Toll-Like Receptors (TLR)

Toll-Like Receptor Signaling• Toll receptor initially discovered in Drosophila as

important receptor in dorso-ventral embryonic pattern– Toll mutants refers to the fact that these mutants could not

establish a proper dorsal-ventral axis– Toll in German means ‘great’, apparently this was one of the

words describing the scientists’ enthusiasm after observing the mutant flies

• Hoffman and colleagues showed that Toll-mutant flies susceptible to fungal infections

• Mammalian homologues discovered and designated as Toll-Like Receptors (TLR)

• TLRs recognize specific patterns in pathogens which are not observed in mammals

Toll-Like Receptors

Toll-Like Receptors

http://upload.wikimedia.org/wikipedia/commons/3/3c/Kdo2-lipidA.png

• Large molecules found in outer membrane of Gram- negative bacteria

• Comprised of a lipid and saccharide component• Highly immunogenic• Recognized by TLR4• Can cause septic shock and lead to death• Often referred to as Endotoxin since it is not

secreted but is a byproduct of bacterial lysis• Great variability among different bacterial strains

LipoPolySaccharides (LPS)

• TLR3, TLR7 and TLR8 detect viral nucleic acids

• Found in intracellular membranes since viral nucleic acids are endogenously generated

• Note the TIR domains of these TLRs face the cytosol of the cell

• Poly I:C is an agonist for TLR3 that mimics binding of dsRNA to TLR3

Viral Nucleic Acids

Cytoplasmic tails of TLRs show similarities to IL-1 receptor (TIR)

Common adaptor to TLRs is MyD88

Crucial proline residue in all TLR TIR domains, except TLR3

If substituted with histidine, no signaling occurs

All TLRs likely have a MyD88 pathway (TLR3 is an exception)

TLR4 has a MyD88 independent pathway as well

CytoplasmicTIR domain

TLRs and TIR Domain

MyD88 knockout mice have no response to LPS

MyD88 is essential to all inflammatory signaling pathways

MyD88s, a splice variant of MyD88 down regulates the inflammatory response

MyD88 interacts with TIR domain of TLR and recruits IRAK-4, IRAK-1 and TRAF-6

MyD88 Adaptor

• 4 IRAKs known, IRAK1, IRAK-2, IRAK-M and IRAK-4

• IRAK are serine/threonine kinases• IRAK-4 phosphorylates IRAK-1• IRAK-M plays an inhibitory role in TLR

signaling • IRAK-4 phosphorylates IRAK-1• TRAF6 is a member of the TNF receptor

associated factor (TRAF) family• TRAF6 interacts with IRAK-1 and gets

activated• Release of TRAF6/IRAK-1 ensues

IRAK and TRAF6

• TRAF6/IRAK-1 complex associates with 3 proteins

– TAK1 (TGF-B activated kinase)

– TAB1 (TAK1 binding proteins)

– TAB2 (TAK1 binding proteins)

• Large complex associates with membrane

• Eventually IRAK-1 stays in membrane while TRAF6/TAK1/TAB1/TAB2 move to cytosol

• E2 Ligases such as Ubc13 and Uev1A join further enlarging complex

IRAK and TRAF6 Release

TAK-1 Activation• The enlarged complex that

includes

– TRAF6, TAK1, TAB1, TAB2, Ubc13, Uev1A activate TAK1

• Activated TAK1 phosphorylates IKK complex

• Activated TAK1 can also phosphorylate MAP Kinases

• IKK complex consists of IKK, and /NEMO

• IB phosphorylation results in NF- B translocation to nucleus

MyD88 Independent Pathway MyD88 Knock out mice do not produce inflammatory cytokines such as TNF-

However with TLR4 stimulation NF-B and JNK delayed activity occurs

This strongly suggests the existence of 2 pathways in TLR signaling

a MyD88 dependent pathway (early)

a MyD88 independent pathway (late)

TLR3 stimulation also exchibits a MyD88 independent pathway

TLR4

• Figures obtained from:

• Takeda and Akira, 2004

• Wikipedia

• Kuby, 6th edition

Acknowledgements