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      Extracellular Histones Activate Plasma Membrane Toll-Like Receptor 9 to Trigger Calcium Oscillations in Rat Pancreatic Acinar Tumor Cell AR4-2J

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      Cells
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      extracellular histones, TLR9, calcium oscillations, AR4-2J

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          Abstract

          In acute pancreatitis, histones are released by infiltrating neutrophils, but how histones modulate pancreatic acinar cell function has not been investigated. We have examined histone modulation of rat pancreatic acini and pancreatic acinar tumor cell AR4-2J by calcium imaging. Histones were found to have no effect on calcium in pancreatic acini but blocked calcium oscillations induced by cholecystokinin or acetylcholine. Both mixed (Hx) and individual (H1, H2A, H2B, H3, H4) histones induced calcium oscillations in AR4-2J. RT-PCR and Western blot verified the expression of histone-targeted Toll-like receptor (TLR) 2, 4 and 9. Immunocytochemistry identified TLR2/TLR4 on apical plasma membrane and TLR9 in zymogen granule regions in pancreatic acini. TLR2 was found on neighboring and TLR9 on peripheral plasma membranes, but TLR4 was in the nucleus in AR4-2J clusters. Neither TLR2 agonist zymosan-A nor TLR4 agonist lipopolysaccharide had any effect on calcium, but TLR9 agonist ODN1826 induced calcium oscillations; TLR9 antagonist ODN2088 blocked H4-induced calcium oscillations in AR4-2J, which also disappeared after treatment of AR4-2J with glucocorticoid dexamethasone, with concurrent TLR9 migration from plasma membrane to cell interiors. TLR9 down regulation with siRNA suppressed H4-induced calcium oscillations. These data together suggest that extracellular histones activate plasma membrane TLR9 to trigger calcium oscillations in AR4-2J cells.

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          Crystal structure of the TLR4-MD-2 complex with bound endotoxin antagonist Eritoran.

          TLR4 and MD-2 form a heterodimer that recognizes LPS (lipopolysaccharide) from Gram-negative bacteria. Eritoran is an analog of LPS that antagonizes its activity by binding to the TLR4-MD-2 complex. We determined the structure of the full-length ectodomain of the mouse TLR4 and MD-2 complex. We also produced a series of hybrids of human TLR4 and hagfish VLR and determined their structures with and without bound MD-2 and Eritoran. TLR4 is an atypical member of the LRR family and is composed of N-terminal, central, and C-terminal domains. The beta sheet of the central domain shows unusually small radii and large twist angles. MD-2 binds to the concave surface of the N-terminal and central domains. The interaction with Eritoran is mediated by a hydrophobic internal pocket in MD-2. Based on structural analysis and mutagenesis experiments on MD-2 and TLR4, we propose a model of TLR4-MD-2 dimerization induced by LPS.
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            Intracellular localization of Toll-like receptor 9 prevents recognition of self DNA but facilitates access to viral DNA.

            Toll-like receptors (TLRs) sense infection by detecting molecular structures of microbial origin. TLR3, TLR7 and TLR9 recognize nucleic acids and are localized to intracellular compartments where they normally respond to viral nucleic acids. The purpose for this intracellular localization, however, is not clear. Here we describe a chimeric TLR9 receptor that localized to the cell surface and responded normally to synthetic TLR9 ligands but not to viral nucleic acids. However, the 'relocated' chimeric TLR9 receptor was able to recognize self DNA, which does not stimulate wild-type TLR9. These data demonstrated that intracellular localization of TLR9 was not required for ligand recognition. Instead, localization of the nucleic acid-sensing TLRs is critical in discriminating between self and nonself nucleic acid.
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              TLR signaling in the gut in health and disease.

              The human intestine has evolved in the presence of diverse enteric microflora. TLRs convert the recognition of pathogen-associated molecules in the gut into signals for anti-microbial peptide expression, barrier fortification, and proliferation of epithelial cells. Healing of injured intestinal epithelium and clearance of intramucosal bacteria require the presence of intact TLR signaling. Nucleotide oligomerization domain (Nod)1 and Nod2 are additional pattern recognition receptors that are required for defense against invasive enteric pathogens. Through spatial and functional localization of TLR and Nod molecules, the normal gut maintains a state of controlled inflammation. By contrast, patients with inflammatory bowel disease demonstrate inflammation in response to the normal flora. A subset of these patients carry polymorphisms in TLR and CARD15/NOD2 genes. A better understanding of the delicate regulation of TLR and Nod molecules in the gut may lead to improved treatment for enteric infections and idiopathic inflammatory bowel diseases.
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                Author and article information

                Journal
                Cells
                Cells
                cells
                Cells
                MDPI
                2073-4409
                20 December 2018
                January 2019
                : 8
                : 1
                : 3
                Affiliations
                Institute of Cell Biology, Beijing Normal University, Beijing 100875, China; guohaiyan111@ 123456163.com
                Author notes
                [* ]Correspondence: zjcui@ 123456bnu.edu.cn ; Tel: +86-10-5880-9162
                Author information
                https://orcid.org/0000-0003-4252-4876
                Article
                cells-08-00003
                10.3390/cells8010003
                6356355
                30577532
                2da3476b-b96a-4696-bff5-833783ec0860
                © 2018 by the authors.

                Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ( http://creativecommons.org/licenses/by/4.0/).

                History
                : 12 November 2018
                : 17 December 2018
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                extracellular histones,tlr9,calcium oscillations,ar4-2j
                extracellular histones, tlr9, calcium oscillations, ar4-2j

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