{"gene":"XKR8","run_date":"2026-04-28T23:00:23","timeline":{"discoveries":[{"year":2016,"finding":"XKR8 forms a complex with basigin (BSG) or neuroplastin (NPTN); BSG and NPTN chaperone XKR8 to the plasma membrane, and their absence causes XKR8 to localize intracellularly, abolishing apoptosis-induced phosphatidylserine exposure. The atypical glutamic acid in BSG's transmembrane region is required for BSG association with XKR8. Upon apoptotic signaling, XKR8 is cleaved at the C-terminus and the XKR8/BSG complex forms a higher-order complex, likely a heterotetramer of two XKR8 and two BSG/NPTN molecules.","method":"Co-immunoprecipitation, BSG/NPTN double-knockout cell lines, mutational analysis of BSG transmembrane glutamic acid, native PAGE/size-exclusion to determine complex stoichiometry","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1-2 — reciprocal co-IP, knockout cells, mutagenesis, and complex stoichiometry; replicated in subsequent structural studies","pmids":["27503893"],"is_preprint":false},{"year":2019,"finding":"XKR8 scramblase activity is activated by phosphorylation: kinase inhibitors suppress and phosphatase inhibitors enhance PtdSer exposure; mass spectrometry and mutational analysis identified three phosphorylation sites whose phosphomimic mutations render XKR8 resistant to kinase inhibition. Flippase activity counteracts XKR8-mediated PtdSer exposure, so combined phosphorylation of XKR8 and flippase downregulation cooperate to achieve maximal PtdSer externalization.","method":"Phos-tag PAGE, mass spectrometry, site-directed mutagenesis of phosphorylation sites, kinase/phosphatase inhibitor treatments, flippase gene deletion","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 — in vitro assay, mass spectrometry-identified sites, and mutagenesis; multiple orthogonal methods in a single rigorous study","pmids":["30718401"],"is_preprint":false},{"year":2018,"finding":"XKR8 is the sole caspase-dependent scramblase expressed in mouse hematopoietic cells; Xkr8 knockout mice show strongly reduced PtdSer exposure on apoptotic thymocytes, splenocytes, and neutrophils, impairing their phagocytic clearance by Tim4/MerTK-expressing macrophages in vitro and in vivo, and leading to lupus-like autoimmune disease.","method":"Xkr8 knockout mouse model, flow cytometry for PtdSer exposure (annexin V), in vitro phagocytosis assays with Tim4/MerTK-expressing phagocytes, histological/immunological characterization","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 — clean knockout with defined cellular and in vivo phenotypes, multiple orthogonal readouts; replicated in subsequent papers","pmids":["29440417"],"is_preprint":false},{"year":2021,"finding":"Cryo-EM and X-ray crystallography of the human XKR8-Basigin complex at 3.8 Å revealed a cuboid transmembrane region with 22 charged residues stabilized by salt bridges; phosphatidylcholine binds in a hydrophobic cleft on the extracellular surface; six charged residues arranged top-to-bottom inside the molecule form a translocation pathway for phospholipids. A critical tryptophan at the exoplasmic end of the path gates scramblase activity — its mutation to alanine renders the complex constitutively active.","method":"Cryo-EM, X-ray crystallography, site-directed mutagenesis (W→A), scramblase activity assay","journal":"Nature structural & molecular biology","confidence":"High","confidence_rationale":"Tier 1 — high-resolution structure combined with mutagenesis and functional validation","pmids":["34625749"],"is_preprint":false},{"year":2020,"finding":"XKR8-mediated PtdSer exposure is essential for phagocytic clearance of apoptotic germ cells by Sertoli cells during spermatogenesis; Xkr8-/- male mice are infertile due to impaired PtdSer-dependent 'eat-me' signaling and secondary accumulation of unengulfed dead germ cells.","method":"Xkr8 knockout mouse model, annexin V flow cytometry, in vitro phagocytosis assay with Tim4/MerTK cells, fluorescence and electron microscopy of testes","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 2 — clean KO with defined phenotypic readout, multiple microscopy methods, in vitro mechanistic confirmation","pmids":["31712393"],"is_preprint":false},{"year":2024,"finding":"Cryo-EM structure of the human XKR8-Basigin complex in lipid nanodiscs at 3.66 Å shows that the C-terminal tail of XKR8 engages the cytoplasmic groove of XKR8 via polar and van der Waals interactions, maintaining the inactive state; point mutations disrupting these interactions strongly enhance scramblase activity, establishing that the C-terminal tail functions as a plug that is released upon caspase cleavage or phosphorylation to activate XKR8.","method":"Cryo-EM in lipid nanodiscs, site-directed mutagenesis of C-terminal tail contact residues, scramblase activity assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — high-resolution structure in native-like lipid environment plus mutagenesis with functional validation","pmids":["38364890"],"is_preprint":false},{"year":2023,"finding":"XKR8 is required for developmental axon pruning in the mammalian brain: Xkr8 knockout mice show failure of PtdSer exposure in hippocampal neurons, excess excitatory nerve terminals, increased cortico-cortical and cortico-spinal projections, aberrant hippocampal electrophysiology, and global brain hyperconnectivity, identifying XKR8-mediated phospholipid scrambling as the mechanism labeling developing projections for glial phagocytic elimination.","method":"Xkr8 knockout mouse model, annexin V staining for PtdSer exposure, confocal microscopy of synapse density, axon tracing, electrophysiology (hippocampal recordings)","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 2 — clean KO with multiple orthogonal phenotypic readouts linking XKR8 scramblase activity to a defined developmental process","pmids":["37211968"],"is_preprint":false},{"year":2026,"finding":"During neutrophil extracellular trap (NET) formation, XKR8 is cleaved by caspase-3, disrupting plasma membrane lipid asymmetry; the resulting phospholipid scrambling alters membrane lipid tension, promoting Ca2+ influx through mechanosensitive channels that drives NET formation. Mutation of the caspase-3 cleavage site in XKR8 impairs NET formation, and Xkr8-deficient mice show defective NETs and compromised control of Candida albicans pulmonary infection.","method":"Caspase-3 cleavage site mutagenesis, Xkr8 knockout mice, calcium channel inhibition at different stages, NET quantification, in vivo fungal infection model","journal":"Nature immunology","confidence":"High","confidence_rationale":"Tier 1-2 — mutagenesis of cleavage site, genetic KO with in vivo phenotype, pharmacological dissection of mechanosensitive Ca2+ signaling downstream of scrambling","pmids":["41781710"],"is_preprint":false},{"year":2025,"finding":"In a CRISPR-Cas9 knockout breast cancer model, XKR8 KO specifically suppressed macrophage-mediated efferocytosis and tumor growth in immune-competent (but not immune-deficient) mice, establishing that XKR8-driven apoptotic PtdSer externalization promotes tumor immune evasion via efferocytosis, distinct from the calcium-activated TMEM16F scramblase pathway.","method":"CRISPR/Cas9 knockout, orthotopic tumor implantation in immune-competent and immune-deficient (NOD/SCID, RAG-KO) mice, efferocytosis assays","journal":"Cell death discovery","confidence":"Medium","confidence_rationale":"Tier 2 — genetic KO with immune-competent vs immune-deficient comparison, but single lab","pmids":["41198619"],"is_preprint":false},{"year":2023,"finding":"A nonsense variant (p.W237X) in XKR8 impairs surface localization of XKR8 protein in HEK293T cells, and transgenic mice carrying this variant show late-onset auditory neuropathy with altered XKR8 protein localization in spiral ganglion neurons, linking proper XKR8 membrane trafficking to inner ear neural homeostasis.","method":"Plasmid expression in HEK293T cells, immunolabeling for surface localization, transgenic mouse audiometry (ABR), in situ hybridization and immunolabeling in cochlea","journal":"Journal of translational medicine","confidence":"Medium","confidence_rationale":"Tier 2-3 — cell-based localization assay plus transgenic mouse phenotype; single lab, single variant","pmids":["37101210"],"is_preprint":false}],"current_model":"XKR8 is a 10-transmembrane phospholipid scramblase that resides at the plasma membrane as a complex with the chaperones basigin or neuroplastin; upon apoptotic caspase-3-mediated cleavage (or activating phosphorylation) of its C-terminal plug domain, XKR8 adopts an active heterotetramer conformation, opening a charged transmembrane translocation pathway to irreversibly externalize phosphatidylserine as an 'eat-me' signal for efferocytosis—a process essential for apoptotic cell clearance in diverse tissues, developmental axon pruning, NET formation in neutrophils, and prevention of autoimmunity."},"narrative":{"teleology":[{"year":2016,"claim":"Establishing that XKR8 requires BSG or NPTN as obligate chaperone partners for plasma-membrane delivery and that the activated complex assembles into a heterotetramer resolved the long-standing question of how XKR8 reaches the cell surface and acquires apoptosis-dependent scramblase competence.","evidence":"Reciprocal co-IP, BSG/NPTN double-KO cells, BSG transmembrane glutamic-acid mutagenesis, native PAGE and size-exclusion chromatography","pmids":["27503893"],"confidence":"High","gaps":["Structural basis of heterotetramer assembly unknown at this stage","Role of NPTN versus BSG in tissue-specific contexts not resolved","Activation mechanism downstream of caspase cleavage not molecularly defined"]},{"year":2018,"claim":"Demonstrating that Xkr8 is the sole caspase-dependent scramblase in hematopoietic cells and that its loss causes lupus-like autoimmunity established XKR8 as physiologically essential for apoptotic cell clearance and immune homeostasis.","evidence":"Xkr8 knockout mouse, annexin V flow cytometry, Tim4/MerTK phagocytosis assays, immunological characterization","pmids":["29440417"],"confidence":"High","gaps":["Whether other XKR family members compensate in non-hematopoietic tissues not addressed","Downstream signaling linking impaired efferocytosis to autoimmunity not dissected"]},{"year":2019,"claim":"Identifying phosphorylation as a caspase-independent activation mechanism for XKR8, and showing that flippase inactivation cooperates with XKR8 activation, revealed a dual-switch model for phosphatidylserine externalization.","evidence":"Phos-tag PAGE, mass spectrometry of phosphosites, phosphomimic mutagenesis, kinase/phosphatase inhibitor treatments, flippase gene deletion","pmids":["30718401"],"confidence":"High","gaps":["Identity of the responsible kinase(s) not determined","Whether phosphorylation-mediated activation occurs physiologically during non-apoptotic processes not shown"]},{"year":2020,"claim":"Showing that Xkr8 knockout causes male infertility through failed PtdSer-dependent clearance of apoptotic germ cells by Sertoli cells extended XKR8's physiological role beyond the immune system to reproductive biology.","evidence":"Xkr8 KO mouse testes analysis, electron and fluorescence microscopy, phagocytosis assays","pmids":["31712393"],"confidence":"High","gaps":["Whether XKR8 functions in female reproductive cell clearance not examined","Molecular cues linking Sertoli-cell recognition to XKR8-exposed PtdSer not fully defined"]},{"year":2021,"claim":"The first atomic-resolution structure of the XKR8–BSG complex revealed a cuboid transmembrane architecture with a charged intramolecular translocation pathway and a tryptophan gate, providing a molecular mechanism for how the scramblase translocates phospholipids.","evidence":"Cryo-EM and X-ray crystallography at 3.8 Å, Trp-to-Ala mutagenesis yielding constitutive activity, scramblase assay","pmids":["34625749"],"confidence":"High","gaps":["Structure captured in detergent, not native lipid bilayer","Conformational change accompanying caspase cleavage not visualized","Lipid selectivity of the translocation pathway not mechanistically explained"]},{"year":2023,"claim":"Demonstrating that Xkr8 loss prevents PtdSer-dependent glial pruning of excess synapses and axonal projections in the developing brain established XKR8 as a central effector of developmental neural circuit refinement.","evidence":"Xkr8 KO mouse brain, confocal synapse density analysis, axon tracing, hippocampal electrophysiology","pmids":["37211968"],"confidence":"High","gaps":["Upstream cues triggering XKR8 activation in neurons not identified","Whether phenotype is fully cell-autonomous versus involving non-neuronal XKR8 not resolved"]},{"year":2023,"claim":"Identification of a human nonsense variant (p.W237X) that impairs XKR8 surface localization and causes late-onset auditory neuropathy in transgenic mice linked XKR8 membrane trafficking to inner-ear neural homeostasis.","evidence":"HEK293T surface-localization assay, transgenic mouse ABR audiometry, cochlear immunolabeling","pmids":["37101210"],"confidence":"Medium","gaps":["Single variant in a single lab; independent replication in human cohorts lacking","Mechanism connecting impaired XKR8 localization to spiral ganglion neuron degeneration not defined","Whether scramblase activity is required or merely surface presence unclear"]},{"year":2024,"claim":"A cryo-EM structure in lipid nanodiscs resolved the autoinhibitory mechanism: the XKR8 C-terminal tail plugs the cytoplasmic groove via polar and van der Waals contacts, and disruption of these contacts activates scrambling, unifying caspase cleavage and phosphorylation as convergent plug-release mechanisms.","evidence":"Cryo-EM at 3.66 Å in lipid nanodiscs, C-terminal tail contact-residue mutagenesis, scramblase activity assay","pmids":["38364890"],"confidence":"High","gaps":["Active-state structure after plug removal not yet captured","How phosphorylation physically displaces the plug not structurally resolved"]},{"year":2025,"claim":"Showing that XKR8 knockout in breast cancer cells selectively suppresses macrophage efferocytosis and tumor growth in immune-competent hosts distinguished the apoptotic XKR8 scramblase pathway from TMEM16F in the tumor immune microenvironment.","evidence":"CRISPR KO in breast cancer cells, orthotopic implantation in syngeneic versus immunodeficient mice, efferocytosis assays","pmids":["41198619"],"confidence":"Medium","gaps":["Single tumor model and single lab; generalizability across cancer types not established","Mechanism by which efferocytosis promotes immune evasion not molecularly dissected"]},{"year":2026,"claim":"Revealing that caspase-3-mediated XKR8 cleavage during NETosis disrupts membrane lipid asymmetry, altering lipid tension to open mechanosensitive Ca²⁺ channels, established an unexpected non-apoptotic role for XKR8 scramblase in innate immune defense.","evidence":"Caspase-3 cleavage-site mutagenesis, Xkr8 KO mice, Ca²⁺ channel pharmacology, NET quantification, in vivo Candida albicans pulmonary infection model","pmids":["41781710"],"confidence":"High","gaps":["Identity of the mechanosensitive channel(s) activated by XKR8-driven lipid scrambling not determined","Whether XKR8 participates in other forms of regulated cell death (e.g., pyroptosis, ferroptosis) unknown"]},{"year":null,"claim":"A structure of the fully activated (plug-released) XKR8–BSG complex has not been captured, leaving the conformational transition that opens the lipid translocation pathway and the basis for phospholipid selectivity unresolved.","evidence":"","pmids":[],"confidence":"High","gaps":["No active-state atomic structure available","Lipid headgroup selectivity of the translocation pathway not mechanistically explained","Upstream kinase(s) responsible for activating phosphorylation not identified"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0008289","term_label":"lipid binding","supporting_discovery_ids":[3,5]},{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[1,5,7]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,3,5,9]}],"pathway":[{"term_id":"R-HSA-5357801","term_label":"Programmed Cell Death","supporting_discovery_ids":[2,4,6,7]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[2,7,8]}],"complexes":["XKR8–Basigin heterotetramer","XKR8–Neuroplastin heterotetramer"],"partners":["BSG","NPTN"],"other_free_text":[]},"mechanistic_narrative":"XKR8 is a caspase-activated phospholipid scramblase that irreversibly externalizes phosphatidylserine on apoptotic cells, providing the principal 'eat-me' signal for efferocytic clearance and thereby preventing autoimmunity, supporting spermatogenesis, and enabling developmental axon pruning. XKR8 resides at the plasma membrane as a heteromeric complex with the single-pass chaperones basigin (BSG) or neuroplastin (NPTN), which are required for its surface delivery; upon apoptotic caspase-3 cleavage of a C-terminal autoinhibitory plug domain—or, independently, via activating phosphorylation at three identified sites—the complex reorganizes into a heterotetrameric state that opens a charged intramolecular translocation pathway lined by six polar residues and gated by a conserved tryptophan [PMID:27503893, PMID:34625749, PMID:38364890, PMID:30718401]. Xkr8 knockout mice exhibit defective phosphatidylserine exposure on apoptotic hematopoietic cells leading to lupus-like autoimmune disease, male infertility from failed germ-cell clearance, brain hyperconnectivity from impaired synaptic pruning, and defective neutrophil extracellular trap formation with compromised antifungal defense [PMID:29440417, PMID:31712393, PMID:37211968, PMID:41781710]. A nonsense variant (p.W237X) that impairs XKR8 surface localization causes late-onset auditory neuropathy in a transgenic mouse model, linking XKR8 membrane trafficking to inner-ear neural homeostasis [PMID:37101210]."},"prefetch_data":{"uniprot":{"accession":"Q9H6D3","full_name":"XK-related protein 8","aliases":[],"length_aa":395,"mass_kda":44.7,"function":"Phospholipid scramblase that promotes phosphatidylserine exposure on apoptotic cell surface (PubMed:23845944, PubMed:25231987). Phosphatidylserine is a specific marker only present at the surface of apoptotic cells and acts as a specific signal for engulfment (PubMed:23845944). Required for the clearance of apoptotic cells, such as engulfment of apoptotic germ cells by Sertoli cells, clearance of senescent neutrophils or regulation of bipolar cell numbers in the retina (By similarity). Has no effect on calcium-induced exposure of phosphatidylserine (PubMed:23845944). Promotes myoblast differentiation and survival (PubMed:28881496) (Microbial infection) Incorporated into Ebola virus-like particles, where its phospholipid scramblase activity is required to promote phosphatidylserine exposure on the surface of viral particles (PubMed:29338048). Externalization of phosphatidylserine on the surface of viral particles is required for uptake by host cells (PubMed:29338048)","subcellular_location":"Cell membrane; Cytoplasm, perinuclear region","url":"https://www.uniprot.org/uniprotkb/Q9H6D3/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/XKR8","classification":"Not Classified","n_dependent_lines":12,"n_total_lines":1208,"dependency_fraction":0.009933774834437087},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/XKR8","total_profiled":1310},"omim":[{"mim_id":"621370","title":"X KELL BLOOD GROUP PRECURSOR-RELATED FAMILY, MEMBER 9; XKR9","url":"https://www.omim.org/entry/621370"},{"mim_id":"621368","title":"X KELL BLOOD GROUP PRECURSOR-RELATED FAMILY, MEMBER 4; XKR4","url":"https://www.omim.org/entry/621368"},{"mim_id":"619940","title":"X KELL BLOOD GROUP PRECURSOR-RELATED FAMILY, MEMBER 8; XKR8","url":"https://www.omim.org/entry/619940"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Golgi apparatus","reliability":"Approved"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/XKR8"},"hgnc":{"alias_symbol":["FLJ10307"],"prev_symbol":[]},"alphafold":{"accession":"Q9H6D3","domains":[{"cath_id":"-","chopping":"2-307_314-345_372-380","consensus_level":"medium","plddt":87.2798,"start":2,"end":380}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9H6D3","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9H6D3-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9H6D3-F1-predicted_aligned_error_v6.png","plddt_mean":82.12},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=XKR8","jax_strain_url":"https://www.jax.org/strain/search?query=XKR8"},"sequence":{"accession":"Q9H6D3","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9H6D3.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9H6D3/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9H6D3"}},"corpus_meta":[{"pmid":"27503893","id":"PMC_27503893","title":"Xkr8 phospholipid scrambling complex in apoptotic phosphatidylserine exposure.","date":"2016","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/27503893","citation_count":116,"is_preprint":false},{"pmid":"36424448","id":"PMC_36424448","title":"Targeting Xkr8 via nanoparticle-mediated in situ co-delivery of siRNA and chemotherapy drugs for cancer immunochemotherapy.","date":"2022","source":"Nature nanotechnology","url":"https://pubmed.ncbi.nlm.nih.gov/36424448","citation_count":87,"is_preprint":false},{"pmid":"30718401","id":"PMC_30718401","title":"Phosphorylation-mediated activation of mouse Xkr8 scramblase for phosphatidylserine exposure.","date":"2019","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/30718401","citation_count":57,"is_preprint":false},{"pmid":"29440417","id":"PMC_29440417","title":"Lupus-like autoimmune disease caused by a lack of Xkr8, a caspase-dependent phospholipid scramblase.","date":"2018","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/29440417","citation_count":44,"is_preprint":false},{"pmid":"34625749","id":"PMC_34625749","title":"The tertiary structure of the human Xkr8-Basigin complex that scrambles phospholipids at plasma membranes.","date":"2021","source":"Nature structural & molecular biology","url":"https://pubmed.ncbi.nlm.nih.gov/34625749","citation_count":39,"is_preprint":false},{"pmid":"31712393","id":"PMC_31712393","title":"Infertility Caused by Inefficient Apoptotic Germ Cell Clearance in Xkr8-Deficient Male Mice.","date":"2020","source":"Molecular and cellular biology","url":"https://pubmed.ncbi.nlm.nih.gov/31712393","citation_count":15,"is_preprint":false},{"pmid":"38685385","id":"PMC_38685385","title":"Knocking down of Xkr8 enhances chemotherapy efficacy through modulating tumor immune microenvironment.","date":"2024","source":"Journal of controlled release : official journal of the Controlled Release Society","url":"https://pubmed.ncbi.nlm.nih.gov/38685385","citation_count":11,"is_preprint":false},{"pmid":"37211968","id":"PMC_37211968","title":"Phospholipid scramblase Xkr8 is required for developmental axon pruning via phosphatidylserine exposure.","date":"2023","source":"The EMBO journal","url":"https://pubmed.ncbi.nlm.nih.gov/37211968","citation_count":9,"is_preprint":false},{"pmid":"38364890","id":"PMC_38364890","title":"The role of the C-terminal tail region as a plug to regulate XKR8 lipid scramblase.","date":"2024","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/38364890","citation_count":9,"is_preprint":false},{"pmid":"30559640","id":"PMC_30559640","title":"Xkr8 Modulates Bipolar Cell Number in the Mouse Retina.","date":"2018","source":"Frontiers in neuroscience","url":"https://pubmed.ncbi.nlm.nih.gov/30559640","citation_count":5,"is_preprint":false},{"pmid":"37101210","id":"PMC_37101210","title":"A dominant variant in apoptosis-related gene XKR8 is relevant to hereditary auditory neuropathy.","date":"2023","source":"Journal of translational medicine","url":"https://pubmed.ncbi.nlm.nih.gov/37101210","citation_count":2,"is_preprint":false},{"pmid":"40391322","id":"PMC_40391322","title":"Phospholipid Scramblases TMEM16F and Xkr8 mediate distinct features of Phosphatidylserine (PS) externalization and immune suppression to promote tumor growth.","date":"2025","source":"bioRxiv : the preprint server for biology","url":"https://pubmed.ncbi.nlm.nih.gov/40391322","citation_count":1,"is_preprint":false},{"pmid":"38471639","id":"PMC_38471639","title":"Knocking down of Xkr8 enhances chemotherapy efficacy through modulating tumor immune microenvironment.","date":"2024","source":"Journal of controlled release : official journal of the Controlled Release Society","url":"https://pubmed.ncbi.nlm.nih.gov/38471639","citation_count":1,"is_preprint":false},{"pmid":"41198619","id":"PMC_41198619","title":"Phospholipid scramblases TMEM16F and Xkr8 mediate distinct features of phosphatidylserine (PS) externalization and immune suppression to promote tumor growth.","date":"2025","source":"Cell death discovery","url":"https://pubmed.ncbi.nlm.nih.gov/41198619","citation_count":0,"is_preprint":false},{"pmid":"41781710","id":"PMC_41781710","title":"Lipid asymmetry disruption by XKR8 orchestrates neutrophil extracellular trap formation and inhibits fungal infection.","date":"2026","source":"Nature immunology","url":"https://pubmed.ncbi.nlm.nih.gov/41781710","citation_count":0,"is_preprint":false},{"pmid":"41692920","id":"PMC_41692920","title":"XKR8 Deletion Protects Against Noise-Induced Hearing Loss by Attenuating Apoptosis and Preserving Mitochondrial Bioenergetics in the Cochlea.","date":"2026","source":"Molecular neurobiology","url":"https://pubmed.ncbi.nlm.nih.gov/41692920","citation_count":0,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":9077,"output_tokens":2921,"usd":0.035523},"stage2":{"model":"claude-opus-4-6","input_tokens":6246,"output_tokens":2998,"usd":0.15927},"total_usd":0.194793,"stage1_batch_id":"msgbatch_01BparvgRZ9S2LMidvaoo86K","stage2_batch_id":"msgbatch_01NkBfv9SVZWthhS7j6mU6G3","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2016,\n      \"finding\": \"XKR8 forms a complex with basigin (BSG) or neuroplastin (NPTN); BSG and NPTN chaperone XKR8 to the plasma membrane, and their absence causes XKR8 to localize intracellularly, abolishing apoptosis-induced phosphatidylserine exposure. The atypical glutamic acid in BSG's transmembrane region is required for BSG association with XKR8. Upon apoptotic signaling, XKR8 is cleaved at the C-terminus and the XKR8/BSG complex forms a higher-order complex, likely a heterotetramer of two XKR8 and two BSG/NPTN molecules.\",\n      \"method\": \"Co-immunoprecipitation, BSG/NPTN double-knockout cell lines, mutational analysis of BSG transmembrane glutamic acid, native PAGE/size-exclusion to determine complex stoichiometry\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — reciprocal co-IP, knockout cells, mutagenesis, and complex stoichiometry; replicated in subsequent structural studies\",\n      \"pmids\": [\"27503893\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"XKR8 scramblase activity is activated by phosphorylation: kinase inhibitors suppress and phosphatase inhibitors enhance PtdSer exposure; mass spectrometry and mutational analysis identified three phosphorylation sites whose phosphomimic mutations render XKR8 resistant to kinase inhibition. Flippase activity counteracts XKR8-mediated PtdSer exposure, so combined phosphorylation of XKR8 and flippase downregulation cooperate to achieve maximal PtdSer externalization.\",\n      \"method\": \"Phos-tag PAGE, mass spectrometry, site-directed mutagenesis of phosphorylation sites, kinase/phosphatase inhibitor treatments, flippase gene deletion\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro assay, mass spectrometry-identified sites, and mutagenesis; multiple orthogonal methods in a single rigorous study\",\n      \"pmids\": [\"30718401\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"XKR8 is the sole caspase-dependent scramblase expressed in mouse hematopoietic cells; Xkr8 knockout mice show strongly reduced PtdSer exposure on apoptotic thymocytes, splenocytes, and neutrophils, impairing their phagocytic clearance by Tim4/MerTK-expressing macrophages in vitro and in vivo, and leading to lupus-like autoimmune disease.\",\n      \"method\": \"Xkr8 knockout mouse model, flow cytometry for PtdSer exposure (annexin V), in vitro phagocytosis assays with Tim4/MerTK-expressing phagocytes, histological/immunological characterization\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean knockout with defined cellular and in vivo phenotypes, multiple orthogonal readouts; replicated in subsequent papers\",\n      \"pmids\": [\"29440417\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Cryo-EM and X-ray crystallography of the human XKR8-Basigin complex at 3.8 Å revealed a cuboid transmembrane region with 22 charged residues stabilized by salt bridges; phosphatidylcholine binds in a hydrophobic cleft on the extracellular surface; six charged residues arranged top-to-bottom inside the molecule form a translocation pathway for phospholipids. A critical tryptophan at the exoplasmic end of the path gates scramblase activity — its mutation to alanine renders the complex constitutively active.\",\n      \"method\": \"Cryo-EM, X-ray crystallography, site-directed mutagenesis (W→A), scramblase activity assay\",\n      \"journal\": \"Nature structural & molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — high-resolution structure combined with mutagenesis and functional validation\",\n      \"pmids\": [\"34625749\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"XKR8-mediated PtdSer exposure is essential for phagocytic clearance of apoptotic germ cells by Sertoli cells during spermatogenesis; Xkr8-/- male mice are infertile due to impaired PtdSer-dependent 'eat-me' signaling and secondary accumulation of unengulfed dead germ cells.\",\n      \"method\": \"Xkr8 knockout mouse model, annexin V flow cytometry, in vitro phagocytosis assay with Tim4/MerTK cells, fluorescence and electron microscopy of testes\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with defined phenotypic readout, multiple microscopy methods, in vitro mechanistic confirmation\",\n      \"pmids\": [\"31712393\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Cryo-EM structure of the human XKR8-Basigin complex in lipid nanodiscs at 3.66 Å shows that the C-terminal tail of XKR8 engages the cytoplasmic groove of XKR8 via polar and van der Waals interactions, maintaining the inactive state; point mutations disrupting these interactions strongly enhance scramblase activity, establishing that the C-terminal tail functions as a plug that is released upon caspase cleavage or phosphorylation to activate XKR8.\",\n      \"method\": \"Cryo-EM in lipid nanodiscs, site-directed mutagenesis of C-terminal tail contact residues, scramblase activity assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — high-resolution structure in native-like lipid environment plus mutagenesis with functional validation\",\n      \"pmids\": [\"38364890\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"XKR8 is required for developmental axon pruning in the mammalian brain: Xkr8 knockout mice show failure of PtdSer exposure in hippocampal neurons, excess excitatory nerve terminals, increased cortico-cortical and cortico-spinal projections, aberrant hippocampal electrophysiology, and global brain hyperconnectivity, identifying XKR8-mediated phospholipid scrambling as the mechanism labeling developing projections for glial phagocytic elimination.\",\n      \"method\": \"Xkr8 knockout mouse model, annexin V staining for PtdSer exposure, confocal microscopy of synapse density, axon tracing, electrophysiology (hippocampal recordings)\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with multiple orthogonal phenotypic readouts linking XKR8 scramblase activity to a defined developmental process\",\n      \"pmids\": [\"37211968\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"During neutrophil extracellular trap (NET) formation, XKR8 is cleaved by caspase-3, disrupting plasma membrane lipid asymmetry; the resulting phospholipid scrambling alters membrane lipid tension, promoting Ca2+ influx through mechanosensitive channels that drives NET formation. Mutation of the caspase-3 cleavage site in XKR8 impairs NET formation, and Xkr8-deficient mice show defective NETs and compromised control of Candida albicans pulmonary infection.\",\n      \"method\": \"Caspase-3 cleavage site mutagenesis, Xkr8 knockout mice, calcium channel inhibition at different stages, NET quantification, in vivo fungal infection model\",\n      \"journal\": \"Nature immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — mutagenesis of cleavage site, genetic KO with in vivo phenotype, pharmacological dissection of mechanosensitive Ca2+ signaling downstream of scrambling\",\n      \"pmids\": [\"41781710\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"In a CRISPR-Cas9 knockout breast cancer model, XKR8 KO specifically suppressed macrophage-mediated efferocytosis and tumor growth in immune-competent (but not immune-deficient) mice, establishing that XKR8-driven apoptotic PtdSer externalization promotes tumor immune evasion via efferocytosis, distinct from the calcium-activated TMEM16F scramblase pathway.\",\n      \"method\": \"CRISPR/Cas9 knockout, orthotopic tumor implantation in immune-competent and immune-deficient (NOD/SCID, RAG-KO) mice, efferocytosis assays\",\n      \"journal\": \"Cell death discovery\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO with immune-competent vs immune-deficient comparison, but single lab\",\n      \"pmids\": [\"41198619\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"A nonsense variant (p.W237X) in XKR8 impairs surface localization of XKR8 protein in HEK293T cells, and transgenic mice carrying this variant show late-onset auditory neuropathy with altered XKR8 protein localization in spiral ganglion neurons, linking proper XKR8 membrane trafficking to inner ear neural homeostasis.\",\n      \"method\": \"Plasmid expression in HEK293T cells, immunolabeling for surface localization, transgenic mouse audiometry (ABR), in situ hybridization and immunolabeling in cochlea\",\n      \"journal\": \"Journal of translational medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — cell-based localization assay plus transgenic mouse phenotype; single lab, single variant\",\n      \"pmids\": [\"37101210\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"XKR8 is a 10-transmembrane phospholipid scramblase that resides at the plasma membrane as a complex with the chaperones basigin or neuroplastin; upon apoptotic caspase-3-mediated cleavage (or activating phosphorylation) of its C-terminal plug domain, XKR8 adopts an active heterotetramer conformation, opening a charged transmembrane translocation pathway to irreversibly externalize phosphatidylserine as an 'eat-me' signal for efferocytosis—a process essential for apoptotic cell clearance in diverse tissues, developmental axon pruning, NET formation in neutrophils, and prevention of autoimmunity.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"XKR8 is a caspase-activated phospholipid scramblase that irreversibly externalizes phosphatidylserine on apoptotic cells, providing the principal 'eat-me' signal for efferocytic clearance and thereby preventing autoimmunity, supporting spermatogenesis, and enabling developmental axon pruning. XKR8 resides at the plasma membrane as a heteromeric complex with the single-pass chaperones basigin (BSG) or neuroplastin (NPTN), which are required for its surface delivery; upon apoptotic caspase-3 cleavage of a C-terminal autoinhibitory plug domain—or, independently, via activating phosphorylation at three identified sites—the complex reorganizes into a heterotetrameric state that opens a charged intramolecular translocation pathway lined by six polar residues and gated by a conserved tryptophan [PMID:27503893, PMID:34625749, PMID:38364890, PMID:30718401]. Xkr8 knockout mice exhibit defective phosphatidylserine exposure on apoptotic hematopoietic cells leading to lupus-like autoimmune disease, male infertility from failed germ-cell clearance, brain hyperconnectivity from impaired synaptic pruning, and defective neutrophil extracellular trap formation with compromised antifungal defense [PMID:29440417, PMID:31712393, PMID:37211968, PMID:41781710]. A nonsense variant (p.W237X) that impairs XKR8 surface localization causes late-onset auditory neuropathy in a transgenic mouse model, linking XKR8 membrane trafficking to inner-ear neural homeostasis [PMID:37101210].\",\n  \"teleology\": [\n    {\n      \"year\": 2016,\n      \"claim\": \"Establishing that XKR8 requires BSG or NPTN as obligate chaperone partners for plasma-membrane delivery and that the activated complex assembles into a heterotetramer resolved the long-standing question of how XKR8 reaches the cell surface and acquires apoptosis-dependent scramblase competence.\",\n      \"evidence\": \"Reciprocal co-IP, BSG/NPTN double-KO cells, BSG transmembrane glutamic-acid mutagenesis, native PAGE and size-exclusion chromatography\",\n      \"pmids\": [\"27503893\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Structural basis of heterotetramer assembly unknown at this stage\",\n        \"Role of NPTN versus BSG in tissue-specific contexts not resolved\",\n        \"Activation mechanism downstream of caspase cleavage not molecularly defined\"\n      ]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Demonstrating that Xkr8 is the sole caspase-dependent scramblase in hematopoietic cells and that its loss causes lupus-like autoimmunity established XKR8 as physiologically essential for apoptotic cell clearance and immune homeostasis.\",\n      \"evidence\": \"Xkr8 knockout mouse, annexin V flow cytometry, Tim4/MerTK phagocytosis assays, immunological characterization\",\n      \"pmids\": [\"29440417\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Whether other XKR family members compensate in non-hematopoietic tissues not addressed\",\n        \"Downstream signaling linking impaired efferocytosis to autoimmunity not dissected\"\n      ]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Identifying phosphorylation as a caspase-independent activation mechanism for XKR8, and showing that flippase inactivation cooperates with XKR8 activation, revealed a dual-switch model for phosphatidylserine externalization.\",\n      \"evidence\": \"Phos-tag PAGE, mass spectrometry of phosphosites, phosphomimic mutagenesis, kinase/phosphatase inhibitor treatments, flippase gene deletion\",\n      \"pmids\": [\"30718401\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Identity of the responsible kinase(s) not determined\",\n        \"Whether phosphorylation-mediated activation occurs physiologically during non-apoptotic processes not shown\"\n      ]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Showing that Xkr8 knockout causes male infertility through failed PtdSer-dependent clearance of apoptotic germ cells by Sertoli cells extended XKR8's physiological role beyond the immune system to reproductive biology.\",\n      \"evidence\": \"Xkr8 KO mouse testes analysis, electron and fluorescence microscopy, phagocytosis assays\",\n      \"pmids\": [\"31712393\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Whether XKR8 functions in female reproductive cell clearance not examined\",\n        \"Molecular cues linking Sertoli-cell recognition to XKR8-exposed PtdSer not fully defined\"\n      ]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"The first atomic-resolution structure of the XKR8–BSG complex revealed a cuboid transmembrane architecture with a charged intramolecular translocation pathway and a tryptophan gate, providing a molecular mechanism for how the scramblase translocates phospholipids.\",\n      \"evidence\": \"Cryo-EM and X-ray crystallography at 3.8 Å, Trp-to-Ala mutagenesis yielding constitutive activity, scramblase assay\",\n      \"pmids\": [\"34625749\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Structure captured in detergent, not native lipid bilayer\",\n        \"Conformational change accompanying caspase cleavage not visualized\",\n        \"Lipid selectivity of the translocation pathway not mechanistically explained\"\n      ]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Demonstrating that Xkr8 loss prevents PtdSer-dependent glial pruning of excess synapses and axonal projections in the developing brain established XKR8 as a central effector of developmental neural circuit refinement.\",\n      \"evidence\": \"Xkr8 KO mouse brain, confocal synapse density analysis, axon tracing, hippocampal electrophysiology\",\n      \"pmids\": [\"37211968\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Upstream cues triggering XKR8 activation in neurons not identified\",\n        \"Whether phenotype is fully cell-autonomous versus involving non-neuronal XKR8 not resolved\"\n      ]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Identification of a human nonsense variant (p.W237X) that impairs XKR8 surface localization and causes late-onset auditory neuropathy in transgenic mice linked XKR8 membrane trafficking to inner-ear neural homeostasis.\",\n      \"evidence\": \"HEK293T surface-localization assay, transgenic mouse ABR audiometry, cochlear immunolabeling\",\n      \"pmids\": [\"37101210\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Single variant in a single lab; independent replication in human cohorts lacking\",\n        \"Mechanism connecting impaired XKR8 localization to spiral ganglion neuron degeneration not defined\",\n        \"Whether scramblase activity is required or merely surface presence unclear\"\n      ]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"A cryo-EM structure in lipid nanodiscs resolved the autoinhibitory mechanism: the XKR8 C-terminal tail plugs the cytoplasmic groove via polar and van der Waals contacts, and disruption of these contacts activates scrambling, unifying caspase cleavage and phosphorylation as convergent plug-release mechanisms.\",\n      \"evidence\": \"Cryo-EM at 3.66 Å in lipid nanodiscs, C-terminal tail contact-residue mutagenesis, scramblase activity assay\",\n      \"pmids\": [\"38364890\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Active-state structure after plug removal not yet captured\",\n        \"How phosphorylation physically displaces the plug not structurally resolved\"\n      ]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Showing that XKR8 knockout in breast cancer cells selectively suppresses macrophage efferocytosis and tumor growth in immune-competent hosts distinguished the apoptotic XKR8 scramblase pathway from TMEM16F in the tumor immune microenvironment.\",\n      \"evidence\": \"CRISPR KO in breast cancer cells, orthotopic implantation in syngeneic versus immunodeficient mice, efferocytosis assays\",\n      \"pmids\": [\"41198619\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Single tumor model and single lab; generalizability across cancer types not established\",\n        \"Mechanism by which efferocytosis promotes immune evasion not molecularly dissected\"\n      ]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Revealing that caspase-3-mediated XKR8 cleavage during NETosis disrupts membrane lipid asymmetry, altering lipid tension to open mechanosensitive Ca²⁺ channels, established an unexpected non-apoptotic role for XKR8 scramblase in innate immune defense.\",\n      \"evidence\": \"Caspase-3 cleavage-site mutagenesis, Xkr8 KO mice, Ca²⁺ channel pharmacology, NET quantification, in vivo Candida albicans pulmonary infection model\",\n      \"pmids\": [\"41781710\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Identity of the mechanosensitive channel(s) activated by XKR8-driven lipid scrambling not determined\",\n        \"Whether XKR8 participates in other forms of regulated cell death (e.g., pyroptosis, ferroptosis) unknown\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"A structure of the fully activated (plug-released) XKR8–BSG complex has not been captured, leaving the conformational transition that opens the lipid translocation pathway and the basis for phospholipid selectivity unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"No active-state atomic structure available\",\n        \"Lipid headgroup selectivity of the translocation pathway not mechanistically explained\",\n        \"Upstream kinase(s) responsible for activating phosphorylation not identified\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\n        \"term_id\": \"GO:0008289\",\n        \"supporting_discovery_ids\": [3, 5]\n      },\n      {\n        \"term_id\": \"GO:0140096\",\n        \"supporting_discovery_ids\": [1, 5, 7]\n      }\n    ],\n    \"localization\": [\n      {\n        \"term_id\": \"GO:0005886\",\n        \"supporting_discovery_ids\": [0, 3, 5, 9]\n      }\n    ],\n    \"pathway\": [\n      {\n        \"term_id\": \"R-HSA-5357801\",\n        \"supporting_discovery_ids\": [2, 4, 6, 7]\n      },\n      {\n        \"term_id\": \"R-HSA-168256\",\n        \"supporting_discovery_ids\": [2, 7, 8]\n      }\n    ],\n    \"complexes\": [\n      \"XKR8–Basigin heterotetramer\",\n      \"XKR8–Neuroplastin heterotetramer\"\n    ],\n    \"partners\": [\n      \"BSG\",\n      \"NPTN\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}