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Biomedical subjects

Zhijian J Chen

Publications and source records attributed to Zhijian J Chen.

At least 19 recordsLinked to original sources
PubMed2006

Essential role of TAK1 in thymocyte development and activation.

The protein kinase TAK1 mediates the activation of NF-kappaB in response to stimulation by proinflammatory cytokines and microbial pathogens in the innate immunity pathways. However, the physiological function of TAK1 in the adaptive immunity pathways is unclear. By engineering mice lacking TAK1 in T cells, here, we show that TAK1 is essential for thymocyte development and activation in vivo. Deletion of TAK1 prevented the maturation of single-positive thymocytes displaying CD4 or CD8, leading to reduction of T cells in the peripheral tissues. Thymocytes lacking TAK1 failed to activate NF-kappaB and JNK and were prone to apoptosis upon stimulation. Our results provide the genetic evidence that TAK1 is required for the activation of NF-kappaB in thymocytes and suggest that TAK1 plays a central role in both innate and adaptive immunity.

Animals↗
PubMed2006

Sorting out Toll signals.

Upon recognition of microbial products, Toll-like receptors (TLRs) recruit distinct combinations of adaptors to induce TLR-specific gene expression. In this issue, Kagan and Medzhitov (2006) demonstrate that the adaptor TIRAP/Mal localizes to the plasma membrane by binding to phosphatidylinositol 4,5-bisphosphate (PIP2). This binding recruits a key adaptor MyD88 to TLR4, suggesting that there is crosstalk between the TLR signaling pathway and phospholipid metabolism.

Adaptor Proteins, Signal Transducing↗
PubMed2006

Activation of IKK by TNFalpha requires site-specific ubiquitination of RIP1 and polyubiquitin binding by NEMO.

The receptor interacting protein kinase 1 (RIP1) is essential for the activation of nuclear factor kappaB (NF-kappaB) by tumor necrosis factor alpha (TNFalpha). Here, we present evidence that TNFalpha induces the polyubiquitination of RIP1 at Lys-377 and that this polyubiquitination is required for the activation of IkappaB kinase (IKK) and NF-kappaB. A point mutation of RIP1 at Lys-377 (K377R) abolishes its polyubiquitination as well as its ability to restore IKK activation in a RIP1-deficient cell line. The K377R mutation of RIP1 also prevents the recruitment of TAK1 and IKK complexes to TNF receptor. Interestingly, polyubiquitinated RIP1 recruits IKK through the binding between the polyubiquitin chains and NEMO, a regulatory subunit of the IKK complex. Mutations of NEMO that disrupt its polyubiquitin binding also abolish IKK activation. These results reveal the biochemical mechanism underlying the essential signaling function of NEMO and provide direct evidence that signal-induced site-specific ubiquitination of RIP1 is required for IKK activation.

Amino Acid Sequence↗
PubMed2006

The specific and essential role of MAVS in antiviral innate immune responses.

The mitochondrial antiviral signaling protein (MAVS) mediates the activation of NFkappaB and IRFs and the induction of interferons in response to viral infection. In vitro studies have also suggested that MAVS is required for interferon induction by cytosolic DNA, but the in vivo evidence is lacking. By generating MAVS-deficient mice, here we show that loss of MAVS abolished viral induction of interferons and prevented the activation of NFkappaB and IRF3 in multiple cell types, except plasmacytoid dendritic cells (pDCs). However, MAVS was not required for interferon induction by cytosolic DNA or by Listeria monocytogenes. Mice lacking MAVS were viable and fertile, but they failed to induce interferons in response to poly(I:C) stimulation and were severely compromised in immune defense against viral infection. These results provide the in vivo evidence that the cytosolic viral signaling pathway through MAVS is specifically required for innate immune responses against viral infection.

Adaptor Proteins, Signal Transducing↗
PubMed2006

Antiviral innate immunity pathways.

Recent studies have uncovered two signaling pathways that activate the host innate immunity against viral infection. One of the pathways utilizes members of the Toll-like receptor (TLR) family to detect viruses that enter the endosome through endocytosis. The TLR pathway induces interferon production through several signaling proteins that ultimately lead to the activation of the transcription factors NF-kappaB, IRF3 and IRF7. The other antiviral pathway uses the RNA helicase RIG-I as the receptor for intracellular viral double-stranded RNA. RIG-I activates NF-kappaB and IRFs through the recently identified adaptor protein MAVS, a CARD domain containing protein that resides in the mitochondrial membrane. MAVS is essential for antiviral innate immunity, but it also serves as a target of Hepatitis C virus (HCV), which employs a viral protease to cleave MAVS off the mitochondria, thereby allowing HCV to escape the host immune system.

Adaptor Proteins, Signal Transducing↗
PubMed2005

Protein ubiquitination: CHIPping away the symmetry.

CHIP is a ubiquitin ligase implicated in the degradation of misfolded proteins. In the November 23 issue of Molecular Cell, identified CHIP as a protein that interacts with the ubiquitin E2 complex Ubc13-Uev1A, which catalyzes the synthesis of Lys-63-linked polyubiquitin chains. Although the ubiquitin ligase activity of CHIP requires its dimerization through the U box domain, the crystal structure of the CHIP-E2 complex reveals that the protomers in the CHIP homodimer adopt distinct conformations such that only one U box of CHIP interacts with Ubc13.

Catalysis↗
PubMed2005

Hepatitis C virus protease NS3/4A cleaves mitochondrial antiviral signaling protein off the mitochondria to evade innate immunity.

Hepatitis C virus (HCV) is a global epidemic manifested mainly by chronic infection. One strategy that HCV employs to establish chronic infection is to use the viral Ser protease NS3/4A to cleave some unknown cellular targets involved in innate immunity. Here we show that the target of NS3/4A is the mitochondrial antiviral signaling protein, MAVS, that activates NF-kappaB and IFN regulatory factor 3 to induce type-I interferons. NS3/4A cleaves MAVS at Cys-508, resulting in the dislocation of the N-terminal fragment of MAVS from the mitochondria. Remarkably, a point mutation of MAVS at Cys-508 renders MAVS resistant to cleavage by NS3/4A, thus maintaining the ability of MAVS to induce interferons in HCV replicon cells. NS3/4A binds to and colocalizes with MAVS in the mitochondrial membrane, and it can cleave MAVS directly in vitro. These results provide an example of host-pathogen interaction in which the virus evades innate immunity by dislodging a pivotal antiviral protein from the mitochondria and suggest that blocking the cleavage of MAVS by NS3/4A may be applied to the prevention and treatment of HCV.

Amino Acid Sequence↗
PubMed2005

Identification and characterization of MAVS, a mitochondrial antiviral signaling protein that activates NF-kappaB and IRF 3.

Viral infection triggers host innate immune responses through activation of the transcription factors NF-kappaB and IRF 3, which coordinately regulate the expression of type-I interferons such as interferon-beta (IFN-beta). Herein, we report the identification of a novel protein termed MAVS (mitochondrial antiviral signaling), which mediates the activation of NF-kappaB and IRF 3 in response to viral infection. Silencing of MAVS expression through RNA interference abolishes the activation of NF-kappaB and IRF 3 by viruses, thereby permitting viral replication. Conversely, overexpression of MAVS induces the expression of IFN-beta through activation of NF-kappaB and IRF 3, thus boosting antiviral immunity. Epistasis experiments show that MAVS is required for the phosphorylation of IRF 3 and IkappaB and functions downstream of RIG-I, an intracellular receptor for viral RNA. MAVS contains an N-terminal CARD-like domain and a C-terminal transmembrane domain, both of which are essential for MAVS signaling. The transmembrane domain targets MAVS to the mitochondria, implicating a new role of mitochondria in innate immunity.

Amino Acid Sequence↗
PubMed2005

The role of ubiquitination in Drosophila innate immunity.

Infection of Drosophila by Gram-negative bacteria triggers a signal transduction pathway (the IMD pathway) culminating in the expression of genes encoding antimicrobial peptides. A key component in this pathway is a Drosophila IkappaB kinase (DmIKK) complex, which stimulates the cleavage and activation of the NF-kappaB transcription factor Relish. Activation of the DmIKK complex requires the MAP3K dTAK1, but the mechanism of dTAK1 activation is not understood. In human cells, the activation of TAK1 and IKK requires the human ubiquitin-conjugating enzymes Ubc13 and UEV1a. Here we demonstrate that the Drosophila homologs of Ubc13 and UEV1a are similarly required for the activation of dTAK1 and the DmIKK complex. Surprisingly, we find that the Drosophila caspase DREDD and its partner dFADD are required for the activation of DmIKK and JNK, in addition to their role in Relish cleavage. These studies reveal an evolutionarily conserved role of ubiquitination in IKK activation, and provide new insights into the hierarchy of signaling components in the Drosophila antibacterial immunity pathway.

Animals↗
PubMed2005

TRAF2: a double-edged sword?

Ubiquitination is best known for its role in targeting proteins for degradation by the proteasome, but evidence of the nonproteolytic functions of ubiquitin is also rapidly accumulating. One example of the regulatory, rather than proteolytic, function of ubiquitin is provided by study of the tumor necrosis factor (TNF) receptor-associated factor (TRAF) proteins, which function as ubiquitin ligases to synthesize lysine 63 (K(63))-linked polyubiquitin chains to mediate protein kinase activation through a proteasome-independent mechanism. Some TRAF proteins, such as TRAF2 and TRAF3, have recently been shown to have a positive role in the canonical pathway that activates nuclear factor kappaB (NF-kappaB) through IkappaB kinase beta (IKKbeta), but a negative role in the noncanonical pathway that activates NF-kappaB through IKKalpha. These opposing roles of TRAF proteins may be linked to their ability to synthesize distinct forms of polyubiquitin chains. Indeed, the TRAF2-interacting protein RIP can mediate IKK activation when it is modified by K(63) polyubiquitin chains, but is targeted to degradation by the proteasome when it is K(48)-polyubiquitinted by the NF-kappaB inhibitor A20. Thus, ubiquitin chains are dynamic switches that can influence signaling outputs in dramatically different ways.

Animals↗
PubMed2005

Ubiquitin signalling in the NF-kappaB pathway.

The transcription factor NF-kappaB (nuclear factor kappa enhancer binding protein) controls many processes, including immunity, inflammation and apoptosis. Ubiquitination regulates at least three steps in the NF-kappaB pathway: degradation of IkappaB (inhibitor of NF-kappaB), processing of NF-kappaB precursors, and activation of the IkappaB kinase (IKK). Recent studies have revealed several enzymes involved in the ubiquitination and deubiquitination of signalling proteins that mediate IKK activation through a degradation-independent mechanism.

Animals↗
PubMed2004

TIFA activates IkappaB kinase (IKK) by promoting oligomerization and ubiquitination of TRAF6.

TRAF6 (tumor necrosis factor receptor-associated factor 6) is a RING (really interesting new gene) domain ubiquitin (Ub) ligase that mediates the activation of protein kinases, such as transforming growth factor beta-activated kinase (TAK1) and IkappaB kinase (IKK), by catalyzing the formation of a unique polyubiquitin chain linked through Lys-63 of Ub. Here, we present evidence that TIFA (TRAF-interacting protein with a forkhead-associated domain, also known as T2BP) activates IKK by promoting the oligomerization and Ub ligase activity of TRAF6. We show that recombinant TIFA protein, but not TRAF6-binding-defective mutant, can activate IKK in crude cytosolic extracts. Furthermore, TIFA activates IKK in an in vitro reconstitution system consisting of purified proteins, including TRAF6, the TAK1 kinase complex, and Ub-conjugating enzyme complex Ubc13-Uev1A. Interestingly, a fraction of recombinant TIFA protein exists as high-molecular-weight oligomers, and only these oligomeric forms of TIFA can activate IKK. Importantly, TIFA induces the oligomerization and polyubiquitination of TRAF6, which leads to the activation of TAK1 and IKK through a proteasome-independent mechanism.

Adaptor Proteins, Signal Transducing↗
PubMed2004

TAB2 and TAB3 activate the NF-kappaB pathway through binding to polyubiquitin chains.

The activation of NF-kappaB and IKK requires an upstream kinase complex consisting of TAK1 and adaptor proteins such as TAB1, TAB2, or TAB3. TAK1 is in turn activated by TRAF6, a RING domain ubiquitin ligase that facilitates the synthesis of lysine 63-linked polyubiquitin chains. Here we present evidence that TAB2 and TAB3 are receptors that bind preferentially to lysine 63-linked polyubiquitin chains through a highly conserved zinc finger (ZnF) domain. Mutations of the ZnF domain abolish the ability of TAB2 and TAB3 to bind polyubiquitin chains, as well as their ability to activate TAK1 and IKK. Significantly, replacement of the ZnF domain with a heterologous ubiquitin binding domain restored the ability of TAB2 and TAB3 to activate TAK1 and IKK. We also show that TAB2 binds to polyubiquitinated RIP following TNFalpha stimulation. These results indicate that polyubiquitin binding domains represent a new class of signaling domains that regulate protein kinase activity through a nonproteolytic mechanism.

Adaptor Proteins, Signal Transducing↗
PubMed2004

Association of ABCA2 expression with determinants of Alzheimer's disease.

With the use of a novel method for detecting differential gene expression, alterations in functional gene clusters related to transport or oxidative stress response and beta-amyloid (Abeta) peptide metabolism were identified in a HEK293 cell line engineered to overexpress the human ATP binding cassette transporter ABCA2. These included fatty acid binding protein, phospholipid binding protein, phospholipid synthesis protein, transporter cofactors, seladin-1, Abeta precursor protein (APP), vimentin, and low-density lipoprotein receptor-related protein. ABCA2 was highly expressed in neuroblastoma cells and colocalized with Abeta and APP. Additionally, increased APP protein levels were detected within ABCA2/APP double-transfected cells, and increased Abeta was detected in the media of ABCA2-transfected cells relative to controls. The transporter was abundant in the temporal and frontal regions of both normal and Alzheimer's disease (AD) brain but was detected at lower concentrations in the parietal, occipital, and cerebellar regions. The ABCA2 transfected cell line expressed resistance to a free radical initiator, confirming involvement in protection against reactive oxygen species and suggesting a further possible link to AD.

ATP-Binding Cassette Transporters↗
PubMed2004

The TRAF6 ubiquitin ligase and TAK1 kinase mediate IKK activation by BCL10 and MALT1 in T lymphocytes.

The CARD domain protein BCL10 and paracaspase MALT1 are essential for the activation of IkappaB kinase (IKK) and NF-kappaB in response to T cell receptor (TCR) stimulation. Here we present evidence that TRAF6 ubiquitin ligase and TAK1 protein kinase mediate IKK activation by BCL10 and MALT1. RNAi-mediated silencing of MALT1, TAK1, TRAF6, and TRAF2 suppressed TCR-dependent IKK activation and interleukin-2 production in T cells. Furthermore, we have reconstituted the pathway from BCL10 to IKK activation in vitro with purified proteins of MALT1, TRAF6, TAK1, and ubiquitination enzymes including Ubc13/Uev1A. We find that a small fraction of BCL10 and MALT1 proteins form high molecular weight oligomers. Strikingly, only these oligomeric forms of BCL10 and MALT1 can activate IKK in vitro. The MALT1 oligomers bind to TRAF6, induce TRAF6 oligomerization, and activate the ligase activity of TRAF6 to polyubiquitinate NEMO. These results reveal an oligomerization --> ubiquitination --> phosphorylation cascade that culminates in NF-kappaB activation in T lymphocytes.

Adaptor Proteins, Signal Transducing↗