Toll-like receptor 8 functions as a negative regulator of neurite outgrowth and inducer of neuronal apoptosis

Toll receptors in Drosophila melanogaster function in morphogenesis and host defense. Mammalian orthologues of Toll, the Toll-like receptors (TLRs), have been studied extensively for their essential functions in controlling innate and adaptive immune responses. We report that TLR8 is dynamically expressed during mouse brain development and localizes to neurons and axons. Agonist stimulation of TLR8 in cultured cortical neurons causes inhibition of neurite outgrowth and induces apoptosis in a dissociable manner. Our evidence indicates that such TLR8-mediated neuronal responses do not involve the canonical TLR-NF-kappaB signaling pathway. These findings reveal novel functions for TLR8 in the mammalian nervous system that are distinct from the classical role of TLRs in immunity.     

Characterization of four Toll related genes during development and immune responses in Anopheles gambiae

Toll-like receptors (TLRs) are a group of evolutionary conserved proteins with diverse biological functions. In Drosophila melanogaster, Toll protein plays an important role in pattern formation in embryogenesis and in antimicrobial immunity in larvae and adults. In insects, Toll and two other related proteins, Tehao and 18-wheeler have been shown to participate in the activation of the innate immune responses to fungal and bacterial pathogens. In this paper we report the cloning and characterization of four TLR gene from malaria vector mosquito Anopheles gambiae, AgToll, AgToll6, AgTrex, and AgToll9, orthologues of DmToll, DmToll6, DmTollo (Toll8) and DmToll9 (CG5528) in Drosophila melanogaster. The expression profiles of these genes during development, in different adult tissues and after immune challenge were examined. As expected for the orthologue of Drosophila Toll, AgToll was found to be expressed highly in the ovary and may play a role in pattern formation during embryogenesis. AgToll9, surprisingly, was found to be highly expressed in the adult gut. The potential roles of these genes in development and immunity were discussed.     

The Drosophila Toll signaling pathway

The identification of the Drosophila melanogaster Toll pathway cascade and the subsequent characterization of TLRs have reshaped our understanding of the immune system. Ever since, Drosophila NF-κB signaling has been actively studied. In flies, the Toll receptors are essential for embryonic development and immunity. In total, nine Toll receptors are encoded in the Drosophila genome, including the Toll pathway receptor Toll. The induction of the Toll pathway by gram-positive bacteria or fungi leads to the activation of cellular immunity as well as the systemic production of certain antimicrobial peptides. The Toll receptor is activated when the proteolytically cleaved ligand Spatzle binds to the receptor, eventually leading to the activation of the NF-κB factors Dorsal-related immunity factor or Dorsal. In this study, we review the current literature on the Toll pathway and compare the Drosophila and mammalian NF-κB pathways.     

Domain combination of the vertebrate-like TLR gene family: implications for their origin and evolution

Domain shuffling, which is an important mechanism in the evolution of multi-domain proteins, has shaped the evolutionary development of the immune system in animals. Toll and Toll-like receptors (TLRs) are a class of proteins that play a key role in the innate and adaptive immune systems. Draft genome sequences provide the opportunity to compare the Toll/TLR gene repertoire among representative metazoans. In this study, we investigated the combination of Toll/interleukin-1 receptor (TIR) and leucine-rich repeat (LRR) domains of metazoan Toll/TLRs. Before Toll with both domains occurred in Cnidaria (sea anemone, Nematostella vectensis), through domain combinations, TIR-only and LRR-only proteins had already appeared in sponges (Amphimedon queenslandica). Although vertebrate-like TIR (V-TIR) domain already appeared in Cnidaria, the vertebrate-like TLR (V-TLR) with both domains appeared much later. The first combination between V-TIR domain and vertebrate-like LRR (V-LRR) domain for V-TLR may have occurred after the divergence of Cnidaria and bilateria. Then, another combination for V-TLR, a recombination of both domains, possibly occurred before or during the evolution of primitive vertebrates. Taken together, two rounds of domain combinations may thus have co-shaped the vertebrate TLRs.     

Innate immunity: cutaneous expression of Toll-like receptors

Toll receptors were first identified as an essential molecule for embryonic patterning in Drosophila and were subsequently shown to be a key in antibacterial and antifungal immunity in adult flies. Toll receptors have been conserved throughout evolution. In mammals, TLRs have been implicated in both inflammatory responses and innate host defense to pathogens. The 11 different TLRs recognize conserved molecular patterns of microbial pathogens termed pathogen-specific molecular patterns (PAMPs), that permit to confer responsiveness to a wide variety of pathogens. Endogenous ligands are also able to activate TLRs. All adult tissue is capable to express at least one of member of TLR family, but a largest repertoire of TLRs is found in tissues exposed to the external environment. The TLR activation induce the NF-kappaB translocation to the nucleus and cytokine secretion. Since the primary function of skin is to provide an effective barrier against outside agression, it is likely that keratinocytes may play a role in a rapid and efficient host defence system, and the fact that keratinocytes are capable of expressing a wide variety of TLRs is subsequently not surprising.     

Structures and recognition modes of toll‐like receptors

Toll‐like receptors (TLRs) recognize common structural patterns in diverse microbial molecules and play central roles in the innate immune response. The structures of extracellular domains and their ligand complexes of several TLRs have been determined by X‐ray crystallography. Here, we discuss recent advances on structures and activation mechanisms of TLRs. Despite the differences in interaction areas of ligand with TLRs, the extracellular domains of TLRs all adopt horseshoe‐shaped structures and the overall M‐shape of the TLR–ligand complexes is strikingly similar. The structural rearrangement information of TLRs sheds new light on their ligand‐recognition and ‐activation mechanisms. Proteins 2016; 85:3–9. © 2016 Wiley Periodicals, Inc.     

Molecular evolution of vertebrate Toll-like receptors: evolutionary rate difference between their leucine-rich repeats and their TIR domains

Toll-like receptors (TLRs) that initiate an innate immune response contain an extracellular leucine rich repeat (LRR) domain and an intracellular Toll IL-receptor (TIR) domain. There are fifteen different TLRs in vertebrates. The LRR domains, which adopt a solenoid structure, usually have higher rates of evolution than do the TIR globular domains. It is important to understand the molecular evolution and functional roles of TLRs from this standpoint. Both pairwise genetic distances and Ka/Ks's (the ratios between non synonymous and synonymous substitution rates) were compared between the LRR domain and the TIR domain of 366 vertebrate TLRs from 96 species (from fish to primates). In fourteen members (TLRs 1, 2, 3, 4, 5, 6, 7, 8, 9, 11/12, 13, 14, 21, and 22/23) the LRR domains evolved significantly more rapidly than did the corresponding TIR domains. The evolutionary rates of the LRR domains are significantly different among these members; LRR domains from TLR3 and TLR7 from primates to fishes have the lowest rate of evolution. In contrast, the fifteenth member, TLR10, shows no significant differences; its TIR domain is not highly conserved. The present results suggest that TLR10 may have a different function in signaling from those other members and that a higher conservation of TLR3 and TLR7 may reflect a more ancient mechanism and/or structure in the innate immune response system. Gene conversions are suggested to have occurred in platypus TLR6 and TLR10. This study provides new insight about structural and functional diversification of vertebrate TLRs.     

Toll-like receptors: mammalian "taste receptors" for a smorgasbord of microbial invaders

In Drosophila, the Toll family of proteins is responsible for the recognition of bacteria and fungi. In mammals, Toll-like receptors (TLRs) are able to recognize and respond to microbial pathogens. Recent findings have defined the relationship between many TLRs and their microbial ligands, as well as the effect of TLR ligation on host defense. These findings have also provided a framework for determining how TLRs may by used to therapeutically modulate immune responses to infection.     

Intracellular vs extracellular recognition of pathogens--common concepts in mammals and flies

There are common themes in innate immune defense systems across the animal and plant kingdoms. Pathogen recognition is commonly based on the identification of microbial molecular patterns by defined receptors and the subsequent activation of signaling pathways that initiate a defense response to fend off the invading microorganism. The existence of mammalian Toll-like receptors (TLRs) and the recent identification of two mammalian nucleotide-binding site leucine-rich repeat (NBS-LRR) proteins (NOD1 and NOD2) as intracellular sensors of bacterial products bring new insights into the possibility of extracellular versus intracellular pathogen recognition and signal transduction depending on the nature of the infection. The homology between TLRs and the Toll system in Drosophila suggests that conserved defense mechanisms are likely to be shared by diverse organisms.     

Toll-like receptors: a family of pattern-recognition receptors in mammals

The innate immune system uses a variety of germline-encoded pattern-recognition receptors that recognize conserved microbial structures or pathogen-associated molecular patterns, such as those that occur in the bacterial cell-wall components peptidoglycan and lipopolysaccharide. Recent studies have highlighted the importance of Toll-like receptors (TLRs) as a family of pattern-recognition receptors in mammals that can discriminate between chemically diverse classes of microbial products. First identified on the basis of sequence similarity with the Drosophila protein Toll, TLRs are members of an ancient superfamily of proteins, which includes related proteins in invertebrates and plants. TLRs activate innate immune defense reactions, such as the release of inflammatory cytokines, but increasing evidence supports an additional critical role for TLRs in orchestrating the development of adaptive immune responses. The sequence similarity between the intracellular domains of the TLRs and the mammalian interleukin-1 and interleukin-18 cytokine receptors reflects the use of a common intracellular signal-transduction cascade triggered by these receptor classes. But more recent findings have demonstrated that there are in fact TLR-specific signaling pathways and cellular responses. Thus, TLRs function as sentinels of the mammalian immune system that can discriminate between diverse pathogen-associated molecular patterns and then elicit pathogen-specific cellular immune responses.     

Toll-like receptor downstream signaling

The family of Toll-like receptors (TLRs) senses conserved structures found in a broad range of pathogens, causing innate immune responses that include the production of inflammatory cytokines, chemokines and interferons. The signal transduction is initiated from the Toll/interleukin-1 receptor (TIR) domain of TLRs after pathogen recognition. Almost all TLRs use a TIR-containing adapter MyD88 to activate a common signaling pathway that results in the activation of NF-kappaB to express cytokine genes relevant to inflammation. Recently, three further TIR-containing adapters have been identified and shown to selectively interact with several TLRs. In particular, activation of the TRIF-dependent pathway confers antiviral responses by inducing anti-viral genes including that encoding interferon-beta. Taken together, these results indicate that the interaction between individual TLRs and the different combinations of adapters directs appropriate responses against distinct pathogens.     

The Aedes aegypti toll pathway controls dengue virus infection

Aedes aegypti, the mosquito vector of dengue viruses, utilizes its innate immune system to ward off a variety of pathogens, some of which can cause disease in humans. To date, the features of insects' innate immune defenses against viruses have mainly been studied in the fruit fly Drosophila melanogaster, which appears to utilize different immune pathways against different types of viruses, in addition to an RNA interference-based defense system. We have used the recently released whole-genome sequence of the Ae. aegypti mosquito, in combination with high-throughput gene expression and RNA interference (RNAi)-based reverse genetic analyses, to characterize its response to dengue virus infection in different body compartments. We have further addressed the impact of the mosquito's endogenous microbial flora on virus infection. Our findings indicate a significant role for the Toll pathway in regulating resistance to dengue virus, as indicated by an infection-responsive regulation and functional assessment of several Toll pathway-associated genes. We have also shown that the mosquito's natural microbiota play a role in modulating the dengue virus infection, possibly through basal-level stimulation of the Toll immune pathway.     

Investigating the effect of key mutations on the conformational dynamics of toll‐like receptor dimers through molecular dynamics simulations and protein structure networks

The Toll‐like receptors (TLRs) are critical components of the innate immune system due to their ability to detect conserved pathogen‐associated molecular patterns, present in bacteria, viruses, and other microorganisms. Ligand detection by TLRs leads to a signaling cascade, mediated by interactions among TIR domains present in the receptors, the bridging adaptors and sorting adaptors. The BB loop is a highly conserved region present in the TIR domain and is crucial for mediating interactions among TIR domain‐containing proteins. Mutations in the BB loop of the Toll‐like receptors, such as the A795P mutation in TLR3 and the P712H mutation (Lps^d mutation) in TLR4, have been reported to disrupt or alter downstream signaling. While the phenotypic effect of these mutations is known, the underlying effect of these mutations on the structure, dynamics and interactions with other TIR domain‐containing proteins is not well understood. Here, we have attempted to investigate the effect of the BB loop mutations on the dimer form of TLRs, using TLR2 and TLR3 as case studies. Our results based on molecular dynamics simulations, protein–protein interaction analyses and protein structure network analyses highlight significant differences between the dimer interfaces of the wild‐type and mutant forms and provide a logical reasoning for the effect of these mutations on adaptor binding to TLRs. Furthermore, it also leads us to propose a hypothesis for the differential requirement of signaling and bridging adaptors by TLRs. This could aid in further understanding of the mechanisms governing such signaling pathways.     

Toll-like receptor downstream signaling

The family of Toll-like receptors (TLRs) senses conserved structures found in a broad range of pathogens, causing innate immune responses that include the production of inflammatory cytokines, chemokines and interferons. The signal transduction is initiated from the Toll/interleukin-1 receptor (TIR) domain of TLRs after pathogen recognition. Almost all TLRs use a TIR-containing adapter MyD88 to activate a common signaling pathway that results in the activation of NF-κB to express cytokine genes relevant to inflammation. Recently, three further TIR-containing adapters have been identified and shown to selectively interact with several TLRs. In particular, activation of the TRIF-dependent pathway confers antiviral responses by inducing anti-viral genes including that encoding interferon-β. Taken together, these results indicate that the interaction between individual TLRs and the different combinations of adapters directs appropriate responses against distinct pathogens.     

Phylogenetic and expression analysis of amphibian Xenopus Toll-like receptors

An anuran amphibian, South African clawed frog (Xenopus laevis), is used to study the immune system, as it possesses a set of acquired immune system represented by T and B lymphocytes and the immunoglobulins. The acquired immune system is impaired throughout the larva and the metamorphosis stage in the amphibians. On the other hand, the role of innate immune system in the tadpole remains unclear. Recently, insect Toll protein homologues, namely, Toll-like receptors (TLRs), have been identified as sensors recognizing microbe-pattern molecules in vertebrates. Whole-genome analysis of Xenopus tropicalis supported the existence of the tlr genes in the frog. In this study, we annotated 20 frog tlr gene nucleotide sequences from the latest genome assembly version 4.1 on the basis of homology and identified cDNAs of the predicted frog TLR proteins. Phylogenetic analysis showed that the repertoire of the frog TLRs consisted of both fish- and mammalian-type TLRs. We showed that the frog TLRs are constitutively expressed in the tadpole as well as in the adult frog. Our results suggest that tadpoles are protected from microbes by the innate system that includes TLRs, despite impaired acquired immune system in tadpoles. This is the first report on the properties of TLRs in the most primitive terrestrial animals like amphibia. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The immune signaling pathways of Manduca sexta

Signal transduction pathways and their coordination are critically important for proper functioning of animal immune systems. Our knowledge of the constituents of the intracellular signaling network in insects mainly comes from genetic analyses in Drosophila melanogaster. To facilitate future studies of similar systems in the tobacco hornworm and other lepidopteran insects, we have identified and examined the homologous genes in the genome of Manduca sexta. Based on 1:1 orthologous relationships in most cases, we hypothesize that the Toll, Imd, MAPK-JNK-p38 and JAK-STAT pathways are intact and operative in this species, as are most of the regulatory mechanisms. Similarly, cellular processes such as autophagy, apoptosis and RNA interference probably function in similar ways, because their mediators and modulators are mostly conserved in this lepidopteran species. We have annotated a total of 186 genes encoding 199 proteins, studied their domain structures and evolution, and examined their mRNA levels in tissues at different life stages. Such information provides a genomic perspective of the intricate signaling system in a non-drosophiline insect.     

Toll receptors in innate immunity.

Innate immunity is the first-line host defense of multicellular organisms that rapidly operates to limit infection upon exposure to infectious agents. In addition, the cells and molecules operating during this early stage of the immune response in vertebrates have a decisive impact on the shaping of the subsequent adaptive response. Genetic studies initially performed in the fruitfly Drosophila and later in mice have revealed the importance of proteins of the Toll family in the innate immune response. We present here our current understanding of the role of this evolutionary ancient family of proteins that are thought to function as cytokine receptors (Toll in Drosophila) or pattern-recognition receptors (TLRs in mammals) and activate similar, albeit non-identical, signal-transduction pathways in flies and mammals.     

Toll-like receptor family and signalling pathway

Toll is a Drosophila gene essential for ontogenesis and anti-microbial resistance. Several orthologues of Toll have been identified and cloned in vertebrates, namely Toll-like receptors (TLRs). Human TLRs are a growing family of molecules involved in innate immunity. TLRs are characterized structurally by a cytoplasmic Toll/interleukin-1 receptor (TIR) domain and by extracellular leucine-rich repeats. TLRs characterized so far activate the MyD88/interleukin-1 receptor-associated kinase (IRAK) signalling pathway. Genetic, gene-transfer and dominant-negative approaches have involved TLR family members (TLR2 and TLR4) in Gram-positive and Gram-negative bacteria recognition and signalling. Accumulating evidence suggests that TLR2 is also involved in signalling-receptor complexes that recognize components of yeast and mycobacteria. However, the definitive roles of other TLRs are still lacking. A systematic approach has been used to determine whether different human leucocyte populations selectively or specifically express TLR mRNA. Based on expression pattern, TLR can be classified as ubiquitous (TLR1), restricted (TLR2, TLR4 and TLR5) and specific (TLR3). Expression and regulation of distinct but overlapping ligand-recognition patterns may underlie the existence of a large, seemingly redundant TLR family. Alternatively, the expression of a TLR in a single cell type may indicate a specific role for this molecule in a restricted setting.     

Toll receptors and pathogen resistance

Toll receptors in insects, mammals and plants are key players that sense the invasion of pathogens. Toll-like receptors (TLRs) in mammals have been established to detect specific components of bacterial and fungal pathogens. Furthermore, recent evidence indicates that TLRs are involved in the recognition of viral invasion. Signalling pathways via TLRs originate from the conserved Toll/IL-1 receptor (TIR) domain. The TIR domain-containing MyD88 acts as a common adaptor that induces inflammatory cytokines; however, there exists a MyD88-independent pathway that induces type I IFNs in TLR4 and TLR3 signalling. Another TIR domain-containing adaptor, TIRAP/Mal has recently been shown to mediate the MyD88-dependent activation in the TLR4 and TLR2 signalling pathway. Thus, individual TLRs may have their own signalling systems that characterize their specific activities.