Autophagy and Burkholderia

Effective Therapeutic Options for Melioidosis: Antibiotics versus Phage Therapy

Melioidosis, also known as Whitmore's disease, is a potentially fatal infection caused by the Gram-negative bacteria Burkholderia pseudomallei with a mortality rate of 10-50%. The condition is a "glanders-like" illness prevalent in Southeast Asian and Northern Australian regions and can affect humans, animals, and sometimes plants. Melioidosis received the epithet "the great mimicker" owing to its vast spectrum of non-specific clinical manifestations, such as localised abscesses, septicaemia, pneumonia, septic arthritis, osteomyelitis, and encephalomyelitis, which often lead to misdiagnosis and ineffective treatment. To date, antibiotics remain the backbone of melioidosis treatment, which includes intravenous therapy with ceftazidime or meropenem, followed by oral therapy with TMP-SMX or amoxicillin/clavulanic acid and supported by adjunctive treatment. However, bacteria have developed resistance to a series of antibiotics, including clinically significant ones, during treatment. Therefore, phage therapy has gained unprecedented interest and has been proposed as an alternative treatment. Although no effective phage therapy has been published, the findings of experimental phage therapies suggest that the concept could be feasible. This article reviews the benefits and limitations of antibiotics and phage therapy in terms of established regimens, bacterial resistance, host specificity, and biofilm degradation.

Hacking the host: exploitation of macrophage polarization by intracellular bacterial pathogens

Macrophages play an integral role in host defenses against intracellular bacterial pathogens. A remarkable plasticity allows for adaptation to the needs of the host to orchestrate versatile innate immune responses to a variety of microbial threats. Several bacterial pathogens have adapted to macrophage plasticity and modulate the classical (M1) or alternative (M2) activation bias towards a polarization state that increases fitness for intracellular survival. Here, we summarize the current understanding of the host macrophage and intracellular bacterial interface; highlighting the roles of M1/M2 polarization in host defense and the mechanisms employed by several important intracellular pathogens to modulate macrophage polarization to favor persistence or proliferation. Understanding macrophage polarization in the context of disease caused by different bacterial pathogens is important for the identification of targets for therapeutic intervention.

Mechanism and medical implications of mammalian autophagy

Autophagy is a highly conserved catabolic process induced under various conditions of cellular stress, which prevents cell damage and promotes survival in the event of energy or nutrient shortage and responds to various cytotoxic insults. Thus, autophagy has primarily cytoprotective functions and needs to be tightly regulated to respond correctly to the different stimuli that cells experience, thereby conferring adaptation to the ever-changing environment. It is now apparent that autophagy is deregulated in the context of various human pathologies, including cancer and neurodegeneration, and its modulation has considerable potential as a therapeutic approach.

The treatment of melioidosis: is there a role for repurposed drugs? A proposal and review

Introduction: Melioidosis is a significant health problem within endemic areas such as Southeast Asia and Northern Australia. The varied presentation of melioidosis and the intrinsic antibiotic resistance of Burkholderia pseudomallei, the causative organism, make melioidosis a difficult infection to manage. Often prolonged courses of antibiotic treatments are required with no guarantee of clinical success.Areas covered: B. pseudomallei is able to enter phagocytic cells, affect immune function, and replicate, via manipulation of the caspase system. An examination of this mechanism, and a look at other factors in the pathogenesis of melioidosis, shows that there are multiple potential points of therapeutic intervention, some of which may be complementary. These include the directed use of antimicrobial compounds, blocking virulence mechanisms, balancing or modulating cytokine responses, and ameliorating sepsis.Expert commentary: There may be therapeutic options derived from drugs in clinical use for unrelated conditions that may have benefit in melioidosis. Key compounds of interest primarily affect the disequilibrium of the cytokine response, and further preclinical work is needed to explore the utility of this approach and encourage the clinical research needed to bring these into beneficial use.

Metformin induced autophagy in diabetes mellitus - Tuberculosis co-infection patients: A case study

Metformin (MET) is a potential combination drug to elevate anti-TB efficacy. However, the clinical effect, especially smear reversion, during metformin applied with anti-tuberculosis and insulin in patients with type 2 DM newly TB co-infection were remain unknown. An observational clinical study was done in DM newly TB co-infection outpatients at Surabaya Paru Hospital. This study evaluated MET therapy, at least 2 months, accompanying with insulin and anti-TB regimens and compared to comparison group. The smear, microtubule-associated Protein1 Light Chain 3B (MAP1LC3B) level, as the presentation of autophagy, Superoxide Dismutase (SOD) level, Interferon (IFN)-γ and Interleukin (IL)-10 levels were evaluated twice. From 42 participants in this study, 22 participants of observation group that received additional MET therapy, 100% had sputum smear reversion after 2-months intensive phase of anti-TB therapy. Whereas 25% of 20 participants of comparison group did not undergo reversion inserts sputum smear. As conclusion, MET has the potential of being an additive combination therapy to enhance the bactericidal effect of anti-TB on DM-TB coinfection patients. Metformin enhances the effects of anti-TB and insulin therapy in increasing the smear reversion by increasing autophagy.

An increase in intracellular p62/NBR1 and persistence of Burkholderia mallei and B. pseudomallei in infected mice linked to autophagy deficiency

Introduction

Burkholderia mallei (B. mallei) and Burkholderia pseudomallei (B. pseudomallei), causative agents of glanders and melioidosis, respectively, are invasive intracellular pathogens that actively multiply in phagocytic and non‐phagocytic cells. Activation of cell‐autonomous autophagy mechanism eliminate intracellular pathogens in which p62 a cytosolic cargo protein is selectively degraded, and an accumulation of this marker occurs if autophagy is deficient. Recurrent, relapsed and reinfection of B. pseudomallei in melioidosis patients in endemic area indicative of lack of complete of clearance and persistence of the pathogen. Reasoning that abundance in the levels of p62 may provide an indication of the intracellular infection, we sought to examine whether increase in intracellular p62 and bacterial burden with Burkholderia infection are linked to autophagy deficiency.

Methods

In this study, we investigated cell culture and mouse models of disease to identify an association between autophagy biomarkers (p62/NBR1) accumulation and intracellular persistence of B. mallei and B. pseudomallei.

Results

We demonstrate, that elevated levels of intracellular p62/NBR1 correlated with bacterial persistence, while pre‐treatment with a pharmacological inducer of autophagy, rapamycin, reduced both intracellular p62, and bacterial survival. Our results showed an elevated p62 levels (2‐5 fold) in spleen and liver cells of Burkholderia‐infected BALB/c mice, as well as in spleen cells of Burkholderia‐infected C57BL/6 mice, suggesting that an increase in p62/NBR1 was due to an autophagy deficiency. Similar to p62, cytosolic LC3‐I levels were also elevated, while the characteristic conversion to the autophagosome‐associated membrane bound form LC3‐II was low in spleens of the infected mice further supporting the conclusion that autophagy was deficient.

Conclusion

Taken together, our results suggest that an increase in intracellular p62/NBR1 may be a potential host cell biomarker of B. mallei or B. pseudomallei infections, and identifying autophagy manipulation may potentially aid to therapeutic approach for complete clearance of the pathogen.

Innate immune response to Burkholderia mallei

Purpose of review

Burkholderia mallei is a facultative intracellular pathogen that causes the highly contagious and often the fatal disease, glanders. With its high rate of infectivity via aerosol and recalcitrance toward antibiotics, this pathogen is considered a potential biological threat agent. This review focuses on the most recent literature highlighting host innate immune response to B. mallei.

Recent findings

Recent studies focused on elucidating host innate immune responses to the novel mechanisms and virulence factors employed by B. mallei for survival. Studies suggest that pathogen proteins manipulate various cellular processes, including host ubiquitination pathways, phagosomal escape, and actin–cytoskeleton rearrangement. Immune-signaling molecules such as Toll-like receptors, nucleotode-binding oligomerization domain, myeloid differentiation primary response protein 88, and proinflammatory cytokines such as interferon-gamma and tumor necrosis factor-α, play key roles in the induction of innate immune responses. Modifications in B. mallei lipopolysaccharide, in particular, the lipid A acyl groups, stimulate immune responses via Toll-like receptor4 activation that may contribute to persistent infection.

Summary

Mortality is high because of septicemia and immune pathogenesis with B. mallei exposure. An effective innate immune response is critical to controlling the acute phase of the infection. Both vaccination and therapeutic approaches are necessary for complete protection against B. mallei.

Inhibition of autolysosome formation in host autophagy by Trypanosoma cruzi infection

Autophagy has emerged as an essential component of the defense system against intracellular pathogens. We demonstrated that Trypanosoma cruzi, an intracellular protozoan parasite, was not eliminated by the host's autophagic machinery despite exposure to the host cell cytoplasm. Puncta of microtubule-associated protein 1 light chain 3 (LC3), an autophagy marker, and LC3-II, a lipidated form of LC3, were significantly increased after infection with T. cruzi, indicating that the parasite activated the early steps of host autophagy and induced autophagosome formation. However, autolysosomes were not observed in the infected cells. In addition, T. cruzi was not enwrapped by autophagosomes, suggesting that the parasite has mechanisms to allow it to evade autophagic capture. The results of this study indicate that host autophagy is incomplete following T. cruzi infection.

Evolution of bopA Gene in Burkholderia: A Case of Convergent Evolution as a Mechanism for Bacterial Autophagy Evasion

Autophagy is an important defense mechanism targeting intracellular bacteria to restrict their survival and growth. On the other hand, several intracellular pathogens have developed an antiautophagy mechanism to facilitate their own replication or intracellular survival. Up to now, no information about the origin or evolution of the antiautophagic genes in bacteria is available. BopA is an effector protein secreted by Burkholderia pseudomallei via the type three secretion system, and it has been shown to play a pivotal role in their escape from autophagy.  The evolutionary origin of bopA was examined in this work. Sequence similarity searches for BopA showed that no homolog of BopA was detected in eukaryotes. However, eukaryotic linear motifs were detected in BopA. The phylogenetic tree of the BopA proteins in our analysis is congruent with the species phylogeny derived from housekeeping genes. Moreover, there was no obvious difference in GC content values of bopA gene and their respective genomes. Integrated information on the taxonomic distribution, phylogenetic relationships, and GC content of the bopA gene of Burkholderia revealed that this gene was acquired via convergent evolution, not from eukaryotic host through horizontal gene transfer (HGT) event. This work has, for the first time, characterized the evolutionary mechanism of bacterial evasion of autophagy. The results of this study clearly demonstrated the role of convergent evolution in the evolution of how bacteria evade autophagy.

Hepatitis B virus dampens autophagy maturation via negative regulation of Rab7 expression

Hepatitis B virus (HBV) infection brings a huge challenge for medical health practitioners. It has been reported that invaded HBV escapes autophagic degradation through inhibiting lysosome maturation following enhanced autophagy formation, which putatively contributes to HBV replication and infection. However, the underlying mechanism by which HBV escapes from autophagic degradation remains elusive. In this study, we monitored the autophagic process using HepG2 cells and mice without or with transient HBV DNA plasmid transfection (pHepG2) or stable HBV infection (HepG2.2.15 cells) in vitro and in vivo. The results of Western blot, transmission electron microscopy and confocal microscopy, confirmed that HBV induced autophagy, while the fusion of autophagosomes with lysosomes was arrested. Furthermore, Rab7, a small GTPase that functions as a molecular switch responsible for the autophagosome-lysosome fusion, was inhibited, suggesting a potential mechanism for HBV-induced inhibition of autophagic degradation. In conclusion, our study proposes a potential mechanism for how HBV escapes autophagic degradation, which might be a novel therapeutic target for controlling HBV infection.

Strategies Used by Bacteria to Grow in Macrophages

Intracellular bacteria are often clinically relevant pathogens that infect virtually every cell type found in host organisms. However, myeloid cells, especially macrophages, constitute the primary cells targeted by most species of intracellular bacteria. Paradoxically, macrophages possess an extensive antimicrobial arsenal and are efficient at killing microbes. In addition to their ability to detect and signal the presence of pathogens, macrophages sequester and digest microorganisms using the phagolysosomal and autophagy pathways or, ultimately, eliminate themselves through the induction of programmed cell death. Consequently, intracellular bacteria influence numerous host processes and deploy sophisticated strategies to replicate within these host cells. Although most intracellular bacteria have a unique intracellular life cycle, these pathogens are broadly categorized into intravacuolar and cytosolic bacteria. Following phagocytosis, intravacuolar bacteria reside in the host endomembrane system and, to some extent, are protected from the host cytosolic innate immune defenses. However, the intravacuolar lifestyle requires the generation and maintenance of unique specialized bacteria-containing vacuoles and involves a complex network of host-pathogen interactions. Conversely, cytosolic bacteria escape the phagolysosomal pathway and thrive in the nutrient-rich cytosol despite the presence of host cell-autonomous defenses. The understanding of host-pathogen interactions involved in the pathogenesis of intracellular bacteria will continue to provide mechanistic insights into basic cellular processes and may lead to the discovery of novel therapeutics targeting infectious and inflammatory diseases.

Interactions between Autophagy and Bacterial Toxins: Targets for Therapy?

Autophagy is a physiological process involved in defense mechanisms for clearing intracellular bacteria. The autophagic pathway is finely regulated and bacterial toxins interact with this process in a complex manner. Bacterial toxins also interact significantly with many biochemical processes. Evaluations of the effects of bacterial toxins, such as endotoxins, pore-forming toxins and adenylate cyclases, on autophagy could support the development of new strategies for counteracting bacterial pathogenicity. Treatment strategies could focus on drugs that enhance autophagic processes to improve the clearance of intracellular bacteria. However, further in vivo studies are required to decipher the upregulation of autophagy and potential side effects limiting such approaches. The capacity of autophagy activation strategies to improve the outcome of antibiotic treatment should be investigated in the future.

Burkholderia pseudomallei type III secretion system cluster 3 ATPase BsaS, a chemotherapeutic target for small-molecule ATPase inhibitors

Melioidosis is an infectious disease of high mortality for humans and other animal species; it is prevalent in tropical regions worldwide. The pathogenesis of melioidosis depends on the ability of its causative agent, the Gram-negative bacterium Burkholderia pseudomallei, to enter and survive in host cells. B. pseudomallei can escape from the phagosome into the cytosol of phagocytic cells where it replicates and acquires actin-mediated motility, avoiding killing by the autophagy-dependent process, LC3 (microtubule-associated protein light chain 3)-associated phagocytosis (LAP). The type III secretion system cluster 3 (TTSS3) facilitates bacterial escape from phagosomes, although the mechanism has not been fully elucidated. Given the recent identification of small-molecule inhibitors of the TTSS ATPase, we sought to determine the potential of the predicted TTSS3 ATPase, encoded by bsaS, as a target for chemotherapeutic treatment of infection. A B. pseudomallei bsaS deletion mutant was generated and used as a control against which to assess the effect of inhibitor treatment. Infection of RAW 264.7 cells with wild-type bacteria and subsequent treatment with the ATPase inhibitor compound 939 resulted in reduced intracellular bacterial survival, reduced escape from phagosomes, and increased colocalization with both LC3 and the lysosomal marker LAMP1 (lysosome-associated membrane protein 1). These changes were similar to those observed for infection of RAW 264.7 cells with the bsaS deletion mutant. We propose that treatment with the ATPase inhibitor compound 939 decreased intracellular bacterial survival through a reduced ability of bacteria to escape from phagosomes and increased killing via LAP. Therefore, small-molecule inhibitors of the TTSS3 ATPase have potential as therapeutic treatments against melioidosis.