Brucella BtpB Manipulates Apoptosis and Autophagic Flux in RAW264.7 Cells

Breaking the cellular defense: the role of autophagy evasion in Francisella virulence

Many pathogens have evolved sophisticated strategies to evade autophagy, a crucial cellular defense mechanism that typically targets and degrades invading microorganisms. By subverting or inhibiting autophagy, these pathogens can create a more favorable environment for their replication and survival within the host. For instance, some bacteria secrete factors that block autophagosome formation, while others might escape from autophagosomes before degradation. These evasion tactics are critical for the pathogens' ability to establish and maintain infections. Understanding the mechanisms by which pathogens avoid autophagy is crucial for developing new therapeutic strategies, as enhancing autophagy could bolster the host's immune response and aid in the elimination of pathogenic bacteria. Francisella tularensis can manipulate host cell pathways to prevent its detection and destruction by autophagy, thereby enhancing its virulence. Given the potential for F. tularensis to be used as a bioterrorism agent due to its high infectivity and ability to cause severe disease, research into how this pathogen evades autophagy is of critical importance. By unraveling these mechanisms, new therapeutic approaches could be developed to enhance autophagic responses and strengthen host defense against this and other similarly evasive pathogens.

Protein disulfide isomerase A4 binds to Brucella BtpB and mediates intracellular NAD+/NADH metabolism in RAW264.7 cells

The Toll/interleukin-1 receptor (TIR) signaling domain is distributed widely in mammalian Toll-like receptors and adaptors, plant nucleotide-binding leucine-rich repeat receptors, and specific bacterial virulence proteins. Proteins that possess TIR domain exhibit NADase activity which is distinct from the canonical signaling function of these domains. However, the effects of bacterial TIR domain proteins on host metabolic switches and the underlying mechanism of NADase activity in these proteins remain unclear. Here, we utilized Brucella TIR domain-containing type IV secretion system effector protein, BtpB, to explore the mechanism of NADase activity in host cells. We showed that using ectopic expression BtpB not only generates depletion of NAD+ but also loss of NADH and ATP in RAW264.7 macrophage cells. Moreover, immunoprecipitation-mass spectrometry, co-immunoprecipitation, and confocal microscope assays revealed that BtpB interacted with host protein disulfide isomerase A4 (PDIA4). The Brucella mutant strain deleted the gene for BtpB, significantly decreased PDIA4 expression. Furthermore, our data revealed that PDIA4 played an important role in regulating intracellular NAD+/NADH levels in macrophages, and PDIA4 overexpression restored the decline of intracellular NAD+ and NADH levels induced by Brucella BtpB. The results provide new insights into the metabolic regulatory activity of TIR domain proteins in the critical human and animal pathogen Brucella.

Brucella mediates autophagy, inflammation, and apoptosis to escape host killing

Brucellosis is a serious zoonosis caused by Brucella spp. infection, which not only seriously jeopardizes the health of humans and mammals, but also causes huge economic losses to the livestock industry. Brucella is a Gram-negative intracellular bacterium that relies primarily on its virulence factors and a variety of evolved survival strategies to replicate and proliferate within cells. Currently, the mechanisms of autophagy, inflammation, and apoptosis in Brucella-infected hosts are not fully understood and require further research and discussion. This review focuses on the relationship between Brucella and autophagy, inflammation, and apoptosis to provide the scientific basis for revealing the pathogenesis of Brucella.

Helicobacter pylori CagA mediated mitophagy to attenuate the NLRP3 inflammasome activation and enhance the survival of infected cells

Helicobacter pylori (H. pylori) is one of the most common bacterial infections in the world, and its key virulence component CagA is the leading cause of gastric cancer. Mitophagy is a form of selective autophagy that eliminates damaged mitochondria and is essential for some viruses and bacteria to evade the immune system. However, the mechanisms by which CagA mediates H. pylori-induced mitophagy and NLRP3 inflammasome activation remain elusive. In this study, we reported that H. pylori primarily uses its CagA to induce mitochondrial oxidative damage, mitochondrial dysfunction, dynamic imbalance, and to block autophagic flux. Inhibition of mitophagy led to an increase in NLRP3 inflammasome activation and apoptosis and a decrease in the viability of H. pylori-infected cells. Our findings suggested that H. pylori induces mitochondrial dysfunction and mitophagy primarily via CagA. It reduces NLRP3 inflammasome activation to evade host immune surveillance and increases the survival and viability of infected cells, potentially leading to gastric cancer initiation and development. Our findings provide new insights into the pathogenesis of H. pylori-induced gastric cancer, and inhibition of mitophagy may be one of the novel techniques for the prevention and treatment of this disease.

A genome-wide CRISPR screen identified host genes essential for intracellular Brucella survival

Brucella is a zoonotic intracellular bacterium that poses threats to human health and economic security. Intracellular infection is a hallmark of the agent Brucella and a primary cause of distress, through which the bacterium regulates the host intracellular environment to promote its own colonization and replication, evading host immunity and pharmaceutical killing. Current studies of Brucella intracellular processes are typically premised on bacterial phenotype such as intracellular bacterial survival, followed by biochemical or molecular biological approaches to reveal detailed mechanisms. While such processes can deepen the understanding of Brucella-host interaction, the insights into host alterations in infection would be easily restricted to known pathways. In the current study, we applied CRISPR Cas9 screen to identify host genes that are most affected by Brucella infection on cell viability at the genomic level. As a result of CRISPR screening, we firstly identified that knockout of the negatively selected genes GOLGA6L6, DEFB103B, OR4F29, and ERCC6 attenuate the viability of both the host cells and intracellular Brucella, suggesting these genes to be potential therapeutic targets for Brucella control. In particular, knockout of DEFB103B diminished Brucella intracellular survival by altering host cell autophagy. Conversely, knockout of positive screening genes promoted intracellular proliferation of Brucella. In summary, we screened host genes at the genomic level throughout Brucella infection, identified host genes that are previously not recognized to be involved in Brucella infection, and provided targets for intracellular infection control.IMPORTANCEBrucella is a Gram-negative bacterium that infects common mammals causing arthritis, myalgia, neuritis, orchitis, or miscarriage and is difficult to cure with antibiotics due to its intracellular parasitism. Therefore, unraveling the mechanism of Brucella-host interactions will help controlling Brucella infections. CRISPR-Cas9 is a gene editing technology that directs knockout of individual target genes by guided RNA, from which genome-wide gene-knockout cell libraries can be constructed. Upon infection with Brucella, the cell library would show differences in viability as a result of the knockout and specific genes could be revealed by genomic DNA sequencing. As a result, genes affecting cell viability during Brucella infection were identified. Further testing of gene function may reveal the mechanisms of Brucella-host interactions, thereby contributing to clinical therapy.

Regulation of the Gene for Alanine Racemase Modulates Amino Acid Metabolism with Consequent Alterations in Cell Wall Properties and Adhesive Capability in Brucella spp

Brucella, a zoonotic facultative intracellular pathogenic bacterium, poses a significant threat both to human health and to the development of the livestock industry. Alanine racemase (Alr), the enzyme responsible for alanine racemization, plays a pivotal role in regulating virulence in this bacterium. Moreover, Brucella mutants with alr gene deletions (Δalr) exhibit potential as vaccine candidates. However, the mechanisms that underlie the detrimental effects of alr knockouts on Brucella pathogenicity remain elusive. Here, initially, we conducted a bioinformatics analysis of Alr, which demonstrated a high degree of conservation of the protein within Brucella spp. Subsequent metabolomics studies unveiled alterations in amino acid pathways following deletion of the alr gene. Furthermore, alr deletion in Brucella suis S2 induced decreased resistance to stress, antibiotics, and other factors. Transmission electron microscopy of simulated macrophage intracellular infection revealed damage to the cell wall in the Δalr strain, whereas propidium iodide staining and alkaline phosphatase and lactate dehydrogenase assays demonstrated alterations in cell membrane permeability. Changes in cell wall properties were revealed by measurements of cell surface hydrophobicity and zeta potential. Finally, the diminished adhesion capacity of the Δalr strain was shown by immunofluorescence and bacterial enumeration assays. In summary, our findings indicate that the alr gene that regulates amino acid metabolism in Brucella influences the properties of the cell wall, which modulates bacterial adherence capability. This study is the first demonstration that Alr impacts virulence by modulating bacterial metabolism, thereby providing novel insights into the pathogenic mechanisms of Brucella spp.

Alr Gene in Brucella suis S2: Its Role in Lipopolysaccharide Biosynthesis and Bacterial Virulence in RAW264.7

Brucella suis, the causative agent of brucellosis, poses a significant public health and animal husbandry threat. However, the role of the alanine racemase (alr) gene, which encodes alanine racemase in Brucella, remains unclear. Here, we analyzed an alr deletion mutant and a complemented strain of Brucella suis S2. The knockout strain displayed an unaltered, smooth phenotype in acriflavine agglutination tests but lacked the core polysaccharide portion of lipopolysaccharide (LPS). Genes involved in the LPS synthesis were significantly upregulated in the deletion mutant. The alr deletion strain exhibited reduced intracellular viability in the macrophages, increased macrophage-mediated killing, and upregulation of the apoptosis markers. Bcl2, an anti-apoptotic protein, was downregulated, while the pro-apoptotic proteins, Bax, Caspase-9, and Caspase-3, were upregulated in the macrophages infected with the deletion strain. The infected macrophages showed increased mitochondrial membrane permeability, Cytochrome C release, and reactive oxygen species, activating the mitochondrial apoptosis pathway. These findings revealed that alanine racemase was dispensable in B. suis S2 but influenced the strain's rough features and triggered the mitochondrial apoptosis pathway during macrophage invasion. The deletion of the alr gene reduced the intracellular survival and virulence. This study enhances our understanding of the molecular mechanism underlying Brucella's survival and virulence and, specifically, how alr gene affects host immune evasion by regulating bacterial LPS biosynthesis.