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The cytokine interleukin-1β (IL-1β) has pivotal roles in antimicrobial immunity, but also incites inflammatory pathology. Bioactive IL-1β is released following proteolytic maturation of the pro-IL-1β precursor by caspase-1 inflammasomes. UBE2L3/UBCH7, a conserved ubiquitin conjugating enzyme, promotes pro-IL-1β ubiquitylation and proteasomal disposal. However, UBE2L3 actions in vivo and ubiquitin ligases involved in this process are unknown. Here we report that deletion of Ube2l3 in mice markedly reduces pro-IL-1β turnover in macrophages, leading to excessive mature IL-1β production, neutrophilic inflammation and disease symptoms following inflammasome activation. A family-wide siRNA screen identified two ubiquitin ligases, TRIP12 and AREL1, which we show add K27-, K29- and K33- poly-ubiquitin chains on lysine residues in the ‘pro’ domain and destabilise pro-IL-1β. Mutation of ubiquitylation sites increased pro-IL-1β stability, but did not affect proteolysis by caspase-1. The extent of mature IL-1β production is therefore determined by precursor abundance, and UBE2L3, TRIP12 and AREL1 limit inflammation by shrinking the cellular pool of pro-IL-1β. Our study has uncovered fundamental processes governing IL-1β homeostasis and provided molecular insights that could be exploited to mitigate its adverse actions in disease.

Summary: Pathogenic bacteria can alter host gene expression through post-translational modifications of histones. We show that a natural colonizer, Streptococcus pneumoniae, induces specific histone modifications, including robust dephosphorylation of histone H3 on serine 10 (H3S10), during infection of respiratory epithelial cells. The bacterial pore-forming toxin pneumolysin (PLY), along with the pyruvate oxidase SpxB responsible for H2O2 production, play important roles in the induction of this modification. The combined effects of PLY and H2O2 trigger host signaling that culminates in H3S10 dephosphorylation, which is mediated by the host cell phosphatase PP1. Strikingly, S. pneumoniae infection induces dephosphorylation and subsequent activation of PP1 catalytic activity. Colonization of PP1 catalytically deficient cells results in impaired intracellular S. pneumoniae survival and infection. Interestingly, PP1 activation and H3S10 dephosphorylation are not restricted to S. pneumoniae and appear to be general epigenomic mechanisms favoring intracellular survival of pathogenic bacteria. : Dong et al. show that Streptococcus pneumoniae infection induces histone H3 dephosphorylation in lung epithelial cells, through two bacterial factors, PLY and SpxB. Infection-triggered activation of the host phosphatase PP1 is required for this histone modification and efficient intracellular infection. Keywords: histone modification, Streptococcus pneumoniae, H3S10 dephosphorylation, protein phosphatase 1, bacterial infection, pneumolysin, SpxB

We would like to acknowledge and thank Pascale Cossart for her valuable insight and support throughout the project and for her contribution towards the acquisition of financial support. We also thank Michael G. Connor for his help in the quantification and analysis of microscopy data.; International audience; The NAD-dependent deacetylase Sirtuin-2 (SIRT2) functions in diverse cellular processes including the cell cycle, metabolism, and has important roles in tumorigenesis and bacterial infection. SIRT2 predominantly resides in the cytoplasm but can also function in the nucleus. Consequently, SIRT2 localisation and its interacting partners may greatly impact its function and need to be defined more clearly. In this study we used mass spectrometry to determine the interactomes of SIRT2 in whole cells and in specific cellular fractions; cytoplasm, nucleus and chromatin. Using this approach, we identified novel interacting partners of SIRT2. These included a number of proteins that function in nuclear import. We show that multiple importins interact with and contribute to the basal nuclear shuttling of SIRT2 and that one of these, IPO7 is required for SIRT2 mediated H3K18 deacetylation in response to bacterial infection. Furthermore, we reveal that the unstructured C-terminus of SIRT2 negatively regulates importin-binding and nuclear transport. This study demonstrates that SIRT2 is actively transported into the nucleus via a process regulated by its C-terminus and provides a resource of SIRT2 interacting partners.

For many intracellular bacterial pathogens manipulating host cell survival is essential for maintaining a replicative niche, and is a common strategy used to promote infection. The bacterial pathogenListeria monocytogenesis well known to hijack host machinery for its own benefit, such as targeting the host histone H3 for modification by SIRT2. However, in what way this modification benefits infection, as well as the molecular players involved, remain unknown. Here we show that SIRT2 activity supportsListeriaintracellular survival by maintaining genome integrity and host cell viability. This protective effect is dependent on H3K18 deacetylation, which safeguards the host genome by counteracting infection-induced DNA damage. Mechanistically, infection causes SIRT2 to interact with the nucleic acid binding protein TDP-43 and localise to genomic R-loops, where H3K18 deacetylation occurs. This work highlights novel functions of TDP-43 and R-loops during bacterial infection and identifies the mechanism through whichL. monocytogenesco-opts SIRT2 to allow efficient infection.

International audience; During infection, the foodborne bacterial pathogen Listeria monocytogenes dynamically influences the gene expression profile of host cells. Infection-induced transcriptional changes are a typical feature of the host-response to bacteria and contribute to the activation of protective genes such as inflammatory cytokines. However, by using specialized virulence factors, bacterial pathogens can target signaling pathways, transcription factors, and epigenetic mechanisms to alter host gene expression, thereby reprogramming the response to infection. Therefore, the transcriptional profile that is established in the host is delicately balanced between antibacterial responses and pathogenesis, where any change in host gene expression might significantly influence the outcome of infection. In this review, we discuss the known transcriptional and epigenetic processes that are engaged during Listeria monocytogenes infection, the virulence factors that can remodel them, and the impact these processes have on the outcome of infection.

Inflammasomes - molecular platforms for caspase-1 activation - have emerged as common hubs for a number of pathways that detect and respond to bacterial pathogens. Caspase-1 activation results in the secretion of bioactive IL-1β and IL-18 and pyroptosis, and thus launches a systemic immune and inflammatory response. In this review we discuss signal transduction leading to 'canonical' and 'non-canonical' activation of caspase-1 through the involvement of upstream caspases. Recent studies have identified a growing number of regulatory networks involving guanylate binding proteins, protein kinases, ubiquitylation and necroptosis related pathways that modulate inflammasome responses and immunity to bacterial infection. By being able to respond to extracellular, vacuolar and cytosolic bacteria, their cytosolic toxins or ligands for cell surface receptors, inflammasomes have emerged as important sentinels of infection.

The Escherichia coli eukaryote-like serine/threonine kinase, encoded by yeaG, is expressed in response to diverse stresses, including nitrogen (N) starvation. A role for yeaG in bacterial stress response is unknown. Here we reveal for the first time that wild-type E. coli displays metabolic heterogeneity following sustained periods of N starvation, with the metabolically active population displaying compromised viability. In contrast, such heterogeneity in metabolic activity is not observed in an E. coli ∆yeaG mutant, which continues to exist as a single and metabolically active population and thus displays an overall compromised ability to survive sustained periods of N starvation. The mechanism by which yeaG acts, involves the transcriptional repression of two toxin/antitoxin modules, mqsR/mqsA and dinJ/yafQ. This, consequently, has a positive effect on the expression of rpoS, the master regulator of the general bacterial stress response. Overall, results indicate that yeaG is required to fully execute the rpoS-dependent gene expression program to allow E. coli to adapt to sustained N starvation and unravels a novel facet to the regulatory basis that underpins adaptive response to N stress.