HOIL1 cleavage by MALT1, the knives are out

The paracaspase MALT1 functions as a bifunctional regulator of lymphocyte activation following the engagement of antigen receptors. First, MALT1 scaffolds the CARMA1-BCL10-MALT1 (CBM) signaling complex in charge of activating the NF-kB transcription factor. Second, MALT1 proteolytic activity governs NF-kB fine-tuning and the homeostasis of the immune system. MALT1 is also constitutively activated in the activated B-cell like (ABC) subset of diffuse large B-cell lymphoma (DLBCL), and the discovery that its chemical inhibition is toxic has opened new perspectives of treatment. Yet, the nature of MALT1 substrates continues to be elucidated. Herein, we review the recent identification of the linear ubiquitin assembly chain complex (LUBAC) element HOIL1 as a new substrate for MALT1 in lymphocytes and lymphoma. We discuss how this processing may affect NF-kB signaling and impact on lymphocyte homeostasis

Ubiquitinylation et déubiquitinylation dans la régulation de l activation du facteur de transcription NF-kB

L activation de la signalisation NF- B par de nombreux immunorécepteurs met en jeu un large signalosome. Afin de propager cette signalisation en réponse à différents stimuli, l oligomérisation d adaptateurs pourvus de domaines d interaction protéine-protéine est nécessaire. Alors que certains adapteurs sont communs d autres sont spécifiques à un immunorécepteur donné. Une des principales caractéristiques partagées par toutes ses protéines est leur capacité à être poly-ubiquitinylé de façon non-dégradative afin d aboutir à une activation optimale de NF- B. Ce projet avait pour objectif d identifier de nouvelles déubiquitinylases impliquées dans la signalisation NF- B. C est ainsi que nous avons identifié USP34 comme étant un régulateur négatif de la signalisation NF- B induite par le TCR dans des cellules Jurkats, une lignée de lymphocytes T immortalisés. Nos données suggèrent un modèle dans lequel USP34 permet d éviter l activation excessive de NF- B, en agissant directement ou indirectement sur les dimères NF- B/I Ba, en aval d IKK, et en modulant l affinité du facteur de transcription pour l ADN. Parallèlement, l étude du microenvironnement des membranes endocellulaires responsables du recrutement des signalosomes formés en réponse à l activation du TCR, du TNFR et du CD40 a permis l identification d une protéine - clé de la signalisation NF- B, la MTDH. Cette protéine du RE s est révélée être un relais déterminant pour l activation d IKK et donc la propagation du signal NF- B.Large signalosome assembly is a prerequisite for NF- B signaling upon engagement of various immunoreceptors. Adaptor proteins containing protein-protein interaction domains oligomerise in response to such stimuli in order to propagate signaling. Each immunoreceptor uses distinct adaptors, as well as common ones, to achieve that. The main characteristic shared by these proteins is their ability to undergo poly-ubiquitinylation in a non-degradative manner, leading to optimal NF- B activation. In this work, we aimed to identify novel deubiquitinylating enzymes that control ubiquitinylation status. That is how USP34 came up to be a negative regulator of NF- B signaling in TCR-activated Jurkat cells, a T lymphocyte cell line. Our data suggest a model whereby USP34 prevents excessive NF- B activation by acting rather late, directly or indirectly on the NF- B:I Ba dimers, downstream of IKK, altering transcription factor DNA binding affinity. In parallel, studies of the endocellular membrane microenvironment that hosts mature signalosomes in response to TCR-, TNFR- and CD40 ligation led to the identification of an ER-residing protein, Metadherin (MTDH), which seems to globally integrate signaling before forwarding it to downstream pathway components able to activate IKK.PARIS11-SCD-Bib. électronique (914719901) / SudocSudocFranceF

Triad3a induces the degradation of early necrosome to limit RipK1-dependent cytokine production and necroptosis.

Understanding the molecular signaling in programmed cell death is vital to a practical understanding of inflammation and immune cell function. Here we identify a previously unrecognized mechanism that functions to downregulate the necrosome, a central signaling complex involved in inflammation and necroptosis. We show that RipK1 associates with RipK3 in an early necrosome, independent of RipK3 phosphorylation and MLKL-induced necroptotic death. We find that formation of the early necrosome activates K48-ubiquitin-dependent proteasomal degradation of RipK1, Caspase-8, and other necrosomal proteins. Our results reveal that the E3-ubiquitin ligase Triad3a promotes this negative feedback loop independently of typical RipK1 ubiquitin editing enzymes, cIAPs, A20, or CYLD. Finally, we show that Triad3a-dependent necrosomal degradation limits necroptosis and production of inflammatory cytokines. These results reveal a new mechanism of shutting off necrosome signaling and may pave the way to new strategies for therapeutic manipulation of inflammatory responses

A cardinal role for cathepsin D in co-ordinating the host-mediated apoptosis of macrophages and killing of pneumococci

The bactericidal function of macrophages against pneumococci is enhanced by their apoptotic demise, which is controlled by the anti-apoptotic protein Mcl-1. Here, we show that lysosomal membrane permeabilization (LMP) and cytosolic translocation of activated cathepsin D occur prior to activation of a mitochondrial pathway of macrophage apoptosis. Pharmacological inhibition or knockout of cathepsin D during pneumococcal infection blocked macrophage apoptosis. As a result of cathepsin D activation, Mcl-1 interacted with its ubiquitin ligase Mule and expression declined. Inhibition of cathepsin D had no effect on early bacterial killing but inhibited the late phase of apoptosis-associated killing of pneumococci in vitro. Mice bearing a cathepsin D-/- hematopoietic system demonstrated reduced macrophage apoptosis in vivo, with decreased clearance of pneumococci and enhanced recruitment of neutrophils to control pulmonary infection. These findings establish an unexpected role for a cathepsin D-mediated lysosomal pathway of apoptosis in pulmonary host defense and underscore the importance of apoptosis-associated microbial killing to macrophage function

Infected Cell Killing by HIV-1 Protease Promotes NF-κB Dependent HIV-1 Replication

Acute HIV-1 infection of CD4 T cells often results in apoptotic death of infected cells, yet it is unclear what evolutionary advantage this offers to HIV-1. Given the independent observations that acute T cell HIV-1 infection results in (1) NF-κB activation, (2) caspase 8 dependent apoptosis, and that (3) caspase 8 directly activates NF-κB, we questioned whether these three events might be interrelated. We first show that HIV-1 infected T cell apoptosis, NF-κB activation, and caspase 8 cleavage by HIV-1 protease are coincident. Next we show that HIV-1 protease not only cleaves procaspase 8, producing Casp8p41, but also independently stimulates NF-κB activity. Finally, we demonstrate that the HIV protease cleavage of caspase 8 is necessary for optimal NF-κB activation and that the HIV-1 protease specific cleavage fragment Casp8p41 is sufficient to stimulate HIV-1 replication through NF-κB dependent HIV-LTR activation both in vitro as well as in cells from HIV infected donors. Consequently, the molecular events which promote death of HIV-1 infected T cells function dually to promote HIV-1 replication, thereby favoring the propagation and survival of HIV-1

Participation of the Cell Polarity Protein PALS1 to T-Cell Receptor-Mediated NF-κB Activation

BACKGROUND: Beside their established function in shaping cell architecture, some cell polarity proteins were proposed to participate to lymphocyte migration, homing, scanning, as well as activation following antigen receptor stimulation. Although PALS1 is a central component of the cell polarity network, its expression and function in lymphocytes remains unknown. Here we investigated whether PALS1 is present in T cells and whether it contributes to T Cell-Receptor (TCR)-mediated activation. METHODOLOGY/PRINCIPAL FINDINGS: By combining RT-PCR and immunoblot assays, we found that PALS1 is constitutively expressed in human T lymphocytes as well as in Jurkat T cells. siRNA-based knockdown of PALS1 hampered TCR-induced activation and optimal proliferation of lymphocyte. We further provide evidence that PALS1 depletion selectively hindered TCR-driven activation of the transcription factor NF-κB. CONCLUSIONS: The cell polarity protein PALS1 is expressed in T lymphocytes and participates to the optimal activation of NF-κB following TCR stimulation

CD95 co-stimulation blocks activation of naive T cells by inhibiting T cell receptor signaling

CD95 is a multifunctional receptor that induces cell death or proliferation depending on the signal, cell type, and cellular context. Here, we describe a thus far unknown function of CD95 as a silencer of T cell activation. Naive human T cells triggered by antigen-presenting cells expressing a membrane-bound form of CD95 ligand (CD95L) or stimulated by anti-CD3 and -CD28 antibodies in the presence of recombinant CD95L had reduced activation and proliferation, whereas preactivated, CD95-sensitive T cells underwent apoptosis. Triggering of CD95 during T cell priming interfered with proximal T cell receptor signaling by inhibiting the recruitment of ζ-chain–associated protein of 70 kD, phospholipase-γ, and protein kinase C-θ into lipid rafts, thereby preventing their mutual tyrosine protein phosphorylation. Subsequently, Ca2+ mobilization and nuclear translocation of transcription factors NFAT, AP1, and NF-κB were strongly reduced, leading to impaired cytokine secretion. CD95-mediated inhibition of proliferation in naive T cells could not be reverted by the addition of exogenous interleukin-2 and T cells primed by CD95 co-stimulation remained partially unresponsive upon secondary T cell stimulation. HIV infection induced CD95L expression in primary human antigeen-presenting cells, and thereby suppressed T cell activation, suggesting that CD95/CD95L-mediated silencing of T cell activation represents a novel mechanism of immune evasion

MAGUKs, scaffolding proteins at cell junctions, are substrates of different proteases during apoptosis

A major feature of apoptotic cell death is gross structural changes, one of which is the loss of cell–cell contacts. The caspases, executioners of apoptosis, were shown to cleave several proteins involved in the formation of cell junctions. The membrane-associated guanylate kinases (MAGUKs), which are typically associated with cell junctions, have a major role in the organization of protein–protein complexes at plasma membranes and are therefore potentially important caspase targets during apoptosis. We report here that MAGUKs are cleaved and/or degraded by executioner caspases, granzyme B and several cysteine cathepsins in vitro. When apoptosis was induced by UV-irradiation and staurosporine in different epithelial cell lines, caspases were found to efficiently cleave MAGUKs in these cell models, as the cleavages could be prevented by a pan-caspase inhibitor N-benzyloxycarbonyl-Val-Ala-Asp(OMe)fluoromethylketone. Using a selective lysosomal disrupting agent -leucyl--leucine methyl ester, which induces apoptosis through the lysosomal pathway, it was further shown that MAGUKs are also cleaved by the cathepsins in HaCaT and CaCo-2 cells. Immunohistological data showed rapid loss of MAGUKs at the sites of cell–cell contacts, preceding actual cell detachment, suggesting that cleavage of MAGUKs is an important step in fast and efficient cell detachment

HIV gp120 Induces, NF-κB Dependent, HIV Replication that Requires Procaspase 8

HIV envelope glycoprotein gp120 causes cellular activation resulting in anergy, apoptosis, proinflammatory cytokine production, and through an unknown mechanism, enhanced HIV replication.We describe that the signals which promote apoptosis are also responsible for the enhanced HIV replication. Specifically, we demonstrate that the caspase 8 cleavage fragment Caspase8p43, activates p50/p65 Nuclear Factor kappaB (NF-kappaB), in a manner which is inhibited by dominant negative IkappaBalpha. This caspase 8 dependent NF-kappaB activation occurs following stimulation with gp120, TNF, or CD3/CD28 crosslinking, but these treatments do not activate NF-kappaB in cells deficient in caspase 8. The Casp8p43 cleavage fragment also transactivates the HIV LTR through NF-kappaB, and the absence of caspase 8 following HIV infection greatly inhibits HIV replication.Gp120 induced caspase 8 dependent NF-kappaB activation is a novel pathway of HIV replication which increases understanding of the biology of T-cell death, as well as having implications for understanding treatment and prevention of HIV infection

A −436C>A Polymorphism in the Human FAS Gene Promoter Associated with Severe Childhood Malaria

Human genetics and immune responses are considered to critically influence the outcome of malaria infections including life-threatening syndromes caused by Plasmodium falciparum. An important role in immune regulation is assigned to the apoptosis-signaling cell surface receptor CD95 (Fas, APO-1), encoded by the gene FAS. Here, a candidate-gene association study including variant discovery at the FAS gene locus was carried out in a case-control group comprising 1,195 pediatric cases of severe falciparum malaria and 769 unaffected controls from a region highly endemic for malaria in Ghana, West Africa. We found the A allele of c.−436C>A (rs9658676) located in the promoter region of FAS to be significantly associated with protection from severe childhood malaria (odds ratio 0.71, 95% confidence interval 0.58–0.88, pempirical = 0.02) and confirmed this finding in a replication group of 1,412 additional severe malaria cases and 2,659 community controls from the same geographic area. The combined analysis resulted in an odds ratio of 0.71 (95% confidence interval 0.62–0.80, p = 1.8×10−7, n = 6035). The association applied to c.−436AA homozygotes (odds ratio 0.47, 95% confidence interval 0.36–0.60) and to a lesser extent to c.−436AC heterozygotes (odds ratio 0.73, 95% confidence interval 0.63–0.84), and also to all phenotypic subgroups studied, including severe malaria anemia, cerebral malaria, and other malaria complications. Quantitative FACS analyses assessing CD95 surface expression of peripheral blood mononuclear cells of naïve donors showed a significantly higher proportion of CD69+CD95+ cells among persons homozygous for the protective A allele compared to AC heterozygotes and CC homozygotes, indicating a functional role of the associated CD95 variant, possibly in supporting lymphocyte apoptosis