in Signal Transduction such as TNF-induced Necrosis
Amyloids are fibrous protein aggregates composed of cross-β structures and associated with many neurodegenerative and infective prion diseases. Amyloids can also perform normal cellular functions, such as host interaction, hazard protection, and memory storage. However, this aspect of the function of amyloids is less defined, especially in mammals. In a recent study, we showed that a signaling complex known as the necrosome containing kinases RIP1 and RIP3 is a functional, hetero-oligomeric amyloidal complex (Cell, 2012). Regulated assembly of
the necrosome with a feed forward, gain-of-function mechanism between kinase activation and RIP1/RIP3 complex formation is critical for TNF-induced programmed necrosis. The discovery of cross-β amyloid structures in proteins complexation and signal transduction provides new insights into both the amyloid field and the signaling field. Because proteins with similar amyloid forming motifs are also present in a number of other innate immune signaling proteins, we propose the high-order oligomeric scaffold of amyloids may be used in other biological processes such as NF-κB activation.
Main Collaborators: Francis Chan, University of Massachusetts, Ann McDermott, Columbia University, Tom Walz, Harvard Medical School
in TLR/IL-1R Signal Transduction
In TLR and IL-1R signaling, ligand binding induces both dimerization and higher-order oligomerization of the receptors. MyD88 is recruited to the receptors through homotypic TIR-TIR interactions and facilitates Myddosome assembly with IRAK4 and IRAK1 or IRAK2 through homotypic high-order death domain interactions (Nature, 2010), which leads to phosphorylation and activation of IRAKs. Activated IRAK1 or IRAK2 interacts with TRAF6 and promotes the formation of a proposed extended two dimensional lattice, stimulating the K63-linked ubiquitin ligase activity of TRAF6 (Nat Struct Mol Biol, 2009). Polyubiquitin is recognized by TAB2 or TAB3 and NEMO to enable the recruitment and activation of the kinases TAK1 and IKK (Nature, 2011).
Activated IKK phosphorylates IκB, promoting its degradation and enabling the nuclear translocation of NF-κB. Therefore, as illustrated here, oligomerization, either defined in stoichiometry or open-ended, appears to occur at all different levels of the signaling cascade, from receptors to adaptors, from ubiquitin ligases to kinases, and eventually to activation of transcriptional responses (Sci Signal, 2012).
Main collaborators: Bryant Darnay, MD Anderson Cancer Center, Michael Lenardo, National Institute of Health, Michael Karin, University of California at San Diego
Intracellular Pathogen Sensing and Signaling
Innate immunity is an evolutionarily conserved mechanism that provides the first line of defense against microbial pathogens such as viruses and bacteria. While extracellular pathogens are recognized by transmembrane Toll-like receptors, cytosolic foreign materials are sensed by several families of germline-encoded receptors such as Nod-like receptors, dsRNA helicases, STING, and p200 family proteins such as AIM2, IFI16 and p202 for interferon (IFN) induction, NF-κB activation and inflammasome formation. We are interested in the structural biology of many of these aspects of self-defense. One of our published results depicts the mode of recognition of bacterial cyclic-di-GMP by STING and the subsequent mechanism of TBK1 activation for IFN induction (Mol Cell, 2012).
Main Collaborators: Zhijian (James) Chen, University of Texas Southwestern Medical Center, Katryn Stacey, University of Queensland, Tom Walz, Harvard Medical School
The immunological pathways that we are studying are highly related to many human diseases, in particular, inflammatory diseases and cancer. We have begun to utilize our expertise in biochemistry and structural biology to pursue discovery of small molecule inhibitors against important molecular disease targets in these pathways. As a first step, we recently published our efforts in identifying small molecule inhibitors against the MALT1 paracaspase important for survival of activated B cell-like diffuse large B cell lymphoma (ABC-DLBCL), a chemoresistant form of DLBCL (Cancer Cell, 2012). We developed a MALT1 activity assay and identified chemically diverse MALT1 inhibitors. A selected lead compound in the chemical class of triazoles, MI-2, featured direct binding to MALT1 and suppression of its protease function. MI-2 concentrated within human ABC-DLBCL cells and irreversibly inhibited cleavage of MALT1 substrates. This was accompanied by NF-κB reporter activity suppression, c-REL nuclear localization inhibition, and NF-κB target gene downregulation. Most notably, MI-2 was nontoxic to mice, and displayed selective activity against ABC-DLBCL cell lines in vitro and xenotransplanted ABC-DLBCL tumors in vivo. The compound was also effective against primary human non-germinal center B cell-like DLBCLs ex vivo.
Main Collaborators: Ari Melnick, Weill Cornell Medical College