effects in a WhiB3-dependent manner. Finally, using an in vitro granuloma model, we show that granuloma formation is compromised in the absence of regX3, but restored upon complementation with either regX3 or whiB3. Our findings provide insight into an important role of RegX3 in the network that regulates the survival of M. tuberculosis under acid stress similar to that encountered in its intracellular niche. Our results argue strongly in favor of a role of the RegX3-WhiB3 axis in establishment of M. tuberculosis infection.

Unraveling the role of H3K4 trimethylation and lncRNA HOTAIR in SATB1 and DUSP4-dependent survival of virulent Mycobacterium tuberculosis in macrophages.

The modification of chromatin influences host transcriptional programs during bacterial infection, at times skewing the balance in favor of pathogen survival. To test the role of chromatin modifications during Mycobacterium tuberculosis infection, we analysed genome-wide deposition of H3K4me3 marks in macrophages infected with either avirulent M. tuberculosis H37Ra or virulent H37Rv, by chromatin immunoprecipitation, followed by sequencing. We validated differences in association of H3K4me3 at the loci of special AT-rich sequence binding protein 1 (SATB1) and dual specificity MAP kinase phosphatase 4 (DUSP4) between H37Rv and H37Ra-infected macrophages, and demonstrated their role in regulating bacterial survival in macrophages as well as the expression of chemokines. SATB1 repressed gp91phox (an NADPH oxidase subunit) thereby regulating reactive oxygen species (ROS) generation during infection. Long non-coding RNA HOX transcript antisense RNA (HOTAIR) was upregulated in H37Ra-, but downregulated in H37Rv-infected macrophages. HOTAIR overexpression correlated with deposition of repressive H3K27me3 marks around the TSSs of DUSP4 and SATB1, suggesting that its downregulation favors the transcription of SATB1 and DUSP4. In summary, we have delineated histone modification- and lncRNA-dependent mechanisms regulating gene expression patterns facilitating survival of virulent M. tuberculosis. Our observations raise the possibility of harnessing histone-modifying enzymes to develop host-directed therapies for tuberculosis.

The roles of microRNAs in regulation of autophagy during bacterial infection.

Xenophagy is a selective form of autophagy targeting intracellular pathogens for lysosomal degradation. Accordingly, bacteria have evolved multiple strategies to evade or minimize autophagy and xenophagy to survive and replicate in host cells. MicroRNAs (miRNAs) are small non-coding RNA molecules that play key roles in host cells by modulating immune and inflammatory responses during infection. Accumulating evidence shows that miRNAs influence the outcome of bacterial infection by regulating canonical autophagy and xenophagy responses in host cells. Despite recent advances, we are only just beginning to understand the role miRNAs play in autophagy processes and how it affects the outcome of host-pathogen interactions in various bacterial infections. In this review, we focus on how Mycobacteria, Listeria, and Helicobacter evade host protective immune responses using miRNA-dependent mechanisms to suppress autophagy. These efforts include recent insights into the crosstalk between miRNAs and autophagy pathways, and how these interactions may be targeted in the search for new therapeutics against bacterial infections.

Activating transcription factor 3 modulates the macrophage immune response to Mycobacterium tuberculosis infection via reciprocal regulation of inflammatory genes and lipid body formation.

Infection of macrophages by Mycobacterium tuberculosis elicits an immune response that clears the bacterium. However, the bacterium is able to subvert the innate immune response. Differential expression of transcription factors (TFs) is central to the dynamic balance of this interaction. Among other functions, TFs regulate the production of antibacterial agents such as nitric oxide, pro-inflammatory cytokines and neutral lipids which are stored in lipid bodies (LBs) and favour bacterial survival. Here, we demonstrate that the TF activating transcription factor 3 (ATF3) is upregulated early during infection of macrophages or mice. Depletion of ATF3 enhances mycobacterial survival in macrophages suggesting its host-protective role. ATF3 interacts with chromatin remodelling protein brahma-related gene 1 and both associate with the promoters of interleukin-12p40, interleukin-6 and nitric oxide synthase 2, to activate expression of these genes. Strikingly, ATF3 downregulates LB formation by associating at the promoters of positive regulators of LB formation such as cholesterol 25 hydroxylase and the microRNA-33 locus. ATF3 represses the association of the activating mark, acetyl histone H4 lysine 8 at the promoter of cholesterol 25 hydroxylase. Our study suggests opposing roles of ATF3 in regulation of distinct sets of macrophage genes during infection, converging on a host-protective immune response.

The sensor kinase MtrB of Mycobacterium tuberculosis regulates hypoxic survival and establishment of infection.

Paired two-component systems (TCSs), having a sensor kinase (SK) and a cognate response regulator (RR), enable the human pathogen Mycobacterium tuberculosis to respond to the external environment and to persist within its host. Here, we inactivated the SK gene of the TCS MtrAB, mtrB, generating the strain ΔmtrB We show that mtrB loss reduces the bacterium's ability to survive in macrophages and increases its association with autophagosomes and autolysosomes. Notably, the ΔmtrB strain was markedly defective in establishing lung infection in mice, with no detectable lung pathology following aerosol challenge. ΔmtrB was less able to withstand hypoxic and acid stresses and to form biofilms and had decreased viability under hypoxia. Transcriptional profiling of ΔmtrB by gene microarray analysis, validated by quantitative RT-PCR, indicated down-regulation of the hypoxia-associated dosR regulon, as well as genes associated with other pathways linked to adaptation of M. tuberculosis to the host environment. Using in vitro biochemical assays, we demonstrate that MtrB interacts with DosR (a noncognate RR) in a phosphorylation-independent manner. Electrophoretic mobility shift assays revealed that MtrB enhances the binding of DosR to the hspX promoter, suggesting an unexpected role of MtrB in DosR-regulated gene expression in M. tuberculosis Taken together, these findings indicate that MtrB functions as a regulator of DosR-dependent gene expression and in the adaptation of M. tuberculosis to hypoxia and the host environment. We propose that MtrB may be exploited as a chemotherapeutic target against tuberculosis.

Genome-wide mRNA-miRNA profiling uncovers a role of the microRNA miR-29b-1-5p/PHLPP1 signalling pathway in Helicobacter pylori-driven matrix metalloproteinase production in gastric epithelial cells.

Aberrant expression of microRNAs (miRNAs) is associated with tumour progression, extracellular matrix remodelling, and cell proliferation. miRNAs modulate host gene expression during infection by pathogens such as Helicobacter pylori, which is associated with varying degrees of gastric pathology. In order to gain insight into the regulation of gene expression by miRNAs during H. pylori infection of gastric epithelial cells and its likely downstream consequences, we analysed the transcriptomes and miRnomes of AGS cells infected with H. pylori. In silico analysis of miRNA-mRNA interactions suggested that miR-29b-1-5p was a likely regulator of pathways associated with gastric epithelial cell pathology. We validated PH domain leucine rich phosphatase 1 (PHLPP1), a negative regulator of the Akt signalling pathway, as a target of miR-29b-1-5p. In an in vivo mouse model, we observed that infection with H. pylori was associated with upregulation of miR-29b-1-5p and downregulation of PHLPP1. Transfection with either a mimic or an inhibitor of miR-29b-1-5p confirmed that downregulation of PHLPP1 upregulates Akt-dependent NF-κB signalling leading to activation of matrix metalloproteinases 2 and 9, players in the degradation of extracellular matrix during H. pylori infection. The secreted antigen HP0175 was associated with upregulation of miR-29b-1-5p, regulation of metalloproteinase activity, and migration of AGS cells. Our study suggests that targeting the miR-29b-1-5p/PHLPP1 signalling axis could be a potential host-directed approach for regulating the outcome of H. pylori infection.

Global mapping of MtrA-binding sites links MtrA to regulation of its targets in Mycobacterium tuberculosis.

Mycobacterium tuberculosis employs two-component systems (TCSs) for survival within its host. The TCS MtrAB is conserved among mycobacteria. The response regulator MtrA is essential in M. tuberculosis. The genome-wide chromatin immunoprecipitation (ChIP) sequencing performed in this study suggested that MtrA binds upstream of at least 45 genes of M. tuberculosis, including those involved in cell wall remodelling, stress responses, persistence and regulation of transcription. It binds to the promoter regions and regulates the peptidoglycan hydrolases rpfA and rpfC, which are required for resuscitation from dormancy. It also regulates the expression of whiB4, a critical regulator of the oxidative stress response, and relF, one-half of the toxin-antitoxin locus relFG. We have identified a new consensus 9 bp loose motif for MtrA binding. Mutational changes in the consensus sequence greatly reduced the binding of MtrA to its newly identified targets. Importantly, we observed that overexpression of a gain-of-function mutant, MtrAY102C, enhanced expression of the aforesaid genes in M. tuberculosis isolated from macrophages, whereas expression of each of these targets was lower in M. tuberculosis overexpressing a phosphorylation-defective mutant, MtrAD56N. This result suggests that phosphorylated MtrA (MtrA-P) is required for the expression of its targets in macrophages. Our data have uncovered new MtrA targets that suggest that MtrA is required for a transcriptional response that likely enables M. tuberculosis to persist within its host and emerge out of dormancy when the conditions are favourable.

MicroRNA 26a (miR-26a)/KLF4 and CREB-C/EBPß regulate innate immune signaling, the polarization of macrophages and the trafficking of Mycobacterium tuberculosis to lysosomes during infection.

For efficient clearance of Mycobacterium tuberculosis (Mtb), macrophages tilt towards M1 polarization leading to the activation of transcription factors associated with the production of antibacterial effector molecules such as nitric oxide (NO) and proinflammatory cytokines such as interleukin 1 ß (IL-1ß) and tumor necrosis factor α (TNF-α). At the same time, resolution of inflammation is associated with M2 polarization with increased production of arginase and cytokines such as IL-10. The transcriptional and post-transcriptional mechanisms that govern the balance between M1 and M2 polarization, and bacteria-containing processes such as autophagy and trafficking of Mtb to lysosomes, are incompletely understood. Here we report for the first time, that the transcription factor KLF4 is targeted by microRNA-26a (miR-26a). During Mtb infection, downregulation of miR-26a (observed both ex vivo and in vivo) facilitates upregulation of KLF4 which in turn favors increased arginase and decreased iNOS activity. We further demonstrate that KLF4 prevents trafficking of Mtb to lysosomes. The CREB-C/EBPß signaling axis also favors M2 polarization. Downregulation of miR-26a and upregulation of C/ebpbeta were observed both in infected macrophages as well as in infected mice. Knockdown of C/ebpbeta repressed the expression of selected M2 markers such as Il10 and Irf4 in infected macrophages. The importance of these pathways is substantiated by observations that expression of miR-26a mimic or knockdown of Klf4 or Creb or C/ebpbeta, attenuated the survival of Mtb in macrophages. Taken together, our results attribute crucial roles for the miR-26a/KLF4 and CREB-C/EBPßsignaling pathways in regulating the survival of Mtb in macrophages. These studies expand our understanding of how Mtb hijacks host signaling pathways to survive in macrophages, and open up new exploratory avenues for host-targeted interventions.

MicroRNA in innate immunity and autophagy during mycobacterial infection.

The fine-tuning of innate immune responses is an important aspect of host defenses against mycobacteria. MicroRNAs (miRNAs), small non-coding RNAs, play essential roles in regulating multiple biological pathways including innate host defenses against various infections. Accumulating evidence shows that many miRNAs regulate the complex interplay between mycobacterial survival strategies and host innate immune pathways. Recent studies have contributed to understanding the role of miRNAs, the levels of which can be modulated by mycobacterial infection, in tuning host autophagy to control bacterial survival and innate effector function. Despite considerable efforts devoted to miRNA profiling over the past decade, further work is needed to improve the selection of appropriate biomarkers for tuberculosis. Understanding the roles and mechanisms of miRNAs in regulating innate immune signaling and autophagy may provide insights into new therapeutic modalities for host-directed anti-mycobacterial therapies. Here, we present a comprehensive review of the recent literature regarding miRNA profiling in tuberculosis and the roles of miRNAs in modulating innate immune responses and autophagy defenses against mycobacterial infections.

Targeting multiple response regulators of Mycobacterium tuberculosis augments the host immune response to infection.

The genome of M. tuberculosis (Mtb) encodes eleven paired two component systems (TCSs) consisting of a sensor kinase (SK) and a response regulator (RR). The SKs sense environmental signals triggering RR-dependent gene expression pathways that enable the bacterium to adapt in the host milieu. We demonstrate that a conserved motif present in the C-terminal domain regulates the DNA binding functions of the OmpR family of Mtb RRs. Molecular docking studies against this motif helped to identify two molecules with a thiazolidine scaffold capable of targeting multiple RRs, and modulating their regulons to attenuate bacterial replication in macrophages. The changes in the bacterial transcriptome extended to an altered immune response with increased autophagy and NO production, leading to compromised survival of Mtb in macrophages. Our findings underscore the promise of targeting multiple RRs as a novel yet unexplored approach for development of new anti-mycobacterial agents particularly against drug-resistant Mtb.