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The eukaryotic transcriptional regulator Nuclear Factor kappa B (NF-κB) plays a central role in the defense to pathogens. Despite this, few studies have analyzed NF-κB activity in single cells during infection. Here, we investigated at the single cell level how NF-κB nuclear localization - a proxy for NF-κB activity - oscillates in infected and uninfected fibroblasts co-existing in cultures exposed to Salmonella enterica serovar Typhimurium. Fibroblasts were used due to the capacity of S. Typhimurium to persist in this cell type. Real-time dynamics of NF-κB was examined in microfluidics, which prevents cytokine accumulation. In this condition, infected (ST+) cells translocate NF-κB to the nucleus at higher rate than the uninfected (ST-) cells. Surprisingly, in non-flow (static) culture conditions, ST- fibroblasts exhibited higher NF-κB nuclear translocation than the ST+ population, with these latter cells turning refractory to external stimuli such as TNF-α or a second infection. Sorting of ST+ and ST- cell populations confirmed enhanced expression of NF-κB target genes such as IL1B, NFKBIA, TNFAIP3, and TRAF1 in uninfected (ST-) fibroblasts. These observations proved that S. Typhimurium dampens the NF-κB response in the infected fibroblast. Higher expression of SOCS3, encoding a "suppressor of cytokine signaling," was also observed in the ST+ population. Intracellular S. Typhimurium subverts NF-κB activity using protein effectors translocated by the secretion systems encoded by pathogenicity islands 1 (T1) and 2 (T2). T1 is required for regulating expression of SOCS3 and all NF-κB target genes analyzed whereas T2 displayed no role in the control of SOCS3 and IL1B expression. Collectively, these data demonstrate that S. Typhimurium attenuates NF-κB signaling in fibroblasts, an effect only perceptible when ST+ and ST- populations are analyzed separately. This tune-down in a central host defense might be instrumental for S. Typhimurium to establish intracellular persistent infections.

Several transcription factors (TFs) oscillate, periodically relocating between the cytoplasm and the nucleus. NF-κB, which plays key roles in inflammation and cancer, displays oscillations whose biological advantage remains unclear. Recent work indicated that NF-κB displays sustained oscillations that can be entrained, that is, reach a persistent synchronized state through small periodic perturbations. We show here that for our GFP-p65 knock-in cells NF-κB behaves as a damped oscillator able to synchronize to a variety of periodic external perturbations with no memory. We imposed synchronous dynamics to prove that transcription of NF-κB-controlled genes also oscillates, but mature transcript levels follow three distinct patterns. Two sets of transcripts accumulate fast or slowly, respectively. Another set, comprising chemokine and chemokine receptor mRNAs, oscillates and resets at each new stimulus, with no memory of the past. We propose that TF oscillatory dynamics is a means of segmenting time to provide renewing opportunity windows for decision.

The eukaryotic transcriptional regulator Nuclear Factor kappa B (NF-κB) plays a central role in the defense to pathogens. Despite this, few studies have analyzed NF-κB activity in single cells during infection. Here, we investigated at the single cell level how NF-κB nuclear localization – a proxy for NF-κB activity – oscillates in infected and uninfected fibroblasts co-existing in cultures exposed toSalmonella entericaserovar Typhimurium. Fibroblasts were used due to the capacity ofS. Typhimurium to persist in this cell type. Real-time dynamics of NF-κB was examined in microfluidics, which prevents cytokine accumulation. In this condition, infected (ST+) cells translocate NF-κB to the nucleus at higher rate than the uninfected (ST-) cells. Surprisingly, in non-flow (static) culture conditions, ST- fibroblasts exhibited higher NF-κB nuclear translocation than the ST+ population, with these latter cells turning refractory to external stimuli such as TNF-α or a second infection. Sorting of ST+ and ST- cell populations confirmed enhanced expression of NF-κB target genes such asIL1B, NFKBIA, TNFAIP3, andTRAF1in uninfected (ST-) fibroblasts. These observations proved thatS. Typhimurium dampens the NF-κB response in the infected fibroblast. Higher expression ofSOCS3, encoding a “suppressor of cytokine signaling,” was also observed in the ST+ population. IntracellularS. Typhimurium subverts NF-κB activity using protein effectors translocated by the secretion systems encoded by pathogenicity islands 1 (T1) and 2 (T2). T1 is required for regulating expression ofSOCS3and all NF-κB target genes analyzed whereas T2 displayed no role in the control ofSOCS3andIL1Bexpression. Collectively, these data demonstrate thatS. Typhimurium attenuates NF-κB signaling in fibroblasts, an effect only perceptible when ST+ and ST- populations are analyzed separately. This tune-down in a central host defense might be instrumental forS. Typhimurium to establish intracellular persistent infections. [ABSTRACT FROM AUTHOR]

NF-κB is a transcription factor that upon activation undergoes cycles of cytoplasmic-to-nuclear and nuclear-to-cytoplasmic transport, giving rise to so called “oscillations”. In turn, oscillations tune the transcriptional output. Since a detailed understanding of oscillations requires a systems biology approach, we developed a method to acquire and analyze large volumes of data on NF-κB dynamics in single cells. We measured the time evolution of the nuclear to total ratio of GFP-p65 in knock-in mouse embryonic fibroblasts using time-lapse imaging. We automatically produced a precise segmentation of nucleus and cytoplasm based on an accurate estimation of the signal and image background. Finally, we defined a set of quantifiers that describe the oscillatory dynamics, which are internally normalized and can be used to compare data recorded by different labs. Using our method, we analyzed NF-κB dynamics in over 2000 cells exposed to different concentrations of TNF- α α. We reproduced known features of the NF-κB system, such as the heterogeneity of the response in the cell population upon stimulation and we confirmed that a fraction of the responding cells does not oscillate. We also unveiled important features: the second and third oscillatory peaks were often comparable to the first one, a basal amount of nuclear NF-κB could be detected in unstimulated cells, and at any time a small fraction of unstimulated cells showed spontaneous random activation of the NF-κB system. Our work lays the ground for systematic, high-throughput, and unbiased analysis of the dynamics of transcription factors that can shuttle between the nucleus and other cell compartments. [ABSTRACT FROM AUTHOR]