Target Analysis - Toll-like receptors
Toll-Like Receptors - Meet The Family
In 1985 the Drosophila melanogaster Toll gene was discovered. Originally identified as having a developmental role, it was later found that the protein encoded by Toll is involved in the synthesis of antimicrobial peptides, and therefore vital to the functioning of the fly's immune system. TLR1, the first reported human TLR, was discovered by Nomura et al in 1994. This was followed by the discovery by Bruce A Beutler's group that mice lacking functional TLR4, another member of the family, were unable to respond to challenge with bacterial lipopolysaccharide (LPS), suggesting a role for these receptors in 'recognition and response' to pathogenic threat. In all, ten human TLRs have been identified, each with their own ligand specificities and downstream signalling pathways, adding a further layer of complexity to the function of TLRs. Additionally, these receptors were found to exist on very specific cell types, depending on their roles in innate immunity.
TLRs are type I transmembrane receptors with a single membrane-spanning domain, as well as an extracellular ligand-binding domain that contains leucine-rich repeats, and an intracellular Toll/interleukin-1 receptor domain. Each of the human TLRs, acting either as homo- or heterodimers, recognises a specific set of pathogen-associated molecular patterns (PAMPs) essential for pathogen survival. Table 1 shows examples of PAMPs and the TLRs they activate.
TLRs are non-catalytic, and ligand binding induces conformational changes that lead to cytosolic signal transduction via a number of adaptor molecules such as MyD88, an adaptor common to all TLRs. Subsequent activation of protein kinases such as IRAK1 leads to signal amplification, and ultimatly result in a stimulatory or inhibitory effect on gene expression and the production of cytokines.
Figure 1 shows an example of a Toll signalling pathway, resulting from recognition of LPS from Gram negative bacteria such as Escherichia coli by TLR4. LPS bound to LPS-binding protein (LBP) is presented to CD14. CD14 manoeuvres the LPS-LBP complex to TLR4, and LPS, in combination with MD22, activates TLR4 signalling. Following ligand binding, the cytoplasmic domain of TLR4 recruits an adaptor molecule such as MyD88 or TRIF, which activates protein kinases and propagates the signal.
Whilst TLRs are able to recognize viral PAMPs, the situation is rather more complex. Some viral proteins are able to subvert TLR pathways, aiding viral entry into cells, whereas others actually suppress TLR function.
Interestingly, it is not only molecules from externally-derived pathogens that are able to elicit Toll-signalling. A number of host molecules have also been identified as ligands for TLRs. Breakdown products from the extracellular matrix, such as hyaluron, intracellular components released when cells rupture, and products of proteolytic cascades are all able to stimulate TLRs, suggesting their role in sensing tissue damage signals caused by disease or injury. Recognition of these products by TLRs leads to the activation/recruitment of immune cells and cytokines that repair the tissue damage. There are already drugs in the pipeline for tissue regeneration applications, such as Clinquest's TLR3 agonist CQ-07001, currently in preclinical trials.
