Metabolic reprogramming: an innate cellular defence mechanism against intracellular bacteria?

International audienceThe limited metabolic resources of a cell represent an intriguing 'conflict of interest' during host-pathogen interactions, as the battle for nutrients might determine the outcome of an infection. To adapt their metabolic needs, innate immune cells such as monocytes, macrophages or dendritic cells reprogram their metabolism upon activation by microbial compounds. In turn, infection by intracellular bacteria provokes metabolic alterations of the host cell that benefit the pathogen. Here, we discuss the state-of-the-art knowledge on metabolic reprogramming of host cells upon activation or infection with intracellular bacteria. The study of the host-driven and pathogen-driven metabolic alterations that seem to co-exist during infection is an emerging field that will define the metabolic pathways that might be targeted to combat infection

Modulation of Host Autophagy during Bacterial Infection: Sabotaging Host Munitions for Pathogen Nutrition.

International audienceCellular homeostasis requires the balanced regulation of anabolic and catabolic processes. While anabolic metabolism consumes energy to build up cellular components, catabolic processes break down organic matters in order to provide energy for the cell and its anabolic processes. Autophagy is a highly conserved and regulated catabolic process by which the eukaryotic cell degrades unnecessary, undesirable, or dysfunctional cellular components, including organelles (1–3). Autophagy is induced by a variety of extra-and intracellular stress stimuli, such as nutrient starvation, oxidative stress, or accumulation of damaged organelles or toxic protein aggregates. Initiation of autophagy first leads to the formation of cup-shaped structures known as phagophores that engulf the undesirable or damaged cellular components. Subsequent elongation of phagophores form double-membrane vesicles called autophagosomes, which deliver their cargo to lysosomes where the content is degraded and recycled (1–3). Autophagy plays a central role in quality control of organelles and proteins, and additionally is a key mechanism to maintain cellular energy levels and nutrient homeostasis during starvation, promoting the recycling and salvage of cellular nutrients. Furthermore, the cellular autophagic machinery is also used to remove invading intracellular pathogens, a process called xenophagy (1, 2). In this case, phagophores engulf invading microbes forming autophagosomes and steering them toward lysosomal degradation. Thus, xenophagy is an innate immune mechanism against bacterial infection that has been shown to be essential to restrict intracellular growth of many bacteria such as Salmonella enterica serovar Typhimurium (4), Mycobacterium tuberculosis (5, 6), Listeria monocytogenes (7), or Group A Streptococcus (8)

Danger-associated metabolic modifications during bacterial infection of macrophages

International audienceIn this review, we propose that certain modifications in cellular metabolism might function as danger signals triggering inflammasome-mediated immune responses. We propose to call them danger-associated metabolic modifications (DAMMs). As intracellular bacteria can actively modulate macrophage metabolism for their benefit, infected host cells might sense bacteria-induced metabolic alterations and activate immune reactions. Here we report the known metabolic interactions that occur during infection of macrophages by intracellular bacteria and discuss the possible emergence of DAMMs upon bacteria-induced alterations of cellular metabolism

Legionella pneumophila restrains autophagy by modulating the host's sphingolipid metabolism

Punctum to: Rolando M, et al. Legionella pneumophila S1P-lyase targets host sphingolipid metabolism and restrains autophagy. Proc Natl Acad Sci USA (2016) 113(7):1901- 6; http://dx.doi.org/10.1073/pnas.1522067113.International audienceSphingolipids are bioactive molecules playing a key role as membrane components, but they are also central regulators of many intracellular processes including macroautophagy/autophagy. In particular, sphingosine-1-phosphate (S1P) is a critical mediator that controls the balance between sphingolipid-induced autophagy and cell death. S1P levels are adjusted via S1P synthesis, dephosphorylation or degradation, catalyzed by SGPL1 (sphingosine-1-phosphate lyase 1). Intracellular pathogens are able to modulate many different host cell pathways to allow their replication. We have found that infection of eukaryotic cells with the human pathogen Legionella pneumophila triggers a change in the host cell sphingolipid metabolism and specifically affects the levels of sphingosine. Indeed, L. pneumophila secretes a protein highly homologous to eukaryotic SGPL1 (named LpSPL). We solved the crystal structure of LpSPL and showed that it encodes lyase activity, targets the host's sphingolipid metabolism, and plays a role in starvation-induced autophagy during L. pneumophila infection to promote intracellular survival

Legionella and mitochondria, an intriguing relationship

International audienceLegionella pneumophila is the causative agent of Legionnaires’ disease, a severe pneumonia. L. pneumophila injects via a type-IV-secretion-system (T4SS) more than 300 bacterial proteins into macrophages, its main host cell in humans. Certain of these bacterial effectors target organelles in the infected cell and hijack multiple processes to facilitate all steps of the intracellular life cycle of this pathogen. In this review, we discuss the interplay between L. pneumophila, an intracellular bacterium fully armed with virulence tools, and mitochondria, the extraordinary eukaryotic organelles playing prominent roles in cellular bioenergetics, cell-autonomous immunity and cell death. We present and discuss key findings concerning the multiple interactions of L. pneumophila with mitochondria during infection and the mechanisms employed by T4SS effectors that target mitochondrial functions to subvert infected cells

From amoeba to macrophages: exploring the molecular mechanisms of Legionella pneumophila infection in both hosts.

International audienceLegionella pneumophila is a Gram-negative bacterium and the causative agent of Legionnaires' disease. It replicates within amoeba and infects accidentally human macrophages. Several similarities are seen in the L. pneumophila-infection cycle in both hosts, suggesting that the tools necessary for macrophage infection may have evolved during co-evolution of L. pneumophila and amoeba. The establishment of the Legionella-containing vacuole (LCV) within the host cytoplasm requires the remodeling of the LCV surface and the hijacking of vesicles and organelles. Then L. pneumophila replicates in a safe intracellular niche in amoeba and macrophages. In this review we will summarize the existing knowledge of the L. pneumophila infection cycle in both hosts at the molecular level and compare the factors involved within amoeba and macrophages. This knowledge will be discussed in the light of recent findings from the Acanthamoeba castellanii genome analyses suggesting the existence of a primitive immune-like system in amoeba

Targeting of host organelles by pathogenic bacteria: a sophisticated subversion strategy.

International audienceMany bacterial pathogens have evolved the ability to subvert and exploit host functions in order to enter and replicate in eukaryotic cells. For example, bacteria have developed specific mechanisms to target eukaryotic organelles such as the nucleus, the mitochondria, the endoplasmic reticulum and the Golgi apparatus. In this Review, we highlight the most recent advances in our understanding of the mechanisms that bacterial pathogens use to target these organelles. We also discuss how these strategies allow bacteria to manipulate host functions and to ultimately enable bacterial infection

Manipulation of Autophagy by Bacterial Pathogens Impacts Host Immunity

utophagy is a highly conserved catabolic process, degrading unnecessary or damaged components in the eukaryotic cell to maintain cellular homeostasis, but it is also an intrinsic cellular defence mechanism to remove invading pathogens. A crosstalk between autophagy and innate or adaptive immune responses has been recently reported, whereby autophagy influences both, innate and adaptive immunity like the production and secretion of pro-inflammatory cytokines or MHC class II antigen presentation to T cells. Pathogenic bacteria have evolved diverse strategies to manipulate autophagy, mechanisms that also impact host immune responses at different levels. Here we discuss the influence of autophagy on self-autonomous, innate and adaptive immunity and then focus on how bacterial mechanisms that shape autophagy may impact the host immune system

High-content assay to measure mitochondrial function and bacterial vacuole size in infected human primary macrophages

International audienceRegulation of bioenergetics and cell death are pivotal mitochondrial functions determining the responses of macrophages to infection. Here, we provide a protocol to investigate mitochondrial functions during infection of macrophages by intracellular bacteria. We describe steps for quantifying mitochondrial polarization, cell death, and bacterial infection in infected, living, human primary macrophages at the single-cell level. We also detail the use of the pathogen Legionella pneumophila as model. This protocol can be adapted to investigate mitochondrial functions in other settings. For complete details on the use and execution of this protocol, please refer to Escoll et al. (2021).1

Legionella pneumophila CsrA regulates a metabolic switch from amino acid to glycerolipid metabolism

International audienceLegionella pneumophila CsrA plays a crucial role in the life-stage-specific expression of virulence phenotypes and metabolic activity. However, its exact role is only partly known. To elucidate how CsrA impacts L. pneumophila metabolism we analysed the CsrA depended regulation of metabolic functions by comparative 13C-isotopologue profiling and oxygen consumption experiments of a L. pneumophila wild-type (wt) strain and its isogenic csrA- mutant. We show that a csrA- mutant has significantly lower respiration rates when serine, alanine, pyruvate, α-ketoglutarate or palmitate is the sole carbon source. By contrast, when grown in glucose or glycerol, no differences in respiration were detected. Isotopologue profiling uncovered that the transfer of label from [U-13C3]serine via pyruvate into the citrate cycle and gluconeogenesis was lower in the mutant as judged from the labelling patterns of protein-derived amino acids, cell-wall-derived diaminopimelate, sugars and amino sugars and 3-hydroxybutyrate derived from polyhydroxybutyrate (PHB). Similarly, the incorporation of [U-13C6]glucose via the glycolysis/Entner-Doudoroff (ED) pathway but not via the pentose phosphate pathway was repressed in the csrA- mutant. On the other hand, fluxes due to [U-13C3]glycerol utilization were increased in the csrA- mutant. In addition, we showed that exogenous [1,2,3,4-13C4]palmitic acid is efficiently used for PHB synthesis via 13C2-acetyl-CoA. Taken together, CsrA induces serine catabolism via the tricarboxylic acid cycle and glucose degradation via the ED pathway, but represses glycerol metabolism, fatty acid degradation and PHB biosynthesis, in particular during exponential growth. Thus, CsrA has a determining role in substrate usage and carbon partitioning during the L. pneumophila life cycle and regulates a switch from amino acid usage in replicative phase to glycerolipid usage during transmissive growth