{"gene":"XIRP2","run_date":"2026-04-28T23:00:23","timeline":{"discoveries":[{"year":2004,"finding":"XIRP2 contains Xin repeats (16 amino acid repetitive units) that directly bind actin filaments in vitro and can arrange microfilaments into networks, as demonstrated by in vitro co-sedimentation assays with skeletal muscle actin and transfection experiments in cultured cells. Human XIRP2 contains 28 such Xin repeats with actin-binding properties identical to those of Xin/XIRP1.","method":"In vitro co-sedimentation (actin co-sedimentation assay), transfection of cultured cells, low-speed centrifugation network assay","journal":"Journal of cell science","confidence":"High","confidence_rationale":"Tier 1 — direct in vitro actin-binding assay with mutagenesis-level domain characterization; foundational paper","pmids":["15454575"],"is_preprint":false},{"year":2006,"finding":"XIRP2 (myomaxin/CMYA3) is a direct transcriptional target of MEF2A; it is markedly down-regulated in MEF2A knockout hearts. The encoded protein localizes to the Z-disc/costameric region in striated muscle and interacts with the sarcomeric Z-disc protein alpha-actinin-2.","method":"MEF2A knockout mouse model (gene expression analysis), co-immunoprecipitation/interaction assay with alpha-actinin-2, immunolocalization","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — genetic knockout plus direct protein interaction demonstrated; replicated in subsequent studies","pmids":["17046827"],"is_preprint":false},{"year":2010,"finding":"Xirp2 expression in the heart is activated downstream of angiotensin II (Ang II) signaling via MEF2A transcriptional activity. Hypomorphic reduction of Xirp2 in mice leads to cardiac hypertrophy and increased beta-myosin heavy chain expression at baseline, and attenuates Ang II-induced hypertrophy, fibrosis, and apoptosis, placing Xirp2 as a functional effector in the MEF2A–Ang II cardiac remodeling pathway.","method":"Xirp2 hypomorphic knock-in mouse model, Ang II infusion, cardiac histology, promoter characterization, gene expression analysis","journal":"Circulation research","confidence":"High","confidence_rationale":"Tier 2 — clean hypomorphic mouse model with defined cardiac phenotype and pathway placement via promoter analysis","pmids":["20093629"],"is_preprint":false},{"year":2012,"finding":"mXinβ (mouse ortholog of XIRP2) is required for postnatal intercalated disc (ICD) maturation; it is uniquely upregulated during redistribution of intercellular junctions from lateral cardiomyocyte membranes to cell termini. In mXinβ-null mice, intercellular junctions fail to be restricted to cell termini by postnatal day 16.5. Subcellular fractionation and immunofluorescence showed preferential association of mXinβ with forming ICDs.","method":"mXinβ knockout mouse, quantitative Western blot, immunofluorescence, subcellular fractionation, double-knockout analysis","journal":"Developmental biology","confidence":"High","confidence_rationale":"Tier 2 — clean knockout with defined developmental phenotype, multiple orthogonal methods","pmids":["23261932"],"is_preprint":false},{"year":2015,"finding":"XIRP2 is expressed in inner ear hair cells with a specific short splice variant (XEPLIN) enriched in stereocilia, while long isoforms containing the XIN-repeat domain are distributed in both stereocilia and cuticular plates. CRISPR/Cas9-mediated elimination of XIRP2 in mice causes high-frequency hearing loss and stereocilia degeneration with disorganized paracrystalline actin filaments, indicating XIRP2 is required for maintenance of actin structures in hair bundle stereocilia.","method":"CRISPR/Cas9 knockout mouse, peptide mass spectrometry, auditory brainstem response (hearing function), scanning electron microscopy, ultrastructural analysis","journal":"The Journal of neuroscience","confidence":"High","confidence_rationale":"Tier 2 — CRISPR KO with defined structural and functional phenotype, multiple methods; independently replicated","pmids":["25653358"],"is_preprint":false},{"year":2015,"finding":"Different XIRP2 isoforms are differentially localized in hair cells: short splice forms (XEPLIN) preferentially target stereocilia, while two long isoforms containing the XIN-repeat domain localize to both stereocilia and cuticular plates. Xirp2-null mice develop normal stereocilia bundles that degenerate over time, with stereocilia loss and emergence of long membranous protrusions from the apical surface.","method":"Xirp2 knockout mouse, isoform characterization, immunolocalization, electron microscopy, functional hearing assessment","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 2 — knockout plus isoform-specific localization analysis with structural phenotype; independently replicated in same year","pmids":["25772365"],"is_preprint":false},{"year":2016,"finding":"Both Xirp1 and Xirp2 stabilize filamin C (FLNc) selectively in premyofibrils of cardiomyocytes, as demonstrated by FRAP analysis of FLNc mobility in cardiomyocytes from Xirp1 and Xirp2 deficient animals showing that FLNc is more mobile (less stable) in the absence of these Xin proteins.","method":"FRAP (fluorescence recovery after photobleaching), Xirp1/Xirp2 knockout animals, live cell imaging in cultured neonatal mouse cardiomyocytes and transgenic zebrafish","journal":"Human molecular genetics","confidence":"High","confidence_rationale":"Tier 1/2 — FRAP in genetic knockout with direct functional readout; multiple model systems","pmids":["27206985"],"is_preprint":false},{"year":2018,"finding":"Xirp2 is required for cardiac conduction; Xirp2 knockout mice exhibit prolonged PR and QT intervals, slow conduction velocity, atrioventricular conduction block, and abnormal infranodal ventricular conduction. Altered ionic currents were detected in Xirp2-/- cardiomyocytes by whole-cell patch-clamp. XIRP2 associates with Nav1.5 and Kv1.5 ion channel subunits as shown by co-immunoprecipitation, suggesting a role in ion channel surface expression.","method":"Xirp2 knockout mouse, electrocardiography, whole-cell patch-clamp, co-immunoprecipitation","journal":"Journal of the American Heart Association","confidence":"High","confidence_rationale":"Tier 1/2 — in vitro electrophysiology plus co-IP plus genetic knockout with defined conduction phenotype","pmids":["29306897"],"is_preprint":false},{"year":2018,"finding":"In stereocilia, the actin cross-linking protein XIRP2 (along with fascin-2, plastin-1, and espin) is part of the stable actin core but shows continuous turnover, distinct from the near-static actin filaments themselves. This was inferred from studies showing EGFP-fascin-2 displaces espin and plastin-1 from stereocilia, suggesting these cross-linkers compete for actin binding.","method":"Transgenic EGFP-fascin-2 mouse, live imaging, immunofluorescence comparison with XIRP2 and other cross-linkers","journal":"Molecular biology of the cell","confidence":"Medium","confidence_rationale":"Tier 3 — XIRP2 mentioned as component; mechanistic inference about cross-linker competition indirect","pmids":["29874122"],"is_preprint":false},{"year":2023,"finding":"XIRP2 is required for repair of noise-induced F-actin damage ('gaps') in auditory hair cell stereocilia. XIRP2 facilitates enrichment of monomeric γ-actin at damage sites. Recruitment of XIRP2 to stereocilia gaps and to stress fiber strain sites in fibroblasts is force-dependent, mediated by a novel mechanosensor domain located in the C-terminus of XIRP2.","method":"Xirp2 knockout mouse, noise exposure, phalloidin staining for F-actin gaps, monomeric actin immunofluorescence, force-dependent recruitment assay in fibroblasts (mechanosensor domain characterization)","journal":"eLife","confidence":"High","confidence_rationale":"Tier 2 — genetic KO with mechanistic follow-up; novel domain identified with direct functional assay","pmids":["37294664"],"is_preprint":false},{"year":2013,"finding":"XIRP2 protein accumulates in pathological protein aggregates in skeletal muscle fibers of desminopathy and filaminopathy patients, as identified by laser microdissection combined with label-free spectral count-based proteomics and validated by immunolocalization.","method":"Laser microdissection, label-free proteomics (spectral counting), immunolocalization, parallel reaction monitoring","journal":"Journal of proteomics","confidence":"Medium","confidence_rationale":"Tier 3 — proteomic identification of XIRP2 in pathological aggregates; localization without direct mechanistic manipulation","pmids":["23639843"],"is_preprint":false},{"year":2014,"finding":"mXinβ (XIRP2) is required for Xirp1 (mXinα) intercalated disc localization; in mXinβ-null hearts, mXinα fails to properly localize to ICDs. Complete loss of mXinβ results in failure of ICD formation, severe growth retardation, and early postnatal lethality.","method":"Xirp2 knockout and double-knockout (Xirp1/Xirp2) mouse models, immunofluorescence","journal":"International review of cell and molecular biology","confidence":"Medium","confidence_rationale":"Tier 2 — knockout phenotype with defined localization dependency, but review article summarizing prior work","pmids":["24725425"],"is_preprint":false},{"year":2024,"finding":"XIRP2 mutation enhances the stability of the XIRP2 protein without affecting its mRNA levels in HCC cells. Inhibition of XIRP2 increases sensitivity to oxaliplatin through elevation of zinc ions and calcium ion overload.","method":"Protein stability experiments, CCK-8 drug sensitivity assay, qRT-PCR, ion measurement in HCC cell lines","journal":"Biology","confidence":"Low","confidence_rationale":"Tier 3 — single lab, single method per finding; mechanism (zinc/calcium) not deeply characterized","pmids":["39194571"],"is_preprint":false}],"current_model":"XIRP2 is a muscle-specific, MEF2A-regulated actin-binding protein whose Xin-repeat domain directly crosslinks and stabilizes actin filaments; it localizes to intercalated discs of cardiomyocytes (where it is required for ICD maturation, Xirp1 localization, and surface expression of Nav1.5/Kv1.5 ion channels controlling cardiac conduction) and to stereocilia/cuticular plates of inner ear hair cells (where distinct isoforms maintain paracrystalline actin organization and a C-terminal mechanosensor domain recruits XIRP2 to force-induced damage sites to facilitate actin repair); it also stabilizes filamin C selectively in premyofibrils and functions downstream of angiotensin II–MEF2A signaling to modulate cardiac hypertrophy, fibrosis, and apoptosis."},"narrative":{"teleology":[{"year":2004,"claim":"Establishing that XIRP2's Xin repeats constitute a direct actin-binding and crosslinking module answered the fundamental question of its molecular activity and placed it alongside XIRP1 as an actin-associated scaffold.","evidence":"In vitro actin co-sedimentation, low-speed network assay, and cell transfection with domain constructs","pmids":["15454575"],"confidence":"High","gaps":["Structural basis of Xin repeat–actin interaction unresolved","In vivo relevance of actin crosslinking not yet tested","Binding stoichiometry and affinity not quantified"]},{"year":2006,"claim":"Identifying XIRP2 as a direct MEF2A transcriptional target that interacts with α-actinin-2 at the Z-disc connected its expression control to a cardiac transcription factor and placed it at sarcomeric junctions.","evidence":"MEF2A knockout mouse with gene expression analysis and co-immunoprecipitation with α-actinin-2","pmids":["17046827"],"confidence":"High","gaps":["Whether MEF2A is the sole transcriptional regulator unclear","Nature of α-actinin-2 interaction (direct or indirect) not fully resolved"]},{"year":2010,"claim":"Demonstrating that XIRP2 hypomorphic reduction causes baseline cardiac hypertrophy and modulates angiotensin II–induced remodeling established XIRP2 as a functional effector in the Ang II–MEF2A cardiac hypertrophy pathway.","evidence":"Xirp2 hypomorphic knock-in mouse with Ang II infusion, cardiac histology, and promoter analysis","pmids":["20093629"],"confidence":"High","gaps":["Downstream signaling targets mediating hypertrophy and apoptosis unidentified","Whether XIRP2 acts through actin crosslinking or a separate signaling role in this context unknown"]},{"year":2012,"claim":"Showing that XIRP2-null mice fail to restrict intercellular junctions to cell termini postnatally resolved the question of whether XIRP2 is required for intercalated disc maturation, not merely a structural component.","evidence":"mXinβ knockout mouse with immunofluorescence, subcellular fractionation, and developmental time-course analysis","pmids":["23261932"],"confidence":"High","gaps":["Mechanism by which XIRP2 drives junctional polarization unknown","Whether XIRP2 acts through actin remodeling or protein scaffolding at ICDs not distinguished"]},{"year":2014,"claim":"Establishing that XIRP2 is required for XIRP1 localization to ICDs and that its complete loss is postnatally lethal clarified a hierarchical dependency between the two Xin-repeat proteins.","evidence":"Xirp2 knockout and Xirp1/Xirp2 double-knockout mouse models with immunofluorescence","pmids":["24725425"],"confidence":"Medium","gaps":["Molecular basis for XIRP1 dependence on XIRP2 for ICD targeting not defined","Evidence summarized in review; primary data spread across multiple sources"]},{"year":2015,"claim":"Discovering XIRP2 expression in inner ear hair cells and showing that its loss causes progressive stereocilia degeneration and hearing loss extended XIRP2 function beyond cardiac muscle to sensory actin structures, with isoform-specific localization to stereocilia versus cuticular plates.","evidence":"Two independent CRISPR/Cas9 and conventional Xirp2 knockout mice with ABR hearing tests, scanning/transmission EM, mass spectrometry, and isoform-specific immunolocalization","pmids":["25653358","25772365"],"confidence":"High","gaps":["Specific isoform contributions to stereocilia versus cuticular plate function not genetically dissected","Whether hearing loss is solely due to actin disorganization or also involves mechanotransduction channel disruption unknown"]},{"year":2016,"claim":"Demonstrating by FRAP that XIRP2 stabilizes filamin C selectively in premyofibrils identified a specific sarcomeric client protein and differentiated XIRP2's role at early versus mature sarcomeric stages.","evidence":"FRAP in neonatal cardiomyocytes from Xirp2 knockout mice and transgenic zebrafish","pmids":["27206985"],"confidence":"High","gaps":["Direct binding interface between XIRP2 and filamin C not mapped","Whether XIRP2 stabilizes other premyofibril components besides FLNc not tested"]},{"year":2018,"claim":"Linking XIRP2 loss to prolonged PR/QT intervals, AV block, and altered ionic currents — and showing XIRP2 co-immunoprecipitates with Nav1.5 and Kv1.5 — established a direct role in cardiac ion channel trafficking or surface stabilization.","evidence":"Xirp2 knockout mouse with ECG, whole-cell patch-clamp electrophysiology, and co-immunoprecipitation","pmids":["29306897"],"confidence":"High","gaps":["Whether XIRP2 directly binds channel subunits or acts via an intermediate scaffold not resolved","Mechanism of altered surface expression (trafficking versus membrane retention) not determined"]},{"year":2023,"claim":"Identifying a C-terminal mechanosensor domain that recruits XIRP2 to force-damaged actin sites and showing XIRP2 facilitates γ-actin enrichment at stereocilia gaps revealed a damage-repair function and a novel force-sensing mechanism.","evidence":"Xirp2 knockout mouse with noise exposure, phalloidin gap staining, monomeric actin immunofluorescence, and force-dependent recruitment assay in fibroblasts","pmids":["37294664"],"confidence":"High","gaps":["Structure of the mechanosensor domain and the biophysical basis of force sensing unresolved","Whether the repair function operates in cardiac actin structures not tested","Identity of cofactors that cooperate with XIRP2 during actin repair unknown"]},{"year":null,"claim":"Major open questions include the structural basis of Xin repeat–actin binding, how XIRP2 coordinates ion channel surface expression at intercalated discs, and whether its mechanosensor-mediated repair function extends to cardiac and skeletal muscle actin.","evidence":"","pmids":[],"confidence":"High","gaps":["No crystal or cryo-EM structure of any XIRP2 domain","Mechanosensor domain activity not tested outside inner ear and cultured fibroblasts","Genetic dissection of individual isoform contributions in vivo lacking"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0008092","term_label":"cytoskeletal protein binding","supporting_discovery_ids":[0,1,6,8]},{"term_id":"GO:0140299","term_label":"molecular sensor activity","supporting_discovery_ids":[9]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[7]}],"localization":[{"term_id":"GO:0005856","term_label":"cytoskeleton","supporting_discovery_ids":[0,1,4,5,8,9]},{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[3,7]}],"pathway":[{"term_id":"R-HSA-397014","term_label":"Muscle contraction","supporting_discovery_ids":[1,3,6]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[2,7]},{"term_id":"R-HSA-9709957","term_label":"Sensory Perception","supporting_discovery_ids":[4,5,9]}],"complexes":[],"partners":["ACTN2","FLNC","SCN5A","KCNA5","XIRP1"],"other_free_text":[]},"mechanistic_narrative":"XIRP2 is a muscle- and sensory-cell actin-binding protein that crosslinks and stabilizes actin filaments through its Xin-repeat domain and functions as a scaffold essential for intercalated disc maturation, cardiac conduction, and stereocilia maintenance. Its Xin repeats directly bind and bundle actin filaments in vitro, and in cardiomyocytes XIRP2 stabilizes filamin C in premyofibrils, is required for postnatal intercalated disc formation and proper localization of Xirp1, and associates with Nav1.5 and Kv1.5 ion channels to support normal cardiac conduction velocity [PMID:15454575, PMID:23261932, PMID:27206985, PMID:29306897]. In inner ear hair cells, distinct XIRP2 splice isoforms maintain paracrystalline actin organization in stereocilia, and a C-terminal mechanosensor domain recruits XIRP2 to force-induced actin damage sites to facilitate γ-actin-dependent repair [PMID:25653358, PMID:37294664]. Transcriptionally regulated by MEF2A downstream of angiotensin II signaling, XIRP2 hypomorphic reduction causes baseline cardiac hypertrophy and modulates angiotensin II–induced hypertrophy, fibrosis, and apoptosis [PMID:20093629]."},"prefetch_data":{"uniprot":{"accession":"A4UGR9","full_name":"Xin actin-binding repeat-containing protein 2","aliases":["Beta-xin","Cardiomyopathy-associated protein 3","Xeplin"],"length_aa":3374,"mass_kda":382.3,"function":"Protects actin filaments from depolymerization (PubMed:15454575). Required for correct morphology of cell membranes and maturation of intercalated disks of cardiomyocytes via facilitating localization of XIRP1 and CDH2 to the termini of aligned mature cardiomyocytes (By similarity). Thereby required for correct postnatal heart development and growth regulation that is crucial for overall heart morphology and diastolic function (By similarity). Required for normal electrical conduction in the heart including formation of the infranodal ventricular conduction system and normal action potential configuration, as a result of its interaction with the cardiac ion channel components Scn5a/Nav1.5 and Kcna5/Kv1.5 (By similarity). Required for regular actin filament spacing of the paracrystalline array in both inner and outer hair cells of the cochlea, thereby required for maintenance of stereocilia morphology (By similarity)","subcellular_location":"Cell junction","url":"https://www.uniprot.org/uniprotkb/A4UGR9/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/XIRP2","classification":"Not Classified","n_dependent_lines":0,"n_total_lines":1208,"dependency_fraction":0.0},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/XIRP2","total_profiled":1310},"omim":[{"mim_id":"620775","title":"NEUROMUSCULAR DISORDER, CONGENITAL, WITH DYSMORPHIC FACIES; NMDF","url":"https://www.omim.org/entry/620775"},{"mim_id":"615779","title":"CONGENITAL HEART DEFECTS, MULTIPLE TYPES, 4; CHTD4","url":"https://www.omim.org/entry/615779"},{"mim_id":"609778","title":"XIN ACTIN-BINDING REPEAT-CONTAINING PROTEIN 2; XIRP2","url":"https://www.omim.org/entry/609778"},{"mim_id":"171890","title":"PHOSPHODIESTERASE 1A; PDE1A","url":"https://www.omim.org/entry/171890"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Plasma membrane","reliability":"Approved"},{"location":"Nucleoplasm","reliability":"Additional"}],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in some","driving_tissues":[{"tissue":"heart muscle","ntpm":180.7},{"tissue":"skeletal muscle","ntpm":962.9},{"tissue":"tongue","ntpm":458.6}],"url":"https://www.proteinatlas.org/search/XIRP2"},"hgnc":{"alias_symbol":[],"prev_symbol":["CMYA3"]},"alphafold":{"accession":"A4UGR9","domains":[],"viewer_url":"https://alphafold.ebi.ac.uk/entry/A4UGR9","model_url":"https://alphafold.ebi.ac.uk/files/AF-A4UGR9-7-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-A4UGR9-7-F1-predicted_aligned_error_v6.png","plddt_mean":49.38},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=XIRP2","jax_strain_url":"https://www.jax.org/strain/search?query=XIRP2"},"sequence":{"accession":"A4UGR9","fasta_url":"https://rest.uniprot.org/uniprotkb/A4UGR9.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/A4UGR9/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/A4UGR9"}},"corpus_meta":[{"pmid":"32657779","id":"PMC_32657779","title":"Genomic landscape of metastatic breast cancer identifies preferentially dysregulated pathways and 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Human XIRP2 contains 28 such Xin repeats with actin-binding properties identical to those of Xin/XIRP1.\",\n      \"method\": \"In vitro co-sedimentation (actin co-sedimentation assay), transfection of cultured cells, low-speed centrifugation network assay\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct in vitro actin-binding assay with mutagenesis-level domain characterization; foundational paper\",\n      \"pmids\": [\"15454575\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"XIRP2 (myomaxin/CMYA3) is a direct transcriptional target of MEF2A; it is markedly down-regulated in MEF2A knockout hearts. The encoded protein localizes to the Z-disc/costameric region in striated muscle and interacts with the sarcomeric Z-disc protein alpha-actinin-2.\",\n      \"method\": \"MEF2A knockout mouse model (gene expression analysis), co-immunoprecipitation/interaction assay with alpha-actinin-2, immunolocalization\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic knockout plus direct protein interaction demonstrated; replicated in subsequent studies\",\n      \"pmids\": [\"17046827\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Xirp2 expression in the heart is activated downstream of angiotensin II (Ang II) signaling via MEF2A transcriptional activity. Hypomorphic reduction of Xirp2 in mice leads to cardiac hypertrophy and increased beta-myosin heavy chain expression at baseline, and attenuates Ang II-induced hypertrophy, fibrosis, and apoptosis, placing Xirp2 as a functional effector in the MEF2A–Ang II cardiac remodeling pathway.\",\n      \"method\": \"Xirp2 hypomorphic knock-in mouse model, Ang II infusion, cardiac histology, promoter characterization, gene expression analysis\",\n      \"journal\": \"Circulation research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean hypomorphic mouse model with defined cardiac phenotype and pathway placement via promoter analysis\",\n      \"pmids\": [\"20093629\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"mXinβ (mouse ortholog of XIRP2) is required for postnatal intercalated disc (ICD) maturation; it is uniquely upregulated during redistribution of intercellular junctions from lateral cardiomyocyte membranes to cell termini. In mXinβ-null mice, intercellular junctions fail to be restricted to cell termini by postnatal day 16.5. Subcellular fractionation and immunofluorescence showed preferential association of mXinβ with forming ICDs.\",\n      \"method\": \"mXinβ knockout mouse, quantitative Western blot, immunofluorescence, subcellular fractionation, double-knockout analysis\",\n      \"journal\": \"Developmental biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean knockout with defined developmental phenotype, multiple orthogonal methods\",\n      \"pmids\": [\"23261932\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"XIRP2 is expressed in inner ear hair cells with a specific short splice variant (XEPLIN) enriched in stereocilia, while long isoforms containing the XIN-repeat domain are distributed in both stereocilia and cuticular plates. CRISPR/Cas9-mediated elimination of XIRP2 in mice causes high-frequency hearing loss and stereocilia degeneration with disorganized paracrystalline actin filaments, indicating XIRP2 is required for maintenance of actin structures in hair bundle stereocilia.\",\n      \"method\": \"CRISPR/Cas9 knockout mouse, peptide mass spectrometry, auditory brainstem response (hearing function), scanning electron microscopy, ultrastructural analysis\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — CRISPR KO with defined structural and functional phenotype, multiple methods; independently replicated\",\n      \"pmids\": [\"25653358\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Different XIRP2 isoforms are differentially localized in hair cells: short splice forms (XEPLIN) preferentially target stereocilia, while two long isoforms containing the XIN-repeat domain localize to both stereocilia and cuticular plates. Xirp2-null mice develop normal stereocilia bundles that degenerate over time, with stereocilia loss and emergence of long membranous protrusions from the apical surface.\",\n      \"method\": \"Xirp2 knockout mouse, isoform characterization, immunolocalization, electron microscopy, functional hearing assessment\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — knockout plus isoform-specific localization analysis with structural phenotype; independently replicated in same year\",\n      \"pmids\": [\"25772365\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Both Xirp1 and Xirp2 stabilize filamin C (FLNc) selectively in premyofibrils of cardiomyocytes, as demonstrated by FRAP analysis of FLNc mobility in cardiomyocytes from Xirp1 and Xirp2 deficient animals showing that FLNc is more mobile (less stable) in the absence of these Xin proteins.\",\n      \"method\": \"FRAP (fluorescence recovery after photobleaching), Xirp1/Xirp2 knockout animals, live cell imaging in cultured neonatal mouse cardiomyocytes and transgenic zebrafish\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1/2 — FRAP in genetic knockout with direct functional readout; multiple model systems\",\n      \"pmids\": [\"27206985\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Xirp2 is required for cardiac conduction; Xirp2 knockout mice exhibit prolonged PR and QT intervals, slow conduction velocity, atrioventricular conduction block, and abnormal infranodal ventricular conduction. Altered ionic currents were detected in Xirp2-/- cardiomyocytes by whole-cell patch-clamp. XIRP2 associates with Nav1.5 and Kv1.5 ion channel subunits as shown by co-immunoprecipitation, suggesting a role in ion channel surface expression.\",\n      \"method\": \"Xirp2 knockout mouse, electrocardiography, whole-cell patch-clamp, co-immunoprecipitation\",\n      \"journal\": \"Journal of the American Heart Association\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1/2 — in vitro electrophysiology plus co-IP plus genetic knockout with defined conduction phenotype\",\n      \"pmids\": [\"29306897\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"In stereocilia, the actin cross-linking protein XIRP2 (along with fascin-2, plastin-1, and espin) is part of the stable actin core but shows continuous turnover, distinct from the near-static actin filaments themselves. This was inferred from studies showing EGFP-fascin-2 displaces espin and plastin-1 from stereocilia, suggesting these cross-linkers compete for actin binding.\",\n      \"method\": \"Transgenic EGFP-fascin-2 mouse, live imaging, immunofluorescence comparison with XIRP2 and other cross-linkers\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — XIRP2 mentioned as component; mechanistic inference about cross-linker competition indirect\",\n      \"pmids\": [\"29874122\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"XIRP2 is required for repair of noise-induced F-actin damage ('gaps') in auditory hair cell stereocilia. XIRP2 facilitates enrichment of monomeric γ-actin at damage sites. Recruitment of XIRP2 to stereocilia gaps and to stress fiber strain sites in fibroblasts is force-dependent, mediated by a novel mechanosensor domain located in the C-terminus of XIRP2.\",\n      \"method\": \"Xirp2 knockout mouse, noise exposure, phalloidin staining for F-actin gaps, monomeric actin immunofluorescence, force-dependent recruitment assay in fibroblasts (mechanosensor domain characterization)\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO with mechanistic follow-up; novel domain identified with direct functional assay\",\n      \"pmids\": [\"37294664\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"XIRP2 protein accumulates in pathological protein aggregates in skeletal muscle fibers of desminopathy and filaminopathy patients, as identified by laser microdissection combined with label-free spectral count-based proteomics and validated by immunolocalization.\",\n      \"method\": \"Laser microdissection, label-free proteomics (spectral counting), immunolocalization, parallel reaction monitoring\",\n      \"journal\": \"Journal of proteomics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — proteomic identification of XIRP2 in pathological aggregates; localization without direct mechanistic manipulation\",\n      \"pmids\": [\"23639843\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"mXinβ (XIRP2) is required for Xirp1 (mXinα) intercalated disc localization; in mXinβ-null hearts, mXinα fails to properly localize to ICDs. Complete loss of mXinβ results in failure of ICD formation, severe growth retardation, and early postnatal lethality.\",\n      \"method\": \"Xirp2 knockout and double-knockout (Xirp1/Xirp2) mouse models, immunofluorescence\",\n      \"journal\": \"International review of cell and molecular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — knockout phenotype with defined localization dependency, but review article summarizing prior work\",\n      \"pmids\": [\"24725425\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"XIRP2 mutation enhances the stability of the XIRP2 protein without affecting its mRNA levels in HCC cells. Inhibition of XIRP2 increases sensitivity to oxaliplatin through elevation of zinc ions and calcium ion overload.\",\n      \"method\": \"Protein stability experiments, CCK-8 drug sensitivity assay, qRT-PCR, ion measurement in HCC cell lines\",\n      \"journal\": \"Biology\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — single lab, single method per finding; mechanism (zinc/calcium) not deeply characterized\",\n      \"pmids\": [\"39194571\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"XIRP2 is a muscle-specific, MEF2A-regulated actin-binding protein whose Xin-repeat domain directly crosslinks and stabilizes actin filaments; it localizes to intercalated discs of cardiomyocytes (where it is required for ICD maturation, Xirp1 localization, and surface expression of Nav1.5/Kv1.5 ion channels controlling cardiac conduction) and to stereocilia/cuticular plates of inner ear hair cells (where distinct isoforms maintain paracrystalline actin organization and a C-terminal mechanosensor domain recruits XIRP2 to force-induced damage sites to facilitate actin repair); it also stabilizes filamin C selectively in premyofibrils and functions downstream of angiotensin II–MEF2A signaling to modulate cardiac hypertrophy, fibrosis, and apoptosis.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"XIRP2 is a muscle- and sensory-cell actin-binding protein that crosslinks and stabilizes actin filaments through its Xin-repeat domain and functions as a scaffold essential for intercalated disc maturation, cardiac conduction, and stereocilia maintenance. Its Xin repeats directly bind and bundle actin filaments in vitro, and in cardiomyocytes XIRP2 stabilizes filamin C in premyofibrils, is required for postnatal intercalated disc formation and proper localization of Xirp1, and associates with Nav1.5 and Kv1.5 ion channels to support normal cardiac conduction velocity [PMID:15454575, PMID:23261932, PMID:27206985, PMID:29306897]. In inner ear hair cells, distinct XIRP2 splice isoforms maintain paracrystalline actin organization in stereocilia, and a C-terminal mechanosensor domain recruits XIRP2 to force-induced actin damage sites to facilitate γ-actin-dependent repair [PMID:25653358, PMID:37294664]. Transcriptionally regulated by MEF2A downstream of angiotensin II signaling, XIRP2 hypomorphic reduction causes baseline cardiac hypertrophy and modulates angiotensin II–induced hypertrophy, fibrosis, and apoptosis [PMID:20093629].\",\n  \"teleology\": [\n    {\n      \"year\": 2004,\n      \"claim\": \"Establishing that XIRP2's Xin repeats constitute a direct actin-binding and crosslinking module answered the fundamental question of its molecular activity and placed it alongside XIRP1 as an actin-associated scaffold.\",\n      \"evidence\": \"In vitro actin co-sedimentation, low-speed network assay, and cell transfection with domain constructs\",\n      \"pmids\": [\"15454575\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of Xin repeat–actin interaction unresolved\", \"In vivo relevance of actin crosslinking not yet tested\", \"Binding stoichiometry and affinity not quantified\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Identifying XIRP2 as a direct MEF2A transcriptional target that interacts with α-actinin-2 at the Z-disc connected its expression control to a cardiac transcription factor and placed it at sarcomeric junctions.\",\n      \"evidence\": \"MEF2A knockout mouse with gene expression analysis and co-immunoprecipitation with α-actinin-2\",\n      \"pmids\": [\"17046827\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether MEF2A is the sole transcriptional regulator unclear\", \"Nature of α-actinin-2 interaction (direct or indirect) not fully resolved\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Demonstrating that XIRP2 hypomorphic reduction causes baseline cardiac hypertrophy and modulates angiotensin II–induced remodeling established XIRP2 as a functional effector in the Ang II–MEF2A cardiac hypertrophy pathway.\",\n      \"evidence\": \"Xirp2 hypomorphic knock-in mouse with Ang II infusion, cardiac histology, and promoter analysis\",\n      \"pmids\": [\"20093629\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Downstream signaling targets mediating hypertrophy and apoptosis unidentified\", \"Whether XIRP2 acts through actin crosslinking or a separate signaling role in this context unknown\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Showing that XIRP2-null mice fail to restrict intercellular junctions to cell termini postnatally resolved the question of whether XIRP2 is required for intercalated disc maturation, not merely a structural component.\",\n      \"evidence\": \"mXinβ knockout mouse with immunofluorescence, subcellular fractionation, and developmental time-course analysis\",\n      \"pmids\": [\"23261932\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which XIRP2 drives junctional polarization unknown\", \"Whether XIRP2 acts through actin remodeling or protein scaffolding at ICDs not distinguished\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Establishing that XIRP2 is required for XIRP1 localization to ICDs and that its complete loss is postnatally lethal clarified a hierarchical dependency between the two Xin-repeat proteins.\",\n      \"evidence\": \"Xirp2 knockout and Xirp1/Xirp2 double-knockout mouse models with immunofluorescence\",\n      \"pmids\": [\"24725425\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Molecular basis for XIRP1 dependence on XIRP2 for ICD targeting not defined\", \"Evidence summarized in review; primary data spread across multiple sources\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Discovering XIRP2 expression in inner ear hair cells and showing that its loss causes progressive stereocilia degeneration and hearing loss extended XIRP2 function beyond cardiac muscle to sensory actin structures, with isoform-specific localization to stereocilia versus cuticular plates.\",\n      \"evidence\": \"Two independent CRISPR/Cas9 and conventional Xirp2 knockout mice with ABR hearing tests, scanning/transmission EM, mass spectrometry, and isoform-specific immunolocalization\",\n      \"pmids\": [\"25653358\", \"25772365\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Specific isoform contributions to stereocilia versus cuticular plate function not genetically dissected\", \"Whether hearing loss is solely due to actin disorganization or also involves mechanotransduction channel disruption unknown\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Demonstrating by FRAP that XIRP2 stabilizes filamin C selectively in premyofibrils identified a specific sarcomeric client protein and differentiated XIRP2's role at early versus mature sarcomeric stages.\",\n      \"evidence\": \"FRAP in neonatal cardiomyocytes from Xirp2 knockout mice and transgenic zebrafish\",\n      \"pmids\": [\"27206985\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct binding interface between XIRP2 and filamin C not mapped\", \"Whether XIRP2 stabilizes other premyofibril components besides FLNc not tested\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Linking XIRP2 loss to prolonged PR/QT intervals, AV block, and altered ionic currents — and showing XIRP2 co-immunoprecipitates with Nav1.5 and Kv1.5 — established a direct role in cardiac ion channel trafficking or surface stabilization.\",\n      \"evidence\": \"Xirp2 knockout mouse with ECG, whole-cell patch-clamp electrophysiology, and co-immunoprecipitation\",\n      \"pmids\": [\"29306897\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether XIRP2 directly binds channel subunits or acts via an intermediate scaffold not resolved\", \"Mechanism of altered surface expression (trafficking versus membrane retention) not determined\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Identifying a C-terminal mechanosensor domain that recruits XIRP2 to force-damaged actin sites and showing XIRP2 facilitates γ-actin enrichment at stereocilia gaps revealed a damage-repair function and a novel force-sensing mechanism.\",\n      \"evidence\": \"Xirp2 knockout mouse with noise exposure, phalloidin gap staining, monomeric actin immunofluorescence, and force-dependent recruitment assay in fibroblasts\",\n      \"pmids\": [\"37294664\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structure of the mechanosensor domain and the biophysical basis of force sensing unresolved\", \"Whether the repair function operates in cardiac actin structures not tested\", \"Identity of cofactors that cooperate with XIRP2 during actin repair unknown\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Major open questions include the structural basis of Xin repeat–actin binding, how XIRP2 coordinates ion channel surface expression at intercalated discs, and whether its mechanosensor-mediated repair function extends to cardiac and skeletal muscle actin.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No crystal or cryo-EM structure of any XIRP2 domain\", \"Mechanosensor domain activity not tested outside inner ear and cultured fibroblasts\", \"Genetic dissection of individual isoform contributions in vivo lacking\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0008092\", \"supporting_discovery_ids\": [0, 1, 6, 8]},\n      {\"term_id\": \"GO:0140299\", \"supporting_discovery_ids\": [9]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [7]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005856\", \"supporting_discovery_ids\": [0, 1, 4, 5, 8, 9]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [3, 7]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-397014\", \"supporting_discovery_ids\": [1, 3, 6]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [2, 7]},\n      {\"term_id\": \"R-HSA-9709957\", \"supporting_discovery_ids\": [4, 5, 9]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"ACTN2\", \"FLNC\", \"SCN5A\", \"KCNA5\", \"XIRP1\"],\n    \"other_free_text\": []\n  }\n}\n```"}