Toll-Like Receptors Flashcards
Loss of TLR4 from mice protects the mice from endotoxic shock
TRUE OR FALSE
TRUE
Toll-Like Receptors (TLRs)
- Innate immune cells detect pathogens via “ pathogen associates molecular patterns” (PAMPs)
- Molecules or structures that are specific for microbes and viruses and not visible on host cells.
- PAMPs are recognized by “pattern recognition receptors” (PRR):
Unlike T cell or B cell receptors PRRs are invariant germline encoded receptors.
Expressed on innate immune cells and in some cases T cells, B cells and endothelial cells.
Several classes of PRR exist:
- Toll like receptors
- Nod like receptors
- C- type lectin receptors
- DNA receptors
- RNA receptors (RIG-I, Mda5)
TLR functions:
Pathogen killing:
- Phagocytosis
- Production of oxygen reactive species
Recruitment of immune cells to the site of infection and activation of other immune cells:
- Production of prostaglandins
- Production of chemokines
- Production of pro-inflammatory cytokines
TLRs in disease:
Infection – TLRs play important roles in activating the host response to pathogens
Cancer – TLR activation occurs during the killing of cancer cells by immune cells; this process is subverted by cancer cells during tumor development
Autoimmune disorders – Aberrant activation of TLRs can contribute to the development of autoimmune disorders
Diabetes Diabetes – Associated with chronic inflammation; TLR driven activation of macrophages present in the adipose tissue have been suggested to be responsible for this
Cardiovascular disease – Increased TLR activation contributes to the development of foam cells (specialized macrophages that are present in atherosclerotic plaques)
TLR structure:
Part of the IL-1 receptor family
The number of TLR genes is species dependent: 10 in human, 12 in mice
Purple sea urchin has 253 TLRs
Different Toll like receptors are found if different locations.
Different TLRs recognize different types of PAMPs
2 Domains:
Leucine Rich Repeat Domain (Outside Cell) - Responsible for ligand binding
TIR Domain (Inside Cell) - Activation of Signalling
- Toll/interleukin-1 receptor homology domain
- Found in both TLRs and IL-1 receptors
- Also present in TLR/IL-1 adaptor proteins
- Mediates the interaction between TLRs and adaptor proteins; critical to initiate downstream signaling
Interleukin 1beta function
Interleukin 1s are a family of related cytokines
Interleukin 1beta is the best understood.
- Activates gene expression
- Stimulates endothelial cells to express adhesion molecules and chemokines such as MCP1
- Stimulates cytokine production
- Promotes Th17 cell polarization
- IL-1 injection promotes fever, IL-6 production, and increase neutrophil numbers and activates the acute phase response
TLR Signalling:
TLRs activate signaling via either Myd88 or Trif dependent signaling
All TLRs except TLR3 couple via Myd88
- ForTLR2 and TLR4 Myd88 recruitment is promoted a 2nd adaptor, Mal
TLR3 and TLR4 can couple via Trif
- Recruitment of Trif to TLR4 is mediated by Tram
TLR adaptor proteins:
- Myd88
- Mal
- Trif
- Tram
- SARM
Myd88 dependent signaling:
IRAK4 is recruited to Myd88 via interactions between the death domain of Myd88 and IRAK4
IRAK4 recruits and phosphorylates IRAK1 and IRAK2; This activates the kinase activity of IRAK1. IRAK2 lacks key catalytic residues in the kinase active site and is inactive. Kinase dead mutants of IRAK1 can mediate signaling, suggestion the major role of IRAK1 and 2 is as scaffold proteins.
IRAK4 is essential for Myd88 dependent signaling
IRAK1 and 2 promote the formation of a ubiquitin signaling complex
TRUE OR FALSE
TRUE
IRAK1/2 interact with Traf6
This results in the addition of Lys63 polyubiquitin chains to both Traf6 and IRAK1.
While Traf6 has E3 ligase activity, the physiological E3 ligase in TLR signaling in not clear.
Lys 63 chains are added by Ubc13/UVE1A
Linear polyubiquitin are the added by the Lubac complex
Ub binding domains in TLR signalling:
Key ubiquitin binding domains:
NFZ – binds Lys63 polyubiquitin chains – present in Tab2/3
UBAN – binds linear polyubiquitin chains – present in nemo
Activation of Tak1 in TLR signaling:
Tak1 complexes consist of the kinase Tak1 and the accessory subunits Tab1, Tab2 and Tab3.
Tak1 is in the MAPK Kinase Kinase family
Tab1 pseudophosphatase and is involved in regulating Tak1 activity in the cell.
Tab2 and Tab3 have an C-terminal NFZ domains that bind specifically to Lys63 polyubiquitin
The IKK complex is recruits to linear polyubiquitin chains via its nemo subunit
This allows activation of IKKb by Tak1
NFkappaB activation:
The NFkB pathway is made up of the NFkB transcription factor, IkB inhibitory proteins and IKappaB Kinases (IKK)s
Activated downstream of multiple stimuli not just TLR signaling
Multiple activation mechanisms including the “canonical” activation pathway that is involved in TLR signaling
Before activation NFkB dimers are bound to IkB which localizes them to the cytoplasm
On activation the IKK complex phosphorylates IkB
This promotes IkB ubiquitination with K48 linked chains and its degradation via the proteasome
The released NFkB dimer can the translocate to the nucleus
IKKg / nemo is able to bind to polyubiquitin chains via a UBAN domain
This has a 100 fold selectivity for linear ubiquitin chains over K63 linked chains
Tak1 activation ERK1/2 occurs via IKK:
Before activation Tpl2 is held in an inactive complex made up of Tpl2, the p105 NFkB subunit and Abin2
Abin2 recruits the complex to Lys63 polyubquitin chains via its Ub binding domain
This allows phosphorylation of p105 by IKKb, which results in the degradation of both p105 and Abin2.
This releases active Tpl2 which the activated the ERK1/2 cascade
Trif dependent signaling
Both Myd88 and Trif can activate the same signaling pathways
For TLR4 deletion of both Myd88 and Trif is required to completely block TLR4 mediated MAPK and NFkB activation
Trif can activate Tbk1 via Traf3
Tbk1 activates the transcription factor IRF3
IRF3 stimulates the production of interferon b (IFNb)
Once secreted IFNb can restimulate cells to induce the transcription of type interferon genes
- Induces an anti-viral sate in cells
