Efficient one-pot synthesis of CXCL14 and its derivative using an N-sulfanylethylanilide peptide as a peptide thioester equivalent and their biological evaluation

Identification of Low-Density Lipoprotein Receptor-Related Protein 1 as a CXCL14 Receptor Using Chemically Synthesized Tetrafunctional Probes

CXCL14 is a primordial CXC-type chemokine that transports CpG oligodeoxynucleotides (ODN) into endosomes and lysosomes in dendritic cells, thereby leading to the activation of the Toll-like receptor 9 (TLR9)-mediated innate immune system. However, the underlying molecular mechanism by which the CXCL14-CpG ODN complex enters cells remains elusive. Herein, we describe the chemical synthesis of CXCL14-derived photoaffinity probes and their application to the identification of target receptors for CXCL14 using quantitative proteomics. By utilizing native chemical ligation and maleimide-thiol coupling chemistry, we synthesized site-specifically modified CXCL14-based photoaffinity probes that contain photoreactive 2-aryl-5-carboxytetrazole (ACT) and a hydrazine-labile cleavable linker. CXCL14-based probes were found to be capable of binding CpG ODN to immune cells, whose bioactivities were comparable to native CXCL14. Application of CXCL14-derived probes to quantitative proteomic experiments enabled the identification of dozens of target receptor candidates for CXCL14 in mouse macrophage-derived RAW264.7 cells, and we discovered that low-density lipoprotein receptor-related protein 1 (LRP1) is a novel receptor for CXCL14 by competitive proteome profiling. We further showed that disruption of LRP1 affected the incorporation of the CXCL14-CpG ODN complex in the cells. Overall, this report highlights the power of synthetic CXCL14-derived photoaffinity probes combined with chemical proteomics to discover previously unidentified receptors for CXCL14, which could promote an understanding of the molecular functions of CXCL14 and the elaborate machinery of innate immune systems.

Identification of Functional Domains of CXCL14 Involved in High-Affinity Binding and Intracellular Transport of CpG DNA

Some CXC chemokines, including CXCL14, transport CpG oligodeoxynucleotides (ODNs) into dendritic cells (DCs), thereby activating TLR9. The molecular basis of this noncanonical function of CXC chemokines is not well understood. In this study, we investigated the CpG ODN binding and intracellular transport activities of various CXC chemokines and partial peptides of CXCL14 in mouse bone marrow-derived dendritic cells. CXCL14, CXCL4, and CXCL12 specifically bound CpG ODN, but CXCL12 failed to transport it into cells at low dose. CXCL14 N-terminal peptides 1-47, but not 1-40, was capable of transporting CpG ODN into the cell, resulting in an increase in cytokine production. However, both the 1-47 and 1-40 peptides bound CpG ODN. By contrast, CXCL14 peptides 13-50 did not possess CpG ODN binding capacity or transport activity. The chimeric peptides CXCL12 (1-22)-CXCL14 (13-47) bound CpG ODN but failed to transport it. These results suggest that amino acids 1-12 and 41-47 of CXCL14 are required for binding and intracellular transport of CpG ODN, respectively. We found that an anti-CXCL14 Ab blocked cell-surface binding and internalization of the CpG ODN/CXCL14 complex. On the basis of these findings, we propose that CXCL14 has two functional domains, one involved in DNA recognition and the other in internalization of CXCL14-CpG DNA complex via an unidentified CXCL14 receptor, which together are responsible for eliciting the CXCL14/CpG ODN-mediated TLR9 activation. These domains could play roles in CXCL14-related diseases such as arthritis, obesity-induced diabetes, and various types of carcinoma.

Copper-Mediated Deprotection of Thiazolidine and Selenazolidine Derivatives Applied to Native Chemical Ligation

Cupric sulfate efficiently opens thiazolidine and selenazolidine rings, producing a protected N-terminal cysteine or selenocysteine derivative without the use of inert gas or solvent. This is a clear advantage over methods that use water-soluble palladium salts, which fail to react with the selenazolidine ring. This copper-mediated reaction proceeds with monovalent or divalent copper ions, and disulfide bond formation followed by ring-opening promotes the process. This copper-mediated reaction, which is compatible with the standard native chemical ligation conditions, was applied to the synthesis of the 77-mer CXCL14 protein.

CXCL14 Acts as a Specific Carrier of CpG DNA into Dendritic Cells and Activates Toll-like Receptor 9-mediated Adaptive Immunity

CXCL14 is a primordial chemokine that plays multiple roles in tumor suppression, autoimmune arthritis, and obesity-associated insulin resistance. However, the underlying molecular mechanisms are unclear. Here, we show that CXCL14 transports various types of CpG oligodeoxynucleotide (ODN) into the endosomes and lysosomes of bone marrow-derived dendritic cells (DCs), thereby activating Toll-like receptor 9 (TLR9). A combination of CpG ODN (ODN2395) plus CXCL14 induced robust production of IL-12 p40 by wild-type, but not Tlr9-knockout, DCs. Consistent with this, ODN2395-mediated activation of DCs was significantly attenuated in Cxcl14-knockout mice. CXCL14 bound CpG ODN with high affinity at pH 7.5, but not at pH 6.0, thereby enabling efficient delivery of CpG ODN to TLR9 in the endosome/lysosome. Furthermore, the CXCL14-CpG ODN complex specifically bound to high affinity CXCL14 receptors on DCs. Thus, CXCL14 serves as a specific carrier of CpG DNA to sensitize TLR9-mediated immunosurveillance.

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CXCL14 specifically binds CpG DNA with high affinity.

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CXCL14/CpG DNA complex is efficiently transported into dendritic cells.

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CXCL14/CpG DNA induces cytokine production via Toll-like receptor 9.

We discovered that CXCL14 specifically binds CpG DNAs with high affinity and transports them into the endosomes and lysosomes of dendritic cells (DCs). Consequently, Toll-like receptor 9 (Tlr9) in DCs was activated, thereby leading to robust production of IL-12 p40 and IL-6. These activities of CXCL14 were not observed in Tlr9-deficient DCs. Moreover, CpG DNA (ODN2395)-mediated activation of DCs was significantly attenuated in Cxcl14-knockout mice. Therefore, CXCL14 plays an important role in the Tlr9-mediated immunosurveillance against pathogens and cancers. From the clinical point of view, CXCL14/CpG DNA could be useful as a new type of vaccine adjuvant.

N-Sulfanylethylaminooxybutyramide (SEAoxy): A Crypto-Thioester Compatible with Fmoc Solid-Phase Peptide Synthesis

An N-sulfanylethylaminooxybutyramide (SEAoxy) has been developed as a novel thioester equivalent for native chemical ligation. SEAoxy peptide was straightforwardly synthesized by conventional Fmoc solid-phase peptide synthesis without a problem. Moreover, SEAoxy peptide could be directly applied to native chemical ligation owing to the intramolecular N-to-S acyl shift that releases the peptide-thioester in situ. This methodology was successfully applied to the synthesis of two bioactive peptides.

CXCL14 as an emerging immune and inflammatory modulator

CXCL14, a relatively novel chemokine, is a non-ELR (glutamic acid-leucine-arginine) chemokine with a broad spectrum of biological activities. CXCL14 mainly contributes to the regulation of immune cell migration, also executes antimicrobial immunity. The identity of the receptor for CXCL14 still remains obscure and therefore the intracellular signaling pathway is not entirely delineated. The present review summarizes the contribution of CXCL14 in these two aspects and discusses the biological mechanisms regulating CXCL14 expression and potential CXCL14 mediated functional implications in a variety of cells.