Induction of ER stress in macrophages of tuberculosis granulomas

Roles of HIF-1α signaling in Mycobacterium tuberculosis infection: New targets for anti-TB therapeutics?

Tuberculosis (TB), a deadly infectious disease induced by Mycobacterium tuberculosis (Mtb), continues to be a global public health issue that kill millions of patents every year. Despite significant efforts have been paid to identify effective TB treatments, the emergence of drug-resistant strains of the disease and the presence of comorbidities in TB patients urges us to explore the detailed mechanisms involved in TB immunity and develop more effective innovative anti-TB strategies. HIF-1α, a protein involved in regulating cellular immune responses during TB infection, has been highlighted as a promising target for the development of novel strategies for TB treatment due to its critical roles in anti-TB host immunity. This review provides a summary of current research progress on the roles of HIF-1α in TB infection, highlighting its importance in regulating the host immune response upon Mtb infection and summarizing the influences and mechanisms of HIF-1α on anti-TB immunological responses of host cells. This review also discusses the various challenges associated with developing HIF-1α as a target for anti-TB therapies, including ensuring specificity and avoiding off-target effects on normal cell function, determining the regulation and expression of HIF-1α in TB patients, and developing drugs that can inhibit HIF-1α. More deep understanding of the molecular mechanisms involved in HIF-1α signaling, its impact on TB host status, and systematic animal testing and clinical trials may benefit the optimization of HIF-1α as a novel therapeutic target for TB.

YY1 Contributes to the Inflammatory Responses of Mycobacterium tuberculosis-Infected Macrophages Through Transcription Activation-Mediated Upregulation TLR4

Tuberculosis (TB) is a chronic respiratory infectious disease and is induced by Mycobacterium tuberculosis (M.tb) infection. Macrophages serve as the cellular home in immunoreaction against M.tb infection, which is tightly regulated through Toll-like receptor 4 (TLR4) expression. Therefore, this study is designed to explore the role and mechanism of TLR4 in mycobacterial injury in human macrophages (THP-1 cells) after M.tb infection. Cell proliferation and apoptosis were assessed using MTT, EdU, and flow cytometry assays. ELISA kits were utilized to assess the levels of Interleukin-6 (IL-6), IL-1β, and tumor necrosis factor α (TNF-α). The binding between Yin-Yang-1 (YY1) and TLR4 promoter was predicted by JASPAR and verified using Chromatin immunoprecipitation (ChIP) and dual-luciferase reporter assays. M.tb infection might repress THP-1 cell proliferation, and induce cell apoptosis and inflammatory response in a multiplicity of infection (MOI)-dependent manner. Moreover, M.tb infection increased the expression of TLR4 in HTP-1 cells in an MOI-dependent way, and its downregulation might overturn M.tb infection-mediated HTP-1 cell damage and inflammatory response. At the molecular level, YY1 was a transcription factor of TLR4 and promoted TLR4 transcription via binding to its promoter region. Besides, YY1 might activate the NF-kB signaling pathway via regulating TLR4. Meanwhile, TLR4 inhibitor BAY11-7082 might overturn the repression effect of TLR4 on M.tb-infected HTP-1 cell damage. YY1-activated TLR4 might aggravate mycobacterial injury in human macrophages after M.tb infection by the NF-kB pathway, providing a promising therapeutic target for TB treatment.

miR-4687-5p Affects Intracellular Survival of Mycobacterium tuberculosis through Its Regulation of NRAMP1 Expression in A549 Cells

Tuberculosis (TB), as one of the leading causes of death, poses a serious predicament to the world. MicroRNAs (miRNAs) play a role in the post-transcriptional regulation of gene expression. It has been reported that the expression of miRNAs changes upon mycobacterial infection; the screening and identification of miRNAs regulating the expression of genes could benefit our understanding of TB pathogenesis and generate effective strategies for its control and prevention. In this study, luciferase assays showed that miR-4687-5p is bound to the 3'-untranslated region of natural resistance-associated macrophage protein 1 (NRAMP1). Additionally, we found a significant increase in miR-4687-5p expression in Mycobacterium tuberculosis (Mtb)-infected A549 cells. Concomitantly, we detected a reduced level of NRAMP1 expression, suggesting that NRAMP1 is one of the targets of miR-4687-5p. Infection experiments evidenced that the transfection of miR-4687-5p induced a decrease in NRAMP1 expression and increased intracellular Mtb loads post-infection, indicating that miR-4687-5p promotes the intracellular survival of Mtb through its downregulation of the NRAMP1 protein level. We also found that the transfection of miR-4687-5p induced increased apoptosis and decreased cell proliferation post-infection with Mtb. The results presented in our study suggest that miR-4687-5p may be indicative of the susceptibility of Mtb infection to humans and could act as a potential therapeutic target for tuberculosis treatment.

The uncharted territory of host-pathogen interaction in tuberculosis

Mycobacterium tuberculosis (M.tb) effectively manipulates the host processes to establish the deadly respiratory disease, Tuberculosis (TB). M.tb has developed key mechanisms to disrupt the host cell health to combat immune responses and replicate efficaciously. M.tb antigens such as ESAT-6, 19kDa lipoprotein, Hip1, and Hsp70 destroy the integrity of cell organelles (Mitochondria, Endoplasmic Reticulum, Nucleus, Phagosomes) or delay innate/adaptive cell responses. This is followed by the induction of cellular stress responses in the host. Such cells can either undergo various cell death processes such as apoptosis or necrosis, or mount effective immune responses to clear the invading pathogen. Further, to combat the infection progression, the host secretes extracellular vesicles such as exosomes to initiate immune signaling. The exosomes can contain M.tb as well as host cell-derived peptides that can act as a double-edged sword in the immune signaling event. The host-symbiont microbiota produces various metabolites that are beneficial for maintaining healthy tissue microenvironment. In juxtaposition to the above-mentioned mechanisms, M.tb dysregulates the gut and respiratory microbiome to support its replication and dissemination process. The above-mentioned interconnected host cellular processes of Immunometabolism, Cellular stress, Host Microbiome, and Extracellular vesicles are less explored in the realm of exploration of novel Host-directed therapies for TB. Therefore, this review highlights the intertwined host cellular processes to control M.tb survival and showcases the important factors that can be targeted for designing efficacious therapy.

Adjunctive Integrated Stress Response Inhibition Accelerates Tuberculosis Clearance in Mice

Despite numerous advances in tuberculosis (TB) drug development, long treatment durations have led to the emergence of multidrug resistance, which poses a major hurdle to global TB control. Shortening treatment time therefore remains a top priority. Host-directed therapies that promote bacterial clearance and/or lung health may improve the efficacy and treatment duration of tuberculosis antibiotics. We recently discovered that inhibition of the integrated stress response, which is abnormally activated in tuberculosis and associated with necrotic granuloma formation, reduced bacterial numbers and lung inflammation in mice. Here, we evaluated the impact of the integrated stress response (ISR) inhibitor ISRIB, administered as an adjunct to standard tuberculosis antibiotics, on bacterial clearance, relapse, and lung pathology in a mouse model of tuberculosis. Throughout the course of treatment, ISRIB robustly lowered bacterial burdens compared to the burdens with standard TB therapy alone and accelerated the time to sterility in mice, as demonstrated by significantly reduced relapse rates after 4 months of treatment. In addition, mice receiving adjunctive ISRIB tended to have reduced lung necrosis and inflammation. Together, our findings identify the ISR pathway as a promising therapeutic target with the potential to shorten TB treatment durations and improve lung health. IMPORTANCE Necrosis of lung lesions is a hallmark of tuberculosis (TB) that promotes bacterial growth, dissemination, and transmission. This process is driven by the persistent hyperactivation of the integrated stress response (ISR) pathway. Here, we show that adjunctive ISR inhibition during standard antibiotic therapy accelerates bacterial clearance and reduces immunopathology in a clinically relevant mouse model of TB, suggesting that host-directed therapies that de-escalate these pathological stress responses may shorten TB treatment durations. Our findings present an important conceptual advance toward overcoming the challenge of improving TB therapy and lowering the global burden of disease.

Dual oxidase 1 is dispensable during Mycobacterium tuberculosis infection in mice

Introduction

Mycobacterium tuberculosis (Mtb) is the primary cause of human tuberculosis (TB) and is currently the second most common cause of death due to a singleinfectious agent. The first line of defense against airborne pathogens, including Mtb, is the respiratory epithelium. One of the innate defenses used by respiratory epithelial cells to prevent microbial infection is an oxidative antimicrobial system consisting of the proteins, lactoperoxidase (LPO) and Dual oxidase 1 (Duox1), the thiocyanate anion (SCN-) and hydrogen peroxide (H2O2), which together lead to the generation of antimicrobial hypothiocyanite (OSCN-) in the airway lumen. OSCN- kills bacteria and viruses in vitro, but the role of this Duox1-based system in bacterial infections in vivo remains largely unknown. The goal of this study was to assess whether Duox1 contributes to the immune response against the unique respiratory pathogen, Mtb.

Methods

Duox1-deficient (Duox1 KO) and wild-type (WT) mice were infected with Mtb aerosols and bacterial titers, lung pathology, cytokines and immune cell recruitment were assessed.

Results and discussion

Mtb titers in the lung, spleen and liver were not different 30 days after infection between WT and Duox1 KO mice. Duox1 did not affect lung histology assessed at days 0, 30, and 90 post-Mtb infection. Mtb-infected Duox1 KO animals exhibited enhanced production of certain cytokines and chemokines in the airway; however, this response was not associated with significantly higher numbers of macrophages or neutrophils in the lung. B cell numbers were lower, while apoptosis was higher in the pulmonary lesions of Mtb-infected Duox1 KO mice compared to infected WT animals. Taken together, these data demonstrate that while Duox1 might influence leukocyte recruitment to inflammatory cell aggregates, Duox1 is dispensable for the overall clinical course of Mtb lung infection in a mouse model.

Autophagy is induced by swine acute diarrhea syndrome coronavirus through the cellular IRE1-JNK-Beclin 1 signaling pathway after an interaction of viral membrane-associated papain-like protease and GRP78

Autophagy plays an important role in the infectious processes of diverse pathogens. For instance, cellular autophagy could be harnessed by viruses to facilitate replication. However, it is still uncertain about the interplay of autophagy and swine acute diarrhea syndrome coronavirus (SADS-CoV) in cells. In this study, we reported that SADS-CoV infection could induce a complete autophagy process both in vitro and in vivo, and an inhibition of autophagy significantly decreased SADS-CoV production, thus suggesting that autophagy facilitated the replication of SADS-CoV. We found that ER stress and its downstream IRE1 pathway were indispensable in the processes of SADS-CoV-induced autophagy. We also demonstrated that IRE1-JNK-Beclin 1 signaling pathway, neither PERK-EIF2S1 nor ATF6 pathways, was essential during SADS-CoV-induced autophagy. Importantly, our work provided the first evidence that expression of SADS-CoV PLP2-TM protein induced autophagy through the IRE1-JNK-Beclin 1 signaling pathway. Furthermore, the interaction of viral PLP2-TMF451-L490 domain and substrate-binding domain of GRP78 was identified to activate the IRE1-JNK-Beclin 1 signaling pathway, and thus resulting in autophagy, and in turn, enhancing SADS-CoV replication. Collectively, these results not only showed that autophagy promoted SADS-CoV replication in cultured cells, but also revealed that the molecular mechanism underlying SADS-CoV-induced autophagy in cells.

Mycobacterium tuberculosis PE_PGRS1 promotes mycobacteria intracellular survival via reducing the concentration of intracellular free Ca2+ and suppressing endoplasmic reticulum stress

Mycobacterium tuberculosis (M. tuberculosis) is the causative agent of tuberculosis (TB). And the PE_PGRS family members of M. tuberculosis are closely associated with virulence and antigen presentation but with function largely elusive. PE_PGRS1(Rv0109) contained 7 Ca²⁺ binding domains of GGXGXD/NXUX (X is any amino acid), which can reduce intracellular Ca²⁺ surge. In addition, PE_PGRS1 can mitigate the activation of PERK branch in endoplasmic reticulum (ER) stress by down-regulating the expression of CHOP, Bip, p-PERK, p-eIF2α, and ATF4. Interestingly, we found that two splicing variations of Bax/Bcl-2, Baxβ, and Bcl-2α, were differentially expressed after infection with Ms_PE_PGRS1, and may be involved in the regulation of apoptosis. Hence, this study identified that PE_PGRS1 is a novel calcium-associated protein that can decrease intracellular Ca²⁺ levels and the PERK axis. And the weakening of the PERK-eIF2α-ATF4 axis reduces THP-1 macrophages apoptosis, promotes the survival of mycobacteria in macrophages.

Gut dysbacteriosis attenuates resistance to Mycobacterium bovis infection by decreasing Cyclooxygenase 2 to inhibit endoplasmic reticulum stress

AbstractThe role of gut microbiota has been described as an important influencer of the immune system. Gut-lung axis is critical in the prevention of mycobacterium infection, but the specific mechanism by which dysbiosis affects tuberculosis have not been reported. In this study, we attempted to provide more information on how the gut-lung axis contributes to Mycobacterium bovis (M. bovis) infection. Mice pre-treated with broad-spectrum antibiotics cocktail (Abx) to induce gut dysbiosis. Interestingly, dysbiosis of microbes showed a significant increase in the bacterial burden in lungs, and inhibited the level of COX-2. After fecal transplantation, cyclooxygenase 2（COX-2） expression was restored and the inflammatory lesion in the lung was reduced. Further research found that the deficiency of COX-2 inhibited endoplasmic reticulum stress (ER-stress). This mechanism was completed by COX-2 interaction with BIP. Moreover, we found a positive feedback mechanism by which blocking ER-stress could reduce COX-2 levels via the NF-κB pathway. Taken together, we reveal for the first time gut dysbacteriosis exacerbates M. bovis disease by limiting COX-2 /ER-stress pathway. The finding strengthens the foundation of gut microbiota-targeted therapy for tuberculosis treatment.

Induction of Endoplasmic Reticulum Stress by CdhM Mediates Apoptosis of Macrophage During Mycobacterium tuberculosis Infection

The normal operation of the endoplasmic reticulum (ER) is critical for cells and organisms. However, ER stress, caused by imbalanced protein folding, occurs frequently, which perturbs the function of the ER and even results in cell apoptosis eventually. Many insults can induce ER stress; pathogen infection is one of them. Most of the genes involved in ER stress have been reported to be upregulated in Mycobacterium tuberculosis (Mtb) granulomas of humans and mice, implicating that infection with Mtb can induce ER stress. However, little is known about the molecular mechanism of Mtb induction of ER stress. Here, we reveal that Mycobacterium protein CDP-diglyceride hydrolase of Mycobacteriumn (CdhM) could target the ER and cause abnormal ER morphology and cell death. RNA-seq analysis suggests that most of the ER stress-involved genes were modulated by CdhM. Further assessed by biochemical experiments, the transcription and protein levels of ER stress markers BiP and CHOP, as well as the levels of XBP1 splicing and eIF2α phosphorylation, were significantly increased by CdhM, confirming that CdhM could induce ER stress alone or during infection. A single conserved amino acid mutant of CdhM, including L44A, G96A, H150A, and W175A, was incapable of inducing ER stress, which indicates that induction of ER stress by CdhM is specific and functional. Furthermore, CdhM-induced ER stress could also promote apoptosis of macrophages during Mtb infection. Overexpression of CdhM conferred a significant benefit for Mtb replication by releasing Mtb into extracellular during infection of macrophage in vitro, as presented in CFU assays. Overall, our study identified a novel Mtb effector protein CdhM which may promote Mtb dissemination and proliferation by induction of ER stress and apoptosis and provided new insight into the physiological significance of induction of ER stress in tuberculosis (TB) granulomas.

Hiding in Plain Sight: Formation and Function of Stress Granules During Microbial Infection of Mammalian Cells

Stress granule (SG) formation is a host cell response to stress-induced translational repression. SGs assemble with RNA-binding proteins and translationally silent mRNA. SGs have been demonstrated to be both inhibitory to viruses, as well as being subverted for viral roles. In contrast, the function of SGs during non-viral microbial infections remains largely unexplored. A handful of microbial infections have been shown to result in host SG assembly. Nevertheless, a large body of evidence suggests SG formation in hosts is a widespread response to microbial infection. Diverse stresses caused by microbes and their products can activate the integrated stress response in order to inhibit translation initiation through phosphorylation of the eukaryotic translation initiation factor 2α (eIF2α). This translational response in other contexts results in SG assembly, suggesting that SG assembly can be a general phenomenon during microbial infection. This review explores evidence for host SG formation in response to bacterial, fungal, and protozoan infection and potential functions of SGs in the host and for adaptations of the pathogen.

Effectors Targeting the Unfolded Protein Response during Intracellular Bacterial Infection

The unfolded protein response (UPR) is a homeostatic response to endoplasmic reticulum (ER) stress within eukaryotic cells. The UPR initiates transcriptional and post-transcriptional programs to resolve ER stress; or, if ER stress is severe or prolonged, initiates apoptosis. ER stress is a common feature of bacterial infection although the role of the UPR in host defense is only beginning to be understood. While the UPR is important for host defense against pore-forming toxins produced by some bacteria, other bacterial effector proteins hijack the UPR through the activity of translocated effector proteins that facilitate intracellular survival and proliferation. UPR-mediated apoptosis can limit bacterial replication but also often contributes to tissue damage and disease. Here, we discuss the dual nature of the UPR during infection and the implications of UPR activation or inhibition for inflammation and immunity as illustrated by different bacterial pathogens.

Meta-Analysis of Peripheral Blood Transcriptome Datasets Reveals a Biomarker Panel for Tuberculosis in Patients Infected With HIV

Biomarkers are critical for rapid diagnosis of tuberculosis (TB) and could benefit patients with AIDS where diagnosis of TB co-infection is challenging. Meta-analysis is an approach to combine the results of the studies with standard statistical method by weighting each study with different sample size. This study aimed to use meta-analysis to integrate transcriptome datasets from different studies and screen for TB biomarkers in patients who were HIV-positive. Five datasets were subjected to meta-analysis on whole-blood transcriptomes from 640 patients infected with HIV. A total of 293 differentially expressed genes (DEGs) were identified as significant (P<0.0001) using the random effective model to integrate the statistical results from each study. DEGs were enriched in biological processes related to TB, such as “Type I interferon signaling” and “stimulatory C-type lectin receptor signaling”. Eighteen DEGs had at least a two-fold change in expression between patients infected with HIV who were TB-positive and those who were TB-negative. GBP4, SERPING1, ATF3 and CDKBN3 were selected as a biomarker panel to perform multivariable logistic regression analysis on TB status and relative gene expression levels. The biomarker panel showed excellent accuracy (AUC>0.90 for HIV+TB) in clinical trial and suggests that meta-analysis is an efficient method to integrate transcriptome datasets from different studies.

The integrated stress response mediates necrosis in murine Mycobacterium tuberculosis granulomas

The mechanism by which only some individuals infected with Mycobacterium tuberculosis develop necrotic granulomas with progressive disease while others form controlled granulomas that contain the infection remains poorly defined. Mice carrying the sst1-suscepible (sst1S) genotype develop necrotic inflammatory lung lesions, similar to human tuberculosis (TB) granulomas, which are linked to macrophage dysfunction, while their congenic counterpart (B6) mice do not. In this study we report that (a) sst1S macrophages developed aberrant, biphasic responses to TNF characterized by superinduction of stress and type I interferon pathways after prolonged TNF stimulation; (b) the late-stage TNF response was driven via a JNK/IFN-β/protein kinase R (PKR) circuit; and (c) induced the integrated stress response (ISR) via PKR-mediated eIF2α phosphorylation and the subsequent hyperinduction of ATF3 and ISR-target genes Chac1, Trib3, and Ddit4. The administration of ISRIB, a small-molecule inhibitor of the ISR, blocked the development of necrosis in lung granulomas of M. tuberculosis-infected sst1S mice and concomitantly reduced the bacterial burden. Hence, induction of the ISR and the locked-in state of escalating stress driven by the type I IFN pathway in sst1S macrophages play a causal role in the development of necrosis in TB granulomas. Interruption of the aberrant stress response with inhibitors such as ISRIB may offer novel host-directed therapy strategies.

Mycobacterium tuberculosis Rv0580c Impedes the Intracellular Survival of Recombinant Mycobacteria, Manipulates the Cytokines, and Induces ER Stress and Apoptosis in Host Macrophages via NF-κB and p38/JNK Signaling

The Mycobacterium tuberculosis (M. tb) genome encodes a large number of hypothetical proteins, which need to investigate their role in physiology, virulence, pathogenesis, and host interaction. To explore the role of hypothetical protein Rv0580c, we constructed the recombinant Mycobacterium smegmatis (M. smegmatis) strain, which expressed the Rv0580c protein heterologously. We observed that Rv0580c expressing M. smegmatis strain (Ms_Rv0580c) altered the colony morphology and increased the cell wall permeability, leading to this recombinant strain becoming susceptible to acidic stress, oxidative stress, cell wall-perturbing stress, and multiple antibiotics. The intracellular survival of Ms_Rv0580c was reduced in THP-1 macrophages. Ms_Rv0580c up-regulated the IFN-γ expression via NF-κB and JNK signaling, and down-regulated IL-10 expression via NF-κB signaling in THP-1 macrophages as compared to control. Moreover, Ms_Rv0580c up-regulated the expression of HIF-1α and ER stress marker genes via the NF-κB/JNK axis and JNK/p38 axis, respectively, and boosted the mitochondria-independent apoptosis in macrophages, which might be lead to eliminate the intracellular bacilli. This study explores the crucial role of Rv0580c protein in the physiology and novel host-pathogen interactions of mycobacteria.

Transcriptome Analysis Reveals Unfolded Protein Response Was Induced During the Early Stage of Burkholderia pseudomallei Infection in A549 Cells

Burkholderia pseudomallei is a zoonotic pathogen that usually affects patients' lungs and causes serious melioidosis. The interaction of B. pseudomallei with its hosts is complex, and cellular response to B. pseudomallei infection in humans still remains to be elucidated. In this study, transcriptomic profiling of B. pseudomallei-infected human lung epithelial A549 cells was performed to characterize the cellular response dynamics during the early infection (EI) stage. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses were performed by using the online databases DAVID 6.8 and KOBAS 3.0. Real-time quantitative PCR and western blot were used for validation experiments. Compared with the negative control group (NC), a set of 36 common genes varied over time with a cut-off level of 1.5-fold change, and a P-value < 0.05 was identified. Bioinformatics analysis indicated that the PERK-mediated unfolded protein response (UPR) was enriched as the most noteworthy biological process category, which was enriched as a branch of UPR in the signaling pathway of protein processing in the endoplasmic reticulum. Other categories, such as inflammatory responses, cell migration, and apoptosis, were also focused. The molecular chaperone Bip (GRP78), PERK, and PERK sensor-dependent phosphorylation of eIF2α (p-eIF2α) and ATF4 were verified to be increasing over time during the EI stage, suggesting that B. pseudomallei infection activated the PERK-mediated UPR in A549 cells. Collectively, these results provide important initial insights into the intimate interaction between B. pseudomallei and lung epithelial cells, which can be further explored toward the elucidation of the cellular mechanisms of B. pseudomallei infections in humans.

Mycobacterium tuberculosis ESX-1-secreted substrate protein EspC promotes mycobacterial survival through endoplasmic reticulum stress-mediated apoptosis

EsxA, secreted by the ESAT-6 secretion system 1 (ESX-1) secretion system, is considered the major Mycobacterium tuberculosis (Mtb) virulence determinant. However, the roles of the individual ESX-1 substrates, such as EspC, remain unclear due to their interdependency for secretion with EsxA. Here, we validated that EspC triggered ER stress-mediated apoptosis in macrophages. The EspC-mediated ER stress was involved in pro-inflammatory cytokines generation, intracellular Ca₂₊ release, and reactive oxygen species accumulation. Mitochondrial transmembrane potential dissipation and mitochondrial outer membrane permeabilization occurred in EspC-treated macrophages, causing apoptosis. Furthermore, ER stress-mediated apoptosis was effectively induced in EspC-overexpressing Mycobacterium smegmatis-infected macrophages and mice. EspC overexpression caused a significant increase in bacterial survival in the macrophages, spleens, and lungs, and accelerated mouse death was observed. Moreover, the increased viability of bacteria in the macrophages was significantly reduced by pretreatment with the apoptosis inhibitor. Overall, our results revealed that EspC is an essential ESX-1 protein for Mtb–host interactions and EspC-induced ER stress-mediated apoptosis may be employed by Mtb to establish and spread infection. Given the critical roles of the ESX systems in Mtb pathogenesis and immunity, our findings offer new perspectives on the complex host-pathogen interactions and mechanisms underlying ESX-1-mediated pathogenesis.

Autophagy, Unfolded Protein Response and Lung Disease

Acute Respiratory Distress Syndrome is a severe disorder affecting thousands of individuals worldwide. The available medical countermeasures do not sufficiently suppress the unacceptable high mortality rates associated with those in need. Thus, intense efforts aim to delineate the function of the lung endothelium, so to deliver new therapeutic approaches against this disease. The present manuscript attempts to shed light on the interrelations between the unfolded protein response and autophagy towards lung disease, to deliver a new line of possible therapeutic approaches against the ferocious Acute Respiratory Distress Syndrome.

Sirtuin inhibits M. tuberculosis -induced apoptosis in macrophage through glycogen synthase kinase-3β

Apoptotic and inflammatory pathways play important roles in Mycobacterium tuberculosis-infected macrophages. Sirt1 is a member of the deacetylase family that is known to promote apoptosis resistance in mammalian cells and was recently reported to regulate mycobacterial immunopathogenesis via inflammatory responses. However, the apoptotic role of Sirt1 in the process of M. tuberculosis infection remains unclear. With the help of mouse peritoneal macrophage samples, we have shown that resveratrol, a Sirt1 activator, inhibited M. tuberculosis-induced apoptosis in peritoneal macrophages. Further, we found that Sirt1 activation prompted M. tuberculosis induced GSK3β phosphorylation. Further investigation into the possible mechanisms of action showed that Sirt1 directly interacted with GSK3β and enhanced GSK3β phosphorylation by promoting its deacetylation. Sirt1 activation inhibited M. tuberculosis growth. Thus, it seemed that Sirt1 acted as a novel regulator of apoptosis signaling in M. tuberculosis infection via its direct effects on GSK3β. Sirt1 may therefore be a new candidate for the prevention and treatment of tuberculosis.

Intestinal Immune Homeostasis and Inflammatory Bowel Disease: A Perspective on Intracellular Response Mechanisms

The pathogenesis of inflammatory bowel disease (IBD) involves perturbation of intestinal immune homeostasis in genetically susceptible individuals. A mutual interplay between intestinal epithelial cells (IECs) and gut resident microbes maintains a homeostatic environment across the gut. An idiopathic gastrointestinal (GI) complication triggers aberrant physiological stress in the epithelium and peripheral myeloid cells, leading to a chronic inflammatory condition. Indeed, events in the endoplasmic reticulum (ER) and mitochondria contribute to orchestrating intracellular mechanisms such as the unfolded protein response (UPR) and oxidative stress, respectively, to resolve aberrant cellular stress. This review highlights the signaling cascades encrypted within ER and mitochondria in IECs and/or myeloid cells to dissipate chronic stress in maintaining intestinal homeostasis.

Signaling pathways that control mRNA translation initiation in macrophages

Translational control in mammalian cells plays a critical role in regulating differentiation, cell growth, cell cycle and response to diverse stresses. Macrophages are one of the most versatile cell types in the body. They are professional phagocytic cells that can be found in almost all tissues and adapt tissue-specific functions. Recent studies highlight the importance of translational control in macrophages during invasion of pathogens, exposure to cytokines and in the context of tissue specific functions. In this review, we summarize the current knowledge regarding the role of mRNA translational control in regulation of macrophages.

Endoplasmic reticulum stress potentiates the autophagy of alveolar macrophage to attenuate acute lung injury and airway inflammation

Endoplasmic reticulum (ER) stress has been reported to play a role in acute lung injury (ALI), yet the in-depth mechanism remains elusive. This study aims to investigate the effect of ER stress-induced autophagy of alveolar macrophage (AM) on acute lung injury (ALI) and airway inflammation using mouse models. ALI models were induced by intranasal instillation of lipopolysaccharide (LPS). The lung weight/body weight (LW/BW) ratio and excised lung gas volume (ELGV) in each group were measured. Mouse bronchoalveolar lavage fluid (BALF) was collected for cell sorting and protein concentration determination. Expression of tumor necrosis factor α (TNF-α) and interleukin-6 (IL-6) in lung tissues and BALF was also detected. Mouse AMs were isolated to observe the autophagy. Expression of GRP78, PERK, LC3I, LC3II and Beclin1 was further determined. The results indicated that tunicamycin (TM) elevated GRP78 and PERK expression of AMs in ALI mice in a dose-dependent manner. Low dosage of TM abated LC3I expression, increased LC3II and Beclin1 expression, triggered ER stress and AM autophagy, and alleviated pathological changes of AMs in ALI mice. Also, in ALI mice, low dosage of TM attenuated goblet cell proliferation of tracheal wall, and declined LW/BW ratio, ELGV, total cells and neutrophils, protein concentrations in BALF, and IL-6 and TNF-α expression in lung tissues and BALF. Collectively, this study suggests that a low dosage of TM-induced ER stress can enhance the autophagy of AM in ALI mice models, thus attenuating the progression of ALI and airway inflammation.

Interleukin-2-Inducible T-Cell Kinase Deficiency Impairs Early Pulmonary Protection Against Mycobacterium tuberculosis Infection

Interleukin-2 (IL-2) inducible T-cell kinase (ITK) is a non-receptor tyrosine kinase highly expressed in T-cell lineages and regulates multiple aspects of T-cell development and function, mainly through its function downstream of the T-cell receptor. Itk deficiency can lead to CD4 lymphopenia and Epstein-Bar virus (EBV)-associated lymphoproliferation and recurrent pulmonary infections in humans. However, the role of the ITK signaling pathway in pulmonary responses in active tuberculosis due to Mtb infection is not known. We show here that human lungs with active tuberculosis exhibit altered T-cell receptor/ITK signaling and that Itk deficiency impaired early protection against Mtb in mice, accompanied by defective development of IL-17A-producing γδ T cells in the lungs. These findings have important implications of human genetics associated with susceptibility to Mtb due to altered immune responses and molecular signals modulating host immunity that controls Mtb activity. Enhancing ITK signaling pathways may be an alternative strategy to target Mtb infection, especially in cases with highly virulent strains in which IL-17A plays an essential protective role.

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.

Reactive oxygen species-mediated endoplasmic reticulum stress response induces apoptosis of Mycobacterium avium-infected macrophages by activating regulated IRE1-dependent decay pathway

Mycobacterium avium, a slow‐growing nontuberculous mycobacterium, causes fever, diarrhoea, loss of appetite, and weight loss in immunocompromised people. We have proposed that endoplasmic reticulum (ER) stress‐mediated apoptosis plays a critical role in removing intracellular mycobacteria. In the present study, we investigated the role of the regulated IRE1‐dependent decay (RIDD) pathway in macrophages during M. avium infection based on its role in the regulation of gene expression. The inositol‐requiring enzyme 1 (IRE1)/apoptosis signal‐regulating kinase 1 (ASK1)/c‐Jun N‐terminal kinase (JNK) signalling pathway was activated in macrophages after infection with M. avium. The expression of RIDD‐associated genes, such as Bloc1s1 and St3gal5, was decreased in M. avium‐infected macrophages. Interestingly, M. avium‐induced apoptosis was significantly suppressed by pretreatment with irestatin (inhibitor of IRE1α) and 4μ8c (RIDD blocker). Macrophages pretreated with N‐acetyl cysteine (NAC) showed decreased levels of reactive oxygen species (ROS), IRE1α, and apoptosis after M. avium infection. The expression of Bloc1s1 and St3gal5 was increased in NAC‐pretreated macrophages following infection with M. avium. Growth of M. avium was significantly increased in irestatin‐, 4μ8c‐, and NAC‐treated macrophages compared with the control. The data indicate that the ROS‐mediated ER stress response induces apoptosis of M. avium‐infected macrophages by activating IRE1α‐RIDD. Thus, activation of IRE1α suppresses the intracellular survival of M. avium in macrophages.

Oxidative and endoplasmic reticulum stress in tuberculous meningitis related seizures

BACKGROUND AND PURPOSE

High oxygen consumption and high polyunsaturated fatty acid content in the brain may render it vulnerable to oxidative stress and endoplasmic reticulum (ER) stress. We report the role of these parameters in tuberculous meningitis (TBM) patients with seizures and correlate these with clinical radiological, and laboratory findings.

METHODS

Serum oxidative stress markers ; Catalase, Superoxide dismutase (SOD), Glutathione (GSH), Protein-carbonyl, Malonaldehyde (MDA) were measured using spectrophotometer and ER stress markers-ATF4, CHOP, XBP1 and GRP-78 using RT-PCR in TBM patients, 29 with seizures, 20 without seizures and 20 matched controls. In 10 patients, sequential estimation of oxidative stress and ER stress markers was also measured.

RESULTS

In comparison to controls, TBM patients had significant difference in the expression of oxidative stress and ER stress markers. Serum MDA (P=0.02), protein-carbonyl (P < 0.01) were significantly higher and SOD (P=0.02) and GSH (P < 0.01) significantly lower in the patients with seizures compared to those without seizures. The ER stress markers were insignificantly elevated in TBM patients with seizures. On sequential evaluation, oxidative stress and ER stress markers increased following seizures and returned to baseline at the time of discharge.

CONCLUSION

The results suggest some role of oxidative stress and ER stress in TBM, but do not predict its outcome.

Screening Mycobacterium tuberculosis Secreted Proteins Identifies Mpt64 as a Eukaryotic Membrane-Binding Bacterial Effector

Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis, is one of the most successful human pathogens. One reason for its success is that Mtb can reside within host macrophages, a cell type that normally functions to phagocytose and destroy infectious bacteria. However, Mtb is able to evade macrophage defenses in order to survive for prolonged periods of time. Many intracellular pathogens secrete virulence factors targeting host membranes and organelles to remodel their intracellular environmental niche. We hypothesized that Mtb secreted proteins that target host membranes are vital for Mtb to adapt to and manipulate the host environment for survival. Thus, we characterized 200 secreted proteins from Mtb for their ability to associate with eukaryotic membranes using a unique temperature-sensitive yeast screen and to manipulate host trafficking pathways using a modified inducible secretion screen. We identified five Mtb secreted proteins that both associated with eukaryotic membranes and altered the host secretory pathway. One of these secreted proteins, Mpt64, localized to the endoplasmic reticulum during Mtb infection of murine and human macrophages and impaired the unfolded protein response in macrophages. These data highlight the importance of secreted proteins in Mtb pathogenesis and provide a basis for further investigation into their molecular mechanisms.IMPORTANCE Advances have been made to identify secreted proteins of Mycobacterium tuberculosis during animal infections. These data, combined with transposon screens identifying genes important for M. tuberculosis virulence, have generated a vast resource of potential M. tuberculosis virulence proteins. However, the function of many of these proteins in M. tuberculosis pathogenesis remains elusive. We have integrated three cell biological screens to characterize nearly 200 M. tuberculosis secreted proteins for eukaryotic membrane binding, host subcellular localization, and interactions with host vesicular trafficking. In addition, we observed the localization of one secreted protein, Mpt64, to the endoplasmic reticulum (ER) during M. tuberculosis infection of macrophages. Interestingly, although Mpt64 is exported by the Sec pathway, its delivery into host cells was dependent upon the action of the type VII secretion system. Finally, we observed that Mpt64 impairs the ER-mediated unfolded protein response in macrophages.

Oxidized low-density lipoprotein (oxLDL) supports Mycobacterium tuberculosis survival in macrophages by inducing lysosomal dysfunction

Type 2 diabetes mellitus (DM) is a major risk factor for developing tuberculosis (TB). TB-DM comorbidity is expected to pose a serious future health problem due to the alarming rise in global DM incidence. At present, the causal underlying mechanisms linking DM and TB remain unclear. DM is associated with elevated levels of oxidized low-density lipoprotein (oxLDL), a pathologically modified lipoprotein which plays a key role during atherosclerosis development through the formation of lipid-loaded foamy macrophages, an event which also occurs during progression of the TB granuloma. We therefore hypothesized that oxLDL could be a common factor connecting DM to TB. To study this, we measured oxLDL levels in plasma samples of healthy controls, TB, DM and TB-DM patients, and subsequently investigated the effect of oxLDL treatment on human macrophage infection with Mycobacterium tuberculosis (Mtb). Plasma oxLDL levels were significantly elevated in DM patients and associated with high triglyceride levels in TB-DM. Strikingly, incubation with oxLDL strongly increased macrophage Mtb load compared to native or acetylated LDL (acLDL). Mechanistically, oxLDL -but not acLDL- treatment induced macrophage lysosomal cholesterol accumulation and increased protein levels of lysosomal and autophagy markers, while reducing Mtb colocalization with lysosomes. Importantly, combined treatment of acLDL and intracellular cholesterol transport inhibitor (U18666A) mimicked the oxLDL-induced lysosomal phenotype and impaired macrophage Mtb control, illustrating that the localization of lipid accumulation is critical. Collectively, these results demonstrate that oxLDL could be an important DM-associated TB-risk factor by causing lysosomal dysfunction and impaired control of Mtb infection in human macrophages.

Tuberculosis (TB) is an infectious disease of the lungs caused by a bacterium, Mycobacterium tuberculosis (Mtb), and is responsible for over a million deaths per year worldwide. Population studies have demonstrated that type 2 diabetes mellitus (DM) is a risk factor for TB as it triples the risk of developing the disease. DM is a metabolic disorder which is generally associated with obesity, and is characterized by resistance to the pancreatic hormone insulin and high blood glucose and lipid levels. As the global incidence of DM is rising at an alarming rate, especially in regions where TB is common, it is important to understand precisely how DM increases the risk of developing TB. Both TB and DM are associated with the development of foamy macrophages, lipid-loaded white blood cells, which can be the result of a specific lipoprotein particle called oxidized low-density lipoprotein (oxLDL). Here, we demonstrated that DM patients have high blood levels of oxLDL, and generating foamy macrophages with oxLDL supported Mtb survival after infection as a result of faulty intracellular cholesterol accumulation. Our results propose a proof of concept for oxLDL as a risk factor for TB development, encouraging future studies on lipid-lowering therapies for TB-DM.

Endoplasmic Reticulum Stress Induces Macrophages to Produce IL-1β During Mycobacterium bovis Infection via a Positive Feedback Loop Between Mitochondrial Damage and Inflammasome Activation

Mycobacterium bovis, the causative agent of tuberculosis in cattle and humans, infects host macrophages and induces endoplasmic reticulum stress (ERS), mitochondrial damage, and interleukin (IL)-1β production. The relationship between these phenotypes is yet to be elucidated. In this study, we investigated the role of ERS in mitochondrial damage and IL-1β production in macrophages during infection with a virulent M. bovis strain. We found that ERS activates the inflammasome via NOD-like receptor family, pyrin domain-containing 3 (NLRP3)-caspase-8 and that IFN-inducible protein absent in melanoma 2 (AIM2) triggered mitochondrial damage. ERS increased reactive oxygen species (ROS), which promoted translocation of the inflammasome to the mitochondria. NLRP3, but not AIM2, was involved in the ERS-induced cleavage of caspase-8 and Bid, leading to mitochondrial damage, which was required for the production of mature IL-1β. Our data suggest that ERS induces macrophages to produce mature IL-1β during infection with virulent M. bovis through a positive feedback loop between mitochondrial damage and inflammasome activation. To the best of our knowledge, this is the first evidence of the involvement of ERS and mitochondrial damage in inflammasome activation during M. bovis infection.

The Immune Escape Mechanisms of Mycobacterium Tuberculosis

Epidemiological data from the Center of Disease Control (CDC) and the World Health Organization (WHO) statistics in 2017 show that 10.0 million people around the world became sick with tuberculosis. Mycobacterium tuberculosis (MTB) is an intracellular parasite that mainly attacks macrophages and inhibits their apoptosis. It can become a long-term infection in humans, causing a series of pathological changes and clinical manifestations. In this review, we summarize innate immunity including the inhibition of antioxidants, the maturation and acidification of phagolysosomes and especially the apoptosis and autophagy of macrophages. Besides, we also elaborate on the adaptive immune response and the formation of granulomas. A thorough understanding of these escape mechanisms is of major importance for the prevention, diagnosis and treatment of tuberculosis.

The C/EBP Homologous Protein (CHOP) Transcription Factor Functions in Endoplasmic Reticulum Stress-Induced Apoptosis and Microbial Infection

Apoptosis is a form of cell death by which the body maintains the homeostasis of the internal environment. Apoptosis is an initiative cell death process that is controlled by genes and is mainly divided into endogenous pathways (mitochondrial pathway), exogenous pathways (death receptor pathway), and apoptotic pathways induced by endoplasmic reticulum (ER) stress. The homeostasis imbalance in ER results in ER stress. Under specific conditions, ER stress can be beneficial to the body; however, if ER protein homeostasis is not restored, the prolonged activation of the unfolded protein response may initiate apoptotic cell death via the up-regulation of the C/EBP homologous protein (CHOP). CHOP plays an important role in ER stress-induced apoptosis and this review focuses on its multifunctional roles in that process, as well as its role in apoptosis during microbial infection. We summarize the upstream and downstream pathways of CHOP in ER stress induced apoptosis. We also focus on the newest discoveries in the functions of CHOP-induced apoptosis during microbial infection, including DNA and RNA viruses and some species of bacteria. Understanding how CHOP functions during microbial infection will assist with the development of antimicrobial therapies.

Rv3033, as an Emerging Anti-apoptosis Factor, Facilitates Mycobacteria Survival via Inhibiting Macrophage Intrinsic Apoptosis

Apoptosis inhibition is a critical strategy of mycobacteria facilitating its survival in macrophages, but the underlying mechanism is not completely understood. In this study, we found that Rv3033, a secreted virulence factor of mycobacteria, played an important role in bacillary survival within macrophages. Forced over-expressed of Rv3033 in macrophages could efficiently resist mycobacteria-induced early and late apoptosis, accompanied with the obvious increased cellular bacterial burden. By exploring the underlying mechanism, we found that Rv3033 efficiently repressed the intrinsic (caspase-9 meditated), but not the extrinsic (caspase-8 mediated) apoptotic pathway in mycobacteria-infected macrophages. And this repression relied on the orchestrating blockade of both mitochondrial cytochrome c release and endoplasmic reticulum (ER) stress PERK branch activation. Our study uncovered a novel function of mycobacterial virulence factor Rv3033 as an anti-apoptotic protein, which may provide a new target for tuberculosis (TB) treatment.

Characterization of immune responses of human PBMCs infected with Mycobacterium tuberculosis H37Ra: Impact of donor declared BCG vaccination history on immune responses and M. tuberculosis growth

This study characterized the immune responses in early Mycobacterium tuberculosis (Mtb) H37Ra infection of human peripheral blood mononuclear cell (PBMC)-collagen matrix culture and the impact of Bacille Calmette-Guérin (BCG) vaccination history of donor PBMCs on the immune responses to Mtb infection. Aggregates of PBMCs were initially observed on day 3 and the size of aggregates continued to increase on day 8 post-infection, where macrophages and T cell subsets were identified to be present. Similarly, mycobacterial load progressively increased in infected PBMCs during the 8 days of culture but were significantly lower in infected PBMCs from BCG vaccinated (BCG+) donors compared to unvaccinated (BCG-) donors. The levels of INF-γ, TNF-α, IL-4, IL-6, IL-10 and IL-17 in the supernatants of Mtb-infected PBMCs peaked at day 3 and decreased on days 5 and 8. The levels of these cytokines except IL-10 were significantly lower in Mtb-infected PBMCs from BCG+ donors compared to infected PBMCs from BCG- donors. The percentages of activated naïve Th cells, activated effector memory Th cells and activated central memory Tc cells were significantly higher in Mtb-infected PBMCs compared to uninfected PBMCs at day 8 post-infection. Further, the proportion of activated central memory Tc cells was significantly higher in infected PBMCs from BCG+ donors compared to the BCG- donors. This study highlights the possibility that BCG vaccination may confound results that utilize human PBMCs to study Mtb infection.

Storage lipid studies in tuberculosis reveal that foam cell biogenesis is disease-specific

Foam cells are lipid-laden macrophages that contribute to the inflammation and tissue damage associated with many chronic inflammatory disorders. Although foam cell biogenesis has been extensively studied in atherosclerosis, how these cells form during a chronic infectious disease such as tuberculosis is unknown. Here we report that, unlike the cholesterol-laden cells of atherosclerosis, foam cells in tuberculous lung lesions accumulate triglycerides. Consequently, the biogenesis of foam cells varies with the underlying disease. In vitro mechanistic studies showed that triglyceride accumulation in human macrophages infected with Mycobacterium tuberculosis is mediated by TNF receptor signaling through downstream activation of the caspase cascade and the mammalian target of rapamycin complex 1 (mTORC1). These features are distinct from the known biogenesis of atherogenic foam cells and establish a new paradigm for non-atherogenic foam cell formation. Moreover, they reveal novel targets for disease-specific pharmacological interventions against maladaptive macrophage responses.

The PGRS Domain of Mycobacterium tuberculosis PE_PGRS Protein Rv0297 Is Involved in Endoplasmic Reticulum Stress-Mediated Apoptosis through Toll-Like Receptor 4

The genome of Mycobacterium tuberculosis, the causal organism of tuberculosis (TB), encodes a unique protein family known as the PE/PPE/PGRS family, present exclusively in the genus Mycobacterium and nowhere else in the living kingdom, with largely unexplored functions. We describe the functional significance of the PGRS domain of Rv0297, a member of this family. In silico analyses revealed the presence of intrinsically disordered stretches and putative endoplasmic reticulum (ER) localization signals in the PGRS domain of Rv0297 (Rv0297PGRS). The PGRS domain aids in ER localization, which was shown by infecting macrophage cells with M. tuberculosis and by overexpressing the protein by transfection in macrophage cells followed by activation of the unfolded protein response, as evident from increased expression of GRP78/GRP94 and CHOP/ATF4, leading to disruption of intracellular Ca²⁺ homeostasis and increased nitric oxide (NO) and reactive oxygen species (ROS) production. The consequent activation of the effector caspase-8 resulted in apoptosis of macrophages, which was Toll-like receptor 4 (TLR4) dependent. Administration of recombinant Rv0297PGRS (rRv0297PGRS) also exhibited similar effects. These results implicate a hitherto-unknown role of the PGRS domain of the PE_PGRS protein family in ER stress-mediated cell death through TLR4. Since this protein is already known to be present at later stages of infection in human granulomas it points to the possibility of it being employed by M. tuberculosis for its dissemination via an apoptotic mechanism.

Apoptosis is generally thought to be a defense mechanism in protecting the host against Mycobacterium tuberculosis in early stages of infection. However, apoptosis during later stages in lung granulomas may favor the bacterium in disseminating the disease. ER stress has been found to induce apoptosis in TB granulomas, in zones where apoptotic macrophages accumulate in mice and humans. In this study, we report ER stress-mediated apoptosis of host cells by the Rv0297-encoded PE_PGRS5 protein of M. tuberculosis exceptionally present in the pathogenic Mycobacterium genus. The PGRS domain of Rv0297 aids the protein in localizing to the ER and induces the unfolded protein response followed by apoptosis of macrophages. The effect of the Rv0297PGRS domain was found to be TLR4 dependent. This study presents novel insights on the strategies employed by M. tuberculosis to disseminate the disease.

Mycobacterium fortuitum-induced ER-Mitochondrial calcium dynamics promotes calpain/caspase-12/caspase-9 mediated apoptosis in fish macrophages

Mycobacterium fortuitum is a natural fish pathogen. It induces apoptosis in headkidney macrophages (HKM) of catfish, Clarias sp though the mechanism remains largely unknown. We observed M. fortuitum triggers calcium (Ca²⁺) insult in the sub-cellular compartments which elicits pro-apototic ER-stress factor CHOP. Alleviating ER-stress inhibited CHOP and attenuated HKM apoptosis implicating ER-stress in the pathogenesis of M. fortuitum. ER-stress promoted calpain activation and silencing the protease inhibited caspase-12 activation. The study documents the primal role of calpain/caspase-12 axis on caspase-9 activation in M. fortuitum-pathogenesis. Mobilization of Ca²⁺ from ER to mitochondria led to increased mitochondrial Ca²⁺ (Ca²⁺)_m load,_, mitochondrial permeability transition (MPT) pore opening, altered mitochondrial membrane potential (ΔΨm) and cytochrome c release eventually activating the caspase-9/-3 cascade. Ultra-structural studies revealed close apposition of ER and mitochondria and pre-treatment with (Ca²⁺)_m-uniporter (MUP) blocker ruthenium red, reduced Ca²⁺ overload suggesting (Ca²⁺)_m fluxes are MUP-driven and the ER-mitochondria tethering orchestrates the process. This is the first report implicating role of sub-cellular Ca²⁺ in the pathogenesis of M. fortuitum. We summarize, the dynamics of Ca²⁺ in sub-cellular compartments incites ER-stress and mitochondrial dysfunction, leading to activation of pro-apoptotic calpain/caspase-12/caspase-9 axis in M. fortuitum-infected HKM.

Reinforcing the Functionality of Mononuclear Phagocyte System to Control Tuberculosis

The mononuclear phagocyte system (MPS) constitutes dendritic cells, monocytes, and macrophages. This system contributes to various functions that are essential for maintaining homeostasis, activation of innate immunity, and bridging it with the adaptive immunity. Consequently, MPS is highly important in bolstering immunity against the pathogens. However, MPS is the frontline cells in destroying Mycobacterium tuberculosis (Mtb), yet the bacterium prefers to reside in the hostile environment of macrophages. Therefore, it may be very interesting to study the struggle between Mtb and MPS to understand the outcome of the disease. In an event when MPS predominates Mtb, the host remains protected. By contrast, the situation becomes devastating when the pathogen tames and tunes the host MPS, which ultimately culminates into tuberculosis (TB). Hence, it becomes extremely crucial to reinvigorate MPS functionality to overwhelm Mtb and eliminate it. In this article, we discuss the strategies to bolster the function of MPS by exploiting the molecules associated with the innate immunity and highlight the mechanisms involved to overcome the Mtb-induced suppression of host immunity. In future, such approaches may provide an insight to develop immunotherapeutics to treat TB.

Phenotypic Heterogeneity in Mycobacterium tuberculosis

The interaction between the host and the pathogen is extremely complex and is affected by anatomical, physiological, and immunological diversity in the microenvironments, leading to phenotypic diversity of the pathogen. Phenotypic heterogeneity, defined as nongenetic variation observed in individual members of a clonal population, can have beneficial consequences especially in fluctuating stressful environmental conditions. This is all the more relevant in infections caused by Mycobacterium tuberculosis wherein the pathogen is able to survive and often establish a lifelong persistent infection in the host. Recent studies in tuberculosis patients and in animal models have documented the heterogeneous and diverging trajectories of individual lesions within a single host. Since the fate of the individual lesions appears to be determined by the local tissue environment rather than systemic response of the host, studying this heterogeneity is very relevant to ensure better control and complete eradication of the pathogen from individual lesions. The heterogeneous microenvironments greatly enhance M. tuberculosis heterogeneity influencing the growth rates, metabolic potential, stress responses, drug susceptibility, and eventual lesion resolution. Single-cell approaches such as time-lapse microscopy using microfluidic devices allow us to address cell-to-cell variations that are often lost in population-average measurements. In this review, we focus on some of the factors that could be considered as drivers of phenotypic heterogeneity in M. tuberculosis as well as highlight some of the techniques that are useful in addressing this issue.

Oleate protects macrophages from palmitate-induced apoptosis through the downregulation of CD36 expression

In obese patients, free fatty acids ectopically accumulated in non-adipose tissues cause cell death. Saturated fatty acids are more deleterious to non-adipose cells, and supplementation with monounsaturated fatty acids has been proposed to rescue cells from saturated fatty acid-induced cytotoxicity; however, the mechanisms are not well understood. To understand the cytoprotective role of monounsaturated fatty acids in lipotoxic cell death of macrophages, we investigated the antagonizing effect of oleate and the underlying mechanisms in palmitate-treated RAW264.7 cells. Palmitate strongly induced apoptosis in macrophages by increasing CD36 expression, which was identified to mediate both endoplasmic reticulum stress and the generation of reactive oxygen species. Co-treatment with oleate significantly reduced CD36 expression and its downstream signaling pathways of apoptosis in palmitate-treated cells. These findings provide a novel mechanism by which oleate protects macrophages from palmitate-induced lipotoxicity.

Role of apoptosis and autophagy in tuberculosis

Tuberculosis (TB) is one of the oldest known human diseases and is transmitted by the bacteria Mycobacterium tuberculosis (Mtb). TB has a rich history with evidence of TB infections dating back to 5,800 bc TB is unique in its ability to remain latent in an individual for decades, with the possibility of later reactivation, causing widespread systemic symptoms. Currently, it is estimated that more than one-third of the world's population (~2 billion people) are infected with Mtb. Prolonged periods of therapy and complexity of treatment regimens, especially in active infection, have led to poor compliance in patients being treated for TB. Therefore, it is vitally important to have a thorough knowledge of the pathophysiology of Mtb to understand the disease progression, as well as to develop novel diagnostic tests and treatments. Alveolar macrophages represent both the primary host cell and the first line of defense against the Mtb infection. Apoptosis and autophagy of macrophages play a vital role in the pathogenesis and also in the host defense against Mtb. This review will outline the role of these two cellular processes in defense against Mtb with particular emphasis on innate immunity and explore developing therapies aimed at altering host responses to the disease.

Calreticulin modulates the intracellular survival of mycobacteria by regulating ER-stress-mediated apoptosis

Endoplasmic reticulum (ER)-stress-mediated apoptosis is a host defense mechanism against Mycobacterium tuberculosis (Mtb) infection. Calreticulin (CRT) is the major calcium-binding chaperone protein. Previous reports have suggested a close relationship between the cell-surface expression of CRT and apoptosis. In this study, the role of CRT during Mtb infection was examined. The results showed that Mtb infection induces CRT production by macrophages and that CRT levels are correlated with the degree of apoptotic cell death. The enhanced production of CRT was associated with the ER stress induced by Mtb infection. A significant increase in CRT translocation from the cytosol to the plasma membrane after 24 h of infection suggested the importance of CRT localization in the induction of apoptosis during Mtb infection. An investigation of the factors associated with CRT translocation and the ability of ectopically expressed CRT to induce apoptosis showed that pretreatment with a reactive oxygen species scavenger decreased Mtb-induced CRT expression, leading to the reduction of CHOP and caspase-3 activation. The intracellular survival of Mtb was significantly higher in macrophages transfected with a CRT-specific small interfering RNA than in control cells. The key role of CRT in inducing apoptosis included its interaction with CXCR1 and TNFR1 in Mtb-infected macrophages. The CRT/CXCR1/TNFR1 complex was shown to induce the extrinsic apoptotic pathway during Mtb infection. Together, these results demonstrate that CRT is critical for the intracellular survival of Mtb, via ER-stress-induced apoptosis, as well as the importance of ER stress-mediated CRT localization in the pathogenesis of tuberculosis.

Listeria monocytogenes induces an interferon-enhanced activation of the integrated stress response that is detrimental for resolution of infection in mice

Type I interferons (IFNs) induce a detrimental response during Listeria monocytogenes (L. monocytogenes) infection. We were interested in identifying mechanisms linking IFN signaling to negative host responses against L. monocytogenes infection. Herein, we found that infection of myeloid cells with L. monocytogenes led to a coordinated induction of type I IFNs and activation of the integrated stress response (ISR). Infected cells did not induce Xbp1 splicing or BiP upregulation, indicating that the unfolded protein response was not triggered. CHOP (Ddit3) gene expression was upregulated during the ISR activation induced by L. monocytogenes. Myeloid cells deficient in either type I IFN signaling or PKR activation had less upregulation of CHOP following infection. CHOP‐deficient mice showed lower expression of innate immune cytokines and were more resistant than wild‐type counterparts following L. monocytogenes infection. These findings indicate that L. monocytogenes infection induces type I IFNs, which activate the ISR through PKR, which contributes to a detrimental outcome in the infected host.

Using structural knowledge in the protein data bank to inform the search for potential host-microbe protein interactions in sequence space: application to Mycobacterium tuberculosis

Background

A comprehensive map of the human-M. tuberculosis (MTB) protein interactome would help fill the gaps in our understanding of the disease, and computational prediction can aid and complement experimental studies towards this end. Several sequence-based in silico approaches tap the existing data on experimentally validated protein-protein interactions (PPIs); these PPIs serve as templates from which novel interactions between pathogen and host are inferred. Such comparative approaches typically make use of local sequence alignment, which, in the absence of structural details about the interfaces mediating the template interactions, could lead to incorrect inferences, particularly when multi-domain proteins are involved.

Results

We propose leveraging the domain-domain interaction (DDI) information in PDB complexes to score and prioritize candidate PPIs between host and pathogen proteomes based on targeted sequence-level comparisons. Our method picks out a small set of human-MTB protein pairs as candidates for physical interactions, and the use of functional meta-data suggests that some of them could contribute to the in vivo molecular cross-talk between pathogen and host that regulates the course of the infection. Further, we present numerical data for Pfam domain families that highlights interaction specificity on the domain level. Not every instance of a pair of domains, for which interaction evidence has been found in a few instances (i.e. structures), is likely to functionally interact. Our sorting approach scores candidates according to how “distant” they are in sequence space from known examples of DDIs (templates). Thus, it provides a natural way to deal with the heterogeneity in domain-level interactions.

Conclusions

Our method represents a more informed application of local alignment to the sequence-based search for potential human-microbial interactions that uses available PPI data as a prior. Our approach is somewhat limited in its sensitivity by the restricted size and diversity of the template dataset, but, given the rapid accumulation of solved protein complex structures, its scope and utility are expected to keep steadily improving.

Electronic supplementary material

The online version of this article (doi:10.1186/s12859-017-1550-y) contains supplementary material, which is available to authorized users.

Alternate splicing of transcripts shape macrophage response to Mycobacterium tuberculosis infection

Transcriptional reprogramming of macrophages upon Mycobacterium tuberculosis (Mtb) infection is widely studied; however, the significance of alternate splicing (AS) in shaping cellular responses to mycobacterial infections is not yet appreciated. Alternate splicing can influence transcript stability or structure, function and localization of corresponding proteins thereby altering protein stoichiometry and physiological consequences. Using comprehensive analysis of a time-series RNA-seq data obtained from human macrophages infected with virulent or avirulent strains of Mtb, we show extensive remodeling of alternate splicing in macrophage transcriptome. The global nature of this regulation was evident since genes belonging to functional classes like trafficking, immune response, autophagy, redox and metabolism showed marked departure in the pattern of splicing in the infected macrophages. The systemic perturbation of splicing machinery in the infected macrophages was apparent as genes involved at different stages of spliceosome assembly were also regulated at the splicing level. Curiously there was a considerable increase in the expression of truncated/non-translatable variants of several genes, specifically upon virulent infections. Increased expression of truncated transcripts correlated with a decline in the corresponding protein levels. We verified the physiological relevance for one such candidate gene RAB8B; whose truncated variant gets enriched in H37Rv infected cells. Upon tweaking relative abundance of longer or shorter variants of RAB8B transcripts by specialized transduction, mycobacterial targeting to lysosomes could be promoted or blocked respectively, which also resulted in corresponding changes in the bacterial survival. Our results show RAB8B recruitment to the mycobacterial phagosomes is required for phagosome maturation. Thus the abundance of truncated RAB8B variant helps virulent Mtb survival by limiting the RAB8B levels in the cells, a mechanism which we subsequently verified in human primary macrophages. Taken together we demonstrate alternate splicing as a new locus of intervention by Mtb and provide attractive alternative to exploit for novel drug targets against Mtb.

Upregulation of autophagy genes and the unfolded protein response in human heart failure

The cellular environment of the mammalian heart constantly is challenged with environmental and intrinsic pathological insults, which affect the proper folding of proteins in heart failure. The effects of damaged or misfolded proteins on the cell can be profound and result in a process termed "proteotoxicity". While proteotoxicity is best known for its role in mediating the pathogenesis of neurodegenerative diseases such as Alzheimer's disease, its role in human heart failure also has been recognized. The UPR involves three branches, including PERK, ATF6, and IRE1. In the presence of a misfolded protein, the GRP78 molecular chaperone that normally interacts with the receptors PERK, ATF6, and IRE-1 in the endoplasmic reticulum detaches to attempt to stabilize the protein. Mouse models of cardiac hypertrophy, ischemia, and heart failure demonstrate increases in activity of all three branches after removing GRP78 from these internal receptors. Recent studies have linked elevated PERK and CHOP in vitro with regulation of ion channels linked with human systolic heart failure. With this in mind, we specifically investigated ventricular myocardium from 10 patients with a history of conduction system defects or arrhythmias for expression of UPR and autophagy genes compared to myocardium from non-failing controls. We identified elevated Chop, Atf3, and Grp78 mRNA, along with XBP-1-regulated Cebpa mRNA, indicative of activation of the UPR in human heart failure with arrhythmias.

Mycobacterium bovis Induces Endoplasmic Reticulum Stress Mediated-Apoptosis by Activating IRF3 in a Murine Macrophage Cell Line

Mycobacterium bovis (M. bovis) is highly adapted to macrophages and has developed multiple mechanisms to resist intracellular assaults. However, the host cells in turn deploy a multipronged defense mechanism to control bacterial infection. Endoplasmic reticulum (ER) stress-mediated apoptosis is one such primary defense mechanism. However, the role of interferon regulatory factor 3 (IRF3) between ER stress and apoptosis during M. bovis infection is unknown. Here, we demonstrate that M. bovis effectively induced apoptosis in murine macrophages. Caspase-12, caspase-9, and caspase-3 were activated over a 48 h infection period. The splicing of XBP-1 mRNA and the level of phosphorylation of eIF2α, indicators of ER stress, significantly increased at early time points after M. bovis infection. The expansion of the ER compartment, a morphological hallmark of ER stress, was observed at 6 h. Pre-treatment of Raw 264.7 cells with 4-PBA (an ER stress-inhibitor) reduced the activation of the ER stress indicators, caspase activation and its downstream poly (ADP-ribose) polymerase (PARP) cleavage, phosphorylation of TBK1 and IRF3 and cytoplasmic co-localization of STING and TBK1. M. bovis infection led to the interaction of activated IRF3 and cytoplasmic Bax leading to mitochondrial damage. Role of IRF3 in apoptosis was further confirmed by blocking this molecule with BX-795 that showed significant reduction expression of caspase-8 and caspase-3. Intracellular survival of M. bovis increased in response to 4-PBA and BX-795. These findings indicate that STING-TBK1-IRF3 pathway mediates a crosstalk between ER stress and apoptosis during M. bovis infection, which can effectively control intracellular bacteria.

Roles of endoplasmic reticulum stress-mediated apoptosis in M1-polarized macrophages during mycobacterial infections

Alteration of macrophage function has an important regulatory impact on the survival of intracellular mycobacteria. We found that macrophages infected with attenuated Mycobacterium tuberculosis (Mtb) strain H37Ra had elevated expression of M1-related molecules, whereas the M2 phenotype was dominant in macrophages infected with virulent Mtb H37Rv. Further, the TLR signalling pathway played an important role in modulating macrophage polarization against Mtb infection. Interestingly, endoplasmic reticulum (ER) stress was significantly increased in M1 polarized macrophages and these macrophages effectively removed intracellular Mtb, indicating that ER stress may be an important component of the host immune response to Mtb in M1 macrophages. This improved understanding of the mechanisms that regulate macrophage polarization could provide new therapeutic strategies for tuberculosis.

Necrotic Cell Sensor Clec4e Promotes a Proatherogenic Macrophage Phenotype Through Activation of the Unfolded Protein Response

Background:

Atherosclerotic lesion expansion is characterized by the development of a lipid-rich necrotic core known to be associated with the occurrence of complications. Abnormal lipid handling, inflammation, and alteration of cell survival or proliferation contribute to necrotic core formation, but the molecular mechanisms involved in this process are not properly understood. C-type lectin receptor 4e (Clec4e) recognizes the cord factor of Mycobacterium tuberculosis but also senses molecular patterns released by necrotic cells and drives inflammation.

Methods:

We hypothesized that activation of Clec4e signaling by necrosis is causally involved in atherogenesis. We addressed the impact of Clec4e activation on macrophage functions in vitro and on the development of atherosclerosis using low-density lipoprotein receptor–deficient ( Ldlr −/− ) mice in vivo.

Results:

We show that Clec4e is expressed within human and mouse atherosclerotic lesions and is activated by necrotic lesion extracts. Clec4e signaling in macrophages inhibits cholesterol efflux and induces a Syk-mediated endoplasmic reticulum stress response, leading to the induction of proinflammatory mediators and growth factors. Chop and Ire1a deficiencies significantly limit Clec4e-dependent effects, whereas Atf3 deficiency aggravates Clec4e-mediated inflammation and alteration of cholesterol efflux. Repopulation of Ldlr −/− mice with Clec4e −/− bone marrow reduces lipid accumulation, endoplasmic reticulum stress, and macrophage inflammation and proliferation within the developing arterial lesions and significantly limits atherosclerosis.

Conclusions:

Our results identify a nonredundant role for Clec4e in coordinating major biological pathways involved in atherosclerosis and suggest that it may play similar roles in other chronic inflammatory diseases.

Understanding the pathophysiology of the human TB lung granuloma using in vitro granuloma models

Tuberculosis remains a major human health threat that infects one in three individuals worldwide. Infection with Mycobacterium tuberculosis is a standoff between host and bacteria in the formation of a granuloma. This review will introduce a variety of bacterial and host factors that impact individual granuloma fates. The authors describe advances in the development of in vitro granuloma models, current evidence surrounding infection and granuloma development, and the applicability of existing in vitro models in the study of human disease. In vitro models of infection help improve our understanding of pathophysiology and allow for the discovery of other potential models of study.