The inflammasome.
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Adaptor Proteins, Signal Transducing↗
Biomedical subjects
Jürg Tschopp
Publications and source records attributed to Jürg Tschopp.
Cellular FLIP long form augments caspase activity and death of T cells through heterodimerization with and activation of caspase-8.
Caspase activity is required not only for the death of T cells, but also for their activation. A delicate balance of caspase activity is thus required during T cell activation at a level that will not drive cell death. How caspase activity is initiated and regulated during T cell activation is not known. One logical candidate for this process is cellular FLIP long form (c-FLIP(L)), because it can block caspase-8 recruitment after Fas (CD95) ligation as well as directly heterodimerize with and activate caspase-8. The current findings demonstrate that after T cell activation, caspase-8 and c-FLIP(L) associate in a complex enriched for active caspases. This occurs coincidently with the cleavage of two known caspase-8 substrates, c-FLIP(L) and receptor interacting protein 1. Caspase activity is higher in wild-type CD8(+) than CD4(+) effector T cells. Increased expression of c-FLIP(L) results in augmented caspase activity in resting and effector T cells to levels that provoke cell death, especially of the CD8 subset. c-FLIP(L) is thus not only an inhibitor of cell death by Fas, it can also act as a principal activator of caspases independently of Fas.
Animals↗
Identification of proteoglycans as the APRIL-specific binding partners.
B cell activating factor of the tumor necrosis factor (TNF) family (BAFF) and a proliferation-inducing ligand (APRIL) are closely related ligands within the TNF superfamily that play important roles in B lymphocyte biology. Both ligands share two receptors--transmembrane activator and calcium signal--modulating cyclophilin ligand interactor (TACI) and B cell maturation antigen (BCMA)--that are predominantly expressed on B cells. In addition, BAFF specifically binds BAFF receptor, whereas the nature of a postulated APRIL-specific receptor remains elusive. We show that the TNF homology domain of APRIL binds BCMA and TACI, whereas a basic amino acid sequence (QKQKKQ) close to the NH2 terminus of the mature protein is required for binding to the APRIL-specific "receptor." This interactor was identified as negatively charged sulfated glycosaminoglycan side chains of proteoglycans. Although T cell lines bound little APRIL, the ectopic expression of glycosaminoglycan-rich syndecans or glypicans conferred on these cells a high binding capacity that was completely dependent on APRIL's basic sequence. Moreover, syndecan-1-positive plasma cells and proteoglycan-rich nonhematopoietic cells displayed high specific, heparin-sensitive binding to APRIL. Inhibition of BAFF and APRIL, but not BAFF alone, prevented the survival and/or the migration of newly formed plasma cells to the bone marrow. In addition, costimulation of B cell proliferation by APRIL was only effective upon APRIL oligomerization. Therefore, we propose a model whereby APRIL binding to the extracellular matrix or to proteoglycan-positive cells induces APRIL oligomerization, which is the prerequisite for the triggering of TACI- and/or BCMA-mediated activation, migration, or survival signals.
Animals↗
NLRs join TLRs as innate sensors of pathogens.
Pathogen-recognition receptors (PRRs) are key components of immune systems and are involved in innate effector mechanisms and activation of adaptive immunity. Since their discovery in vertebrates, Toll-like receptors (TLRs) have become the focus of extensive research that has revealed their significance in the regulation of many facets of our immune system. What makes TLRs so central and fascinating is their ability to recognize microbes and directly initiate specific signal transduction cascades that alert the host defences. In this review, we discuss the function and biology of a new family of PRRs, the NACHT-LRRs (NLRs), which include both nucleotide-binding oligomerization domains (NODs) and NALPs [NACHT-, LRR- and pyrin domain (PYD)-containing proteins], and underline some intriguing similarities between NLRs and TLRs that emphasize the role of NLRs as a complementary system for host-microbe interactions.
Adaptor Proteins, Signal Transducing↗
The RIP kinases: crucial integrators of cellular stress.
Since the discovery of the first member ten years ago, the receptor-interacting protein (RIP) family kinases have emerged as essential sensors of cellular stress. The different members integrate both extracellular stress signals transmitted by various cell-surface receptors and signals emanating from intracellular stress. The cascades of events initiated by activated RIPs are complex. Not only are pro-survival, inflammatory and immune responses triggered by RIP kinases via the activation of transcription factors such as NF-kappaB and AP-1, but opposing, death-inducing programs can also be initiated by the RIP kinases. Hence, RIP kinases are crucial regulators of cell survival and cell death.
Amino Acid Sequence↗
Identification of bacterial muramyl dipeptide as activator of the NALP3/cryopyrin inflammasome.
Activation of caspase-1 and subsequent processing and secretion of the pro-inflammatory cytokine IL-1beta is triggered upon assembly of the inflammasome complex. It is generally believed that bacterial lipopolysaccharides (LPS) are activators of the inflammasome through stimulation of Toll-like receptor 4 (TLR4). Like TLRs, NALP3/Cryopyrin, which is a key component of the inflammasome, contains Leucine-Rich-Repeats (LRRs). LRRs are frequently used to sense bacterial components, thus raising the possibility that bacteria directly activate the inflammasome. Here, we show that bacterial peptidoglycans (PGN), but surprisingly not LPS, induce NALP3-mediated activation of caspase-1 and maturation of proIL-1beta. Activation is independent of TLRs because the PGN degradation product muramyl dipeptide (MDP), which is not sensed by TLRs, is the minimal-activating structure. Macrophages from a patient with Muckle-Wells syndrome, an autoinflammatory disease associated with mutations in the NALP3/Cryopyrin gene, show increased IL-1beta secretion in the presence of MDP. The activation of the NALP3-inflammasome by MDP may be the basis of the potent adjuvant activity of MDP.
Acetylmuramyl-Alanyl-Isoglutamine↗
Inflammatory caspases: linking an intracellular innate immune system to autoinflammatory diseases.
Caspases not only play an essential role during apoptotic cell death, but a subfamily of them-the inflammatory caspases-are associated with immune responses to microbial pathogens. Activation of inflammatory caspases, such as caspase-1 and caspase-5, occurs upon assembly of an intracellular complex, designated the inflammasome. This results in the cleavage and activation of the proinflammatory cytokines IL-1beta and IL-18. Mutations in one of the scaffold proteins of the inflammasome, NALP3/Cryopyrin, are associated with autoinflammatory disorders underscoring the importance of regulating inflammatory caspase activation.
Animals↗
Cellular FLIP long form-transgenic mice manifest a Th2 cytokine bias and enhanced allergic airway inflammation.
Cellular FLIP long form (c-FLIP(L)) is a caspase-defective homologue of caspase-8 that blocks apoptosis by death receptors. The expression of c-FLIP(L) in T cells can also augment extracellular signal-regulated kinase phosphorylation after TCR ligation via the association of c-FLIP(L) with Raf-1. This contributes to the hyperproliferative capacity of T cells from c-FLIP(L)-transgenic mice. In this study we show that activated CD4(+) T cells from c-FLIP(L)-transgenic mice produce increased amounts of Th2 cytokines and decreased amounts of Th1 cytokines. This correlates with increased serum concentrations of the Th2-dependent IgG1 and IgE. The Th2 bias of c-FLIP(L)-transgenic CD4(+) T cells parallels impaired NF-kappa B activity and increased levels of GATA-3, which contribute, respectively, to decreased IFN-gamma and increased Th2 cytokines. The Th2 bias of c-FLIP(L)-transgenic mice extends to an enhanced sensitivity to OVA-induced asthma. Taken together, these results show that c-FLIP(L) can influence cytokine gene expression to promote Th2-driven allergic reaction, in addition to its traditional role of blocking caspase activation induced by death receptors.
Adjuvants, Immunologic↗
The PIDDosome, a protein complex implicated in activation of caspase-2 in response to genotoxic stress.
Apoptosis is triggered by activation of initiator caspases upon complex-mediated clustering of the inactive zymogen, as occurs in the caspase-9-activating apoptosome complex. Likewise, caspase-2, which is involved in stress-induced apoptosis, is recruited into a large protein complex, the molecular composition of which remains elusive. We show that activation of caspase-2 occurs in a complex that contains the death domain-containing protein PIDD, whose expression is induced by p53, and the adaptor protein RAIDD. Increased PIDD expression resulted in spontaneous activation of caspase-2 and sensitization to apoptosis by genotoxic stimuli. Because PIDD functions in p53-mediated apoptosis, the complex assembled by PIDD and caspase-2 is likely to regulate apoptosis induced by genotoxins.
Adaptor Proteins, Signal Transducing↗
RIP1 is an essential mediator of Toll-like receptor 3-induced NF-kappa B activation.
Stimulation of Toll-like receptors (TLRs) initiates potent innate immune responses through Toll-interleukin 1 receptor (TIR) domain-containing adaptors such as MyD88 and Trif. Analysis of Trif-deficient mice has shown that TLR3-dependent activation of the transcription factor NF-kappa B by the TLR3 ligand double-stranded RNA is Trif dependent. Here we investigated the 'downstream' signaling events that regulate TLR3-dependent Trif-induced NF-kappa B activation. Trif recruited the kinases receptor interacting protein (RIP)-1 and RIP3 through its RIP homotypic interaction motif. In the absence of RIP1, TLR3-mediated signals activating NF-kappa B, but not the kinase JNK or interferon-beta, were abolished, suggesting that RIP1 mediates Trif-induced NF-kappa B activation. In contrast, the presence of RIP3 negatively regulated the Trif-RIP1-induced NF-kappa B pathway. Therefore, in contrast to other TLRs, which use interleukin 1 receptor-associated kinase (IRAK) proteins to activate NF-kappa B, TLR 3-induced NF-kappa B activation is dependent on RIP kinases.
Adaptor Proteins, Vesicular Transport↗
The anti-apoptotic factor Bcl-2 can functionally substitute for the B cell survival but not for the marginal zone B cell differentiation activity of BAFF.
The TNF family ligand B cell-activating factor (BAFF, BLyS, TALL-1) is an essential factor for B cell development. BAFF binds to three receptors, BAFF-R, transmembrane activator and CAML interactor (TACI), and B cell maturation antigen (BCMA), but only BAFF-R is required for successful survival and maturation of splenic B cells. To test whether the effect of BAFF is due to the up-regulation of anti-apoptotic factors, TACI-Ig-transgenic mice, in which BAFF function is inhibited, were crossed with transgenic mice expressing FLICE-inhibitory protein (FLIP) or Bcl-2 in the B cell compartment. FLIP expression did not rescue B cells, while enforced Bcl-2 expression restored peripheral B cells and the ability to mount T-dependent antibody responses. However, many B cells retained immaturity markers and failed to express normal amounts of CD21. Marginal zone B cells were not restored and the T-independent IgG3, but not IgM, response was impaired in the TACI-IgxBcl-2 mice. These results suggest that BAFF is required not only to inhibit apoptosis of maturating B cells, but also to promote differentiation events, in particular those leading to the generation of marginal zone B cells.
Animals↗
Life and death decisions: secondary complexes and lipid rafts in TNF receptor family signal transduction.
Signaling by receptors in the TNF receptor (TNFR) superfamily mediate biological outcomes ranging from inflammation to apoptosis and other forms of programmed cell death. How receptor signaling mediates these divergent responses is just beginning to be understood. Here, we discuss how receptor submembrane localization and the formation of alternate signaling complexes can alter the fate of cells stimulated through TNFRs with a death domain, also known as "death receptors."
Animals↗
NALP3 forms an IL-1beta-processing inflammasome with increased activity in Muckle-Wells autoinflammatory disorder.
Mutations within the NALP3/cryopyrin/CIAS1 gene are responsible for three autoinflammatory disorders: Muckle-Wells syndrome, familial cold autoinflammatory syndrome, and CINCA. The NALP3 protein is homologous to NALP1, which is a component of the inflammasome, a molecular platform that activates the proinflammatory caspases-1 and -5. NALP3 (and other members of the NALP family) lacks the C-terminal, CARD-containing sequence of NALP1, and its role in caspase activation is unclear. Here, we report that NALP2 and NALP3 associate with ASC, the CARD-containing protein Cardinal, and caspase-1 (but not caspase-5), thereby forming an inflammasome with high proIL-1beta-processing activity. Macrophages from Muckle-Wells patients spontaneously secrete active IL-1beta. Increased inflammasome activity is therefore likely to be the molecular basis of the symptoms associated with NALP3-dependent autoinflammatory disorders.
Adaptor Proteins, Signal Transducing↗
BAFF production by antigen-presenting cells provides T cell co-stimulation.
The B cell-activating factor from the tumor necrosis factor family (BAFF) is an important regulator of B cell immunity. Recently, we demonstrated that recombinant BAFF also provides a co-stimulatory signal to T cells. Here, we studied expression of BAFF in peripheral blood leukocytes and correlated this expression with BAFF T cell co-stimulatory function. BAFF is produced by antigen-presenting cells (APC). Blood dendritic cells (DC) as well as DC differentiated in vitro from monocytes or CD34+ stem cells express BAFF mRNA. Exposure to bacterial products further up-regulates BAFF production in these cells. A low level of BAFF transcription, up-regulated upon TCR stimulation, was also detected in T cells. Functionally, blockade of endogenous BAFF produced by APC and, to a lesser extent, by T cells inhibits T cell activation. Altogether, this indicates that BAFF may regulate T cell immunity during APC-T cell interactions and as an autocrine factor once T cells have detached from the APC.
B-Cell Activating Factor↗
N-terminal fragment of c-FLIP(L) processed by caspase 8 specifically interacts with TRAF2 and induces activation of the NF-kappaB signaling pathway.
Caspase 8 is required not only for death receptor-mediated apoptosis but also for lymphocyte activation in the immune system. FLIP(L), the long-splice form of c-FLIP, is one of the specific substrates for caspase 8, and increased expression of FLIP(L) promotes activation of the NF-kappaB signaling pathway. The synthetic caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp(OMe)-fluoromethylketone (zVAD-fmk) markedly blocked NF-kappaB activation induced by overexpression of FLIP(L). FLIP(L) is specifically processed by caspase 8 into N-terminal FLIP(p43) and C-terminal FLIP(p12). Only FLIP(p43) was able to induce NF-kappaB activation as efficiently as FLIP(L), and FLIP(p43)-induced NF-kappaB activation became insensitive to zVAD-fmk. In caspase 8-deficient cells, FLIP(p43) provoked NF-kappaB activation only when procaspase 8 or caspase 8(p43) was complemented. FLIP(p43)-induced NF-kappaB activation was profoundly blocked by the dominant-negative TRAF2. Moreover, endogenous TRAF2 interacted specifically with FLIP(p43), and the formation of the FLIP(p43)-caspase 8-TRAF2 tertiary complex was a prerequisite to induction of NF-kappaB activation. zVAD-fmk prevented the recruitment of TRAF2 into the death-inducing signaling complex. Thus, our present results demonstrate that FLIP(p43) processed by caspase 8 specifically interacts with TRAF2 and subsequently induces activation of the NF-kappaB signaling pathway.
Amino Acid Chloromethyl Ketones↗
Ubiquitination of the Epstein-Barr virus-encoded latent membrane protein 1 depends on the integrity of the TRAF binding site.
The latent membrane protein 1 (LMP1) encoded by the Epstein-Barr virus functions as a constitutively activated receptor of the tumor necrosis factor receptor family. LMP1 is a short-lived protein that is ubiquitinated and degraded by the proteasome. We have previously shown that LMP1 recruits the adapter protein tumor necrosis factor receptor-associated factor 3 (TRAF3) to lipid rafts. To test if TRAFs are involved in LMP1's ubiquitination, we have mutated the LMP1 CTAR1 site that has been identified as a TRAF binding site. We show that the CTAR1 mutant (CTAR1(-)) is expressed after transfection at a similar level to wild-type LMP1, and behaves as wild-type LMP1 with respect to membrane localization. However, CTAR1(-) does not bind TRAF3. We demonstrate that ubiquitination of CTAR1(-) is significantly reduced when compared to wild-type LMP1. In addition, the expression of wild-type LMP1 induces the ubiquitination, an effect that is significantly reduced when the CTAR1(-) is expressed. Taken together, our results suggest that TRAF proteins are involved in the ubiquitination of LMP1, and that their binding to LMP1 may facilitate their own ubiquitination.
Binding Sites↗
Induction of TNF receptor I-mediated apoptosis via two sequential signaling complexes.
Apoptosis induced by TNF-receptor I (TNFR1) is thought to proceed via recruitment of the adaptor FADD and caspase-8 to the receptor complex. TNFR1 signaling is also known to activate the transcription factor NF-kappa B and promote survival. The mechanism by which this decision between cell death and survival is arbitrated is not clear. We report that TNFR1-induced apoptosis involves two sequential signaling complexes. The initial plasma membrane bound complex (complex I) consists of TNFR1, the adaptor TRADD, the kinase RIP1, and TRAF2 and rapidly signals activation of NF-kappa B. In a second step, TRADD and RIP1 associate with FADD and caspase-8, forming a cytoplasmic complex (complex II). When NF-kappa B is activated by complex I, complex II harbors the caspase-8 inhibitor FLIP(L) and the cell survives. Thus, TNFR1-mediated-signal transduction includes a checkpoint, resulting in cell death (via complex II) in instances where the initial signal (via complex I, NF-kappa B) fails to be activated.
Adaptor Proteins, Signal Transducing↗
BAFF and the regulation of B cell survival.
The TNF family member BAFF is a fundamental survival factor for B cells. BAFF binds to three receptors, only one of which, BAFF-R, does not cross-react with the BAFF-related ligand APRIL. The survival function of BAFF on B cells is mediated mainly by BAFF-R and is particularly effective in transitional B cells. BAFF depletion leads to a considerable decrease in mature B cells, without apparent effect on B cell genesis. Consistently, BAFF overexpression results in an expanded B cell compartment and autoimmunity in mice. Elevated amounts of BAFF can be found in the serum of patients suffering from autoimmune diseases. The BAFF system is a promising target for the treatment of autoimmune diseases.
Animals↗