TLR4

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Toll-like receptor 4

PDB rendering based on 2z64.
Available structures
PDB Ortholog search: PDBe, RCSB
Identifiers
Symbols TLR4 ; ARMD10; CD284; TLR-4; TOLL
External IDs OMIM603030 MGI96824 HomoloGene41317 ChEMBL: 5255 GeneCards: TLR4 Gene
RNA expression pattern
PBB GE TLR4 221060 s at tn.png
More reference expression data
Orthologs
Species Human Mouse
Entrez 7099 21898
Ensembl ENSG00000136869 ENSMUSG00000039005
UniProt O00206 Q9QUK6
RefSeq (mRNA) NM_003266 NM_021297
RefSeq (protein) NP_003257 NP_067272
Location (UCSC) Chr 9:
120.47 – 120.48 Mb
Chr 4:
66.83 – 66.93 Mb
PubMed search [1] [2]

Toll-like receptor 4 is a protein that in humans is encoded by the TLR4 gene.[1][2] TLR 4 is a toll-like receptor. It detects lipopolysaccharide from Gram-negative bacteria and is thus important in the activation of the innate immune system. TLR 4 has also been designated as CD284 (cluster of differentiation 284). The molecular weight of TLR 4 is approximately 95 kDa.

Function[edit]

The protein encoded by this gene is a member of the Toll-like receptor (TLR) family, which plays a fundamental role in pathogen recognition and activation of innate immunity. TLRs are highly conserved from Drosophila to humans and share structural and functional similarities. They recognize pathogen-associated molecular patterns (PAMPs) that are expressed on infectious agents, and mediate the production of cytokines necessary for the development of effective immunity.

The various TLRs exhibit different patterns of expression. This receptor is most abundantly expressed in placenta, and in myelomonocytic subpopulation of the leukocytes.

It cooperates with LY96 and CD14 to mediate in signal transduction events induced by lipopolysaccharide (LPS)[3] found in most gram-negative bacteria. Mutations in this gene have been associated with differences in LPS responsiveness.

Several transcript variants of this gene have been found, but the protein-coding potential of most of them is uncertain.[4]

Signaling pathway of toll-like receptors. Dashed grey lines represent unknown associations

Interactions[edit]

TLR 4 has been shown to interact with:

Intracellular trafficking of TLR4 is dependent on the GTPase Rab-11a, and knock down of Rab-11a results in hampered TLR4 recruitment to E. coli-containing phagosomes and subsequent reduced signal transduction through the MyD88-independent pathway.[12]

Clinical significance[edit]

A 2005 study [3] suggested a link between the TLR 4 receptor and binge drinking; when researchers manipulated the genes responsible for the expression of TLR 4 and GABA receptors in rodents that had been bred and trained to drink excessively, the animals showed a "profound reduction" in drinking behaviours. Additionally, it has been shown that ethanol, even in the absence of LPS, can activate TLR4 signaling pathways.[13]

Various single nucleotide polymorphisms (SNPs) of the TLR4 in humans have been identified [14]and for some of them an association with increased susceptibility to Gram-negative bacterial infections [15]or faster progression and a more severe course of sepsis in critically ill patients was reported. [16]

A 2014 study found high levels of TLR4 molecules and M2 tumor-associated macrophages and increased susceptibility to cancer growth in mice deprived of sleep. Mice genetically modified so that they could not produce TLR4 molecules showed normal cancer growth.[17]

Drugs targeting TLR4[edit]

Toll-like receptor 4 has been shown to be important for the long-term side-effects of opioid analgesic drugs. Various μ-opioid receptor ligands have been tested and found to also possess action as agonists or antagonists of TLR4, with opioid agonists such as morphine being TLR4 agonists, while opioid antagonists such as naloxone were found to be TLR4 antagonists. Activation of TLR4 leads to downstream release of inflammatory modulators including TNF-α and Interleukin-1, and constant low-level release of these modulators is thought to reduce the efficacy of opioid drug treatment with time, and be involved in both the development of tolerance to opioid analgesic drugs,[18][19] and in the emergence of side-effects such as hyperalgesia and allodynia that can become a problem following extended use of opioid drugs.[20][21] Drugs that block the action of TNF-α or IL-1β have been shown to increase the analgesic effects of opioids and reduce the development of tolerance and other side-effects,[22][23] and this has also been demonstrated with drugs that block TLR4 itself. Interestingly the response of TLR4 to opioid drugs has been found to be enantiomer-independent, so the "unnatural" enantiomers of opioid drugs such as morphine and naloxone, which lack affinity for opioid receptors, still produce the same activity at TLR4 as their "normal" enantiomers.[24][25] This means that the unnatural enantiomers of opioid antagonists, such as (+)-naloxone, can be used to block the TLR4 activity of opioid analgesic drugs, while leaving the μ-opioid receptor mediated analgesic activity unaffected.[26])[25][27] This may also be the mechanism behind the beneficial effect of ultra-low dose naltrexone on opioid analgesia.[28]

Morphine causes inflammation by binding to the protein lymphocyte antigen 96, which, in turn, causes the protein to bind to Toll-like receptor 4 (TLR4).[29] The morphine-induced TLR4 activation attenuates pain suppression by opioid and enhances the development of opioid tolerance and addiction, drug abuse, and other negative side effects such as respiratory depression. Drug candidates that target TLR4 may improve opioid-based pain management therapies.[30]

Agonists[edit]

Antagonists[edit]

References[edit]

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Further reading[edit]

External links[edit]