Interleukin-17 Receptor A
The first IL-17 receptor (IL-17RA) was identified in 1995 by using the viral homolog, herpesvirus saimiri gene 13 (HVS13) (Yao et al. 1995). An Fc-HVS13 fusion protein was used to isolate cDNA from mouse T cells which encode a protein that binds all its orthologous forms. Following the discovery of mouse IL-17RA, Yao et al. isolated cDNA encoding the human IL-17RA from an activated peripheral blood leukocyte cDNA library by using mouse cDNA as a probe. The predicted 866-amino-acid type I membrane glycoprotein was 69% identical and 82% similar to the mouse sequence. It was not recognized to be related to any of the other known cytokine receptors and did not possess similarity to any other known protein nor any recognizable domains. Hence, IL-17RA and its ligand, IL-17A, represented a distinct highly conserved receptor-ligand pair. The study also showed low affinity of IL-17RA for IL-17A, lower than the concentration required to mediate the cellular responses, and suggested existence of an additional subunit that ensures high-affinity binding and/or signaling (Yao et al. 1997). Subsequently, a series of other IL-17R family members, IL-17RB to IL17RE, were discovered, using sequence similarity searches of genome databases (Moseley et al. 2003). The evidence now suggests that IL-17RA is the common signaling subunit used by at least four ligands – IL-17A and IL-17F bind to the IL-17RA/IL-17RC heterodimer, IL-17C binds to the IL-17RA/IL-17RE dimer, and IL-17E signals through the IL-17RA/IL-17RB complex (Gaffen 2009). IL-17RA was found to be ubiquitously expressed in all cell types examined, although the level of its expression varies widely and the mechanisms underlying the regulation of IL-17RA gene expression are scarcely elucidated.
IL-17RA Gene Mapping
IL-17RA Protein Structure
Functional IL-17RA is a type I membrane glycoprotein consisting g of a 293-amino-acid extracellular domain, a 21-amino-acid carboxy-proximal transmembrane domain, and a 525-amino-acid cytoplasmic tail. Extracellular domain contains seven potential N-linked glycosylation sites at glutamine 49, 54, 67, 206, 225, 242, and 265 and five intra-chain disulfide bonds between cysteines at positions 43–50, 57–16, 185–196, 245–276, and 290–294 (Yao et al. 1997; http://www.uniprot.org/uniprot/Q96F46). In SDS-polyacrylamide gel electrophoresis, IL-17R precipitates as a 128–158 kDa glycoprotein and as 105–107 kDa nascent, non-glycosylated protein (Yao et al. 1997). Moreover, the structural conformation of IL-17RA analysis indicated that the ectodomains contain 17β-strands, five helices, and three turn domains, while intracellular tail shows five β-strands, 11 helices, and two turn domains (Krstic et al. 2015).
Two fibronectin III-like (FNIII) regions at the extracellular domain are common to all five members of the IL-17R family, whereas the cytoplasmic IL-17RA tail contains several docking regions for the interaction with the different signaling effectors: conserved SEF/IL-17R (SEFIR) domain, which has homology to TOR domains found in Toll and IL-1 receptors (TIR) (Huang et al. 2015), and unique to human IL-17RA TIR-like loop (TILL) domain, located downstream of SEFIR motif. This TILL domain seems to be critical for the activation of downstream signaling pathways by IL-17A, since specific mutations in this motif were shown to impair cells’ response to IL-17A (Gaffen 2009). Moreover, following SEFIR domains, a 92-amino-acid long C-terminus motif called SEFEX also appeared to be required for the IL17RA-mediated signal transduction (Onishi et al. 2010); finally, a C/CCAAT/enhancer-binding protein (C/EBPβ) activation domain (CBAD) is described in the C-terminus domain (Fig. 1).
Signal Transduction Initiated by IL-17R
Namely, CBAD is unique to IL-17RA domain too, because it is not involved in the activation of NF-κB and mitogen-activated protein kinase (MAPK) pathways (Liu et al. 2015).
The 2.3 Å resolution crystal structure of the compact unit of IL-17RA includes the complete SEFIR domain and an additional α-helical C-terminal extension. SEFIR domains of IL-17RA and IL-17RB are different in protein topology and inflexion. Namely, in SEFIR of IL-17RA, long insertion between strand βC and helix αC is well ordered, creating a helix (αCC’ ins) and a flexible loop (CC’). Position of DD’ loop in the IL-17RA SEFIR domain is shifted about 12 Å to accommodate the αCC’ins without forming any nodes, in comparison to structure of IL-17RB SEFIR domain. It has been suggested that helix αC is crucial for interaction with Act1 protein and gene expression triggered by IL-17. Thus, the structure of helix αC and downstream motifs of SEFIR create a binding site for activation of Act1 (Zhang et al. 2014; Onishi et al. 2010). It has been revealed that heterotypic SEFIR–SEFIR association dependent on key functional residues in Act1 is also mapped as part of helix αC, which is conserved in IL-17RA and IL-17C. On the other side, homodimerization of Act1 is dependent on helix αB’ (Zhang et al. 2014).
Act1 contains two tumor necrosis factor receptor-associated factor (TRAF)-binding motifs (residues EESE (35 to 42) and EESE (333 to 337)), a helix–loop–helix (HLH) domain (residues 135 to 190), a SEFIR domain (residues 394 to 574), and a coil–coiled (C-C) domain (residues 470 to 500) (Doyle et al. 2012). Surprisingly, Act1 recruitment is not sufficient for downstream signaling activation, whereas ubiquitination of TRAF6 correlates tightly with functional receptors (Onishi et al. 2010). Besides its role as adaptor protein, a non-adaptor function of Act1 has recently been identified, exerted through direct binding to the promoter region of IL-17A-sensing genes, thus directly regulating their transcription (Velichko et al. 2016).
After IL-17 stimulation, Act1 is engaged to the IL-17R complex through homotypic interactions of the SEFIR domains. Mapping of domain and blocking by a cell-permeable decoy peptide approach demonstrated that a coiled–coiled (CC) loop presented in the SEFIR domain is crucial for the interaction of Act1 with IL-17RA. In case of IL-17RB, the crystal structure analyses showed that residues located within the SEFIR domain of IL-17RB (Leu419 and Leu422) and within the CC loop of Act1 (Leu474, His475, Lys477, and Tyr478) are crucial for this homotypic interaction between IL-17RB and Act1 (Gu et al. 2013). Further, it has recently been shown that SEFIR domain of orphan receptor IL-17RD targets TIR adaptor proteins to inhibit Toll-like receptor (TLR) downstream signaling. Namely, it has been observed that IL-17RD negatively regulates TLR-induced responses, while deficiency of IL-17RD expression leads to enhanced pro-inflammatory signaling in response to TLR stimulation. Therefore, a paradigm of the IL-17R and TLR family member’s cross regulation has been suggested (Mellet et al. 2015).
Recruitment of Act-1 to IL-17RA, IL-17RB, and IL-17RC activates the NF-κB, MAPKs, and C/EBP pathways, leading to the induction of target genes stimulated by IL-17 cytokines (Boisson et al. 2013). Recruitment of Act1 by IL-17RA consequently initiates ubiquitination of TRAF6 (TNF receptor-associated factor 6) which leads to activation of transforming growth factor-β (TGF-β)-activated kinase 1 (TAK1) and further to activation of IKK and NF-κB. Namely, U-box-like E3 ligase ubiquitinates TRAF6 by utilizing the Ubc13–Uev1A E2 complex in a Lys63-linked non-degradative way. The U-box domain of Act1 is essential for IL-17-induced NF-κB activation. Subsequently, engagement of TRAF6 activates the “canonical NF-κB pathway” and MAPK signaling pathways, such as extracellular signal-regulated kinase (ERK), p38, and c-Jun N-terminal kinase (JNK). Recently, Syk has been identified as an upstream signaling molecule in IL-17A-induced Act1-TRAF6 interaction in keratinocytes (Wu et al. 2015).
Also, inducible phosphorylation of Act1, mediated by IKKi (IKKε) and serine/threonine protein kinase TBK1, can engage TRAF2 and TRAF5 units, which promote an mRNA stability pathway via the splicing regulatory protein SF2/ASF or human antigen R (HuR) complexes (Gaffen et al. 2014). Proteins TGF-β-activated kinase 2 (TAB2) and TAB3 in complex with TAK1 can be involved in NF-κB activation, regulated by miR-23b (Liu et al. 2015). Interestingly, for the activation of MAPKs and mRNA stabilization but not for NF-κB activation by IL-17A, IL-17RA acts in complex with Act1, TRAF5, TRAF2, and alternative splicing factor which is controlled by inducible IκB kinase. Moreover, significance of cross talk of Act1 and heat shock protein 90 (hsp90) cross talk has been described. Namely, inhibition of molecular chaperone hsp90 can lead to reduction of Act1 recruitment and defect IL-17RA signaling (Liu et al. 2015). Single-nucleotide polymorphism of Act1 gene (rs33980500) results in decreased binding of Act1 to TRAF6. This key mutation in Act1 could lead to increased interaction of the IL-17R with TRAF2/TRAF5 units and modulates IL-17 signaling (Doyle et al. 2012).
IL-17RA signaling activates “canonical NF-κB pathway,” triggering p50 and p65 subunits that result in phosphorylation and degradation of inhibitory NF-κB unit, IκBα. Contrary, it has been observed that IL-17A/IL-17RA complex stimulates the expression of IκBz, a unit shared with TLR/IL-1R pathways and required for IL-6 production. Also, it has been suggested that IL-17RA can activate “noncanonical NF-κB pathway.” Increasing evidences demonstrated that in comparison to TLR/IL-1R and TNFα, IL-17RA signaling is weaker activator of NF-κB. IL-17RA also activates MAPKs which trigger activator protein 1 (AP1) and subsequently results in mRNA stabilization of several proinflammatory cytokines and chemokines. Namely, MAPKs can phosphorylate destabilizing proteins (tristetraprolin), annulling their degradative capacity. Besides, it has been indicated that for IL-17RA-mediated stabilization of mRNA, there is a requirement for Act1 and not for TRAF6, although this mechanism is not fully understood.
C/EBPβ and C/EBPδ are units of C/EBP 6 member family, which can be targets of IL-17RA signaling. SEFIR and the TILL regions induce ERK to phosphorylate threonine 188 within C/EBPβ, while CBAD can trigger GSK3β (glycogen synthase kinase 3β) to phosphorylate the threonine 179 within C/EBPβ. On the other side, Act1, SEFIR, and TILL domains are all required for the induction C/EBPδ.
Additionally, on the basis on IL-17RA-induced expression of IL-6, it has been suggested that IL-17R signaling can be involved in the activation of phosphatidylinositide 3 kinases (PI3K) and Janus kinase/signal transducers and activators of transcription (JAK/STAT) pathways (Liu et al. 2015).
The physiological effects of IL-17 are mediated through its receptor in target cells. Specific structural and functional properties of the IL-17 receptor and their understanding may indicate novel drug compounds or therapeutic approaches (Gaffen et al. 2014). Although the IL-17 cytokine family and IL-17Rs represent a unique ligand–receptor signaling system which is highly conserved across vertebrate evolution, and with little homology with any other known receptors or ligands, detailed mechanisms of IL-17 signaling have not been revealed.
Intriguingly, IL-17A and IL-17RA in humans are associated with pathology in numerous autoimmune and inflammatory conditions, such as rheumatoid arthritis, multiple sclerosis, psoriasis, Crohn’s disease, systemic lupus erythematosus, asthma, Behçet’s disease, and hyper IgE syndrome, which suggest the importance IL-17A targeting for the treatment of abovementioned diseases. For example, the human anti-IL17RA monoclonal antibody brodalumab (AMG 827) treatments are ongoing in phase III of clinical trials (Wasilewska et al. 2016).
The authors summarized literature data elucidating molecular features of IL-17RA from gene to mature protein. We also provided insights into regulatory mechanisms, structural protein conformation, including ligand–receptor interaction, and an overview of signal pathways. It is believed that the better understanding of molecular aspects of IL-17RA from gene expression to their structural ligand interaction is essential for the development of new therapeutic opportunities for the prevention and treatment of inflammatory diseases.
We apologize to those colleagues whose work, although relevant to the issues dealt within this chapter, has not been included due to space limitations. Also we appreciate OMIM (http://www.omim.org/) for the valuable compiled information. This work was supported by Grant no. 175062 from the Ministry of Education, Science and Technological Development of the Republic of Serbia.
- Zhang B, Liu C, Qian W, Han Y, Li X, Deng J. Structure of the unique SEFIR domain from human interleukin 17 receptor A reveals a composite ligand-binding site containing a conserved α-helix for Act1 binding and IL-17 signaling. Acta Crystallogr D Biol Crystallogr. 2014;70(Pt 5):1476–83.PubMedCrossRefPubMedCentralGoogle Scholar