Oral Presentation Lorne Infection and Immunity 2018

Understanding TLR-induced Myddosome formation and signalling (#19)

Dominic De Nardo 1 2 , Kathrine R Balka 1 , Yamel Cardona Gloria 3 , Vikram R Rao 4 , Eicke Latz 5 6 7 8 , Seth L Masters 1 2 8
  1. Inflammation Division, The Walter and Eliza Hall Institute of Medical Research, Parkville, VIC, Australia
  2. Department of Medical Biology, The University of Melbourne, Parkville , VIC, Australia
  3. Interfaculty Institute for Cell Biology, Department of Immunology, University of Tübingen , Tübingen, Germany
  4. Inflammation and Immunology, Pfizer Inc., Cambridge, Massachusetts, USA
  5. Institute of Innate Immunity, University Hospital, University of Bonn, Bonn, Germany
  6. Department of Infectious Diseases and Immunology, University of Massachusetts Medical School, Worcester, Massachusetts , USA
  7. German Center for Neurodegenerative Diseases, Bonn , Germany
  8. Equal contribution, .

The Toll-like receptors (TLRs) are a family of pattern recognition receptors that are expressed either on the cell surface or within intracellular endosomal membranes of innate immune cells, such as macrophages. TLRs sense highly conserved microbial molecular patterns, representing danger signals. The recognition of these danger signals is mediated by the leucine-rich repeat ectodomain of the TLRs, which subsequently initiates signal transduction pathways by virtue of their intracellular Toll/Interleukin-1-receptor (TIR) domains via the recruitment of TIR-containing adapter proteins, e.g. MyD88. The MyD88-dependent pathway leads to the activation of specific transcription factors (e.g. NFkB, IRF5, AP-1), evoking production of potent inflammatory cytokines and chemokines to elicit an inflammatory response to infection. Signal transduction downstream of TLRs is thought to be initiated through formation of a large oligomeric signalling complex containing molecules of MyD88 and members of the IL-1R-associated kinase (IRAK) family, termed the “Myddosome”. Assembly of the Myddosome is mediated via the respective N-terminal death domains (DDs), promoting the interaction and activation of IRAK4 and IRAK1 or IRAK2. Activation of IRAK4 is essential for subsequent activation of IRAK1, as well as later activation of IRAK2. IRAK4 is the central kinase mediating induction of MyD88-dependent signalling pathways as both mice and humans with IRAK4 deficiency display greatly impaired responses to microbial infection.

 

We have found that IRAK4 can play two essential roles within the Myddosome that are critical for MyD88-dependent TLR cytokine responses. Taking both genetic and pharmacological approaches we show that the kinase activity of IRAK4 is dispensable for TLR-mediated NFkB signalling but essential for production of inflammatory cytokines in mouse macrophages. We have found that loss of IRAK4 kinase activity, whilst inhibiting cytokine production, stabilises Myddosome assembly leading to stronger interactions of MyD88 and IRAK proteins. This suggests a model in which IRAK4 plays a dual role; as both a scaffold protein, to facilitate NFkB activation; and a kinase, required for effective TLR-induced cytokine production.