IL17RD
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Interleukin 17 receptor D (also known as Sef) is a
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...
that in humans is encoded by the IL17RD
gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...
. This gene encodes a membrane protein belonging to the interleukin-17 receptor (IL-17R) protein family. Alternate splicing generates multiple transcript variants encoding distinct isoforms. IL-17RD has been described to limit fibroblast growth factor receptor (
FGFR The fibroblast growth factor receptors (FGFR) are, as their name implies, receptors that bind to members of the fibroblast growth factor (FGF) family of proteins. Some of these receptors are involved in pathological conditions. For example, a poi ...
) signaling and to be a part of the IL-17 receptor signaling complex.


Identification

IL-17RD was initially discovered during a large-scale ''in situ'' hybridization screen for genes regulating zebrafish embryogenesis. It was identified as a part of a synexpression group (genes with similar spatio-temporal expression) with negative regulators of fibroblast growth factor ( FGF) and termed Sef (similar expression to FGF genes). The name was later changed to IL-17RD due to its sequence similarity to other IL-17 receptors. It was further determined that IL-17RD co-immunoprecipitates with FGF receptor (FGFR) and inhibits FGF signaling at the level of signal transduction and not by interfering with the ligand or its binding to FGFR.


Structure

IL-17RD is a
type I transmembrane protein A single-pass membrane protein also known as single-spanning protein or bitopic protein is a transmembrane protein that spans the lipid bilayer only once. These proteins may constitute up to 50% of all transmembrane proteins, depending on the or ...
containing extracellular Ig-like domain followed by a fibronectin type III domain, a short transmembrane domain of ~20 amino acids, and an intracellular SEFIR domain  which was identified in IL-17 receptors and some soluble factors involved in IL-17 signaling. The SEFIR domain contains a region with sequence similarity to the TIR domain, which is characteristic of Toll-like receptors (TLRs), receptors of the interleukin 1 family, and adaptor proteins involved in the signaling pathways of these receptors. The regions within SEFIR that can be found in the TIR domain include box 1 and box 2.


IL-17RD in development

IL-17RD (Sef) was identified as part of a group of genes involved in FGF signaling in zebrafish and ''Xenopus laevis'' embryo. Injection of 1-cell stage embryo with ''sef'' mRNA lead to ventralization of the embryo, a similar effect observed after injection with XFD (a dominant negative of FGF receptor), suggesting its function as a negative regulator of FGF receptor signaling. Co-immunoprecipitation assay revealed that the intracellular part, but not the SEFIR domain, is critical for IL-17RD association with FGFR. One of the pathways activated by stimulation of FGFR is Ras/MAPK (the rest being PI3/AKT and PLCγ). Injection of embryos with high amounts of Ras, Raf or MEK causes cell cycle arrest, which can be rescued by co-injection of IL-17RD, further supporting the role of IL-17RD in negative regulation of FGFR signaling. Moreover, IL-17RD appears to regulate FGF signaling at the level of downstream signaling, not the receptor, since overexpression of FGF or FGFR does not cause cell cycle arrest. Taken together IL-17RD seems to negatively regulate FGFR signaling by limiting MAPK signaling via its intracellular domain.


IL-17RD in inflammation


IL-17 signaling

The IL-17 receptor family belongs to a group of structurally similar receptors with a distinctive SEFIR (Sef and IL-17R) domain. The founding member, IL-17RA, along with IL-17RC serve as a receptor complex for IL-17. IL-17 is a proinflammatory cytokine mainly produced by Th17 subset of T cells and plays an important role in extracellular pathogen elimination as well as several autoinflammatory diseases (such as psoriasis or rheumatoid arthritis). IL-17RD has been reported to associate and co-localize with IL-17RA, mediate IL-17 signaling, and interact with TRAF6 (an IL-17 downstream molecule). Moreover, deletion of IL-17RD intracellular domain has a dominant negative effect and suppresses IL-17 signaling. In contrast, deletion of extracellular domain had no effect on IL-17 signaling. However, full-body IL-17RD knockout mice do not present with any apparent phenotype. This might be accounted for by the presence of IL-17RC which to an extent substitutes IL-17RD. It is important to note, however, that IL-17RC or IL-17RD deletion fails to protect against imiquimod-induced psoriasis.


TLR signaling

Since the SEFIR domain contains a TIR domain, the possible role of IL-17RD in TLR signaling was investigated. One study discovered that IL-17RD interacts with TIR adaptor proteins (such as MyD88, Mal, and TRIF) following TLR stimulation. Additionally, this interaction was abolished in IL-17RD which lacks the SEFIR domain. The study concluded that IL-17RD targets TLR-induced pro-inflammatory pathways and inhibits signaling upstream of NF-κB and IRF3.


TNF signaling

One study reported that TNF induces IL-17RD expression, which then serves as a feedback loop inhibiting the activation of TNF-activated NF-κB. Another study focusing on renal cells describes IL-17RD to associate with TNFR2, but not TNFR1, to augment NF-κB activation. The contrasting results suggest different roles of IL-17RD in various tissues.


References


Further reading

* {{Interleukin receptor modulators Genes Human proteins