Cellular & Molecular Immunology (2018) 15, 8–11 & 2018 CSI and USTC All rights reserved 2042-0226/18 $32.00 www.nature.com/cmi RESEARCH HIGHLIGHT Ways and waves of MALT1 paracaspase activation Laura Israël and Frédéric Bornancin Cellular & Molecular Immunology (2018) 15, 8–11; doi:10.1038/cmi.2017.77; published online 7 August 2017 ucosa-associated lymphoid tissue invariant natural killer T (iNKT) cells catalytic function of MALT1 was finally Mlymphoma translocation protein 1 activates caspase-1 by recruiting MALT1 elucidated.6 MALT1 was identified as a (MALT1), also known as paracaspase 1, in a TRAF6-dependent manner that Cys-dependent, Arg-specificprotease, is a ubiquitous protease with homology might bypass CBM formation.2 In the leading to the discovery of several to caspases.1 The function of MALT1 is second study, Thome and colleagues MALT1 proteolytic substrates.5 Further- best understood in immune cells, in show that two Kaposi’ssarcoma-more, constitutive MALT1 protease activ- particular lymphocytes, in which associated herpes virus (KSHV) proteins ity was discovered in diffuse large B-cell MALT1 controls signaling downstream promote NF-κB activation via binding to lymphomas (DLBCL) of the activated of antigen receptors, for example, MALT1 and activating its paracaspase B-cell (ABC) subtype, which are charac- nuclear-factor-kappa-B (NF-κB) and activity, likely in a CBM-independent terized by chronic stimulation of the mitogen-activated protein kinase manner.3 These findings are highlighted B-cell receptor pathway, as a result of pathways. Activation of MALT1 below and show how emerging concepts somatic mutations in some of the paracaspase activity has been shown to continue to push the current boundaries signaling components. In particular, require the assembly of a tripartite of our understanding of the function CARD11 gain-of-function mutations signaling complex termed CBM of MALT1. were characterized,7 which contributed (CARD11/BCL10/MALT1), which is MALT1 was discovered some 20 years to the establishment that CBM assembly formed following the phosphorylation ago as a proto-oncogenic translocation is a keystone for MALT1 protease activa- and conformational change of caspase product that accounted for antibiotic tion (Figure 1). recruitmentdomain11(CARD11)that resistance in MALT lymphoma patients.4 The recent work by Li and colleagues, is induced by antigen-receptor Subsequently, the CARMA protein family published in the June issue of The stimulation. This complex then recruits was identified, and MALT1 was shown to Journal of Clinical Investigation,2 showed theE3ligasetumornecrosisfactor be an essential component of so-called that OX40, a TNF superfamily receptor receptor-associated factor 6 (TRAF6), CBM complexes composed of CARMA1, (TNFRSF4), can activate the MALT1 which leads to the activation of the also known as CARD11, B-cell lym- protease in iNKT cells. Although nor- inhibitor-of-kappa-B kinase (IκK) phoma 10 (BCL10) and MALT1, which mally associated with T-cell survival, complex, an essential step in the release assemble upon antigen-receptor-driven effector differentiation and memory gen- of transcriptionally active NF-κB. Now, stimulation in lymphocytes.5 In addition eration, this study uncovered that OX40 according to recent studies, there might to CARMA1, other CARD containing signaling in iNKT cells instead leads to be alternative mechanisms for MALT1 proteins, for example, CARD9, CARMA2 caspase-1-dependent programmed cell activation that do not rely on the (CARD14) and CARMA3 (CARD10), death, also known as pyroptosis. recruitment into CBM complexes. In have been shown to build similar CBM Furthermore, in TRAF6-deficient the first study, Li and colleagues show complexes in response to various ligands iNKT cells, OX40 failed to induce that the OX40 signaling pathway in in several cell types. They are involved, caspase-1 activation, as shown by the either downstream of receptors/co-recep- absence of caspase-1 cleavage using tors displaying immune-receptor tyro- immunoblotting and flow cytometry Novartis Institutes for BioMedical Research, sine-based activation motifs in their methods. Next, using an immunopreci- Novartis Campus, Basel CH-4002, Switzerland cytoplasmic domain, through a cascade pitation approach with FACS-sorted Correspondence: Dr Frédéric Bornancin, Novartis of phosphorylation-induced recruitment iNKT cells stimulated with OX40 Institutes for BioMedical Research, Novartis Cam- fi pus, Basel CH-4002, Switzerland. of signaling molecules, or downstream of ligand in vitro, the authors identi ed E-mail: [email protected] G-protein-coupled receptors by unknown BCL10 and MALT1 as TRAF6 interac- Received: 30 June 2017; Accepted: 1 July 2017 mechanisms. In 2008, the long elusive tors. By overexpressing TRAF6, MALT1 paracaspase activation LIsraëlandFBornancin 9 receptor pathway represents an interest- ing case to investigate because it is known to recruit several TRAF isoforms via its scaffold protein, ACT1 (NF-κB- activating protein 1), including TRAF2 and TRAF6(ref. 13) (Figure 2). Another recent study by Thome and colleagues,3 published in the March issue of Leukemia,unveiledakeyrolefor MALT1 in primary effusion lymphoma (PEL), which is a subtype of non-Hodg- kin’s B-cell lymphoma caused by KSHV. Inhibiting the constitutive protease func- tion of MALT1 in PEL induced a switch from the latent to the lytic stage of viral infection, leading to PEL cell death Figure 1 The waves of MALT1 activation. MALT1 is recruited into CARD/BCL10/MALT1 in vitro and to reduced tumor size (CBM) complexes after stimulation of antigen receptors or under chronic stimulatory conditions resulting from mutations in signaling components of the pathway, for example, in vivo using a xenograft model. Remark- gain-of-function (GoF) mutations in CARD11. CBM assembly triggers MALT1 paracaspase ably, PEL cell lines do not express Bruton activity. MALT1 cleaves several substrates including BCL10. MALT1 can also auto-cleave, tyrosine kinase (BTK) and, consistently, particularly at R149,8 which is promoted by TRAF6.9 This process represents a feed-forward were not sensitive to the BTK inhibitor mechanism that releases a highly functional MALT1 species. MALT1 auto proteolysis, and ibrutinib, in contrast to HBL-1 cells, 9 cleavage of BCL10 by MALT1 may contribute to downregulation of the pathway. which are an example of an ABC- DLBCL lymphoma cell line that responds MALT1 and caspase-1 in 293T cells, therefore, classical CBM complex assem- to both BTK and MALT1 inhibition. they observed that TRAF6 fails to co- bly appears unlikely. OX40-induced Furthermore, these cell lines do not immunoprecipitate with caspase-1 in recruitment of protein kinase C together express detectable amounts of CARMA1, the absence of MALT1, suggesting with CBM molecules into a TRAF2 CARMA3 or CARD9. Together, this find- that MALT1 is a critical component for complex was proposed previously;11 ing led the authors propose that MALT1 TRAF6-mediated activation of caspase-1. however, the mechanism has remained activation in PEL likely occurs indepen- Binding of BCL10 to MALT1 also incompletely characterized. In the mean- dently of the BTK-CARMA axis. To appears to be key because deletion of time, new insights have been obtained. identify mechanisms that activate MALT1 the N-terminal region of MALT1—a First, shorter isoforms of CARD proteins in latently infected PEL cells, they region previously shown to contain key were reported to be able to interact with screened an expression library of 86 open determinants of BCL10 binding—abro- TRAF2.12 Second, the E3 ligase activity of reading frames (ORFs) encoded by the gated recruitment of caspase-1 into the TRAF6 was shown to be activated down- KSHV genome and found ORFs K13 and MALT1-TRAF6 complex. Additionally, stream of OX40.10 Third, a recent report K15, which are two known NF-κB-indu- a MALT1 protease inhibitor could indicated that TRAF6 induces self- cing proteins expressed during viral block the cleavage of caspase-1 in a cleavage of MALT1, which has an impact latency, to be the most potent hits. Both concentration-dependent manner, thus on the activation cycle of MALT1 K13 and K15 promoted MALT1 activa- suggesting the involvement of MALT1 (Figure 1). That study also raised tion in a concentration-dependent man- paracaspase activity in caspase-1 activa- the possibility that TRAF6 might be ner and induced MALT1 mono- tion. Collectively, these data suggested able to activate the MALT1 protease ubiquitination, which was previously that in iNKT cells, activation of the directly, independently of CBM complex shown to positively correlate with para- OX40 pathway recruits MALT1-BCL10 formation.9 Together, these recent caspase activity.14 Using a pull-down through TRAF6, and MALT1 then acti- insights suggest that MALT1, either approach, they provided evidence that vates caspase-1 to induce cellular pyrop- directly or after release from assembly the cytoplasmic K13 protein activates tosis. Whether cleavage of caspase-1 is into a CBM complex, or even as part of a MALT1 and consequently NF-κBvia directly performed by MALT1 or if there CBM complex—perhaps involving a binding to its protease domain, either is an intermediate regulator, however, short CARD isoform12 —might be able directly or via an additional binding remains to be clarified. to reach signaling hubs coated with partner. The transmembrane K15 protein, This study is the first report of a TNF TRAF proteins, such as those down- which does not bind MALT1, most likely superfamily receptor that can trigger stream of TNF ligand family receptors, activates it via recruitment of additional MALT1 paracaspase activity. Notably, such as OX40.10 Expanding along these signaling proteins through its cytoplasmic the cytoplasmic domain of OX40 bears lines, other pathways might be able to SH2-binding motif. How K13 and K15, TRAF2 recruitment motifs,10 and, recruit MALT1. For instance, the IL-17 respectively, activate MALT1 remains to Cellular & Molecular Immunology MALT1 paracaspase activation LIsraëlandFBornancin 10 Figure 2 Toward novel ways of MALT1 activation?. ‘The path of reference’ represents the classical pathway of MALT1 activation as depicted in Figure 1.