{"gene":"LY96","run_date":"2026-04-28T18:30:27","timeline":{"discoveries":[{"year":1999,"finding":"MD-2 (LY96) is physically associated with TLR4 on the cell surface and is required for LPS responsiveness; transfection of TLR4 alone does not confer LPS signaling, but co-expression with MD-2 does.","method":"Cell transfection, co-immunoprecipitation, functional LPS signaling assay","journal":"The Journal of experimental medicine","confidence":"High","confidence_rationale":"Tier 2 — original discovery paper, reciprocal association demonstrated, replicated by many subsequent labs","pmids":["10359581"],"is_preprint":false},{"year":2001,"finding":"MD-2 directly binds bacterial LPS with an apparent KD of ~65 nM, independent of LBP and CD14; LBP competes with MD-2 for LPS binding.","method":"Recombinant human MD-2 production, multiple LPS-binding assays including competitive binding","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — direct in vitro binding assay with five independent methods and KD determination","pmids":["11500507"],"is_preprint":false},{"year":2001,"finding":"MD-2 exists primarily as disulfide-linked oligomers; monomeric MD-2 preferentially binds TLR4 and confers LPS responsiveness more efficiently than multimeric forms. Intermolecular disulfide bonds (>2) stabilize the MD-2 multimer.","method":"Recombinant protein production, SDS-PAGE, site-directed mutagenesis, functional NF-κB reporter assay","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 — biochemical reconstitution with mutagenesis and functional validation","pmids":["11593030"],"is_preprint":false},{"year":2001,"finding":"Human MD-2 confers species-specific LPS recognition on TLR4: hMD-2 paired with mTLR4 confers responsiveness to lipid A but not lipid IVa, demonstrating that MD-2 directly determines the fine specificity of LPS recognition.","method":"Chimeric receptor transfection, NF-κB activation assay with lipid A and lipid IVa","journal":"International immunology","confidence":"High","confidence_rationale":"Tier 2 — epistasis by chimeric receptor swap, replicated and confirmed by later structural studies","pmids":["11717200"],"is_preprint":false},{"year":2001,"finding":"N-linked glycosylation of MD-2 at Asn26 and Asn114 is required for full LPS-induced signaling (IL-8, JNK activation); the double glycosylation mutant fails to support LPS-induced NF-κB activation or IL-8 secretion, though cell surface expression of MD-2 is not dependent on these sites.","method":"Site-directed mutagenesis, cross-linking assay, luciferase reporter assay, JNK phosphorylation","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — mutagenesis with multiple functional readouts","pmids":["11706042"],"is_preprint":false},{"year":2001,"finding":"Gln22 of mouse MD-2 is essential for species-specific LPS-mimetic signaling by taxol but not for LPS signaling, demonstrating that MD-2 is responsible for taxol's species-specific activity.","method":"Site-directed mutagenesis of MD-2, NF-κB activation assay in transfected HEK293 cells","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 1 — mutagenesis pinpointing a single residue with functional validation","pmids":["11123270"],"is_preprint":false},{"year":2002,"finding":"MD-2 is essential for correct intracellular distribution and cell surface expression of TLR4; in MD-2−/− embryonic fibroblasts, TLR4 is retained in the Golgi apparatus rather than reaching the plasma membrane.","method":"MD-2 knockout mouse generation, subcellular fractionation/immunofluorescence of TLR4 localization, LPS challenge","journal":"Nature immunology","confidence":"High","confidence_rationale":"Tier 2 — genetic KO with direct localization experiment and in vivo phenotype, independently confirmed","pmids":["12055629"],"is_preprint":false},{"year":2002,"finding":"MD-2 physically associates with both TLR4 and TLR2 (more weakly with TLR2), enables TLR2 to respond to LPS and lipid A, and enhances TLR2-mediated responses to Gram-negative bacteria and various bacterial ligands.","method":"Transfection of TLR2/TLR4 with MD-2, co-immunoprecipitation, chemokine production assay","journal":"Journal of endotoxin research","confidence":"Medium","confidence_rationale":"Tier 3 — single Co-IP and functional assay, single lab","pmids":["11521063"],"is_preprint":false},{"year":2002,"finding":"Monomeric recombinant MD-2 binds TLR4 in solution; MD-2 multimerization is stabilized by more than two intermolecular disulfide bonds; monomeric form is the active TLR4-binding species.","method":"In vitro binding assay, SDS-PAGE, site-directed mutagenesis of Cys residues, NF-κB reporter assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — direct biochemical reconstitution with mutagenesis","pmids":["11976338"],"is_preprint":false},{"year":2003,"finding":"The intrachain disulfide bond between Cys95 and Cys105 of MD-2 is critical for LPS responsiveness; substitution of either alone abolishes activity while substituting both partially restores it; most Cys residues lie on the surface and form inter/intrachain disulfide bridges.","method":"Site-directed mutagenesis of all 7 Cys residues, NF-κB reporter assay, structural analysis","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 — systematic mutagenesis of all Cys residues with functional readout","pmids":["12642668"],"is_preprint":false},{"year":2004,"finding":"MD-2 forms a stable monomeric bioactive complex with endotoxin monomer (generated via CD14), which at picomolar concentrations delivers endotoxin to TLR4 and activates cells; TLR4-dependent cell activation requires sequential transfer of endotoxin through LBP→CD14→MD-2→TLR4.","method":"Purification of endotoxin-MD-2 complex, cell activation assay at picomolar concentrations, competitive inhibition with excess MD-2","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 — reconstitution of purified complex with functional validation and mechanistic ordering","pmids":["15010525"],"is_preprint":false},{"year":2004,"finding":"Basic amino acid clusters Lys89-Arg90-Lys91 and Lys125-Lys125 on the surface of MD-2 are required for LPS signaling; these residues lie at the edge of the beta-sheet sandwich near the hydrophobic pocket. MD-2 adopts a beta-sandwich fold predicted by structural modeling and confirmed by CD spectroscopy.","method":"Structural homology modeling, CD spectroscopy, site-directed mutagenesis, functional LPS signaling assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — mutagenesis with functional validation combined with structural model confirmed by spectroscopy","pmids":["15111623"],"is_preprint":false},{"year":2005,"finding":"Monomeric MD-2 (but not multimeric) binds TLR4 with apparent Kd of ~12 nM; LPS antagonist E5564 inhibits cellular activation by competitively preventing LPS binding to MD-2; endogenous soluble MD-2 in human serum (~50 nM) is required for TLR4-mediated LPS responses.","method":"Binding affinity measurement, competitive inhibition assay, depletion of soluble MD-2 from serum, TLR4-Fc fusion protein blocking","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal biochemical approaches, KD measured, competitive inhibition demonstrated","pmids":["16272300"],"is_preprint":false},{"year":2005,"finding":"MD-2 is the principal molecular target for LPS-dependent antagonism by under-acylated LPS (tetra-acylated P. gingivalis LPS and penta-acylated msbB LPS); antagonism occurs at soluble MD-2 and competitive binding to MD-2's LPS-binding site is the main mechanism.","method":"Immunoprecipitation of sCD14 and sMD-2, competitive binding, HEK293 reconstituted TLR4 system","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal assays in reconstituted system identifying MD-2 as key target","pmids":["16177092"],"is_preprint":false},{"year":2005,"finding":"MD-2 amino acid regions 57–79 and 108–135, specifically residues Thr57, Val61, and Glu122, determine the agonist vs. antagonist activity of lipid IVa in a species-specific manner.","method":"Human/mouse chimeric MD-2 expression, site-directed mutagenesis, NF-κB activation assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — systematic chimeric and point mutagenesis with functional readout","pmids":["16407172"],"is_preprint":false},{"year":2006,"finding":"MD-2 residue Gly59 is critical for LPS binding outside the 119–132 region; Phe126 and Gly129 of MD-2 regulate ligand-induced TLR4 receptor clustering independently of LPS binding; receptor clustering and dissociation depend on TLR4 signaling and endosomal acidification.","method":"MD-2 alanine scanning mutagenesis, LPS binding assay, TLR4 clustering assay by microscopy, endosomal acidification inhibition","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 1 — mutagenesis separating LPS binding from receptor clustering, multiple assays","pmids":["16670331"],"is_preprint":false},{"year":2006,"finding":"MD-2 has a hydrophobic binding pocket that is also recognized by the fluorescent probe bis-ANS with sub-10 nM affinity; the bis-ANS binding site overlaps with the LPS binding site near Trp of MD-2; photoincorporation of bis-ANS inhibits LPS responsiveness.","method":"Fluorescence binding assay, UV cross-linking/photoincorporation, NF-κB reporter assay","journal":"FASEB journal","confidence":"High","confidence_rationale":"Tier 1 — direct binding measurement with KD and functional inhibition by covalent probe","pmids":["16940155"],"is_preprint":false},{"year":2007,"finding":"Crystal structure of human MD-2 alone and in complex with tetra-acylated lipid IVa at 2.0 and 2.2 Å: MD-2 has a deep hydrophobic cavity between two beta-sheets; four acyl chains of lipid IVa are fully enclosed in the cavity; phosphorylated glucosamine moieties sit at the cavity entrance.","method":"X-ray crystallography at 2.0 and 2.2 Å resolution","journal":"Science","confidence":"High","confidence_rationale":"Tier 1 — atomic-resolution crystal structure, foundational structural paper","pmids":["17569869"],"is_preprint":false},{"year":2008,"finding":"Paclitaxel binds human MD-2 in a dose-dependent and anti-MD-2 antibody-inhibitable manner; species specificity of paclitaxel TLR4 activation is determined by murine MD-2 (not TLR4); murine MD-2 Phe126 acts as a bridge for TLR4·MD-2 dimerization; paclitaxel binding pocket on MD-2 is characterized computationally.","method":"ELISA-based binding assay, chimeric receptor transfection, NF-κB activation assay, molecular docking","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — direct binding demonstrated with antibody competition, chimeric receptor epistasis, multiple cell lines","pmids":["18650420"],"is_preprint":false},{"year":2008,"finding":"Taxanes (paclitaxel and docetaxel) bind human MD-2 at a site overlapping with LPS and bis-ANS, inhibiting LPS signaling in human TLR4/MD-2 system; circular dichroism reveals conformational changes in human MD-2 upon taxane binding.","method":"Competitive fluorescence displacement, CD spectroscopy, molecular docking, NF-κB reporter assay","journal":"FEBS letters","confidence":"High","confidence_rationale":"Tier 1 — direct binding with competitive assay, conformational analysis by CD, functional validation","pmids":["18977229"],"is_preprint":false},{"year":2008,"finding":"Discrete regions of MD-2 (residues 57–66 and 82–89) and TLR4 LRR14 in the C-terminus are required for lipid IVa-induced signaling; electrostatic surface potential changes in both MD-2 and TLR4 enable lipid IVa signaling; a single TLR4 residue in the glycan-free flank confers ability to respond to lipid IVa.","method":"Chimeric horse/human MD-2 and TLR4 expression, site-directed mutagenesis, NF-κB reporter assay","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 1 — systematic chimeric and point mutagenesis of both MD-2 and TLR4 identifying specific mechanistic residues","pmids":["18606678"],"is_preprint":false},{"year":2009,"finding":"Crystal structure of the TLR4-MD-2-LPS complex at atomic resolution reveals an M-shaped 2:2:2 heterohexamer; five of six LPS lipid chains are buried in MD-2's hydrophobic pocket; the sixth chain is exposed and contacts conserved TLR4 phenylalanines; LPS phosphate groups form ionic interactions with positively charged residues on TLR4 and MD-2 to drive dimerization; MD-2 F126 loop undergoes localized conformational change supporting the interface.","method":"X-ray crystallography of TLR4-MD-2-LPS complex","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 — high-resolution crystal structure of full ternary complex, most highly cited paper in corpus","pmids":["19252480"],"is_preprint":false},{"year":2009,"finding":"Thiol-reactive compounds (fluorescent maleimides, auranofin, JTT-705) form covalent bonds with the free Cys133 of MD-2 and inhibit LPS-induced TLR4 signaling; Cys133 lies within the hydrophobic LPS-binding pocket and its modification blocks LPS signaling in vitro and in vivo.","method":"Covalent labeling, mass spectrometry identification of Cys133, NF-κB reporter assay, in vivo TNF-α production assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — covalent chemical probe identifying specific residue, in vitro and in vivo validation","pmids":["19473973"],"is_preprint":false},{"year":2009,"finding":"Both mouse TLR4 and mouse MD-2 are required for lipid IVa activation; ionic interactions between the 4'-phosphate of lipid IVa and positively charged mouse TLR4 residues Lys367 and Arg434 (absent in human) at the dimerization interface drive species-specific agonism; charge reversal mutations convert mouse to human-like responses and vice versa.","method":"Stable TLR4 cell lines, purified monomeric MD-2, MD-2-deficient macrophages, site-directed mutagenesis, computational modeling","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — mutagenesis with multiple functional assays including primary macrophages, replicated computationally","pmids":["20018893"],"is_preprint":false},{"year":2009,"finding":"Morphine and other opioids non-stereoselectively bind to the LPS-binding pocket of MD-2 (in silico docking) and activate TLR4 signaling in vitro; this activity is blocked by classical TLR4 antagonists and by naloxone non-stereoselectively.","method":"In silico docking to MD-2 pocket, in vitro TLR4 signaling assay, TLR4 KO mouse, pharmacological blockade in vivo","journal":"Brain, behavior, and immunity","confidence":"Medium","confidence_rationale":"Tier 2 — in silico + in vitro + in vivo convergent data, but docking is computational; confirmed by TLR4 KO","pmids":["19679181"],"is_preprint":false},{"year":2010,"finding":"MD-2 residues Tyr42, Arg69, Asp122, and Leu125 determine species-specific lipid IVa activation; residues 122 and 125 reside at the dimerization interface near the pocket entrance affecting receptor dimerization; residues 42 and 69 are at the MD-2/TLR4 interaction surface affecting binding angle.","method":"Systematic site-directed mutagenesis of human and mouse MD-2, NF-κB activation assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — systematic mutagenesis identifying specific mechanistic residues for species specificity","pmids":["20592019"],"is_preprint":false},{"year":2010,"finding":"A novel alternatively spliced isoform of human MD-2, MD-2 short (MD-2s), lacking exon 2, is glycosylated and secreted, binds LPS and TLR4, but fails to activate NF-κB; MD-2s competitively inhibits MD-2 binding to TLR4 and negatively regulates LPS-induced TLR4 signaling; it is upregulated by IFN-γ, IL-6, and TLR4 stimulation.","method":"Molecular cloning, expression, co-immunoprecipitation, NF-κB reporter assay, competitive binding","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 — characterization of endogenous splice variant with multiple mechanistic assays","pmids":["20435923"],"is_preprint":false},{"year":2011,"finding":"Intracellular TLR4/MD-2 in macrophages (those lacking PRAT4A-dependent cell surface expression) can sense phagocytosed bacteria and activate unique LPS-dependent gene sets (MyD88-dependent chemokines and co-stimulatory molecules) but not TRIF-dependent type I IFN production.","method":"PRAT4A KO macrophages, flow cytometry for surface TLR4, cytokine measurement, heat-killed bacteria stimulation","journal":"International immunology","confidence":"High","confidence_rationale":"Tier 2 — genetic KO with specific localization and functional dissection of intracellular vs surface TLR4/MD-2","pmids":["21712422"],"is_preprint":false},{"year":2011,"finding":"Endotoxin·albumin complexes transfer endotoxin monomers to MD-2 and MD-2·TLR4(ecd) with KD ~4 nM and activate TLR4-dependent cells independently of CD14, identifying albumin as an alternate endotoxin carrier to MD-2.","method":"Purified component binding assay, radiolabeled endotoxin transfer, cell activation assay","journal":"Innate immunity","confidence":"High","confidence_rationale":"Tier 1 — reconstitution with purified components and KD measurement","pmids":["21994253"],"is_preprint":false},{"year":2012,"finding":"Crystal structures of mouse TLR4/MD-2/LPS and TLR4/MD-2/lipid IVa at 2.5 and 2.7 Å reveal that lipid IVa in mouse complex occupies nearly the same space as LPS and forms an agonistic 2:2:2 complex; human MD-2 binds lipid IVa in an entirely different antagonistic orientation.","method":"X-ray crystallography of mouse TLR4/MD-2/LPS and TLR4/MD-2/lipid IVa complexes","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 — atomic-resolution crystal structures providing direct structural explanation for species specificity","pmids":["22532668"],"is_preprint":false},{"year":2013,"finding":"SAA3 (serum amyloid A3) directly binds MD-2 (not TLR4) with KD ~2.2 μM, activates p38 and NF-κB signaling via TLR4/MD-2/MyD88-dependent pathway, stimulates cell migration and IL-6/TNF-α production; this was demonstrated using synthetic peptides free of LPS contamination.","method":"Surface plasmon resonance, FLAG-tag co-precipitation, baculovirus coinfection, MyD88 KO cells, cytokine measurement","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 — SPR binding with KD, co-precipitation, and genetic epistasis with KO cells","pmids":["23858030"],"is_preprint":false},{"year":2013,"finding":"Globotetraosylceramide (Gb4) binds directly to TLR4-MD-2 (demonstrated by co-precipitation with recombinant MD-2 and native PAGE) and competes with LPS, attenuating LPS toxicity; A4galt-deficient mice lacking Gb4 show higher LPS sensitivity.","method":"Co-precipitation with recombinant MD-2, native PAGE, A4galt KO mouse, docking model","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 — direct binding assay with recombinant MD-2 plus genetic KO mouse with in vivo validation","pmids":["23471986"],"is_preprint":false},{"year":2014,"finding":"PTX3 (long pentraxin 3) directly binds MD-2 in vitro and requires TLR4/MD-2-mediated TRIF-dependent signaling for antifungal immune protection; MD-2-deficient mice phenocopy TLR4-deficient mice in susceptibility to Aspergillus; PTX3-opsonized conidia activate TLR4/MD-2/TRIF/IL-10 pathway.","method":"In vitro binding assay, Md2 KO mouse, adoptive transfer, cytokine measurement","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 — direct binding, KO mouse epistasis, adoptive transfer rescue","pmids":["25049357"],"is_preprint":false},{"year":2015,"finding":"Small molecule L6H21 inserts into the hydrophobic pocket of MD-2, forming hydrogen bonds with Arg90 and Tyr102, suppresses LPS-induced MAPK/NF-κB signaling in macrophages, and protects septic mice; MD-2 KO mice are protected from LPS shock, validating MD-2 as the therapeutic target.","method":"Molecular docking, SPR, ELISA, fluorescence assay, Western blot, MD-2 KO mouse, sepsis model","journal":"British journal of pharmacology","confidence":"High","confidence_rationale":"Tier 2 — direct binding measured by SPR, specific residues identified, KO genetic validation, in vivo confirmation","pmids":["26076332"],"is_preprint":false},{"year":2016,"finding":"Neoseptin-3 peptidomimetics bind as an asymmetric dimer within the MD-2 hydrophobic pocket (crystal structure at 2.57 Å), activate TLR4/MD-2 independently of CD14, and trigger canonical MyD88- and TRIF-dependent signaling, demonstrating that strong TLR4/MD-2 agonists need not mimic LPS structure.","method":"Chemical synthesis, crystal structure of mTLR4/MD-2/Neoseptin-3 at 2.57 Å, NF-κB reporter assay, MyD88/TRIF signaling assays","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 — crystal structure plus functional validation in multiple signaling assays","pmids":["26831104"],"is_preprint":false},{"year":2017,"finding":"Soluble CD83 (sCD83) binds MD-2 as its high-affinity binding partner on monocytes, alters TLR4 signaling by rapidly degrading IRAK-1, and induces anti-inflammatory mediators (IDO, IL-10, PGE2 via COX-2), leading to T cell unresponsiveness.","method":"Binding partner identification, co-immunoprecipitation, IRAK-1 degradation Western blot, cytokine measurement, T cell proliferation assay","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 — identification of MD-2 as sCD83 receptor with mechanistic downstream signaling characterization","pmids":["28193829"],"is_preprint":false},{"year":2018,"finding":"HMGB1 interacts with TLR4/MD-2 in a two-stage process: the A-box domain binds TLR4 with high affinity (appreciable dissociation rate) while the B-box domain binds MD-2 with low affinity but very slow dissociation rate; A-box alone antagonizes HMGB1 by competitively blocking TLR4 interaction.","method":"Surface plasmon resonance (SPR) with recombinant proteins, domain-specific interaction mapping","journal":"Molecular medicine","confidence":"High","confidence_rationale":"Tier 2 — SPR with full kinetic characterization of domain-specific interactions","pmids":["30134799"],"is_preprint":false},{"year":2020,"finding":"Heme binds MD-2 and activates TLR4 signaling requiring MD-2, TLR4, and CD14; MD-2 residues W23 and Y34 form a heme activation site (distinct from LPS site); W23A reduces heme-NF-κB activity 39% and Y34A by 78%; LPS activation is unaffected by these mutants.","method":"Heme-agarose/biotin-heme pulldown of recombinant MD-2, UV/visible spectroscopy, HEK293 transfection, NF-κB luciferase reporter, site-directed mutagenesis","journal":"Frontiers in immunology","confidence":"High","confidence_rationale":"Tier 1 — direct binding assay combined with mutagenesis identifying specific activation-site residues","pmids":["32695117"],"is_preprint":false},{"year":2021,"finding":"Crystal structure of mouse TLR4-MD-2 with C16-sulfatide at atomic resolution reveals three C16-sulfatide molecules bound to the MD-2 hydrophobic pocket, inducing an active 2:2 dimer conformation similar to LPS; short-chain sulfatides activate mouse TLR4-MD-2 (MyD88 and TRIF) while antagonizing human TLR4-MD-2, with activity dependent on the sulfate group and inversely related to fatty acid chain length.","method":"Crystal structure of mouse TLR4-MD-2/sulfatide, NF-κB reporter assay, TNF-α/IFN ELISA, MyD88/TRIF KO macrophages","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 — crystal structure plus genetic KO functional validation","pmids":["34290146"],"is_preprint":false},{"year":2021,"finding":"Zebrafish LY96 (ly96) encodes an MD-2 ortholog expressed in macrophage-like innate immune cells; zebrafish Md-2 and Tlr4ba form a functional complex that activates NF-κB in response to LPS; ly96 loss-of-function perturbs LPS-induced cytokine production in larval zebrafish.","method":"Single-cell RNA-seq, functional NF-κB reporter assay in co-transfected cells, zebrafish loss-of-function mutants, cytokine measurement","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 — functional complex reconstitution plus genetic KO in vivo with phenotypic readout","pmids":["33472906"],"is_preprint":false},{"year":2023,"finding":"Disulfiram (DSF) inhibits TLR4 signaling by covalently modifying Cys133 of MD-2, blocking LPS sensing and dimerization; DSF suppresses neuroinflammation and dopaminergic neuron loss in a mouse model of Parkinson's disease in a TLR4-dependent manner.","method":"Covalent modification assay, mutagenesis of Cys133, TLR4 dimerization assay, macrophage cytokine assay, MPTP mouse model of Parkinson's disease","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 — specific covalent modification of identified residue with mechanistic validation in vitro and in vivo","pmids":["37487070"],"is_preprint":false},{"year":2004,"finding":"Low or absent expression of MD-2 in human airway epithelia explains their LPS unresponsiveness; adenoviral delivery of MD-2 or exogenous recombinant MD-2 increases LPS responsiveness >100-fold; bacterial products and TNF-α + IFN-γ can induce MD-2 mRNA in these cells.","method":"Adenoviral MD-2 transduction, recombinant MD-2 addition, NF-κB-luciferase assay, HBD-2 mRNA induction","journal":"American journal of physiology. Lung cellular and molecular physiology","confidence":"High","confidence_rationale":"Tier 2 — loss-of-expression mechanism identified with rescue by MD-2 restoration","pmids":["15121639"],"is_preprint":false},{"year":2002,"finding":"MD-2 and TLR4 are required for mmLDL-induced macrophage spreading (actin polymerization); CHO cells transfected with TLR4/MD-2 but not TLR4 alone or TLR2 show elevated F-actin response to mmLDL; CD14 is also involved in mmLDL binding.","method":"CHO cell transfection, J774 CD14-deficient mutant, C3H/HeJ macrophages, F-actin assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — genetic mutant cells plus transfection reconstitution showing specific requirement for MD-2 co-expression","pmids":["12424240"],"is_preprint":false},{"year":2006,"finding":"IFN-γ regulates MD-2 promoter activity through the JAK-STAT pathway; a STAT inhibitor (SOCS3) blocks IFN-γ-mediated MD-2 promoter activation; T-cell cytokines (IFN-γ, TNF-α) sensitize intestinal epithelial cells to LPS by upregulating MD-2.","method":"MD-2 promoter cloning, reporter assay, SOCS3 overexpression, cytokine treatment, RT-PCR, Western blot","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — promoter cloning with reporter assay and dominant-negative STAT inhibition","pmids":["11923281"],"is_preprint":false},{"year":2009,"finding":"CpG methylation and histone deacetylation in the MD-2 promoter epigenetically silence MD-2 expression in intestinal epithelial cells; inhibition of methylation (5-azacytidine) or deacetylation (trichostatin A) restores MD-2 mRNA expression.","method":"Bisulfite sequencing of MD-2 promoter, 5-azacytidine and trichostatin A treatment, MD-2 mRNA measurement","journal":"Innate immunity","confidence":"High","confidence_rationale":"Tier 2 — direct bisulfite sequencing with pharmacological validation","pmids":["19710105"],"is_preprint":false},{"year":2007,"finding":"Soluble MD-2 is a type II acute-phase protein: its mRNA and protein are upregulated in mouse liver after acute-phase induction, secreted by human hepatocytes, and upregulated by IL-6; sMD-2 opsonizes Gram-negative bacteria and accelerates/enhances phagocytosis by neutrophils.","method":"Acute-phase response induction in mice, hepatocyte secretion assay, IL-6 stimulation, opsonization-phagocytosis assay","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 — in vivo and in vitro validation of acute-phase regulation and opsonin function","pmids":["18056837"],"is_preprint":false},{"year":2005,"finding":"RP105/MD-1 directly interacts with TLR4/MD-2 and inhibits LPS binding to the TLR4/MD-2 signaling complex; RP105 is a specific physiological inhibitor of TLR4 signaling in dendritic cells and macrophages.","method":"Co-immunoprecipitation, LPS binding competition assay, HEK293 and primary cell functional assays","journal":"Journal of endotoxin research","confidence":"High","confidence_rationale":"Tier 2 — direct interaction demonstrated with competitive LPS binding assay and primary cell validation","pmids":["16303092"],"is_preprint":false},{"year":2006,"finding":"Trypsin proteolytically cleaves MD-2 at multiple trypsin cleavage sites in intestinal epithelial cells, causing desensitization to LPS; endogenous MD-2 is predominantly retained in the ER calnexin-calreticulin cycle in normal intestinal epithelium.","method":"Biochemical proteolysis assay, subcellular fractionation (ER localization), LPS responsiveness assay, IBD tissue analysis","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 — direct biochemical evidence of proteolysis with functional consequence and localization data","pmids":["16547263"],"is_preprint":false},{"year":2015,"finding":"MD-2 residues Arg90 and Tyr102 mediate nickel/cobalt-induced TLR4 activation; nickel and cobalt activate human TLR4/MD-2 through TLR4 histidine residues (H456/H458 for cobalt) and require MD-2 for signal transduction, triggering both MyD88- and TRIF-dependent pathways.","method":"Site-directed mutagenesis of MD-2 and TLR4, NF-κB reporter assay, MyD88/TRIF pathway analysis","journal":"PloS one","confidence":"High","confidence_rationale":"Tier 1 — mutagenesis of both MD-2 and TLR4 identifying specific mechanistic residues","pmids":["25803856"],"is_preprint":false},{"year":2013,"finding":"Monophosphoryl lipid A (sMLA/MPLA) does not efficiently drive TLR4/MD-2 heterotetramer formation compared to diphosphoryl lipid A, explaining its weak MyD88 signaling; MD-2 F126A mutant confirms that heterotetramer formation is required for full sMLA signaling activity.","method":"MTS510 antibody staining for heterotetramer detection, TRAF6 recruitment assay, MD-2 F126A mutagenesis, NF-κB/MAPK activation assays","journal":"PloS one","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal assays linking heterotetramer formation to signaling output","pmids":["23638128"],"is_preprint":false}],"current_model":"LY96 (MD-2) is a secreted glycoprotein that non-covalently associates with the ectodomain of TLR4 and serves as the direct LPS-binding subunit of the TLR4/MD-2 receptor complex: it contains a deep hydrophobic beta-sandwich cavity that accommodates the acyl chains of lipid A, transfers endotoxin monomers (received sequentially from LBP and CD14) to the TLR4 ectodomain, and induces LPS-driven dimerization of two TLR4-MD-2 units into an M-shaped 2:2:2 heterohexamer that triggers MyD88- and TRIF-dependent inflammatory signaling; MD-2 also determines species-specific ligand discrimination (lipid IVa, taxol, sulfatides), acts as an opsonin and acute-phase protein in soluble form, and is regulated epigenetically by CpG methylation and by IFN-γ/JAK-STAT-dependent promoter activation."},"narrative":{"teleology":[{"year":1999,"claim":"The fundamental question of how TLR4 senses LPS was resolved by demonstrating that TLR4 alone is insufficient and requires a physically associated accessory protein, MD-2, for LPS responsiveness.","evidence":"Co-immunoprecipitation and functional LPS signaling assays in transfected cells","pmids":["10359581"],"confidence":"High","gaps":["Mechanism of MD-2–TLR4 association unknown","Whether MD-2 directly contacts LPS or acts allosterically was unresolved","Stoichiometry of the signaling complex undefined"]},{"year":2001,"claim":"MD-2 was established as the direct LPS-binding subunit of the complex, binding LPS at ~65 nM Kd independently of LBP/CD14, while its monomeric form was identified as the preferential TLR4-binding species and species-specific ligand discrimination was mapped to MD-2 rather than TLR4.","evidence":"Recombinant MD-2 LPS-binding assays, SDS-PAGE oligomer analysis, site-directed mutagenesis, chimeric human/mouse MD-2–TLR4 NF-κB assays, and glycosylation mutagenesis","pmids":["11500507","11593030","11717200","11706042","11123270"],"confidence":"High","gaps":["Three-dimensional structure of MD-2 unknown","Mechanism of LPS transfer from CD14 to MD-2 unresolved","Structural basis for species-specific discrimination unclear"]},{"year":2002,"claim":"MD-2 was shown to be required not only for signaling but also for TLR4 surface trafficking—without MD-2, TLR4 is retained in the Golgi—and to participate in recognition of minimally modified LDL, broadening its ligand scope beyond endotoxin.","evidence":"MD-2 knockout mouse with TLR4 subcellular localization analysis; CHO transfection with F-actin/mmLDL assays","pmids":["12055629","12424240"],"confidence":"High","gaps":["Chaperone or escort mechanism for MD-2-dependent TLR4 trafficking uncharacterized","Whether MD-2 directly binds mmLDL lipids was not demonstrated"]},{"year":2003,"claim":"Systematic mutagenesis of all seven cysteines revealed that the Cys95–Cys105 intrachain disulfide is critical for LPS responsiveness, establishing the disulfide architecture that stabilizes the functional fold.","evidence":"Complete Cys-to-Ala mutagenesis with NF-κB reporter assays","pmids":["12642668"],"confidence":"High","gaps":["No crystal structure yet to visualize disulfide geometry","Contribution of intermolecular disulfides to oligomer regulation not fully resolved"]},{"year":2004,"claim":"The sequential LPS transfer pathway LBP→CD14→MD-2→TLR4 was reconstituted, showing that a stable monomeric endotoxin·MD-2 complex activates TLR4 at picomolar concentrations, and MD-2 expression level was identified as the limiting factor for LPS responsiveness in airway epithelia.","evidence":"Purified endotoxin·MD-2 complex activation assays; adenoviral MD-2 rescue in human airway epithelial cells; mutagenesis of basic residue clusters","pmids":["15010525","15121639","15111623"],"confidence":"High","gaps":["Structural mechanism of CD14-to-MD-2 lipid transfer unknown","Kinetics of sequential transfer not measured"]},{"year":2005,"claim":"Quantitative binding studies established MD-2's Kd for TLR4 (~12 nM), identified MD-2 as the molecular target for LPS antagonism by under-acylated lipids, mapped species-specificity determinant residues (Thr57, Val61, Glu122), and revealed that RP105/MD-1 inhibits TLR4 signaling by directly interacting with TLR4/MD-2.","evidence":"Competitive binding assays, serum sMD-2 depletion, chimeric MD-2 mutagenesis, co-immunoprecipitation of RP105","pmids":["16272300","16177092","16407172","16303092"],"confidence":"High","gaps":["No atomic structure of full complex to rationalize antagonist/agonist switch","RP105/MD-1 interaction interface not mapped at residue level"]},{"year":2006,"claim":"Functional dissection separated LPS binding from receptor clustering (Phe126, Gly129), identified a hydrophobic pocket probe (bis-ANS) overlapping the LPS site, characterized IFN-γ/JAK-STAT-dependent transcriptional regulation, and showed trypsin proteolysis desensitizes intestinal MD-2.","evidence":"Alanine scanning mutagenesis, fluorescence binding, MD-2 promoter reporter assays with SOCS3 inhibition, biochemical proteolysis","pmids":["16670331","16940155","11923281","16547263"],"confidence":"High","gaps":["No crystal structure yet for full-length complex","In vivo relevance of trypsin cleavage in intestinal tolerance not confirmed genetically"]},{"year":2007,"claim":"The crystal structure of human MD-2 alone and with lipid IVa (2.0–2.2 Å) revealed the deep hydrophobic β-sandwich cavity that fully encloses four acyl chains, providing the first atomic-level explanation for lipid binding and antagonist accommodation, and soluble MD-2 was identified as an acute-phase opsonin.","evidence":"X-ray crystallography; acute-phase induction in mice, hepatocyte secretion, opsonization-phagocytosis assay","pmids":["17569869","18056837"],"confidence":"High","gaps":["Structure of agonist (hexa-acylated LPS) complex not yet solved","How the sixth acyl chain protrudes to contact TLR4 was not visible"]},{"year":2008,"claim":"Taxol was shown to bind directly to human MD-2 at the LPS-overlapping pocket, inducing conformational changes; species specificity of taxol TLR4 activation was mapped to murine MD-2 Phe126, and electrostatic surface potential changes in MD-2 and TLR4 enabling lipid IVa signaling were delineated.","evidence":"ELISA binding, competitive fluorescence displacement, CD spectroscopy, chimeric horse/human MD-2/TLR4 mutagenesis","pmids":["18650420","18977229","18606678"],"confidence":"High","gaps":["No crystal structure of taxol-MD-2 complex","Structural basis of conformational change upon non-lipid ligand binding unresolved"]},{"year":2009,"claim":"The landmark crystal structure of the TLR4–MD-2–LPS heterohexamer revealed the M-shaped 2:2:2 signaling complex: five acyl chains buried in the MD-2 pocket, the sixth exposed chain contacting TLR4*, and ionic LPS phosphate interactions driving dimerization; Cys133 in the pocket was identified as a druggable thiol target, and epigenetic silencing of MD-2 by CpG methylation was demonstrated.","evidence":"X-ray crystallography of TLR4–MD-2–LPS; covalent Cys133 labeling with mass spectrometry and in vivo TNF-α assay; bisulfite sequencing with pharmacological reactivation","pmids":["19252480","19473973","19710105","20018893"],"confidence":"High","gaps":["Dynamics of the sixth acyl chain exposure during activation not captured","Whether epigenetic silencing is reversible in vivo not tested"]},{"year":2010,"claim":"A splice variant (MD-2s) lacking exon 2 was characterized as a natural dominant-negative regulator that binds LPS and TLR4 but fails to signal, and additional species-specificity residues (Tyr42, Arg69, Asp122, Leu125) at the dimerization interface and TLR4 contact surface were mapped.","evidence":"Molecular cloning and co-immunoprecipitation of MD-2s; systematic mutagenesis of human/mouse MD-2","pmids":["20435923","20592019"],"confidence":"High","gaps":["In vivo function of MD-2s isoform not tested in knockout models","Full structural model of MD-2s fold and its inability to promote dimerization lacking"]},{"year":2011,"claim":"Intracellular TLR4/MD-2 (in the absence of surface expression) was shown to sense phagocytosed bacteria and activate MyD88-dependent but not TRIF-dependent genes, establishing compartment-specific signaling, and albumin was identified as an alternative endotoxin carrier to MD-2.","evidence":"PRAT4A KO macrophages with cytokine profiling; purified endotoxin·albumin transfer assay with Kd measurement","pmids":["21712422","21994253"],"confidence":"High","gaps":["Whether intracellular MD-2 source is recycled or newly synthesized is unknown","Physiological relevance of albumin-mediated transfer versus CD14-mediated transfer not quantified in vivo"]},{"year":2012,"claim":"Crystal structures of mouse TLR4/MD-2 with LPS and lipid IVa provided the structural explanation for species-specific agonism: lipid IVa adopts an agonistic orientation in mouse MD-2 (similar to LPS) but an entirely different antagonistic orientation in human MD-2.","evidence":"X-ray crystallography of mouse TLR4/MD-2/LPS and TLR4/MD-2/lipid IVa at 2.5–2.7 Å","pmids":["22532668"],"confidence":"High","gaps":["How the pocket selects between orientations at the biophysical level remains unclear","No structure of human MD-2 with agonist hexa-acyl LPS for direct comparison"]},{"year":2013,"claim":"The ligand scope of MD-2 was further expanded to endogenous ligands (SAA3 binding at ~2.2 μM Kd, Gb4 competing with LPS), and the requirement for heterotetramer formation for full MyD88 signaling was quantified using monophosphoryl lipid A and MD-2 F126A.","evidence":"SPR for SAA3, co-precipitation and native PAGE for Gb4, A4galt KO mouse; MTS510 antibody heterotetramer assay","pmids":["23858030","23471986","23638128"],"confidence":"High","gaps":["Structural basis for SAA3-MD-2 interaction unknown","Whether Gb4 inserts into the hydrophobic pocket or binds the surface not resolved"]},{"year":2015,"claim":"MD-2 was validated as a druggable target through small-molecule L6H21 binding to the hydrophobic pocket (Arg90, Tyr102), and the pocket was shown to mediate nickel/cobalt sensing through the same residues, revealing MD-2 as a sensor for transition metal allergens.","evidence":"SPR, molecular docking, MD-2 KO mouse sepsis model; site-directed mutagenesis for nickel/cobalt","pmids":["26076332","25803856"],"confidence":"High","gaps":["Crystal structure of L6H21-MD-2 complex not obtained","Physiological relevance of nickel sensing through MD-2 in human contact allergy not directly demonstrated"]},{"year":2016,"claim":"The crystal structure of neoseptin-3 (a peptidomimetic) bound as an asymmetric dimer within the MD-2 pocket demonstrated that structurally unrelated non-lipid molecules can fully activate TLR4/MD-2, decoupling agonism from LPS-like structure.","evidence":"Crystal structure of mTLR4/MD-2/neoseptin-3 at 2.57 Å with NF-κB and MyD88/TRIF signaling assays","pmids":["26831104"],"confidence":"High","gaps":["Whether neoseptin-3 induces identical downstream gene programs as LPS not determined","Human TLR4/MD-2 response to neoseptin-3 not tested"]},{"year":2017,"claim":"Soluble CD83 was identified as a new MD-2-binding immunomodulator that co-opts the TLR4/MD-2 axis to degrade IRAK-1 and induce tolerogenic mediators (IDO, IL-10, PGE2), establishing MD-2 as a receptor for immunosuppressive signaling.","evidence":"Binding partner identification, co-immunoprecipitation, IRAK-1 degradation, T cell proliferation assay","pmids":["28193829"],"confidence":"High","gaps":["Binding affinity and stoichiometry of sCD83–MD-2 not quantified","Whether sCD83 occupies the hydrophobic pocket or binds externally is unknown"]},{"year":2018,"claim":"The two-stage binding model for HMGB1 was established: HMGB1 A-box binds TLR4 (high affinity, fast off-rate) while B-box binds MD-2 (low affinity, very slow off-rate), explaining how A-box peptide antagonizes HMGB1-driven inflammation.","evidence":"SPR kinetics with domain-specific recombinant proteins","pmids":["30134799"],"confidence":"High","gaps":["Structural model of HMGB1-B-box/MD-2 interface lacking","Whether B-box occupies the lipid pocket not determined"]},{"year":2020,"claim":"A distinct heme-activation site on MD-2 (W23, Y34) was identified that is separate from the LPS-binding pocket, demonstrating that MD-2 harbors at least two functionally independent ligand-recognition surfaces.","evidence":"Heme-agarose pulldown of recombinant MD-2, UV/Vis spectroscopy, W23A/Y34A mutagenesis with NF-κB reporter","pmids":["32695117"],"confidence":"High","gaps":["Structural basis of the heme site not determined crystallographically","Whether heme and LPS can simultaneously activate MD-2 not tested"]},{"year":2021,"claim":"Crystal structures of mouse TLR4-MD-2 with C16-sulfatide revealed three sulfatide molecules filling the pocket in an agonistic dimer geometry, extending the structural repertoire of pocket-bound endogenous lipids, and zebrafish ly96 was shown to be a functional MD-2 ortholog required for LPS-induced cytokine production.","evidence":"Crystal structure of mouse TLR4-MD-2/sulfatide; zebrafish loss-of-function mutant with NF-κB assay","pmids":["34290146","33472906"],"confidence":"High","gaps":["Human TLR4/MD-2 sulfatide complex structure not available","Evolutionary conservation of the MD-2 pocket across vertebrates not systematically addressed"]},{"year":2023,"claim":"Disulfiram was shown to covalently modify Cys133 in the MD-2 pocket, blocking LPS sensing and TLR4 dimerization, and protecting dopaminergic neurons in a Parkinson's disease model, validating Cys133 as a therapeutic target for neuroinflammation.","evidence":"Covalent modification assay, Cys133 mutagenesis, TLR4 dimerization assay, MPTP mouse model","pmids":["37487070"],"confidence":"High","gaps":["Selectivity of disulfiram for MD-2 Cys133 versus other cellular thiols not fully characterized","Long-term safety and efficacy in neurodegeneration models not established"]},{"year":null,"claim":"Key unresolved questions include: the dynamic mechanism by which CD14 transfers endotoxin monomers into the MD-2 pocket, the structural basis for heme recognition at the W23/Y34 site versus the canonical lipid pocket, and how intracellular versus surface TLR4/MD-2 complexes achieve compartment-specific signaling output.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No structure of CD14–MD-2 transfer intermediate","No crystal structure of heme-bound MD-2","Mechanism governing MyD88-only versus TRIF-dependent signaling from different compartments not fully resolved"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0008289","term_label":"lipid binding","supporting_discovery_ids":[1,10,16,17,21,31,37,38]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0,6,26,46]},{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[0,21,34]},{"term_id":"GO:0048018","term_label":"receptor ligand activity","supporting_discovery_ids":[45]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,6,12]},{"term_id":"GO:0005576","term_label":"extracellular region","supporting_discovery_ids":[12,45]},{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[47]},{"term_id":"GO:0005794","term_label":"Golgi apparatus","supporting_discovery_ids":[6]}],"pathway":[{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[0,10,21,27,34,39]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[0,21,30,34,35]}],"complexes":["TLR4/MD-2","TLR4/MD-2/LPS heterohexamer","RP105/MD-1/TLR4/MD-2"],"partners":["TLR4","CD14","LBP","RP105","HMGB1","PTX3","CD83","SAA3"],"other_free_text":[]},"mechanistic_narrative":"LY96 (MD-2) is an essential co-receptor of TLR4 that functions as the direct lipid-binding subunit of the innate immune endotoxin-sensing complex, governing LPS recognition, species-specific ligand discrimination, and downstream inflammatory signaling. MD-2 adopts a β-sandwich fold enclosing a deep hydrophobic pocket that accommodates acyl chains of LPS and diverse ligands (lipid IVa, taxol, sulfatides, heme, nickel ions); five of six LPS acyl chains are buried in this pocket while the sixth is exposed to contact a partner TLR4, driving assembly of the signaling-competent M-shaped 2:2:2 TLR4–MD-2–LPS heterohexamer that activates MyD88- and TRIF-dependent pathways [PMID:17569869, PMID:19252480, PMID:34290146]. Monomeric MD-2 is the active TLR4-binding species (~12 nM Kd), receives endotoxin monomers sequentially from LBP and CD14, and is required for TLR4 surface trafficking; in its absence TLR4 is retained in the Golgi [PMID:12055629, PMID:16272300, PMID:15010525]. MD-2 expression is regulated epigenetically by CpG methylation and transcriptionally by IFN-γ/JAK-STAT signaling; soluble MD-2 circulates as an acute-phase protein that opsonizes Gram-negative bacteria and enhances neutrophil phagocytosis [PMID:19710105, PMID:11923281, PMID:18056837]."},"prefetch_data":{"uniprot":{"accession":"Q9Y6Y9","full_name":"Lymphocyte antigen 96","aliases":["ESOP-1","Protein MD-2"],"length_aa":160,"mass_kda":18.5,"function":"Binds bacterial lipopolysaccharide (LPS) (PubMed:17569869, PubMed:17803912). Cooperates with TLR4 in the innate immune response to bacterial lipopolysaccharide (LPS), and with TLR2 in the response to cell wall components from Gram-positive and Gram-negative bacteria (PubMed:11160242, PubMed:11593030). Enhances TLR4-dependent activation of NF-kappa-B (PubMed:10359581). Cells expressing both LY96 and TLR4, but not TLR4 alone, respond to LPS (PubMed:10359581)","subcellular_location":"Secreted, extracellular space; Secreted","url":"https://www.uniprot.org/uniprotkb/Q9Y6Y9/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/LY96","classification":"Not Classified","n_dependent_lines":6,"n_total_lines":1208,"dependency_fraction":0.004966887417218543},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/LY96","total_profiled":1310},"omim":[{"mim_id":"619990","title":"TRANSMEMBRANE p24 TRAFFICKING PROTEIN 7; TMED7","url":"https://www.omim.org/entry/619990"},{"mim_id":"605243","title":"LYMPHOCYTE ANTIGEN 96; LY96","url":"https://www.omim.org/entry/605243"},{"mim_id":"603030","title":"TOLL-LIKE RECEPTOR 4; TLR4","url":"https://www.omim.org/entry/603030"},{"mim_id":"187380","title":"TENASCIN C; TNC","url":"https://www.omim.org/entry/187380"},{"mim_id":"123886","title":"S100 CALCIUM-BINDING PROTEIN A9; S100A9","url":"https://www.omim.org/entry/123886"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Vesicles","reliability":"Approved"}],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in all","driving_tissues":[{"tissue":"lymphoid tissue","ntpm":61.4}],"url":"https://www.proteinatlas.org/search/LY96"},"hgnc":{"alias_symbol":["MD-2"],"prev_symbol":[]},"alphafold":{"accession":"Q9Y6Y9","domains":[{"cath_id":"2.60.40.770","chopping":"29-156","consensus_level":"medium","plddt":92.4318,"start":29,"end":156}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9Y6Y9","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9Y6Y9-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9Y6Y9-F1-predicted_aligned_error_v6.png","plddt_mean":87.69},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=LY96","jax_strain_url":"https://www.jax.org/strain/search?query=LY96"},"sequence":{"accession":"Q9Y6Y9","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9Y6Y9.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9Y6Y9/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9Y6Y9"}},"corpus_meta":[{"pmid":"19252480","id":"PMC_19252480","title":"The structural basis of lipopolysaccharide recognition by the TLR4-MD-2 complex.","date":"2009","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/19252480","citation_count":1865,"is_preprint":false},{"pmid":"10359581","id":"PMC_10359581","title":"MD-2, a molecule that confers lipopolysaccharide responsiveness on Toll-like receptor 4.","date":"1999","source":"The Journal of experimental medicine","url":"https://pubmed.ncbi.nlm.nih.gov/10359581","citation_count":1576,"is_preprint":false},{"pmid":"12055629","id":"PMC_12055629","title":"Essential role of MD-2 in LPS responsiveness and TLR4 distribution.","date":"2002","source":"Nature immunology","url":"https://pubmed.ncbi.nlm.nih.gov/12055629","citation_count":792,"is_preprint":false},{"pmid":"19679181","id":"PMC_19679181","title":"Evidence that opioids may have toll-like receptor 4 and MD-2 effects.","date":"2009","source":"Brain, behavior, and immunity","url":"https://pubmed.ncbi.nlm.nih.gov/19679181","citation_count":431,"is_preprint":false},{"pmid":"17569869","id":"PMC_17569869","title":"Crystal structures of human MD-2 and its complex with antiendotoxic lipid IVa.","date":"2007","source":"Science (New York, N.Y.)","url":"https://pubmed.ncbi.nlm.nih.gov/17569869","citation_count":370,"is_preprint":false},{"pmid":"12424240","id":"PMC_12424240","title":"Minimally modified LDL binds to CD14, induces macrophage spreading via TLR4/MD-2, and inhibits phagocytosis of apoptotic cells.","date":"2002","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/12424240","citation_count":317,"is_preprint":false},{"pmid":"11923281","id":"PMC_11923281","title":"TLR4 and MD-2 expression is regulated by immune-mediated signals in human intestinal epithelial cells.","date":"2002","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/11923281","citation_count":293,"is_preprint":false},{"pmid":"10644670","id":"PMC_10644670","title":"Mouse toll-like receptor 4.MD-2 complex mediates lipopolysaccharide-mimetic signal transduction by Taxol.","date":"2000","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/10644670","citation_count":282,"is_preprint":false},{"pmid":"15010525","id":"PMC_15010525","title":"Isolation of an endotoxin-MD-2 complex that produces Toll-like receptor 4-dependent cell activation at picomolar concentrations.","date":"2004","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/15010525","citation_count":281,"is_preprint":false},{"pmid":"22532668","id":"PMC_22532668","title":"Structural basis of species-specific endotoxin sensing by innate immune receptor TLR4/MD-2.","date":"2012","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/22532668","citation_count":278,"is_preprint":false},{"pmid":"15051069","id":"PMC_15051069","title":"Innate recognition of lipopolysaccharide by Toll-like receptor 4-MD-2.","date":"2004","source":"Trends in microbiology","url":"https://pubmed.ncbi.nlm.nih.gov/15051069","citation_count":235,"is_preprint":false},{"pmid":"11500507","id":"PMC_11500507","title":"MD-2 binds to bacterial lipopolysaccharide.","date":"2001","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/11500507","citation_count":230,"is_preprint":false},{"pmid":"23408095","id":"PMC_23408095","title":"Recognition of lipid A variants by the TLR4-MD-2 receptor complex.","date":"2013","source":"Frontiers in cellular and infection microbiology","url":"https://pubmed.ncbi.nlm.nih.gov/23408095","citation_count":212,"is_preprint":false},{"pmid":"11706042","id":"PMC_11706042","title":"MD-2 and TLR4 N-linked glycosylations are important for a functional lipopolysaccharide receptor.","date":"2001","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/11706042","citation_count":203,"is_preprint":false},{"pmid":"11717200","id":"PMC_11717200","title":"Human MD-2 confers on mouse Toll-like receptor 4 species-specific lipopolysaccharide recognition.","date":"2001","source":"International immunology","url":"https://pubmed.ncbi.nlm.nih.gov/11717200","citation_count":196,"is_preprint":false},{"pmid":"16785528","id":"PMC_16785528","title":"Analysis of TLR4 polymorphic variants: new insights into TLR4/MD-2/CD14 stoichiometry, structure, and signaling.","date":"2006","source":"Journal of immunology (Baltimore, Md. : 1950)","url":"https://pubmed.ncbi.nlm.nih.gov/16785528","citation_count":187,"is_preprint":false},{"pmid":"11593030","id":"PMC_11593030","title":"Secreted MD-2 is a large polymeric protein that efficiently confers lipopolysaccharide sensitivity to Toll-like receptor 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2 — original discovery paper, reciprocal association demonstrated, replicated by many subsequent labs\",\n      \"pmids\": [\"10359581\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"MD-2 directly binds bacterial LPS with an apparent KD of ~65 nM, independent of LBP and CD14; LBP competes with MD-2 for LPS binding.\",\n      \"method\": \"Recombinant human MD-2 production, multiple LPS-binding assays including competitive binding\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct in vitro binding assay with five independent methods and KD determination\",\n      \"pmids\": [\"11500507\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"MD-2 exists primarily as disulfide-linked oligomers; monomeric MD-2 preferentially binds TLR4 and confers LPS responsiveness more efficiently than multimeric forms. Intermolecular disulfide bonds (>2) stabilize the MD-2 multimer.\",\n      \"method\": \"Recombinant protein production, SDS-PAGE, site-directed mutagenesis, functional NF-κB reporter assay\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — biochemical reconstitution with mutagenesis and functional validation\",\n      \"pmids\": [\"11593030\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Human MD-2 confers species-specific LPS recognition on TLR4: hMD-2 paired with mTLR4 confers responsiveness to lipid A but not lipid IVa, demonstrating that MD-2 directly determines the fine specificity of LPS recognition.\",\n      \"method\": \"Chimeric receptor transfection, NF-κB activation assay with lipid A and lipid IVa\",\n      \"journal\": \"International immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — epistasis by chimeric receptor swap, replicated and confirmed by later structural studies\",\n      \"pmids\": [\"11717200\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"N-linked glycosylation of MD-2 at Asn26 and Asn114 is required for full LPS-induced signaling (IL-8, JNK activation); the double glycosylation mutant fails to support LPS-induced NF-κB activation or IL-8 secretion, though cell surface expression of MD-2 is not dependent on these sites.\",\n      \"method\": \"Site-directed mutagenesis, cross-linking assay, luciferase reporter assay, JNK phosphorylation\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis with multiple functional readouts\",\n      \"pmids\": [\"11706042\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Gln22 of mouse MD-2 is essential for species-specific LPS-mimetic signaling by taxol but not for LPS signaling, demonstrating that MD-2 is responsible for taxol's species-specific activity.\",\n      \"method\": \"Site-directed mutagenesis of MD-2, NF-κB activation assay in transfected HEK293 cells\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis pinpointing a single residue with functional validation\",\n      \"pmids\": [\"11123270\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"MD-2 is essential for correct intracellular distribution and cell surface expression of TLR4; in MD-2−/− embryonic fibroblasts, TLR4 is retained in the Golgi apparatus rather than reaching the plasma membrane.\",\n      \"method\": \"MD-2 knockout mouse generation, subcellular fractionation/immunofluorescence of TLR4 localization, LPS challenge\",\n      \"journal\": \"Nature immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO with direct localization experiment and in vivo phenotype, independently confirmed\",\n      \"pmids\": [\"12055629\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"MD-2 physically associates with both TLR4 and TLR2 (more weakly with TLR2), enables TLR2 to respond to LPS and lipid A, and enhances TLR2-mediated responses to Gram-negative bacteria and various bacterial ligands.\",\n      \"method\": \"Transfection of TLR2/TLR4 with MD-2, co-immunoprecipitation, chemokine production assay\",\n      \"journal\": \"Journal of endotoxin research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — single Co-IP and functional assay, single lab\",\n      \"pmids\": [\"11521063\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Monomeric recombinant MD-2 binds TLR4 in solution; MD-2 multimerization is stabilized by more than two intermolecular disulfide bonds; monomeric form is the active TLR4-binding species.\",\n      \"method\": \"In vitro binding assay, SDS-PAGE, site-directed mutagenesis of Cys residues, NF-κB reporter assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct biochemical reconstitution with mutagenesis\",\n      \"pmids\": [\"11976338\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"The intrachain disulfide bond between Cys95 and Cys105 of MD-2 is critical for LPS responsiveness; substitution of either alone abolishes activity while substituting both partially restores it; most Cys residues lie on the surface and form inter/intrachain disulfide bridges.\",\n      \"method\": \"Site-directed mutagenesis of all 7 Cys residues, NF-κB reporter assay, structural analysis\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — systematic mutagenesis of all Cys residues with functional readout\",\n      \"pmids\": [\"12642668\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"MD-2 forms a stable monomeric bioactive complex with endotoxin monomer (generated via CD14), which at picomolar concentrations delivers endotoxin to TLR4 and activates cells; TLR4-dependent cell activation requires sequential transfer of endotoxin through LBP→CD14→MD-2→TLR4.\",\n      \"method\": \"Purification of endotoxin-MD-2 complex, cell activation assay at picomolar concentrations, competitive inhibition with excess MD-2\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstitution of purified complex with functional validation and mechanistic ordering\",\n      \"pmids\": [\"15010525\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Basic amino acid clusters Lys89-Arg90-Lys91 and Lys125-Lys125 on the surface of MD-2 are required for LPS signaling; these residues lie at the edge of the beta-sheet sandwich near the hydrophobic pocket. MD-2 adopts a beta-sandwich fold predicted by structural modeling and confirmed by CD spectroscopy.\",\n      \"method\": \"Structural homology modeling, CD spectroscopy, site-directed mutagenesis, functional LPS signaling assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis with functional validation combined with structural model confirmed by spectroscopy\",\n      \"pmids\": [\"15111623\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Monomeric MD-2 (but not multimeric) binds TLR4 with apparent Kd of ~12 nM; LPS antagonist E5564 inhibits cellular activation by competitively preventing LPS binding to MD-2; endogenous soluble MD-2 in human serum (~50 nM) is required for TLR4-mediated LPS responses.\",\n      \"method\": \"Binding affinity measurement, competitive inhibition assay, depletion of soluble MD-2 from serum, TLR4-Fc fusion protein blocking\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal biochemical approaches, KD measured, competitive inhibition demonstrated\",\n      \"pmids\": [\"16272300\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"MD-2 is the principal molecular target for LPS-dependent antagonism by under-acylated LPS (tetra-acylated P. gingivalis LPS and penta-acylated msbB LPS); antagonism occurs at soluble MD-2 and competitive binding to MD-2's LPS-binding site is the main mechanism.\",\n      \"method\": \"Immunoprecipitation of sCD14 and sMD-2, competitive binding, HEK293 reconstituted TLR4 system\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal assays in reconstituted system identifying MD-2 as key target\",\n      \"pmids\": [\"16177092\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"MD-2 amino acid regions 57–79 and 108–135, specifically residues Thr57, Val61, and Glu122, determine the agonist vs. antagonist activity of lipid IVa in a species-specific manner.\",\n      \"method\": \"Human/mouse chimeric MD-2 expression, site-directed mutagenesis, NF-κB activation assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — systematic chimeric and point mutagenesis with functional readout\",\n      \"pmids\": [\"16407172\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"MD-2 residue Gly59 is critical for LPS binding outside the 119–132 region; Phe126 and Gly129 of MD-2 regulate ligand-induced TLR4 receptor clustering independently of LPS binding; receptor clustering and dissociation depend on TLR4 signaling and endosomal acidification.\",\n      \"method\": \"MD-2 alanine scanning mutagenesis, LPS binding assay, TLR4 clustering assay by microscopy, endosomal acidification inhibition\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis separating LPS binding from receptor clustering, multiple assays\",\n      \"pmids\": [\"16670331\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"MD-2 has a hydrophobic binding pocket that is also recognized by the fluorescent probe bis-ANS with sub-10 nM affinity; the bis-ANS binding site overlaps with the LPS binding site near Trp of MD-2; photoincorporation of bis-ANS inhibits LPS responsiveness.\",\n      \"method\": \"Fluorescence binding assay, UV cross-linking/photoincorporation, NF-κB reporter assay\",\n      \"journal\": \"FASEB journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct binding measurement with KD and functional inhibition by covalent probe\",\n      \"pmids\": [\"16940155\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Crystal structure of human MD-2 alone and in complex with tetra-acylated lipid IVa at 2.0 and 2.2 Å: MD-2 has a deep hydrophobic cavity between two beta-sheets; four acyl chains of lipid IVa are fully enclosed in the cavity; phosphorylated glucosamine moieties sit at the cavity entrance.\",\n      \"method\": \"X-ray crystallography at 2.0 and 2.2 Å resolution\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — atomic-resolution crystal structure, foundational structural paper\",\n      \"pmids\": [\"17569869\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Paclitaxel binds human MD-2 in a dose-dependent and anti-MD-2 antibody-inhibitable manner; species specificity of paclitaxel TLR4 activation is determined by murine MD-2 (not TLR4); murine MD-2 Phe126 acts as a bridge for TLR4·MD-2 dimerization; paclitaxel binding pocket on MD-2 is characterized computationally.\",\n      \"method\": \"ELISA-based binding assay, chimeric receptor transfection, NF-κB activation assay, molecular docking\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct binding demonstrated with antibody competition, chimeric receptor epistasis, multiple cell lines\",\n      \"pmids\": [\"18650420\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Taxanes (paclitaxel and docetaxel) bind human MD-2 at a site overlapping with LPS and bis-ANS, inhibiting LPS signaling in human TLR4/MD-2 system; circular dichroism reveals conformational changes in human MD-2 upon taxane binding.\",\n      \"method\": \"Competitive fluorescence displacement, CD spectroscopy, molecular docking, NF-κB reporter assay\",\n      \"journal\": \"FEBS letters\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct binding with competitive assay, conformational analysis by CD, functional validation\",\n      \"pmids\": [\"18977229\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Discrete regions of MD-2 (residues 57–66 and 82–89) and TLR4 LRR14 in the C-terminus are required for lipid IVa-induced signaling; electrostatic surface potential changes in both MD-2 and TLR4 enable lipid IVa signaling; a single TLR4 residue in the glycan-free flank confers ability to respond to lipid IVa.\",\n      \"method\": \"Chimeric horse/human MD-2 and TLR4 expression, site-directed mutagenesis, NF-κB reporter assay\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — systematic chimeric and point mutagenesis of both MD-2 and TLR4 identifying specific mechanistic residues\",\n      \"pmids\": [\"18606678\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Crystal structure of the TLR4-MD-2-LPS complex at atomic resolution reveals an M-shaped 2:2:2 heterohexamer; five of six LPS lipid chains are buried in MD-2's hydrophobic pocket; the sixth chain is exposed and contacts conserved TLR4 phenylalanines; LPS phosphate groups form ionic interactions with positively charged residues on TLR4 and MD-2 to drive dimerization; MD-2 F126 loop undergoes localized conformational change supporting the interface.\",\n      \"method\": \"X-ray crystallography of TLR4-MD-2-LPS complex\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — high-resolution crystal structure of full ternary complex, most highly cited paper in corpus\",\n      \"pmids\": [\"19252480\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Thiol-reactive compounds (fluorescent maleimides, auranofin, JTT-705) form covalent bonds with the free Cys133 of MD-2 and inhibit LPS-induced TLR4 signaling; Cys133 lies within the hydrophobic LPS-binding pocket and its modification blocks LPS signaling in vitro and in vivo.\",\n      \"method\": \"Covalent labeling, mass spectrometry identification of Cys133, NF-κB reporter assay, in vivo TNF-α production assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — covalent chemical probe identifying specific residue, in vitro and in vivo validation\",\n      \"pmids\": [\"19473973\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Both mouse TLR4 and mouse MD-2 are required for lipid IVa activation; ionic interactions between the 4'-phosphate of lipid IVa and positively charged mouse TLR4 residues Lys367 and Arg434 (absent in human) at the dimerization interface drive species-specific agonism; charge reversal mutations convert mouse to human-like responses and vice versa.\",\n      \"method\": \"Stable TLR4 cell lines, purified monomeric MD-2, MD-2-deficient macrophages, site-directed mutagenesis, computational modeling\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis with multiple functional assays including primary macrophages, replicated computationally\",\n      \"pmids\": [\"20018893\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Morphine and other opioids non-stereoselectively bind to the LPS-binding pocket of MD-2 (in silico docking) and activate TLR4 signaling in vitro; this activity is blocked by classical TLR4 antagonists and by naloxone non-stereoselectively.\",\n      \"method\": \"In silico docking to MD-2 pocket, in vitro TLR4 signaling assay, TLR4 KO mouse, pharmacological blockade in vivo\",\n      \"journal\": \"Brain, behavior, and immunity\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in silico + in vitro + in vivo convergent data, but docking is computational; confirmed by TLR4 KO\",\n      \"pmids\": [\"19679181\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"MD-2 residues Tyr42, Arg69, Asp122, and Leu125 determine species-specific lipid IVa activation; residues 122 and 125 reside at the dimerization interface near the pocket entrance affecting receptor dimerization; residues 42 and 69 are at the MD-2/TLR4 interaction surface affecting binding angle.\",\n      \"method\": \"Systematic site-directed mutagenesis of human and mouse MD-2, NF-κB activation assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — systematic mutagenesis identifying specific mechanistic residues for species specificity\",\n      \"pmids\": [\"20592019\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"A novel alternatively spliced isoform of human MD-2, MD-2 short (MD-2s), lacking exon 2, is glycosylated and secreted, binds LPS and TLR4, but fails to activate NF-κB; MD-2s competitively inhibits MD-2 binding to TLR4 and negatively regulates LPS-induced TLR4 signaling; it is upregulated by IFN-γ, IL-6, and TLR4 stimulation.\",\n      \"method\": \"Molecular cloning, expression, co-immunoprecipitation, NF-κB reporter assay, competitive binding\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — characterization of endogenous splice variant with multiple mechanistic assays\",\n      \"pmids\": [\"20435923\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Intracellular TLR4/MD-2 in macrophages (those lacking PRAT4A-dependent cell surface expression) can sense phagocytosed bacteria and activate unique LPS-dependent gene sets (MyD88-dependent chemokines and co-stimulatory molecules) but not TRIF-dependent type I IFN production.\",\n      \"method\": \"PRAT4A KO macrophages, flow cytometry for surface TLR4, cytokine measurement, heat-killed bacteria stimulation\",\n      \"journal\": \"International immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO with specific localization and functional dissection of intracellular vs surface TLR4/MD-2\",\n      \"pmids\": [\"21712422\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Endotoxin·albumin complexes transfer endotoxin monomers to MD-2 and MD-2·TLR4(ecd) with KD ~4 nM and activate TLR4-dependent cells independently of CD14, identifying albumin as an alternate endotoxin carrier to MD-2.\",\n      \"method\": \"Purified component binding assay, radiolabeled endotoxin transfer, cell activation assay\",\n      \"journal\": \"Innate immunity\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstitution with purified components and KD measurement\",\n      \"pmids\": [\"21994253\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Crystal structures of mouse TLR4/MD-2/LPS and TLR4/MD-2/lipid IVa at 2.5 and 2.7 Å reveal that lipid IVa in mouse complex occupies nearly the same space as LPS and forms an agonistic 2:2:2 complex; human MD-2 binds lipid IVa in an entirely different antagonistic orientation.\",\n      \"method\": \"X-ray crystallography of mouse TLR4/MD-2/LPS and TLR4/MD-2/lipid IVa complexes\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — atomic-resolution crystal structures providing direct structural explanation for species specificity\",\n      \"pmids\": [\"22532668\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"SAA3 (serum amyloid A3) directly binds MD-2 (not TLR4) with KD ~2.2 μM, activates p38 and NF-κB signaling via TLR4/MD-2/MyD88-dependent pathway, stimulates cell migration and IL-6/TNF-α production; this was demonstrated using synthetic peptides free of LPS contamination.\",\n      \"method\": \"Surface plasmon resonance, FLAG-tag co-precipitation, baculovirus coinfection, MyD88 KO cells, cytokine measurement\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — SPR binding with KD, co-precipitation, and genetic epistasis with KO cells\",\n      \"pmids\": [\"23858030\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Globotetraosylceramide (Gb4) binds directly to TLR4-MD-2 (demonstrated by co-precipitation with recombinant MD-2 and native PAGE) and competes with LPS, attenuating LPS toxicity; A4galt-deficient mice lacking Gb4 show higher LPS sensitivity.\",\n      \"method\": \"Co-precipitation with recombinant MD-2, native PAGE, A4galt KO mouse, docking model\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct binding assay with recombinant MD-2 plus genetic KO mouse with in vivo validation\",\n      \"pmids\": [\"23471986\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"PTX3 (long pentraxin 3) directly binds MD-2 in vitro and requires TLR4/MD-2-mediated TRIF-dependent signaling for antifungal immune protection; MD-2-deficient mice phenocopy TLR4-deficient mice in susceptibility to Aspergillus; PTX3-opsonized conidia activate TLR4/MD-2/TRIF/IL-10 pathway.\",\n      \"method\": \"In vitro binding assay, Md2 KO mouse, adoptive transfer, cytokine measurement\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct binding, KO mouse epistasis, adoptive transfer rescue\",\n      \"pmids\": [\"25049357\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Small molecule L6H21 inserts into the hydrophobic pocket of MD-2, forming hydrogen bonds with Arg90 and Tyr102, suppresses LPS-induced MAPK/NF-κB signaling in macrophages, and protects septic mice; MD-2 KO mice are protected from LPS shock, validating MD-2 as the therapeutic target.\",\n      \"method\": \"Molecular docking, SPR, ELISA, fluorescence assay, Western blot, MD-2 KO mouse, sepsis model\",\n      \"journal\": \"British journal of pharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct binding measured by SPR, specific residues identified, KO genetic validation, in vivo confirmation\",\n      \"pmids\": [\"26076332\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Neoseptin-3 peptidomimetics bind as an asymmetric dimer within the MD-2 hydrophobic pocket (crystal structure at 2.57 Å), activate TLR4/MD-2 independently of CD14, and trigger canonical MyD88- and TRIF-dependent signaling, demonstrating that strong TLR4/MD-2 agonists need not mimic LPS structure.\",\n      \"method\": \"Chemical synthesis, crystal structure of mTLR4/MD-2/Neoseptin-3 at 2.57 Å, NF-κB reporter assay, MyD88/TRIF signaling assays\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure plus functional validation in multiple signaling assays\",\n      \"pmids\": [\"26831104\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Soluble CD83 (sCD83) binds MD-2 as its high-affinity binding partner on monocytes, alters TLR4 signaling by rapidly degrading IRAK-1, and induces anti-inflammatory mediators (IDO, IL-10, PGE2 via COX-2), leading to T cell unresponsiveness.\",\n      \"method\": \"Binding partner identification, co-immunoprecipitation, IRAK-1 degradation Western blot, cytokine measurement, T cell proliferation assay\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — identification of MD-2 as sCD83 receptor with mechanistic downstream signaling characterization\",\n      \"pmids\": [\"28193829\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"HMGB1 interacts with TLR4/MD-2 in a two-stage process: the A-box domain binds TLR4 with high affinity (appreciable dissociation rate) while the B-box domain binds MD-2 with low affinity but very slow dissociation rate; A-box alone antagonizes HMGB1 by competitively blocking TLR4 interaction.\",\n      \"method\": \"Surface plasmon resonance (SPR) with recombinant proteins, domain-specific interaction mapping\",\n      \"journal\": \"Molecular medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — SPR with full kinetic characterization of domain-specific interactions\",\n      \"pmids\": [\"30134799\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Heme binds MD-2 and activates TLR4 signaling requiring MD-2, TLR4, and CD14; MD-2 residues W23 and Y34 form a heme activation site (distinct from LPS site); W23A reduces heme-NF-κB activity 39% and Y34A by 78%; LPS activation is unaffected by these mutants.\",\n      \"method\": \"Heme-agarose/biotin-heme pulldown of recombinant MD-2, UV/visible spectroscopy, HEK293 transfection, NF-κB luciferase reporter, site-directed mutagenesis\",\n      \"journal\": \"Frontiers in immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct binding assay combined with mutagenesis identifying specific activation-site residues\",\n      \"pmids\": [\"32695117\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Crystal structure of mouse TLR4-MD-2 with C16-sulfatide at atomic resolution reveals three C16-sulfatide molecules bound to the MD-2 hydrophobic pocket, inducing an active 2:2 dimer conformation similar to LPS; short-chain sulfatides activate mouse TLR4-MD-2 (MyD88 and TRIF) while antagonizing human TLR4-MD-2, with activity dependent on the sulfate group and inversely related to fatty acid chain length.\",\n      \"method\": \"Crystal structure of mouse TLR4-MD-2/sulfatide, NF-κB reporter assay, TNF-α/IFN ELISA, MyD88/TRIF KO macrophages\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure plus genetic KO functional validation\",\n      \"pmids\": [\"34290146\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Zebrafish LY96 (ly96) encodes an MD-2 ortholog expressed in macrophage-like innate immune cells; zebrafish Md-2 and Tlr4ba form a functional complex that activates NF-κB in response to LPS; ly96 loss-of-function perturbs LPS-induced cytokine production in larval zebrafish.\",\n      \"method\": \"Single-cell RNA-seq, functional NF-κB reporter assay in co-transfected cells, zebrafish loss-of-function mutants, cytokine measurement\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — functional complex reconstitution plus genetic KO in vivo with phenotypic readout\",\n      \"pmids\": [\"33472906\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Disulfiram (DSF) inhibits TLR4 signaling by covalently modifying Cys133 of MD-2, blocking LPS sensing and dimerization; DSF suppresses neuroinflammation and dopaminergic neuron loss in a mouse model of Parkinson's disease in a TLR4-dependent manner.\",\n      \"method\": \"Covalent modification assay, mutagenesis of Cys133, TLR4 dimerization assay, macrophage cytokine assay, MPTP mouse model of Parkinson's disease\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — specific covalent modification of identified residue with mechanistic validation in vitro and in vivo\",\n      \"pmids\": [\"37487070\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Low or absent expression of MD-2 in human airway epithelia explains their LPS unresponsiveness; adenoviral delivery of MD-2 or exogenous recombinant MD-2 increases LPS responsiveness >100-fold; bacterial products and TNF-α + IFN-γ can induce MD-2 mRNA in these cells.\",\n      \"method\": \"Adenoviral MD-2 transduction, recombinant MD-2 addition, NF-κB-luciferase assay, HBD-2 mRNA induction\",\n      \"journal\": \"American journal of physiology. Lung cellular and molecular physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — loss-of-expression mechanism identified with rescue by MD-2 restoration\",\n      \"pmids\": [\"15121639\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"MD-2 and TLR4 are required for mmLDL-induced macrophage spreading (actin polymerization); CHO cells transfected with TLR4/MD-2 but not TLR4 alone or TLR2 show elevated F-actin response to mmLDL; CD14 is also involved in mmLDL binding.\",\n      \"method\": \"CHO cell transfection, J774 CD14-deficient mutant, C3H/HeJ macrophages, F-actin assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic mutant cells plus transfection reconstitution showing specific requirement for MD-2 co-expression\",\n      \"pmids\": [\"12424240\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"IFN-γ regulates MD-2 promoter activity through the JAK-STAT pathway; a STAT inhibitor (SOCS3) blocks IFN-γ-mediated MD-2 promoter activation; T-cell cytokines (IFN-γ, TNF-α) sensitize intestinal epithelial cells to LPS by upregulating MD-2.\",\n      \"method\": \"MD-2 promoter cloning, reporter assay, SOCS3 overexpression, cytokine treatment, RT-PCR, Western blot\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — promoter cloning with reporter assay and dominant-negative STAT inhibition\",\n      \"pmids\": [\"11923281\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"CpG methylation and histone deacetylation in the MD-2 promoter epigenetically silence MD-2 expression in intestinal epithelial cells; inhibition of methylation (5-azacytidine) or deacetylation (trichostatin A) restores MD-2 mRNA expression.\",\n      \"method\": \"Bisulfite sequencing of MD-2 promoter, 5-azacytidine and trichostatin A treatment, MD-2 mRNA measurement\",\n      \"journal\": \"Innate immunity\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct bisulfite sequencing with pharmacological validation\",\n      \"pmids\": [\"19710105\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Soluble MD-2 is a type II acute-phase protein: its mRNA and protein are upregulated in mouse liver after acute-phase induction, secreted by human hepatocytes, and upregulated by IL-6; sMD-2 opsonizes Gram-negative bacteria and accelerates/enhances phagocytosis by neutrophils.\",\n      \"method\": \"Acute-phase response induction in mice, hepatocyte secretion assay, IL-6 stimulation, opsonization-phagocytosis assay\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vivo and in vitro validation of acute-phase regulation and opsonin function\",\n      \"pmids\": [\"18056837\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"RP105/MD-1 directly interacts with TLR4/MD-2 and inhibits LPS binding to the TLR4/MD-2 signaling complex; RP105 is a specific physiological inhibitor of TLR4 signaling in dendritic cells and macrophages.\",\n      \"method\": \"Co-immunoprecipitation, LPS binding competition assay, HEK293 and primary cell functional assays\",\n      \"journal\": \"Journal of endotoxin research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct interaction demonstrated with competitive LPS binding assay and primary cell validation\",\n      \"pmids\": [\"16303092\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Trypsin proteolytically cleaves MD-2 at multiple trypsin cleavage sites in intestinal epithelial cells, causing desensitization to LPS; endogenous MD-2 is predominantly retained in the ER calnexin-calreticulin cycle in normal intestinal epithelium.\",\n      \"method\": \"Biochemical proteolysis assay, subcellular fractionation (ER localization), LPS responsiveness assay, IBD tissue analysis\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct biochemical evidence of proteolysis with functional consequence and localization data\",\n      \"pmids\": [\"16547263\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"MD-2 residues Arg90 and Tyr102 mediate nickel/cobalt-induced TLR4 activation; nickel and cobalt activate human TLR4/MD-2 through TLR4 histidine residues (H456/H458 for cobalt) and require MD-2 for signal transduction, triggering both MyD88- and TRIF-dependent pathways.\",\n      \"method\": \"Site-directed mutagenesis of MD-2 and TLR4, NF-κB reporter assay, MyD88/TRIF pathway analysis\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis of both MD-2 and TLR4 identifying specific mechanistic residues\",\n      \"pmids\": [\"25803856\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Monophosphoryl lipid A (sMLA/MPLA) does not efficiently drive TLR4/MD-2 heterotetramer formation compared to diphosphoryl lipid A, explaining its weak MyD88 signaling; MD-2 F126A mutant confirms that heterotetramer formation is required for full sMLA signaling activity.\",\n      \"method\": \"MTS510 antibody staining for heterotetramer detection, TRAF6 recruitment assay, MD-2 F126A mutagenesis, NF-κB/MAPK activation assays\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal assays linking heterotetramer formation to signaling output\",\n      \"pmids\": [\"23638128\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"LY96 (MD-2) is a secreted glycoprotein that non-covalently associates with the ectodomain of TLR4 and serves as the direct LPS-binding subunit of the TLR4/MD-2 receptor complex: it contains a deep hydrophobic beta-sandwich cavity that accommodates the acyl chains of lipid A, transfers endotoxin monomers (received sequentially from LBP and CD14) to the TLR4 ectodomain, and induces LPS-driven dimerization of two TLR4-MD-2 units into an M-shaped 2:2:2 heterohexamer that triggers MyD88- and TRIF-dependent inflammatory signaling; MD-2 also determines species-specific ligand discrimination (lipid IVa, taxol, sulfatides), acts as an opsonin and acute-phase protein in soluble form, and is regulated epigenetically by CpG methylation and by IFN-γ/JAK-STAT-dependent promoter activation.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"LY96 (MD-2) is an essential co-receptor of TLR4 that functions as the direct lipid-binding subunit of the innate immune endotoxin-sensing complex, governing LPS recognition, species-specific ligand discrimination, and downstream inflammatory signaling. MD-2 adopts a β-sandwich fold enclosing a deep hydrophobic pocket that accommodates acyl chains of LPS and diverse ligands (lipid IVa, taxol, sulfatides, heme, nickel ions); five of six LPS acyl chains are buried in this pocket while the sixth is exposed to contact a partner TLR4, driving assembly of the signaling-competent M-shaped 2:2:2 TLR4–MD-2–LPS heterohexamer that activates MyD88- and TRIF-dependent pathways [PMID:17569869, PMID:19252480, PMID:34290146]. Monomeric MD-2 is the active TLR4-binding species (~12 nM Kd), receives endotoxin monomers sequentially from LBP and CD14, and is required for TLR4 surface trafficking; in its absence TLR4 is retained in the Golgi [PMID:12055629, PMID:16272300, PMID:15010525]. MD-2 expression is regulated epigenetically by CpG methylation and transcriptionally by IFN-γ/JAK-STAT signaling; soluble MD-2 circulates as an acute-phase protein that opsonizes Gram-negative bacteria and enhances neutrophil phagocytosis [PMID:19710105, PMID:11923281, PMID:18056837].\",\n  \"teleology\": [\n    {\n      \"year\": 1999,\n      \"claim\": \"The fundamental question of how TLR4 senses LPS was resolved by demonstrating that TLR4 alone is insufficient and requires a physically associated accessory protein, MD-2, for LPS responsiveness.\",\n      \"evidence\": \"Co-immunoprecipitation and functional LPS signaling assays in transfected cells\",\n      \"pmids\": [\"10359581\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of MD-2–TLR4 association unknown\", \"Whether MD-2 directly contacts LPS or acts allosterically was unresolved\", \"Stoichiometry of the signaling complex undefined\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"MD-2 was established as the direct LPS-binding subunit of the complex, binding LPS at ~65 nM Kd independently of LBP/CD14, while its monomeric form was identified as the preferential TLR4-binding species and species-specific ligand discrimination was mapped to MD-2 rather than TLR4.\",\n      \"evidence\": \"Recombinant MD-2 LPS-binding assays, SDS-PAGE oligomer analysis, site-directed mutagenesis, chimeric human/mouse MD-2–TLR4 NF-κB assays, and glycosylation mutagenesis\",\n      \"pmids\": [\"11500507\", \"11593030\", \"11717200\", \"11706042\", \"11123270\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Three-dimensional structure of MD-2 unknown\", \"Mechanism of LPS transfer from CD14 to MD-2 unresolved\", \"Structural basis for species-specific discrimination unclear\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"MD-2 was shown to be required not only for signaling but also for TLR4 surface trafficking—without MD-2, TLR4 is retained in the Golgi—and to participate in recognition of minimally modified LDL, broadening its ligand scope beyond endotoxin.\",\n      \"evidence\": \"MD-2 knockout mouse with TLR4 subcellular localization analysis; CHO transfection with F-actin/mmLDL assays\",\n      \"pmids\": [\"12055629\", \"12424240\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Chaperone or escort mechanism for MD-2-dependent TLR4 trafficking uncharacterized\", \"Whether MD-2 directly binds mmLDL lipids was not demonstrated\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Systematic mutagenesis of all seven cysteines revealed that the Cys95–Cys105 intrachain disulfide is critical for LPS responsiveness, establishing the disulfide architecture that stabilizes the functional fold.\",\n      \"evidence\": \"Complete Cys-to-Ala mutagenesis with NF-κB reporter assays\",\n      \"pmids\": [\"12642668\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No crystal structure yet to visualize disulfide geometry\", \"Contribution of intermolecular disulfides to oligomer regulation not fully resolved\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"The sequential LPS transfer pathway LBP→CD14→MD-2→TLR4 was reconstituted, showing that a stable monomeric endotoxin·MD-2 complex activates TLR4 at picomolar concentrations, and MD-2 expression level was identified as the limiting factor for LPS responsiveness in airway epithelia.\",\n      \"evidence\": \"Purified endotoxin·MD-2 complex activation assays; adenoviral MD-2 rescue in human airway epithelial cells; mutagenesis of basic residue clusters\",\n      \"pmids\": [\"15010525\", \"15121639\", \"15111623\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural mechanism of CD14-to-MD-2 lipid transfer unknown\", \"Kinetics of sequential transfer not measured\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Quantitative binding studies established MD-2's Kd for TLR4 (~12 nM), identified MD-2 as the molecular target for LPS antagonism by under-acylated lipids, mapped species-specificity determinant residues (Thr57, Val61, Glu122), and revealed that RP105/MD-1 inhibits TLR4 signaling by directly interacting with TLR4/MD-2.\",\n      \"evidence\": \"Competitive binding assays, serum sMD-2 depletion, chimeric MD-2 mutagenesis, co-immunoprecipitation of RP105\",\n      \"pmids\": [\"16272300\", \"16177092\", \"16407172\", \"16303092\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No atomic structure of full complex to rationalize antagonist/agonist switch\", \"RP105/MD-1 interaction interface not mapped at residue level\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Functional dissection separated LPS binding from receptor clustering (Phe126, Gly129), identified a hydrophobic pocket probe (bis-ANS) overlapping the LPS site, characterized IFN-γ/JAK-STAT-dependent transcriptional regulation, and showed trypsin proteolysis desensitizes intestinal MD-2.\",\n      \"evidence\": \"Alanine scanning mutagenesis, fluorescence binding, MD-2 promoter reporter assays with SOCS3 inhibition, biochemical proteolysis\",\n      \"pmids\": [\"16670331\", \"16940155\", \"11923281\", \"16547263\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No crystal structure yet for full-length complex\", \"In vivo relevance of trypsin cleavage in intestinal tolerance not confirmed genetically\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"The crystal structure of human MD-2 alone and with lipid IVa (2.0–2.2 Å) revealed the deep hydrophobic β-sandwich cavity that fully encloses four acyl chains, providing the first atomic-level explanation for lipid binding and antagonist accommodation, and soluble MD-2 was identified as an acute-phase opsonin.\",\n      \"evidence\": \"X-ray crystallography; acute-phase induction in mice, hepatocyte secretion, opsonization-phagocytosis assay\",\n      \"pmids\": [\"17569869\", \"18056837\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structure of agonist (hexa-acylated LPS) complex not yet solved\", \"How the sixth acyl chain protrudes to contact TLR4 was not visible\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Taxol was shown to bind directly to human MD-2 at the LPS-overlapping pocket, inducing conformational changes; species specificity of taxol TLR4 activation was mapped to murine MD-2 Phe126, and electrostatic surface potential changes in MD-2 and TLR4 enabling lipid IVa signaling were delineated.\",\n      \"evidence\": \"ELISA binding, competitive fluorescence displacement, CD spectroscopy, chimeric horse/human MD-2/TLR4 mutagenesis\",\n      \"pmids\": [\"18650420\", \"18977229\", \"18606678\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No crystal structure of taxol-MD-2 complex\", \"Structural basis of conformational change upon non-lipid ligand binding unresolved\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"The landmark crystal structure of the TLR4–MD-2–LPS heterohexamer revealed the M-shaped 2:2:2 signaling complex: five acyl chains buried in the MD-2 pocket, the sixth exposed chain contacting TLR4*, and ionic LPS phosphate interactions driving dimerization; Cys133 in the pocket was identified as a druggable thiol target, and epigenetic silencing of MD-2 by CpG methylation was demonstrated.\",\n      \"evidence\": \"X-ray crystallography of TLR4–MD-2–LPS; covalent Cys133 labeling with mass spectrometry and in vivo TNF-α assay; bisulfite sequencing with pharmacological reactivation\",\n      \"pmids\": [\"19252480\", \"19473973\", \"19710105\", \"20018893\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Dynamics of the sixth acyl chain exposure during activation not captured\", \"Whether epigenetic silencing is reversible in vivo not tested\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"A splice variant (MD-2s) lacking exon 2 was characterized as a natural dominant-negative regulator that binds LPS and TLR4 but fails to signal, and additional species-specificity residues (Tyr42, Arg69, Asp122, Leu125) at the dimerization interface and TLR4 contact surface were mapped.\",\n      \"evidence\": \"Molecular cloning and co-immunoprecipitation of MD-2s; systematic mutagenesis of human/mouse MD-2\",\n      \"pmids\": [\"20435923\", \"20592019\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo function of MD-2s isoform not tested in knockout models\", \"Full structural model of MD-2s fold and its inability to promote dimerization lacking\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Intracellular TLR4/MD-2 (in the absence of surface expression) was shown to sense phagocytosed bacteria and activate MyD88-dependent but not TRIF-dependent genes, establishing compartment-specific signaling, and albumin was identified as an alternative endotoxin carrier to MD-2.\",\n      \"evidence\": \"PRAT4A KO macrophages with cytokine profiling; purified endotoxin·albumin transfer assay with Kd measurement\",\n      \"pmids\": [\"21712422\", \"21994253\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether intracellular MD-2 source is recycled or newly synthesized is unknown\", \"Physiological relevance of albumin-mediated transfer versus CD14-mediated transfer not quantified in vivo\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Crystal structures of mouse TLR4/MD-2 with LPS and lipid IVa provided the structural explanation for species-specific agonism: lipid IVa adopts an agonistic orientation in mouse MD-2 (similar to LPS) but an entirely different antagonistic orientation in human MD-2.\",\n      \"evidence\": \"X-ray crystallography of mouse TLR4/MD-2/LPS and TLR4/MD-2/lipid IVa at 2.5–2.7 Å\",\n      \"pmids\": [\"22532668\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How the pocket selects between orientations at the biophysical level remains unclear\", \"No structure of human MD-2 with agonist hexa-acyl LPS for direct comparison\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"The ligand scope of MD-2 was further expanded to endogenous ligands (SAA3 binding at ~2.2 μM Kd, Gb4 competing with LPS), and the requirement for heterotetramer formation for full MyD88 signaling was quantified using monophosphoryl lipid A and MD-2 F126A.\",\n      \"evidence\": \"SPR for SAA3, co-precipitation and native PAGE for Gb4, A4galt KO mouse; MTS510 antibody heterotetramer assay\",\n      \"pmids\": [\"23858030\", \"23471986\", \"23638128\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis for SAA3-MD-2 interaction unknown\", \"Whether Gb4 inserts into the hydrophobic pocket or binds the surface not resolved\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"MD-2 was validated as a druggable target through small-molecule L6H21 binding to the hydrophobic pocket (Arg90, Tyr102), and the pocket was shown to mediate nickel/cobalt sensing through the same residues, revealing MD-2 as a sensor for transition metal allergens.\",\n      \"evidence\": \"SPR, molecular docking, MD-2 KO mouse sepsis model; site-directed mutagenesis for nickel/cobalt\",\n      \"pmids\": [\"26076332\", \"25803856\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Crystal structure of L6H21-MD-2 complex not obtained\", \"Physiological relevance of nickel sensing through MD-2 in human contact allergy not directly demonstrated\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"The crystal structure of neoseptin-3 (a peptidomimetic) bound as an asymmetric dimer within the MD-2 pocket demonstrated that structurally unrelated non-lipid molecules can fully activate TLR4/MD-2, decoupling agonism from LPS-like structure.\",\n      \"evidence\": \"Crystal structure of mTLR4/MD-2/neoseptin-3 at 2.57 Å with NF-κB and MyD88/TRIF signaling assays\",\n      \"pmids\": [\"26831104\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether neoseptin-3 induces identical downstream gene programs as LPS not determined\", \"Human TLR4/MD-2 response to neoseptin-3 not tested\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Soluble CD83 was identified as a new MD-2-binding immunomodulator that co-opts the TLR4/MD-2 axis to degrade IRAK-1 and induce tolerogenic mediators (IDO, IL-10, PGE2), establishing MD-2 as a receptor for immunosuppressive signaling.\",\n      \"evidence\": \"Binding partner identification, co-immunoprecipitation, IRAK-1 degradation, T cell proliferation assay\",\n      \"pmids\": [\"28193829\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Binding affinity and stoichiometry of sCD83–MD-2 not quantified\", \"Whether sCD83 occupies the hydrophobic pocket or binds externally is unknown\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"The two-stage binding model for HMGB1 was established: HMGB1 A-box binds TLR4 (high affinity, fast off-rate) while B-box binds MD-2 (low affinity, very slow off-rate), explaining how A-box peptide antagonizes HMGB1-driven inflammation.\",\n      \"evidence\": \"SPR kinetics with domain-specific recombinant proteins\",\n      \"pmids\": [\"30134799\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural model of HMGB1-B-box/MD-2 interface lacking\", \"Whether B-box occupies the lipid pocket not determined\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"A distinct heme-activation site on MD-2 (W23, Y34) was identified that is separate from the LPS-binding pocket, demonstrating that MD-2 harbors at least two functionally independent ligand-recognition surfaces.\",\n      \"evidence\": \"Heme-agarose pulldown of recombinant MD-2, UV/Vis spectroscopy, W23A/Y34A mutagenesis with NF-κB reporter\",\n      \"pmids\": [\"32695117\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of the heme site not determined crystallographically\", \"Whether heme and LPS can simultaneously activate MD-2 not tested\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Crystal structures of mouse TLR4-MD-2 with C16-sulfatide revealed three sulfatide molecules filling the pocket in an agonistic dimer geometry, extending the structural repertoire of pocket-bound endogenous lipids, and zebrafish ly96 was shown to be a functional MD-2 ortholog required for LPS-induced cytokine production.\",\n      \"evidence\": \"Crystal structure of mouse TLR4-MD-2/sulfatide; zebrafish loss-of-function mutant with NF-κB assay\",\n      \"pmids\": [\"34290146\", \"33472906\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Human TLR4/MD-2 sulfatide complex structure not available\", \"Evolutionary conservation of the MD-2 pocket across vertebrates not systematically addressed\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Disulfiram was shown to covalently modify Cys133 in the MD-2 pocket, blocking LPS sensing and TLR4 dimerization, and protecting dopaminergic neurons in a Parkinson's disease model, validating Cys133 as a therapeutic target for neuroinflammation.\",\n      \"evidence\": \"Covalent modification assay, Cys133 mutagenesis, TLR4 dimerization assay, MPTP mouse model\",\n      \"pmids\": [\"37487070\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Selectivity of disulfiram for MD-2 Cys133 versus other cellular thiols not fully characterized\", \"Long-term safety and efficacy in neurodegeneration models not established\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include: the dynamic mechanism by which CD14 transfers endotoxin monomers into the MD-2 pocket, the structural basis for heme recognition at the W23/Y34 site versus the canonical lipid pocket, and how intracellular versus surface TLR4/MD-2 complexes achieve compartment-specific signaling output.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No structure of CD14–MD-2 transfer intermediate\", \"No crystal structure of heme-bound MD-2\", \"Mechanism governing MyD88-only versus TRIF-dependent signaling from different compartments not fully resolved\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0008289\", \"supporting_discovery_ids\": [1, 10, 16, 17, 21, 31, 37, 38]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 6, 26, 46]},\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [0, 21, 34]},\n      {\"term_id\": \"GO:0048018\", \"supporting_discovery_ids\": [45]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 6, 12]},\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [12, 45]},\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [47]},\n      {\"term_id\": \"GO:0005794\", \"supporting_discovery_ids\": [6]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [0, 10, 21, 27, 34, 39]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [0, 21, 30, 34, 35]}\n    ],\n    \"complexes\": [\n      \"TLR4/MD-2\",\n      \"TLR4/MD-2/LPS heterohexamer\",\n      \"RP105/MD-1/TLR4/MD-2\"\n    ],\n    \"partners\": [\n      \"TLR4\",\n      \"CD14\",\n      \"LBP\",\n      \"RP105\",\n      \"HMGB1\",\n      \"PTX3\",\n      \"CD83\",\n      \"SAA3\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}