Generation and Application of Bioluminescent CD95 Ligand Fusion Proteins

TRAF2 and RIPK1 redundantly mediate classical NFκB signaling by TNFR1 and CD95-type death receptors

This study suggests a modified model of TNFR1-induced complex I-mediated NFκB signaling. Evaluation of a panel of five tumor cell lines (HCT116-PIK3CAmut, SK-MEL-23, HeLa-RIPK3, HT29, D10) with TRAF2 knockout revealed in two cell lines (HT29, HeLa-RIPK3) a sensitizing effect for death receptor-induced necroptosis and in one cell line (D10) a mild sensitization for TNFR1-induced apoptosis. TRAF2 deficiency inhibited death receptor-induced classical NFκB-mediated production of IL-8 only in a subset of cell lines and only partly. TRAF5, furthermore, failed to improve DR-induced NFκB signaling in HCT116-PIK3CAmut and HCT116-PIK3CAmut-TRAF2KO cells. These findings argue for a non-obligatory role of TRAF2 in death receptor-induced classical NFκB signaling. Similar as in TRAF2-deficient cells, TNF- and CD95L-induced NFκB signaling was found to be only poorly affected in RIPK1KO cells and in cells treated with the RIPK1-specific PROTAC LD4172. Intriguingly, however, death receptor-induced NFκB signaling was completely inhibited in HCT116-PIK3CAmut cells double deficient for TRAF2 and RIPK1 and in TRAF2-deficient cells treated with LD4172. Moreover, with exception of recruitment of TRADD, acting upstream to TRAF2 and parallel to RIPK1, TNFR1 signaling complex formation was abrogated in TRAF2-RIPK1 DKO cells. Based on our findings, two distinguishable types of TNFR1-interacting complexes promote TNF-induced NFκB signaling: First, a TRADD-TRAF2/cIAP utilizing complex Ia which becomes evident in RIPK1-deficient cells. Second, a non-modified RIPK1 utilizing complex Ib which acts in TRADD- or TRAF2-deficient cells. Complex Ia and Ib may furthermore interact and cooperate to ubiquitinate RIPK1 resulting in a modified complex Ia/b preventing complex Ia and Ib to convert to the established TNFR1-induced cytotoxic complexes IIa and IIb.

A systems-biology model of the tumor necrosis factor (TNF) interactions with TNF receptor 1 and 2

MOTIVATION

Clustering enables TNF receptors to stimulate intracellular signaling. The differential soluble ligand-induced clustering behavior of TNF receptor 1 (TNFR1) and TNFR2 was modeled. A structured, rule-based model implemented ligand-independent pre-ligand binding assembly domain (PLAD)-mediated homotypic low affinity interactions of unliganded and liganded TNF receptors.

RESULTS

Soluble TNF initiates TNFR1 signaling but not TNFR2 signaling despite receptor binding unless it is secondarily oligomerized. We consider high affinity binding of TNF to signaling-incompetent pre-assembled dimeric TNFR1 and TNFR2 molecules and secondary clustering of liganded dimers to signaling competent ligand-receptor clusters. Published receptor numbers, affinities and measured different activities of clustered receptors validated model simulations for a large range of receptor and ligand concentrations. Different PLAD-PLAD affinities and different activities of receptor clusters explain the observed differences in the TNF receptor stimulating activities of soluble TNF.

AVAILABILITY AND IMPLEMENTATION

All scripts and data are in manuscript and supplement at Bioinformatics online.

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

Analysis of Ligand-Receptor Interactions Using Bioluminescent TNF Superfamily (TNFSF) Ligand Fusion Proteins

Quantitative analysis of the binding of tumor necrosis factor (TNF) superfamily ligands (TNFLs) to TNF receptor superfamily receptors (TNFRs) is of crucial relevance for the understanding of the mechanisms of TNFR activation. Ligand binding studies are also a basic method required for the development and characterization of agonists and antagonists of TNFRs. TNFL-induced formation of fully active TNFR signaling complexes is a complex process. It involves not only reorganization of monomeric and inactive pre-assembled TNFR complexes into trimeric liganded TNFR complexes but also the secondary interaction of the latter. Moreover, various factors, e.g., TNFR modification, special membrane domains, or accessory proteins, may affect TNFL-TNFR interactions in a TNFR type-specific manner. Widely used cell-free methods for the analysis of protein-protein interactions are thus of limited value for the analysis of TNFL-TNFR interactions and makes therefore in this case cellular binding studies to the method of choice. We and others observed that the genetic fusion of monomeric protein domains to the N-terminus of soluble TNFLs has typically no effect on activity and TNFR binding. We exploited this to generate bioluminescent TNFL fusion proteins which allow simple, sensitive, and highly reproducible cellular binding studies for the investigation of TNFL-TNFR interactions. Here, we report detailed protocols for the production of TNFL fusion proteins with the luciferase of Gaussia princeps and the use of these fusion proteins in various types of cellular binding studies.

TRAF2 Controls Death Receptor-Induced Caspase-8 Processing and Facilitates Proinflammatory Signaling

Tumor necrosis factor (TNF) receptor associated factor-2 (TRAF2) knockout (KO) cells were generated to investigate the role of TRAF2 in signaling by TNFR1 and the CD95-type death receptors (DRs) TRAILR1/2 and CD95. To prevent negative selection effects arising from the increased cell death sensitivity of TRAF2-deficient cells, cell lines were used for the generation of the TRAF2 KO variants that were protected from DR-induced apoptosis downstream of caspase-8 activation. As already described in the literature, TRAF2 KO cells displayed enhanced constitutive alternative NFκB signaling and reduced TNFR1-induced activation of the classical NFκB pathway. There was furthermore a significant but only partial reduction in CD95-type DR-induced upregulation of the proinflammatory NFκB-regulated cytokine interleukin-8 (IL8), which could be reversed by reexpression of TRAF2. In contrast, expression of the TRAF2-related TRAF1 protein failed to functionally restore TRAF2 deficiency. TRAF2 deficiency resulted furthermore in enhanced procaspase-8 processing by DRs, but this surprisingly came along with a reduction in net caspase-8 activity. In sum, our data argue for (i) a non-obligate promoting function of TRAF2 in proinflammatory DR signaling and (ii) a yet unrecognized stabilizing effect of TRAF2 on caspase-8 activity.