Antisense regulation

The abundance of PU.1 relative to that of other competing transcription factors dictates lineage fate 'decisions' in developing hematopoietic cells. In Genes & Development, Ebralidze et al. demonstrate regulation of PU.1 expression by antisense transcripts starting from a conserved DNAse I–hypersensitive site (H3) in intron 3 of SPI1, which encodes PU.1. Naturally occurring antisense transcripts are present in both human and mouse hematopoietic cells. Both sense and antisense transcription require an upstream regulatory element that loops to interact with the SPI1 promoter and H3. Knockdown of the antisense transcripts leads to more PU.1 protein expression. Antisense PU.1 blocks the interaction of PU.1 mRNA with the translation elongation factor eEF4A. How the relative abundance of sense and antisense transcription of PU.1 is itself regulated should prove interesting. LAD

Genes Dev. 22, 2085–2092 (2008).

Ifnb induction threshold

Virus infection induces the stochastic assembly of transcription factors NF-κB, IRFs and ATF-2–c-Jun onto the enhancer of a single allele of Ifnb, the gene encoding interferon-β (IFN-β). In Cell, Apostolou and Thanos investigate the mechanism by which virus-mediated Ifnb regulation is initially stochastic and monoallelic. Overexpression of NF-κB, IRFs or ATF-2–c-Jun in cells increases IFN-β production, which indicates that the endogenous factors can be limiting. The Ifng locus–interacting genomic regions contain specialized Alu repeat elements that are putative NF-κB-binding sites. The Alu elements colocalize in some cells even before virus infection, but after infection they are found together with a single transcribed Ifnb locus. Initial monoallelic IFN-β production induces IRF7 production that activates other Ifnb alleles to produce IFN-β. These data provide a model of how virus infection induces stochastic, monoalleleic Ifnb expression, which amplifies to multiallelic Ifnb expression after limiting transcription factors are more abundantly produced. DCB

Cell 134, 85–96 (2008)

Dlx arrests development

The transcriptional programs directing the development of natural killer (NK) cells remain largely unknown. In the Proceedings of the National Academy of Science, Yokoyama and colleagues show that the homeobox transcription factor Distal-less (Dlx) regulates NK cell development. Bone marrow NK cell precursors transiently express three redundant isoforms of Dlx. Dlx3 is the most abundant and is expressed at earlier time points; however, all Dlx expression must be extinguished for NK maturation to occur. Enforced expression of Dlx results in arrested development of both bone marrow and thymic NK populations. Expression of the transcription factors T-bet and Id2, both known to be required for NK cell development, is downregulated in cells overexpressing Dlx. The development of B cells and T cells is likewise impaired after Dlx overexpression. The direct targets of Dlx activity and how Dlx expression is regulated will be the focus of future work. LAD

Proc. Natl. Acad. Sci. USA 105, 10877–10882 (2008)

Twisting T H 1 cells

The transcriptional repressor Twist binds to E boxes in the regulatory regions of target genes. In the Journal of Experimental Medicine, Radbruch and colleagues investigate the function of Twist in CD4+ T cells. Repeated stimulation of T helper type 1 (TH1) cells, but not of T helper type 2 cells or interleukin 17–producing T helper cells, leads to transient Twist expression in a way dependent on interleukin 12 and the transcription factors STAT4, NFAT1 and the NF-κB subunit p65. Effector memory CD4+ T cells overexpressing Twist negatively regulate mRNA encoding IFN-γ, interleukin 2 and tumor necrosis factor, and adoptive transfer of TH1 cells overexpressing Twist attenuates delayed-type hypersensitivity responses in recipient mice. In contrast, knockdown of Twist mRNA in TH1 cells increases inflammation. Thus, Twist negatively autoregulates TH1 inflammatory activity by downregulating TH1-specific genes. DCB

J. Exp. Med. (28 July 2008) doi:10.1084/jem.20072468

Tolerizing oxidation

Whereas necrosis elicits inflammation, caspase-dependent apoptosis engenders immune tolerance. In Immunity, Ferguson and colleagues identify the molecular basis of this difference by showing that reactive oxygen species (ROS) produced by apoptotic cells oxidize HMGB1, a protein released from dying cells, and neutralize its proinflammatory activity. Apoptotic cells treated with agents that scavenge or reduce ROS fail to induce tolerance in a delayed-type hypersensitivity model, whereas necrotic cells treated with an oxidizing agent succeed. Caspase-mediated cleavage of the electron-transport-chain protein p75 NDUSF1 is essential for ROS production, and supernatants of apoptotic cells expressing an uncleavable form of p75 elicit inflammation. Neutralization of HMGB1 or treatment with an oxidizing agent renders supernatants of p75-mutant cells nonimmunogenic. Apoptotic cells lacking HMGB1 or expressing a mutant version of HMGB1 that cannot be oxidized trigger inflammation. Further work is needed to pinpoint the mechanism by which reduced HMGB1 promotes inflammation. CB

Immunity 29, 21–32 (2008)

Two-phase signaling cascade

A complex that includes the TRAF and cIAP E3 ubiquitin ligases, the Ubc13 E2 ubiquitin-conjugating enzyme, and the kinases MEKK1, IKKγ and TAK1 is recruited to CD40. In Science, Karin and colleagues report that dissociation from CD40 is required for the phosphorylation of MEKK1 and TAK1. Within 10 minutes of stimulation with CD40, the complex of MEKK1, TAK1, IKKγ, TRAF2 and Ubc13, although initially bound to CD40, is detected solely in the cytoplasm. Inhibition of cIAP1 and cIAP2 prevents cytoplasmic localization of the complex, lysine 48–linked ubiquitination and proteasome-dependent degradation of TRAF3, and phosphorylation of MEKK1 and TAK1. In contrast, activation of IKKγ occurs independently of cIAP activity. TRAF3-deficient cells show accelerated CD40-induced phosphorylation of MEKK1 and TAK1, even when treated with a cIAP inhibitor. Whether this 'two-step' activation process is operative 'downstream' of other immune receptors is not known, and the biological justification for the spatiotemporal separation of MEKK1 and IKKγ activation is unclear. CB

Science (17 July 2008) doi:10.1126/science.1157340

Written by Christine Borowski, Douglas C. Braaten & Laurie A. Dempsey