Signaling Networks that Regulate the Innate Immune System
Figure 1. Early events in the MyD88 signalling network
The MyD88 signaling network
We have been dissecting the MyD88-dependent signalling network (Fig 1), which is activated by the interaction of bacterial or viral products with Toll-Like receptors (TLR) and inflammatory cytokines of the Interleukin-1 (IL-1) family with their receptors. We recently made the surprising finding that hybrid ubiquitin chains, comprising both Lys63-linked and Met1-linked (or linear) ubiquitin linkages, are formed when the MyD88 and three other innate immune signalling networks are activated. These hybrid ubiquitin chains have critical roles in controlling the recruitment and activation of TAK1 and the canonical IKK complexes [1, 2], which are the “master” kinases of innate immune signalling pathways. We discovered that the formation of Met1-linked ubiquitin chains and their binding to the NEMO component of the IKK complex are required for TAK1 to initiate the activation of IKKβ, in a process completed by IKKβ itself  (Fig 1). We have also found that IKKβ activates Interferon-Regulatory Factor 5 (IRF5) as well as NF-kB . Thus, IKKβ activates the two master transcription factors that control the production of pro-inflammatory cytokines, IRF5 and NF-κB (Fig 1).
We noticed that the interaction of Lys63/Met1-ubiquitin hybrids with ABIN1 restrains the activation of the MyD88 signaling network and prevents the development of lupus nephritis, a severe autoimmune disease . The mice we generated in which ABIN1 is replaced by a mutant unable to bind to the Lys63/Met1-ubiquitin hybrids develop lupus nephritis spontaneously. However, excitingly, we have found that the disease can be completely prevented by crossing to mice in which the IRAK1 or IRAK4 protein kinases have been replaced by catalytically inactive mutants. These studies have validated IRAK1 and IRAK4 as potential targets for the development of drugs to treat lupus . This is important because, in recent years, human polymorphisms in ABIN1 have been shown to predispose to lupus and other autoimmune diseases in eight different human populations. We are also beginning to make progress in understanding the molecular mechanism by which ABIN1 protects against autoimmune disease.
The MyD88 signaling network not only produces pro-inflammatory cytokines to fight infection, but also anti-inflammatory molecules, such as IL-10, which have important roles in terminating inflammation once it has done its job. We discovered that members of the SIK family of protein kinases (SIK1, SIK2 and SIk3) inactivate CRTC3, a co-activator of the transcription factor CREB that is a key driver of IL-10 gene transcription. We identified the first potent inhibitors of the SIKs and found that they transform macrophages from an inflammatory M1 phenotype to the anti-inflammatory M2b phenotype thought to be critical for the resolution of inflammation  (Fig 1). We have generated knock-in mice in which the three SIKs have been replaced by kinase-inactive mutants and crossed them with one another to produce double knock-in mice. We have used macrophages from these mice to provide genetic validation of our earlier results (Darling et al, submitted) and are now investigating whether macrophage driven inflammatory and autoimmune diseases are prevented in SIK knock-in mice to evaluate the therapeutic potential of drugs that inhibit the SIKs.
Over the past year we have made several unexpected findings, which challenge widely held views about how the early signalling events of the MyD88 pathway are initiated. These findings have opened up several exciting new projects in my lab.
The TRIF signalling pathway.
We have become interested in unravelling the early events in the TLR3 signalling network, which is activated by viral double-stranded RNA and signals not through MyD88, but via the distinct adaptor protein TRIF. The activation of this pathway is critical for protection against Herpes Simplex Virus Encephalitis, a devastating disease of the Central Nervous System in young children who have inactivating mutations in proteins that participate in this signalling network. We are particularly interested in discovering how this pathway triggers the activation of the protein kinase TBK1, which is required to activate the transcription factor IRF3 and hence the production of interferon β. We have discovered that TBK1 is activated by a novel mechanism, which we are now trying to elucidate
Relevant recent lab publications cited – members of the Cohen lab are highlighted in boldface type.
Sir Philip Cohen
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