Cysteine methylation disrupts ubiquitin-chain sensing in NF-κB activation.

NF-κB is crucial for innate immune defence against microbial infection. Inhibition of NF-κB signalling has been observed with various bacterial infections. The NF-κB pathway critically requires multiple ubiquitin-chain signals of different natures. The question of whether ubiquitin-chain signalling and its specificity in NF-κB activation are regulated during infection, and how this regulation takes place, has not been explored. Here we show that human TAB2 and TAB3, ubiquitin-chain sensory proteins involved in NF-κB signalling, are directly inactivated by enteropathogenic Escherichia coli NleE, a conserved bacterial type-III-secreted effector responsible for blocking host NF-κB signalling. NleE harboured an unprecedented S-adenosyl-l-methionine-dependent methyltransferase activity that specifically modified a zinc-coordinating cysteine in the Npl4 zinc finger (NZF) domains in TAB2 and TAB3. Cysteine-methylated TAB2-NZF and TAB3-NZF (truncated proteins only comprising the NZF domain) lost the zinc ion as well as the ubiquitin-chain binding activity. Ectopically expressed or type-III-secretion-system-delivered NleE methylated TAB2 and TAB3 in host cells and diminished their ubiquitin-chain binding activity. Replacement of the NZF domain of TAB3 with the NleE methylation-insensitive Npl4 NZF domain resulted in NleE-resistant NF-κB activation. Given the prevalence of zinc-finger motifs and activation of cysteine thiol by zinc binding, methylation of zinc-finger cysteine might regulate other eukaryotic pathways in addition to NF-κB signalling.

Identification of the Essential Role of Viral Bcl-2 for Kaposi's Sarcoma-Associated Herpesvirus Lytic Replication.

UNLABELLED: Kaposi's sarcoma-associated herpesvirus (KSHV) evades host defenses through tight suppression of autophagy by targeting each step of its signal transduction: by viral Bcl-2 (vBcl-2) in vesicle nucleation, by viral FLIP (vFLIP) in vesicle elongation, and by K7 in vesicle maturation. By exploring the roles of KSHV autophagy-modulating genes, we found, surprisingly, that vBcl-2 is essential for KSHV lytic replication, whereas vFLIP and K7 are dispensable. Knocking out vBcl-2 from the KSHV genome resulted in decreased lytic gene expression at the mRNA and protein levels, a lower viral DNA copy number, and, consequently, a dramatic reduction in the amount of progeny infectious viruses, as also described in the accompanying article (A. Gelgor, I. Kalt, S. Bergson, K. F. Brulois, J. U. Jung, and R. Sarid, J Virol 89:5298-5307, 2015). More importantly, the antiapoptotic and antiautophagic functions of vBcl-2 were not required for KSHV lytic replication. Using a comprehensive mutagenesis analysis, we identified that glutamic acid 14 (E14) of vBcl-2 is critical for KSHV lytic replication. Mutating E14 to alanine totally blocked KSHV lytic replication but showed little or no effect on the antiapoptotic and antiautophagic functions of vBcl-2. Our study indicates that vBcl-2 harbors at least three important and genetically separable functions to modulate both cellular signaling and the virus life cycle. IMPORTANCE: The present study shows for the first time that vBcl-2 is essential for KSHV lytic replication. Removal of the vBcl-2 gene results in a lower level of KSHV lytic gene expression, impaired viral DNA replication, and consequently, a dramatic reduction in the level of progeny production. More importantly, the role of vBcl-2 in KSHV lytic replication is genetically separated from its antiapoptotic and antiautophagic functions, suggesting that the KSHV Bcl-2 carries a novel function in viral lytic replication.

SET-domain bacterial effectors target heterochromatin protein 1 to activate host rDNA transcription.

Transcription of rRNA genes (rDNAs) in the nucleolus is regulated by epigenetic chromatin modifications including histone H3 lysine (de)methylation. Here we show that LegAS4, a Legionella pneumophila type IV secretion system (TFSS) effector, is targeted to specific rDNA chromatin regions in the host nucleolus. LegAS4 promotes rDNA transcription, through its SET-domain (named after Drosophila Su(var)3-9, enhancer of zeste [E(z)], and trithorax [trx]) histone lysine methyltransferase (HKMTase) activity. LegAS4's association with rDNA chromatin is mediated by interaction with host HP1α/γ. L. pneumophila infection potently activates rDNA transcription in a TFSS-dependent manner. Other bacteria, including Bordetella bronchiseptica and Burkholderia thailandensis, also harbour nucleolus-localized LegAS4-like HKMTase effectors. The B. thailandensis type III effector BtSET promotes H3K4 methylation of rDNA chromatin, contributing to infection-induced rDNA transcription and bacterial intracellular replication. Thus, activation of host rDNA transcription might be a general bacterial virulence strategy.

Preventing bacterial DNA release and absent in melanoma 2 inflammasome activation by a Legionella effector functioning in membrane trafficking.

Legionella pneumophila, the causative agent of Legionnaires' pneumonia, resides in a distinct vacuole structure called Legionella-containing vacuole (LCV). The LCV resists fusion with the lysosome and permits efficient bacterial replication in host macrophages, which requires a Dot/Icm type IVB secretion system. Dot/Icm-translocated effector SdhA is critical for L. pneumophila intracellular growth and functions to prevent host cell death. Here, we show that the absence of SdhA resulted in elevated caspase-1 activation and IL-1ß secretion as well as macrophage pyroptosis during Legionella infection. These inflammasome activation phenotypes were independent of the established flagellin-NAIP5-NLRC4 axis, but relied on the DNA-sensing AIM2 inflammasome. We further demonstrate that Legionella DNA was released into macrophage cytosol, and this effect was significantly exaggerated by the absence of SdhA. SdhA bears a functional Golgi-targeting GRIP domain that is required for preventing AIM2 inflammasome activation. Ectopically expressed SdhA formed a unique ring-shape membrane structure, further indicating a role in membrane trafficking and maintaining LCV membrane integrity. Our data together suggest a possible link, mediated by the function of SdhA, between LCV trafficking/maturation and suppression of host innate immune detection.

Kaposi's sarcoma-associated herpesvirus K7 modulates Rubicon-mediated inhibition of autophagosome maturation.

Autophagy is an important innate safeguard mechanism for protecting an organism against invasion by pathogens. We have previously discovered that Kaposi's sarcoma-associated herpesvirus (KSHV) evades this host defense through tight suppression of autophagy by targeting multiple steps of autophagy signal transduction. Here, we report that KSHV K7 protein interacts with Rubicon autophagy protein and inhibits the autophagosome maturation step by blocking Vps34 enzymatic activity, further highlighting how KSHV deregulates autophagy-mediated host immunity for its life cycle.

VipD of Legionella pneumophila targets activated Rab5 and Rab22 to interfere with endosomal trafficking in macrophages.

Upon phagocytosis, Legionella pneumophila translocates numerous effector proteins into host cells to perturb cellular metabolism and immunity, ultimately establishing intracellular survival and growth. VipD of L. pneumophila belongs to a family of bacterial effectors that contain the N-terminal lipase domain and the C-terminal domain with an unknown function. We report the crystal structure of VipD and show that its C-terminal domain robustly interferes with endosomal trafficking through tight and selective interactions with Rab5 and Rab22. This domain, which is not significantly similar to any known protein structure, potently interacts with the GTP-bound active form of the two Rabs by recognizing a hydrophobic triad conserved in Rabs. These interactions prevent Rab5 and Rab22 from binding to downstream effectors Rabaptin-5, Rabenosyn-5 and EEA1, consequently blocking endosomal trafficking and subsequent lysosomal degradation of endocytic materials in macrophage cells. Together, this work reveals endosomal trafficking as a target of L. pneumophila and delineates the underlying molecular mechanism.

TXNIP-mediated crosstalk between oxidative stress and glucose metabolism.

Thioredoxin-interacting protein (TXNIP) has emerged as a key player in cancer and diabetes since it targets thioredoxin (TRX)-mediated redox regulation and glucose transporter (GLUT)-mediated metabolism. TXNIP consists of two arrestin (ARR, N-ARR and C-ARR) domains at its amino-terminus and two PPxY (PY) motifs and a di-leucine (LL) motif for endocytosis at its carboxyl-terminus. Here, we report that TXNIP shuffles between TRX and GLUTs to regulate homeostasis of intracellular oxidative stress and glucose metabolism. While TXNIP functions as a gatekeeper of TRX by default, it robustly interacted with class I GLUTs through its C-ARR domain upon increase of intracellular reactive oxygen species. This interaction prompted the surface expression downregulation and lysosomal degradation of GLUTs by its carboxyl-terminal LL endocytic signaling motif to attenuate glucose uptake. Consequently, TXNIP expression significantly limited glucose uptake, leading to the suppression of glycolysis, hexosamine biosynthesis, and the pentose phosphate pathway. Our findings establish a fundamental link between ROS and glucose metabolism through TXNIP and provide a promising target for the drug development against GLUT-related metabolic disorders.

Manipulation of host vesicular trafficking and innate immune defence by Legionella Dot/Icm effectors.

Legionella pneumophila, the causative agent of Legionnaires' disease, infects and replicates in macrophages and amoebas. Following internalization, L. pneumophila resides in a vacuole structure called Legionella-containing vacuole (LCV). The LCV escapes from the endocytic maturation process and avoids fusion with the lysosome, a hallmark of Legionella pathogenesis. Interference with the secretory vesicle transport and avoiding lysosomal targeting render the LCV permissive for L. pneumophila intracellular replication. Central to L. pneumophila pathogenesis is a defect in the organelle trafficking/intracellular multiplication (Dot/Icm) type IV secretion system that translocates a large number of effector proteins into host cells. Many of the Dot/Icm effectors employ diverse and sophisticated biochemical strategies to manipulate the host vesicular transport system, playing an important role in LCV biogenesis and trafficking. Similar to other bacterial pathogens, L. pneumophila also delivers effector proteins to modulate or counteract host innate immune defence pathways such as the NF-κB and apoptotic signalling. This review summarizes the known functions and mechanisms of Dot/Icm effectors that target host membrane trafficking and innate immune defence pathways.

A Legionella type IV effector activates the NF-kappaB pathway by phosphorylating the IkappaB family of inhibitors.

NF-kappaB is critical in innate immune defense responses against invading microbial pathogens. Legionella pneumophila infection of lung macrophages causes Legionnaire's disease with pneumonia symptoms. A set of NF-kappaB-controlled genes involved in inflammation and anti-apoptosis are up-regulated in macrophages upon L. pneumophila infection in a Legionella Dot/Icm type IV secretion system-dependent manner. Among approximately 100 Dot/Icm substrates screened, we identified LegK1 as the sole Legionella protein that harbors a highly potent NF-kappaB-stimulating activity. LegK1 does not affect MAPK and IFN pathways. Activation of the NF-kappaB pathway by LegK1 requires its eukaryotic-like Ser/Thr kinase activity and is independent of upstream components in the NF-kappaB pathway, including TRAFs, NIK, MEKK3, and TAK1. Cell-free reconstitution revealed that LegK1 stimulated NF-kappaB activation in the absence of IKKalpha and IKKbeta, and LegK1 efficiently phosphorylated IkappaBalpha on Ser-32 and Ser-36 both in vitro and in cells. LegK1 seems to mimic the host IKK as LegK1 also directly phosphorylated other IkappaB family of inhibitors including p100 in the noncanonical NF-kappaB pathway. Phosphorylation of p100 by LegK1 led to its maturation into p52. Thus, LegK1 is a bacterial effector that directly activates the host NF-kappaB signaling and likely plays important roles in modulating macrophage defense or inflammatory responses during L. pneumophila infection.

Asian Zika virus strains target CD14+ blood monocytes and induce M2-skewed immunosuppression during pregnancy.

Blood CD14+ monocytes are frontline immunomodulators categorized into classical, intermediate or non-classical subsets, and subsequently differentiated into M1 pro- or M2 anti-inflammatory macrophages on stimulation. Although the Zika virus (ZIKV) rapidly establishes viraemia, the target cells and immune responses, particularly during pregnancy, remain elusive. Furthermore, it is unknown whether African- and Asian-lineage ZIKV have different phenotypic impacts on host immune responses. Using human blood infection, we identified CD14+ monocytes as the primary target for African- or Asian-lineage ZIKV infection. When immunoprofiles of human blood infected with ZIKV were compared, a classical/intermediate monocyte-mediated M1-skewed inflammation by the African-lineage ZIKV infection was observed, in contrast to a non-classical monocyte-mediated M2-skewed immunosuppression by the Asian-lineage ZIKV infection. Importantly, infection of the blood of pregnant women revealed an enhanced susceptibility to ZIKV infection. Specifically, Asian-lineage ZIKV infection of pregnant women's blood led to an exacerbated M2-skewed immunosuppression of non-classical monocytes in conjunction with a global suppression of type I interferon-signalling pathway and an aberrant expression of host genes associated with pregnancy complications. Also, 30 ZIKV+ sera from symptomatic pregnant patients showed elevated levels of M2-skewed immunosuppressive cytokines and pregnancy-complication-associated fibronectin-1. This study demonstrates the differential immunomodulatory responses of blood monocytes, particularly during pregnancy, on infection with different lineages of ZIKV.

Zika Virus NS4A and NS4B Proteins Deregulate Akt-mTOR Signaling in Human Fetal Neural Stem Cells to Inhibit Neurogenesis and Induce Autophagy.

The current widespread outbreak of Zika virus (ZIKV) infection has been linked to severe clinical birth defects, particularly microcephaly, warranting urgent study of the molecular mechanisms underlying ZIKV pathogenesis. Akt-mTOR signaling is one of the key cellular pathways essential for brain development and autophagy regulation. Here, we show that ZIKV infection of human fetal neural stem cells (fNSCs) causes inhibition of the Akt-mTOR pathway, leading to defective neurogenesis and aberrant activation of autophagy. By screening the three structural proteins and seven nonstructural proteins present in ZIKV, we found that two, NS4A and NS4B, cooperatively suppress the Akt-mTOR pathway and lead to cellular dysregulation. Corresponding proteins from the closely related dengue virus do not have the same effect on neurogenesis. Thus, our study highlights ZIKV NS4A and NS4B as candidate determinants of viral pathogenesis and identifies a mechanism of action for their effects, suggesting potential targets for anti-ZIKV therapeutic intervention.

Lpg0393 of Legionella pneumophila is a guanine-nucleotide exchange factor for Rab5, Rab21 and Rab22.

Legionella pneumophila, a human intracellular pathogen, encodes about 290 effector proteins that are translocated into host cells through a secretion machinery. Some of these proteins have been shown to manipulate or subvert cellular processes during infection, but functional roles of a majority of them remain unknown. Lpg0393 is a newly identified Legionella effector classified as a hypothetical protein. Through X-ray crystallographic analysis, we show that Lpg0393 contains a Vps9-like domain, which is structurally most similar to the catalytic core of human Rabex-5 that activates the endosomal Rab proteins Rab5, Rab21 and Rab22. Consistently, Lpg0393 exhibited a guanine-nucleotide exchange factor activity toward the endosomal Rabs. This work identifies the first example of a bacterial guanine-nucleotide exchange factor that is active towards the Rab5 sub-cluster members, implying that the activation of these Rab proteins might be advantageous for the intracellular survival of Legionella.

Autophagy side of MB21D1/cGAS DNA sensor.

The MB21D1/cGAS (Mab-21 domain-containing 1/cyclic GMP-AMP [cGAMP] synthetase), acts as an intracellular pattern recognition receptor (PPR) to sense cytosolic pathogen DNAs and subsequently generates the second messenger cGAMP to initiate the TMEM173/STING pathway for interferon (IFN) production. Intriguingly, we have recently demonstrated crosstalk between the intracellular DNA sensing pathway and autophagy machinery by demonstrating a direct interaction between the MB21D1 DNA sensor and the BECN1/Beclin 1 autophagy protein. This interaction not only suppresses MB21D1 enzymatic activity to halt cGAMP production, but also enhances the autophagy-mediated degradation of cytosolic microbial DNAs. This demonstrates that MB21D1 is the molecular link between the intracellular DNA sensing pathway and the autophagy pathway, ultimately developing well-balanced immune responses against pathogens.

Crosstalk between the cGAS DNA sensor and Beclin-1 autophagy protein shapes innate antimicrobial immune responses.

Robust immune responses are essential for eliminating pathogens but must be metered to avoid prolonged immune activation and potential host damage. Upon recognition of microbial DNA, the cytosolic DNA sensor cyclic GMP-AMP (cGAMP) synthetase (cGAS) produces the second messenger cGAMP to initiate the stimulator of interferon genes (STING) pathway and subsequent interferon (IFN) production. We report that the direct interaction between cGAS and the Beclin-1 autophagy protein not only suppresses cGAMP synthesis to halt IFN production upon double-stranded DNA (dsDNA) stimulation or herpes simplex virus-1 infection, but also enhances autophagy-mediated degradation of cytosolic pathogen DNA to prevent excessive cGAS activation and persistent immune stimulation. Specifically, this interaction releases Rubicon, a negative autophagy regulator, from the Beclin-1 complex, activating phosphatidylinositol 3-kinase class III activity and thereby inducing autophagy to remove cytosolic pathogen DNA. Thus, the cGAS-Beclin-1 interaction shapes innate immune responses by regulating both cGAMP production and autophagy, resulting in well-balanced antimicrobial immune responses.