Metabolic requirements for neutrophil extracellular traps formation.

As part of the innate immune response, neutrophils are at the forefront of defence against infection, resolution of inflammation and wound healing. They are the most abundant leucocytes in the peripheral blood, have a short lifespan and an estimated turnover of 10(10) to 10(11) cells per day. Neutrophils efficiently clear microbial infections by phagocytosis and by oxygen-dependent and oxygen-independent mechanisms. In 2004, a new neutrophil anti-microbial mechanism was described, the release of neutrophil extracellular traps (NETs) composed of DNA, histones and anti-microbial peptides. Several microorganisms, bacterial products, as well as pharmacological stimuli such as PMA, were shown to induce NETs. Neutrophils contain relatively few mitochondria, and derive most of their energy from glycolysis. In this scenario we aimed to analyse some of the metabolic requirements for NET formation. Here it is shown that NETs formation is strictly dependent on glucose and to a lesser extent on glutamine, that Glut-1, glucose uptake, and glycolysis rate increase upon PMA stimulation, and that NET formation is inhibited by the glycolysis inhibitor, 2-deoxy-glucose, and to a lesser extent by the ATP synthase inhibitor oligomycin. Moreover, when neutrophils were exposed to PMA in glucose-free medium for 3 hr, they lost their characteristic polymorphic nuclei but did not release NETs. However, if glucose (but not pyruvate) was added at this time, NET release took place within minutes, suggesting that NET formation could be metabolically divided into two phases; the first, independent from exogenous glucose (chromatin decondensation) and, the second (NET release), strictly dependent on exogenous glucose and glycolysis.

The mitochondrial activity of leukocytes from Artibeus jamaicensis bats remains unaltered after several weeks of flying restriction.

Bats are the only flying mammals known. They have longer lifespan than other mammals of similar size and weight and can resist high loads of many pathogens, mostly viruses, with no signs of disease. These distinctive characteristics have been attributed to their metabolic rate that is thought to be the result of their flying lifestyle. Compared with non-flying mammals, bats have lower production of reactive oxygen species (ROS), and high levels of antioxidant enzymes such as superoxide dismutase. This anti-oxidative vs. oxidative profile may help to explain bat's longer than expected lifespans. The aim of this study was to assess the effect that a significant reduction in flying has on bats leukocytes mitochondrial activity. This was assessed using samples of lymphoid and myeloid cells from peripheral blood from Artibeus jamaicensis bats shortly after capture and up to six weeks after flying deprivation. Mitochondrial membrane potential (Δψm), mitochondrial calcium (mCa2+), and mitochondrial ROS (mROS) were used as key indicators of mitochondrial activity, while total ROS and glucose uptake were used as additional indicators of cell metabolism. Results showed that total ROS and glucose uptake were statistically significantly lower at six weeks of flying deprivation (p < 0.05), in both lymphoid and myeloid cells, however no significant changes in mitochondrial activity associated with flying deprivation was observed (p > 0.05). These results suggest that bat mitochondria are stable to sudden changes in physical activity, at least up to six weeks of flying deprivation. However, decrease in total ROS and glucose uptake in myeloid cells after six weeks of captivity suggest a compensatory mechanism due to the lack of the highly metabolic demands associated with flying.

The Role of Tricarboxylic Acid Cycle Metabolites in Viral Infections.

Host cell metabolism is essential for the viral replication cycle and, therefore, for productive infection. Energy (ATP) is required for the receptor-mediated attachment of viral particles to susceptible cells and for their entry into the cytoplasm. Host cells must synthesize an array of biomolecules and engage in intracellular trafficking processes to enable viruses to complete their replication cycle. The tricarboxylic acid (TCA) cycle has a key role in ATP production as well as in the synthesis of the biomolecules needed for viral replication. The final assembly and budding process of enveloped viruses, for instance, require lipids, and the TCA cycle provides the precursor (citrate) for fatty acid synthesis (FAS). Viral infections may induce host inflammation and TCA cycle metabolic intermediates participate in this process, notably citrate and succinate. On the other hand, viral infections may promote the synthesis of itaconate from TCA cis-aconitate. Itaconate harbors anti-inflammatory, anti-oxidant, and anti-microbial properties. Fumarate is another TCA cycle intermediate with immunoregulatory properties, and its derivatives such as dimethyl fumarate (DMF) are therapeutic candidates for the contention of virus-induced hyper-inflammation and oxidative stress. The TCA cycle is at the core of viral infection and replication as well as viral pathogenesis and anti-viral immunity. This review highlights the role of the TCA cycle in viral infections and explores recent advances in the fast-moving field of virometabolism.

Mitochondria: An Integrative Hub Coordinating Circadian Rhythms, Metabolism, the Microbiome, and Immunity.

There is currently some understanding of the mechanisms that underpin the interactions between circadian rhythmicity and immunity, metabolism and immune response, and circadian rhythmicity and metabolism. In addition, a wealth of studies have led to the conclusion that the commensal microbiota (mainly bacteria) within the intestine contributes to host homeostasis by regulating circadian rhythmicity, metabolism, and the immune system. Experimental studies on how these four biological domains interact with each other have mainly focused on any two of those domains at a time and only occasionally on three. However, a systematic analysis of how these four domains concurrently interact with each other seems to be missing. We have analyzed current evidence that signposts a role for mitochondria as a key hub that supports and integrates activity across all four domains, circadian clocks, metabolic pathways, the intestinal microbiota, and the immune system, coordinating their integration and crosstalk. This work will hopefully provide a new perspective for both hypothesis-building and more systematic experimental approaches.

Mitochondrial Signature in Human Monocytes and Resistance to Infection in C. elegans During Fumarate-Induced Innate Immune Training.

Monocytes can develop immunological memory, a functional characteristic widely recognized as innate immune training, to distinguish it from memory in adaptive immune cells. Upon a secondary immune challenge, either homologous or heterologous, trained monocytes/macrophages exhibit a more robust production of pro-inflammatory cytokines, such as IL-1ß, IL-6, and TNF-α, than untrained monocytes. Candida albicans, ß-glucan, and BCG are all inducers of monocyte training and recent metabolic profiling analyses have revealed that training induction is dependent on glycolysis, glutaminolysis, and the cholesterol synthesis pathway, along with fumarate accumulation; interestingly, fumarate itself can induce training. Since fumarate is produced by the tricarboxylic acid (TCA) cycle within mitochondria, we asked whether extra-mitochondrial fumarate has an effect on mitochondrial function. Results showed that the addition of fumarate to monocytes induces mitochondrial Ca2+ uptake, fusion, and increased membrane potential (Δψm), while mitochondrial cristae became closer to each other, suggesting that immediate (from minutes to hours) mitochondrial activation plays a role in the induction phase of innate immune training of monocytes. To establish whether fumarate induces similar mitochondrial changes in vivo in a multicellular organism, effects of fumarate supplementation were tested in the nematode worm Caenorhabditis elegans. This induced mitochondrial fusion in both muscle and intestinal cells and also increased resistance to infection of the pharynx with E. coli. Together, these findings contribute to defining a mitochondrial signature associated with the induction of innate immune training by fumarate treatment, and to the understanding of whole organism infection resistance.

Immunization with intestinal microbiota-derived Staphylococcus aureus and Escherichia coli reduces bacteria-specific recolonization of the intestinal tract.

A wide array of microorganisms colonizes distinctive anatomical regions of animals, being the intestine the one that harbors the most abundant and complex microbiota. Phylogenetic analyses indicate that it is composed mainly of bacteria, and that Bacterioidetes and Firmicutes are the most represented phyla (>90% of the total eubacteria) in mice and humans. Intestinal microbiota plays an important role in host physiology, contributing to digestion, epithelial cells metabolism, stimulation of intestinal immune responses, and protection against intestinal pathogens. Changes in its composition may affect intestinal homeostasis, a condition known as dysbiosis, which may lead to non-specific inflammation and disease. The aim of this work was to analyze the effect that a bacteria-specific systemic immune response would have on the intestinal re-colonization by that particular bacterium. Bacteria were isolated and identified from the feces of Balb/c mice, bacterial cell-free extracts were used to immunize the same mice from which bacteria came from. Concurrently with immunization, mice were subjected to a previously described antibiotic-based protocol to eliminate most of their intestinal bacteria. Serum IgG and feces IgA, specific for the immunizing bacteria were determined. After antibiotic treatment was suspended, specific bacteria were orally administered, in an attempt to specifically re-colonize the intestine. Results showed that parenteral immunization with gut-derived bacteria elicited the production of both anti-bacterial IgG and IgA, and that immunization reduces bacteria specific recolonization of the gut. These findings support the idea that the systemic immune response may, at least in part, determine the bacterial composition of the gut.

Bioinformatic identification of Mycobacterium tuberculosis proteins likely to target host cell mitochondria: virulence factors?

BACKGROUND: M. tuberculosis infection either induces or inhibits host cell death, depending on the bacterial strain and the cell microenvironment. There is evidence suggesting a role for mitochondria in these processes.On the other hand, it has been shown that several bacterial proteins are able to target mitochondria, playing a critical role in bacterial pathogenesis and modulation of cell death. However, mycobacteria-derived proteins able to target host cell mitochondria are less studied. RESULTS: A bioinformaic analysis based on available genomic sequences of the common laboratory virulent reference strain Mycobacterium tuberculosis H37Rv, the avirulent strain H37Ra, the clinical isolate CDC1551, and M. bovis BCG Pasteur strain 1173P2, as well as of suitable bioinformatic tools (MitoProt II, PSORT II, and SignalP) for the in silico search for proteins likely to be secreted by mycobacteria that could target host cell mitochondria, showed that at least 19 M. tuberculosis proteins could possibly target host cell mitochondria. We experimentally tested this bioinformatic prediction on four M. tuberculosis recombinant proteins chosen from this list of 19 proteins (p27, PE_PGRS1, PE_PGRS33, and MT_1866). Confocal microscopy analyses showed that p27, and PE_PGRS33 proteins colocalize with mitochondria. CONCLUSIONS: Based on the bioinformatic analysis of whole M. tuberculosis genome sequences, we propose that at least 19 out of 4,246 M. tuberculosis predicted proteins would be able to target host cell mitochondria and, in turn, control mitochondrial physiology. Interestingly, such a list of 19 proteins includes five members of a mycobacteria specific family of proteins (PE/PE_PGRS) thought to be virulence factors, and p27, a well known virulence factor. P27, and PE_PGRS33 proteins experimentally showed to target mitochondria in J774 cells. Our results suggest a link between mitochondrial targeting of M. tuberculosis proteins and virulence.

Re-organization of mitochondria at the NK cell immune synapse.

As part of the innate immune response NK cells destroy infected, transformed, or otherwise stressed cells within hours of activation. In contrast, CD4(+) T lymphocytes require a sustained increase in their metabolism in order to cope with the biogenesis of cell components, in a process of proliferation and differentiation into effector cells. Recently, mitochondria have been implied in T lymphocyte immune synapse function but little is known on the role of mitochondria in the NK cell interaction with tumour cells. Here we analysed NK cells mitochondrial membrane potential (Deltapsi(m)) as an indicator of mitochondrial energy status and cellular homeostasis. Upon contact with K562 tumour cells, NK cells undergo Deltapsi(m) depolarization, indicating a rapid consumption of their metabolic energy. Furthermore, pharmacological inhibition of ATP synthesis down-regulates NK cell cytotoxic activity. Confocal- and electron-microscopy analyses showed re-organization of NK cells mitochondria towards the site of interaction with K562 tumour cell (NK cell immune synapse), perhaps as a way to compensate for local energy consumption. Interestingly, mitochondrial re-organization also takes place following NK stimulation with anti-NKGD2 antibodies but not with anti-KIR2DL1 antibodies, suggesting that activating rather than inhibiting cell signalling, triggered by NK cell receptors, is involved in NK cell mitochondria dynamics.