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Caspase-8 scaffolding function and MLKL regulate NLRP3 inflammasome activation downstream of TLR3

Seokwon Kang1, Teresa Fernandes-Alnemri1, Corey Rogers1

  • 1Department of Biochemistry and Molecular Biology, Kimmel Cancer Center, Thomas Jefferson University, Philadelphia, Pennsylvania 19107, USA.

Nature Communications
|June 25, 2015
PubMed

Insights

Toll-like receptor 3 (TLR3) binding to double-stranded RNA (dsRNA) activates the NLRP3 inflammasome through TRIF/RIPK1/FADD-dependent pathways. These pathways involve scaffolding functions of caspase-8 and RIPK1, with RIPK3 and MLKL mediating signal 1 and 2 in the late pathway.

Area of Science:

  • Immunology
  • Molecular Biology
  • Cellular Signaling

Background:

  • Toll-like receptor 2 (TLR2) activates the NLRP3 inflammasome via a MyD88-IRAK1 pathway, requiring priming (signal 1) and potassium depletion (signal 2).
  • The role of Toll-like receptor 3 (TLR3) in inflammasome activation and its downstream signaling pathways remain incompletely understood.

Purpose of the Study:

  • To elucidate the molecular mechanisms by which TLR3 binding to double-stranded RNA (dsRNA) induces NLRP3 inflammasome activation.
  • To differentiate between early and late signaling events and identify key molecular players in TLR3-mediated inflammasome activation.

Main Methods:

  • Utilized macrophages from wildtype and knockout mice lacking key signaling molecules.
  • Investigated the roles of TRIF, RIPK1, RIPK3, MLKL, FADD, and caspase-8 in dsRNA-induced inflammasome activation.
  • Analyzed post-translational modifications and protein-protein interactions.

Main Results:

  • TLR3 activation by dsRNA triggers NLRP3 inflammasome assembly through intermediate and late TRIF/RIPK1/FADD-dependent pathways.
  • Both pathways require the scaffolding function of caspase-8 and RIPK1, but not their catalytic activity.
  • The late pathway additionally requires kinase-active RIPK3 and MLKL for both signal 1 and signal 2, facilitating inflammasome assembly.
  • Cytoplasmic dsRNA activates NLRP3 independently of TRIF, RIPK1, RIPK3, or DRP1 in wildtype macrophages, but requires FADD/caspase-8 to overcome RIPK3 inhibition.

Conclusions:

  • TLR3-mediated dsRNA recognition activates the NLRP3 inflammasome via distinct intermediate and late signaling cascades.
  • These pathways highlight the crucial scaffolding roles of FADD, caspase-8, and RIPK1, and the kinase-dependent functions of RIPK3 and MLKL in inflammasome activation.
  • The findings reveal novel mechanisms of NLRP3 inflammasome regulation by dsRNA, offering insights into innate immune responses.

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