Pan-vertebrate toll-like receptors during evolution

Abstract

Human toll-like receptors (TLRs) recognize pathogen-associated molecular patterns (PAMPs) to raise innate immune responses. The human TLR family was discovered because of its sequence similarity to fruit fly (Drosophila) Toll, which is involved in an anti-fungal response. In this review, we focus on the origin of the vertebrate TLR family highlighted through functional and phylogenetic analyses of TLRs in non-mammalian vertebrates. Recent extensive genome projects revealed that teleosts contain almost all subsets of TLRs that correspond to human TLRs (TLR1, 2, 3, 4, 5, 7, 8, and 9), whereas the urochordate Cionaintestinalis contains only a few TLR genes. Therefore, mammals likely obtained almost all TLR family members at the beginning of vertebrate evolution. This premise is further supported by several functional analyses of non-mammalian TLRs. We have summarized several teleost TLRs with unique properties distinct from mammalian TLRs to outline their specific roles. According to Takifugu rubripes genome project, the puffer fish possesses fish-specific TLR21 and 22. Surprisingly, phylogenetic analyses indicate that TLR21 and 22 emerged during an early period of vertebrate evolution in parallel with other TLRs and that the mammalian ancestor lost TLR21 and 22 during evolution. Our laboratory recently revealed that TLR22 recognizes double-strand RNA and induces interferon production through the TICAM-1 adaptor, as in TLR3, but unlike TLR3, TLR22 localizes to the cell surface. Therefore, differential expression of TLR3 and TLR22, rather than simple redundancy of RNA sensors, may explain the effective protection of fish from RNA virus infection in the water. In this review, we summarize the similarities and differences of the TLR family in various vertebrates and introduce these unique TLRs for a possible application to the field of clinical practices for cancer or virus infection.

Fig. (1). Two double-strand RNA recognition pathways in teleosts

TLR22 localizes at the cell surface and recognizes double-strand RNA derived from viruses. Teleost TLR3 proteins reside in the intracellular compartment and are expected to localize at the early endosome, as in human TLR3. When virions are disrupted and their genome RNA flows out to the extracellular space, one might surmise that the viral double-strand RNA is recognized by TLR22. In another case, when the viral RNA in the cytoplasmic region is exported from the cytoplasm to the intercellular space by exocytosis, the double-strand RNA can be recognized by TLR22. Teloeost TLR3 recognizes viral RNA at the early endosome as in human TLR3. Both TLRs can transmit the signal to induce type I interferon, which exerts anti-virus properties.

Fig. (2). Evolution of the vertebrate TLR family

We expect that in the Cambrian period, the vertebrate ancestor possessed at least nine TLR family members: TLR2, 3, 4, 5, 7, 8, 9, 21, and 22. Those TLRs would have responded to PAMPs such as dsRNA, ssRNA, CpG DNA lipoprotein, peptidoglycan, LPS, flagellin, or other unknown PAMPs. Current TLR members in fish and mammals are expected to have been derived from the TLR family members in the Cambrian common ancestor. During evolution, mammalian ancestors obtained novel TLR members by gene duplication, especially in the TLR2 subfamily. On the other hand, both lineages have lost several members for unknown reasons.