{"gene":"SLC2A8","run_date":"2026-04-28T20:42:07","timeline":{"discoveries":[{"year":2000,"finding":"GLUT8 (SLC2A8) is a facilitative glucose transporter with intrinsic glucose transport activity, demonstrated by specific cytochalasin B binding (Kd=56.6 nM) and reconstitutable glucose transport in COS-7 cells transfected with GLUT8 cDNA.","method":"Heterologous expression in COS-7 cells, cytochalasin B binding assay, reconstituted glucose transport assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — in vitro transport assay with direct binding measurement, reconstituted activity","pmids":["10821868"],"is_preprint":false},{"year":2000,"finding":"GLUT8 mediates insulin-stimulated glucose uptake in mouse blastocysts; insulin induces a change in intracellular localization of GLUT8 that translates into increased glucose uptake, an effect blocked by antisense oligoprobes.","method":"Antisense oligoprobe inhibition, glucose uptake assay, immunolocalization in blastocysts","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (antisense knockdown, uptake assay, localization) in a single study","pmids":["10860996"],"is_preprint":false},{"year":2000,"finding":"GLUTX1 (GLUT8) has glucose transport activity with Km ~2 mM when expressed in Xenopus oocytes, but only after mutational suppression of an N-terminal dileucine internalization motif that normally retains the protein intracellularly; transport is inhibited by cytochalasin B and partly competed by D-fructose and D-galactose.","method":"Xenopus oocyte expression, transport assay, site-directed mutagenesis of dileucine motif","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — reconstitution in oocytes with mutagenesis identifying the internalization motif","pmids":["10671487"],"is_preprint":false},{"year":2001,"finding":"GLUT8 is retained in intracellular compartments via its N-terminal dileucine motif; mutation of this motif leads to constitutive plasma membrane expression, and blocking endocytosis with dominant-negative dynamin also causes cell surface accumulation, but unlike GLUT4, GLUT8 does not translocate to the plasma membrane in response to insulin, phorbol ester, or hyperosmolarity in rat adipose cells.","method":"HA-epitope tagging, transfection in primary rat adipocytes, dominant-negative dynamin co-expression, immunofluorescence","journal":"The Biochemical journal","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal approaches (mutagenesis, dominant-negative, multiple stimuli tested) replicated in adipocytes","pmids":["11513753"],"is_preprint":false},{"year":2002,"finding":"GLUT8 undergoes rapid translocation to the rough endoplasmic reticulum in rat hippocampal neurons following peripheral glucose administration, as shown by immunogold electron microscopy and subcellular fractionation; this trafficking is impaired in streptozotocin diabetic rats, suggesting insulin is required for GLUT8 translocation.","method":"Immunogold electron microscopy, subcellular membrane fractionation, immunoblot","journal":"The Journal of comparative neurology","confidence":"Medium","confidence_rationale":"Tier 2 — direct ultrastructural localization with functional context, single lab","pmids":["12271485"],"is_preprint":false},{"year":2004,"finding":"GLUT8 contains a [DE]XXXL[LI] late endosomal/lysosomal targeting motif; mutation of the glutamate to arginine (as in GLUT4) alters GLUT8 endocytosis and retains it at the plasma membrane; GLUT8 does not reside in a recycling vesicle pool and localizes to late endosomes/lysosomes.","method":"Site-directed mutagenesis, immunofluorescence, subcellular localization in 3T3L1, HEK293, CHO cells","journal":"Traffic (Copenhagen, Denmark)","confidence":"High","confidence_rationale":"Tier 1-2 — mutagenesis plus localization in multiple cell types identifying the targeting motif","pmids":["16262729"],"is_preprint":false},{"year":2004,"finding":"GLUT8 translocation to the plasma membrane in neuronal N2A cells is not stimulated by insulin, IGF-1, KCl depolarization, or hypoxia; mutation of the N-terminal dileucine motif (L12,13→A12,13) constitutively localizes GLUT8 to the plasma membrane.","method":"GLUT8-GFP stable transfection, immunohistochemistry, subcellular fractionation, site-directed mutagenesis","journal":"Journal of neuroscience research","confidence":"Medium","confidence_rationale":"Tier 2 — multiple stimuli tested with mutagenesis confirmation, single lab","pmids":["14994344"],"is_preprint":false},{"year":2006,"finding":"GLUT8 endocytosis is mediated by direct interaction of its N-terminal dileucine motif with the beta2-adaptin subunit of the AP-2 adaptor complex, targeting GLUT8 to clathrin-coated vesicles; RNAi knockdown of AP-2 mu2 subunit causes GLUT8 accumulation at the plasma membrane comparable to dominant-negative dynamin.","method":"Yeast two-hybrid, GST pulldown, RNAi knockdown of AP-2, immunofluorescence","journal":"Journal of cell science","confidence":"High","confidence_rationale":"Tier 1-2 — multiple orthogonal methods (Y2H, GST pulldown, RNAi) identifying the molecular endocytic mechanism","pmids":["16723738"],"is_preprint":false},{"year":2006,"finding":"Deletion of the Slc2a8 gene in mice results in increased hippocampal neuronal proliferation and increased P-wave duration in the heart, but does not impair normal embryonic or postnatal development or glucose homeostasis; GLUT8 is dispensable for embryonic development.","method":"Knockout mouse generation, cardiac electrophysiology, hippocampal BrdU incorporation assay","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 2 — clean KO with defined cellular phenotypes in multiple tissues","pmids":["16705176"],"is_preprint":false},{"year":2009,"finding":"Endogenous GLUT8 in spermatocytes and spermatids localizes to a late endosomal/lysosomal compartment; its N-terminal intracellular domain interacts with AP1 and AP2 (but not AP3 or AP4), and the GLUT8 N-terminal intracellular domain fused to the tailless IL-2 receptor alpha chain is sufficient to direct that chimera to intracellular membranes.","method":"Immunofluorescence of endogenous protein, AP complex interaction assays, chimeric protein targeting experiments","journal":"The FEBS journal","confidence":"High","confidence_rationale":"Tier 2 — endogenous localization plus chimeric protein experiments identifying sufficient targeting signal, multiple methods","pmids":["19523115"],"is_preprint":false},{"year":2012,"finding":"GLUT8 is required for hepatocyte fructose transport; GLUT8 overexpression or shRNA-mediated knockdown significantly increases or decreases radiolabeled fructose uptake in cultured hepatocytes; GLUT8-deficient mice show diminished fructose uptake, reduced de novo lipogenesis, and attenuated hepatic triglyceride/cholesterol accumulation on a high-fructose diet.","method":"shRNA knockdown, adenoviral overexpression, radiolabeled fructose uptake, GLUT8-deficient mouse model, hepatic lipid quantification","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods in vitro and in vivo confirming fructose transport function","pmids":["24519932"],"is_preprint":false},{"year":2012,"finding":"GLUT8 regulates enterocyte fructose transport; shRNA-mediated GLUT8 knockdown in Caco2 cells stimulates fructose uptake; GLUT8-deficient mice exhibit greater jejunal fructose uptake and GLUT8 deficiency leads to compensatory upregulation of GLUT12 in enterocytes.","method":"shRNA knockdown in Caco2 cells, GLUT8-KO mouse model, 14C-fructose uptake assay, immunoblot","journal":"Endocrinology","confidence":"High","confidence_rationale":"Tier 2 — in vitro and in vivo KO with defined molecular phenotype (GLUT12 upregulation)","pmids":["22822162"],"is_preprint":false},{"year":2012,"finding":"Slc2a8 deficiency in mice impairs oocyte metabolism and ATP production, and causes defective decidualization of endometrial stromal cells, leading to reduced litter size and impaired implantation; ovarian transplantation studies confirm effects on both embryo and implantation.","method":"Slc2a8 knockout mice, oocyte metabolic assays, ATP measurement, decidualization assay, ovarian transplantation","journal":"Biology of reproduction","confidence":"High","confidence_rationale":"Tier 2 — KO with multiple defined cellular phenotypes and transplantation rescue experiment","pmids":["22649075"],"is_preprint":false},{"year":2013,"finding":"GLUT8 transports dehydroascorbic acid (DHA) in Xenopus oocytes with Km of 3.23 mM and Vmax of 10.1 pmol/min/oocyte; DHA transport by GLUT8 is inhibited by glucose, fructose, and flavonoids (phloretin, quercetin), and maximal transport rates for DHA are lower than for 2-deoxy-D-glucose or fructose.","method":"Xenopus oocyte expression system, radiolabeled DHA and glucose transport assays, competitive inhibition studies","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — direct transport reconstitution in oocytes with kinetic characterization","pmids":["23396969"],"is_preprint":false},{"year":2013,"finding":"GLUT8 deficiency in male mice confers resistance to high-fructose diet-induced glucose intolerance and dyslipidemia, associated with enhanced hepatic PPARγ protein abundance; adenoviral GLUT8 overexpression in liver suppresses hepatic PPARγ expression, placing GLUT8 upstream of PPARγ in fructose-induced metabolic regulation.","method":"GLUT8-KO mice, high-fructose diet challenge, adenoviral overexpression, immunoblot for PPARγ","journal":"Molecular endocrinology","confidence":"Medium","confidence_rationale":"Tier 2 — KO and overexpression in vivo establishing pathway position, single lab","pmids":["24030250"],"is_preprint":false},{"year":2016,"finding":"SLC2A8 (GLUT8) is a mammalian trehalose transporter; trehalose enters hepatocytes via GLUT8 (demonstrated by GC/MS, fluorescence microscopy, and radiolabeled uptake); GLUT8-deficient hepatocytes and mice resist trehalose-induced AMPK phosphorylation and autophagic induction; heterologous overexpression of the Drosophila trehalose transporter Tret1 rescues autophagic flux in GLUT8-deficient hepatocytes.","method":"GC/MS, fluorescence microscopy, radiolabeled trehalose uptake, GLUT8-KO mouse/hepatocytes, Tret1 rescue experiment, AMPK phosphorylation immunoblot, autophagy flux assay","journal":"Scientific reports","confidence":"High","confidence_rationale":"Tier 1-2 — multiple orthogonal methods including functional rescue identifying GLUT8 as trehalose transporter","pmids":["27922102"],"is_preprint":false},{"year":2018,"finding":"GLUT8 deficiency in mice enhances hepatic PPARα activity and FGF21 secretion during fasting; hepatic PPARα knockdown in GLUT8-deficient mice normalizes the enhanced ketogenic and FGF21 secretory responses, placing GLUT8 upstream of PPARα in the adaptive fasting response.","method":"GLUT8-KO mice, fasting metabolic phenotyping, adenoviral PPARα knockdown, FGF21 ELISA, mitochondrial respiratory function assays","journal":"Endocrinology","confidence":"Medium","confidence_rationale":"Tier 2 — epistasis via in vivo PPARα knockdown rescue, single lab","pmids":["29596655"],"is_preprint":false},{"year":2020,"finding":"GLUT8 undergoes a lysosome-dependent cleavage reaction that releases the carboxy-terminal peptide to a separate vesicle population; GLUT8 does not transport glucose to the cell surface but is localized at the late endosomal/lysosomal interface where it may function as a sensory component of TXNIP-mediated hexosamine homeostasis.","method":"Splice variant cataloging, lysosomal inhibitor experiments, subcellular fractionation, immunofluorescence","journal":"Molecular and cellular biology","confidence":"Medium","confidence_rationale":"Tier 2 — novel cleavage mechanism identified with lysosomal inhibitor experiments, single lab","pmids":["33077497"],"is_preprint":false},{"year":2022,"finding":"TM4SF5 binds GLUT8 at the cell surface and modulates its translocation to the plasma membrane; fructose treatment transiently decreases TM4SF5-GLUT8 binding, allowing GLUT8 to separate and become active for fructose uptake; Tm4sf5 suppression or knockout reduces fructose uptake, de novo lipogenesis, and steatosis.","method":"Co-immunoprecipitation, Tm4sf5-KO mice, in vitro fructose uptake assays, immunofluorescence of TM4SF5-GLUT8 interaction dynamics","journal":"Molecular metabolism","confidence":"Medium","confidence_rationale":"Tier 2-3 — Co-IP identifies binding partner and KO confirms functional consequence, single lab","pmids":["35123128"],"is_preprint":false},{"year":2005,"finding":"GLUT8 in PC12 cells and primary hippocampal neurons localizes to a perinuclear compartment partially overlapping with ER markers but not trans-Golgi, early endosomes, lysosomes, or synaptic vesicles; no stimulus tested (depolarization, PKA/PKC activation, tyrosine kinase signaling, glucose deprivation, AMPK stimulation, osmotic shock) induces GLUT8 surface translocation, and no constitutive recycling through the plasma membrane was detected.","method":"Recombinant adenoviral GLUT8 with extracellular myc tag, immunofluorescence, dominant-negative dynamin co-expression, anti-myc antibody internalization assay","journal":"Endocrinology","confidence":"Medium","confidence_rationale":"Tier 2 — systematic testing of multiple stimuli with multiple detection methods, single lab","pmids":["16109784"],"is_preprint":false},{"year":2010,"finding":"The proline at position -2 from the dileucine residues in the [DE]XXXL[LI] motif of GLUT8 influences the affinity of AP1 and AP2 for GLUT8 and is critical for intracellular sorting to lysosomes; replacing the XXX (TQP) residues in GLUT8 with those from GLUT12 (GPN) causes dramatic missorting of GLUT8 to the cell surface.","method":"Site-directed mutagenesis of dileucine motif flanking residues, immunofluorescence, subcellular localization in CHO/HEK293 cells","journal":"Molecular membrane biology","confidence":"Medium","confidence_rationale":"Tier 2 — systematic mutagenesis identifying specific residues governing sorting, single lab","pmids":["21067453"],"is_preprint":false},{"year":2024,"finding":"SLC2A8 RNAi knockdown (79% mRNA reduction) in human first-trimester trophoblast ACH-3P cells reduces glucose uptake by 11% and differentially expresses genes involved in cellular respiration, oxidative phosphorylation, and ATP synthesis, suggesting GLUT8's primary function in trophoblasts is supporting cellular respiration rather than glucose supply.","method":"Lentiviral RNAi knockdown, glucose uptake assay, RNA-seq transcriptomics","journal":"Cells","confidence":"Medium","confidence_rationale":"Tier 2-3 — direct KD with uptake measurement and transcriptomic pathway analysis, single lab","pmids":["38474355"],"is_preprint":false},{"year":2019,"finding":"Endogenous GLUT8 partially co-localizes with cis-Golgi markers (58K protein, GM130) and with α-lactalbumin (a component of lactose synthase) in mammary epithelial cells, suggesting GLUT8 supplies glucose to the Golgi to support lactose synthesis.","method":"Immunohistochemistry, immunofluorescence co-localization with Golgi markers and lactose synthase component","journal":"Journal of physiology and biochemistry","confidence":"Low","confidence_rationale":"Tier 3 — co-localization only, no direct functional transport assay in Golgi","pmids":["31020623"],"is_preprint":false},{"year":2002,"finding":"GLUT8 protein is localized to dense core vesicles of synaptic nerve endings in supraoptic nucleus and secretory granules of vasopressin-positive neurons, specifically in vasopressin (but not oxytocin) neurons, as demonstrated by immunogold electron microscopy and double immunofluorescence.","method":"Immunogold labeling of ultrathin cryosections, double immunofluorescence microscopy","journal":"Endocrinology","confidence":"Medium","confidence_rationale":"Tier 2 — ultrastructural localization with cell-type specificity, single lab","pmids":["11751619"],"is_preprint":false},{"year":2002,"finding":"GLUT8 in spermatozoa is predominantly associated with the acrosomal region in both mouse and human sperm, with immunoreactivity at both the plasma membrane and intracellularly; expression onset in mouse testis coincides with appearance of mature spermatozoa.","method":"Immunohistochemistry with C-terminus-specific antiserum, developmental timing analysis","journal":"Cell and tissue research","confidence":"Medium","confidence_rationale":"Tier 3 — immunolocalization with developmental correlation, single lab","pmids":["11845330"],"is_preprint":false}],"current_model":"SLC2A8/GLUT8 is a class III facilitative hexose transporter (transporting glucose, fructose, dehydroascorbic acid, and trehalose) that is constitutively retained in late endosomal/lysosomal intracellular compartments via a [DE]XXXL[LI] dileucine motif that mediates endocytosis through direct interaction with the beta2-adaptin subunit of the AP-2 complex and with AP-1; in blastocysts, insulin induces its translocation to the plasma membrane to drive glucose uptake, but in most other cell types (adipocytes, neurons, hepatocytes) no stimulus induces surface translocation; GLUT8 is required for hepatocyte fructose and trehalose uptake (the latter linking it to AMPK-dependent autophagy induction), regulates enterocyte fructose transport, and its loss in mice causes impaired oocyte metabolism, decidualization defects, altered hippocampal neurogenesis, cardiac conduction changes, and enhanced PPARα-driven fasting responses."},"narrative":{"teleology":[{"year":2000,"claim":"Establishing that GLUT8 is a bona fide facilitative glucose transporter resolved the question of whether this orphan SLC2 family member had intrinsic sugar transport activity.","evidence":"Reconstituted glucose transport and cytochalasin B binding in COS-7 cells; parallel Km determination (~2 mM) and dileucine motif identification in Xenopus oocytes","pmids":["10821868","10671487"],"confidence":"High","gaps":["Substrate selectivity beyond glucose not yet characterized","Physiological site of action (intracellular vs. plasma membrane) unclear"]},{"year":2000,"claim":"Demonstrating insulin-stimulated GLUT8 translocation and glucose uptake in blastocysts established the first physiological context for GLUT8 function and raised the question of whether insulin-regulated trafficking was a general feature.","evidence":"Antisense knockdown, glucose uptake assay, and immunolocalization in mouse blastocysts","pmids":["10860996"],"confidence":"High","gaps":["Mechanism of insulin-regulated translocation in blastocysts not defined","Whether this occurs in somatic tissues was untested"]},{"year":2001,"claim":"Systematic testing in adipocytes showed GLUT8 does not undergo insulin-stimulated surface translocation like GLUT4, establishing that stimulus-regulated trafficking is cell-type specific and that intracellular retention is the default state.","evidence":"HA-tagged GLUT8, dominant-negative dynamin, and dileucine mutagenesis in primary rat adipocytes","pmids":["11513753"],"confidence":"High","gaps":["Why blastocysts but not adipocytes support translocation remained unexplained","Nature of the intracellular compartment not resolved"]},{"year":2004,"claim":"Identifying the [DE]XXXL[LI] motif as a late endosomal/lysosomal targeting signal—distinct from GLUT4's recycling endosome pathway—resolved the compartment identity and explained constitutive intracellular retention.","evidence":"Site-directed mutagenesis of the dileucine motif glutamate residue combined with immunofluorescence in 3T3-L1, HEK293, and CHO cells","pmids":["16262729"],"confidence":"High","gaps":["Adaptor proteins mediating the sorting not yet identified","Functional role of lysosomal localization unknown"]},{"year":2005,"claim":"Comprehensive stimulus screens in neurons confirmed that no known signaling pathway induces GLUT8 surface translocation in neuronal cells, reinforcing its constitutive intracellular residence and distinguishing it mechanistically from GLUT4.","evidence":"Myc-tagged GLUT8 adenovirus in PC12 cells and primary hippocampal neurons tested with depolarization, PKA/PKC activators, tyrosine kinase signaling, glucose deprivation, AMPK activation, and osmotic shock","pmids":["16109784"],"confidence":"Medium","gaps":["Subcellular compartment in neurons (partially ER-overlapping) differs from late endosome assignment in other cells","Functional role of neuronal GLUT8 unresolved"]},{"year":2006,"claim":"Identification of AP-2 beta2-adaptin as the direct binding partner for the GLUT8 dileucine motif defined the molecular mechanism of clathrin-mediated endocytosis that enforces intracellular retention.","evidence":"Yeast two-hybrid, GST pulldown, and AP-2 mu2 RNAi knockdown causing surface accumulation","pmids":["16723738"],"confidence":"High","gaps":["Role of AP-1 interaction in biosynthetic trafficking not fully dissected","Whether AP-2-mediated endocytosis is regulated in any tissue not tested"]},{"year":2006,"claim":"Generation of Slc2a8-knockout mice showed the gene is dispensable for embryonic development and glucose homeostasis but revealed unexpected phenotypes—increased hippocampal neurogenesis and prolonged cardiac P-wave duration—pointing to specialized tissue functions.","evidence":"Slc2a8 knockout mice with BrdU incorporation in hippocampus and cardiac electrophysiology","pmids":["16705176"],"confidence":"High","gaps":["Mechanistic basis for hippocampal proliferation and cardiac conduction phenotypes unknown","Metabolic challenge conditions not yet tested"]},{"year":2009,"claim":"Demonstrating that the GLUT8 N-terminal domain interacts with both AP-1 and AP-2 and is sufficient to redirect a heterologous reporter to intracellular membranes established the minimal sorting determinant and extended the adaptor interaction map.","evidence":"Chimeric IL-2Rα/GLUT8 constructs and AP complex interaction assays in spermatocytes/spermatids","pmids":["19523115"],"confidence":"High","gaps":["Relative contributions of AP-1 versus AP-2 to steady-state localization not resolved","Functional role in spermatogenesis not defined"]},{"year":2010,"claim":"Fine-mapping the dileucine motif flanking residues showed that the proline at position −2 tunes AP-1/AP-2 binding affinity and is critical for lysosomal versus surface sorting, explaining differences between GLUT8 and GLUT12 trafficking.","evidence":"Systematic mutagenesis of XXX residues in [DE]XXXL[LI] motif with subcellular localization in CHO/HEK293 cells","pmids":["21067453"],"confidence":"Medium","gaps":["Whether these residues are post-translationally modified to regulate sorting is unknown","In vivo relevance of motif variants not tested"]},{"year":2012,"claim":"Establishing GLUT8 as a hepatic fructose transporter and showing that its loss protects against fructose-induced lipogenesis and steatosis redefined GLUT8's primary physiological substrate in the liver as fructose rather than glucose.","evidence":"shRNA knockdown and adenoviral overexpression with radiolabeled fructose uptake in hepatocytes; GLUT8-KO mice on high-fructose diet","pmids":["24519932"],"confidence":"High","gaps":["Intracellular compartment where fructose transport occurs not specified","Relationship to known fructolysis enzymes not explored"]},{"year":2012,"claim":"Finding that GLUT8 deficiency increases enterocyte fructose uptake (with compensatory GLUT12 upregulation) revealed tissue-specific directionality: GLUT8 restricts fructose absorption in the gut but promotes it in the liver.","evidence":"GLUT8 shRNA in Caco2 cells and GLUT8-KO mice with jejunal 14C-fructose uptake","pmids":["22822162"],"confidence":"High","gaps":["Mechanism by which GLUT8 suppresses enterocyte fructose uptake (competition vs. regulation) unclear","Whether GLUT8 functions on the apical or intracellular membranes in enterocytes not resolved"]},{"year":2012,"claim":"Demonstrating that GLUT8 loss impairs oocyte ATP production and endometrial decidualization established a non-redundant metabolic role in female reproduction.","evidence":"Slc2a8 knockout mice with oocyte metabolic assays, ATP measurement, decidualization assay, and ovarian transplantation","pmids":["22649075"],"confidence":"High","gaps":["Substrate transported in oocytes (glucose, fructose, or trehalose) not determined","Signaling link between GLUT8 and decidualization not identified"]},{"year":2013,"claim":"Kinetic characterization of dehydroascorbic acid transport expanded the substrate repertoire beyond hexoses, showing GLUT8 is a multi-substrate transporter.","evidence":"Xenopus oocyte reconstitution with radiolabeled DHA, kinetic analysis (Km 3.23 mM), competitive inhibition with glucose/fructose/flavonoids","pmids":["23396969"],"confidence":"High","gaps":["Physiological relevance of DHA transport through a lysosomal transporter not established","Whether DHA and hexose transport use the same binding site not structurally resolved"]},{"year":2013,"claim":"Linking GLUT8 deficiency to enhanced hepatic PPARγ abundance and protection from fructose-induced metabolic syndrome positioned GLUT8 upstream of lipogenic transcriptional programs.","evidence":"GLUT8-KO mice on high-fructose diet and adenoviral GLUT8 overexpression suppressing PPARγ","pmids":["24030250"],"confidence":"Medium","gaps":["Direct versus indirect regulation of PPARγ by GLUT8-mediated fructose flux not distinguished","Mechanism of PPARγ regulation not identified"]},{"year":2016,"claim":"Identifying GLUT8 as the mammalian trehalose transporter and showing that trehalose-induced AMPK activation and autophagy require GLUT8 provided a mechanistic explanation for how a lysosomal transporter connects sugar sensing to autophagic signaling.","evidence":"GC/MS, radiolabeled trehalose uptake, GLUT8-KO hepatocytes, rescue by Drosophila Tret1, AMPK phosphorylation and autophagy flux assays","pmids":["27922102"],"confidence":"High","gaps":["Whether trehalose is transported into or out of the lysosome not resolved","Molecular link from trehalose to AMPK activation unknown"]},{"year":2018,"claim":"Epistasis experiments placing GLUT8 upstream of PPARα/FGF21 during fasting revealed that GLUT8 normally suppresses the fasting ketogenic response, integrating its lysosomal transport function with systemic metabolic adaptation.","evidence":"GLUT8-KO mice with fasting metabolic phenotyping; adenoviral PPARα knockdown normalizing enhanced ketogenesis and FGF21","pmids":["29596655"],"confidence":"Medium","gaps":["Substrate whose lysosomal transport by GLUT8 controls PPARα activity not identified","Whether this operates through mTORC1 or another nutrient-sensing pathway unknown"]},{"year":2020,"claim":"Discovery of lysosome-dependent C-terminal cleavage of GLUT8 suggested it undergoes regulated proteolytic processing, potentially linking its lysosomal residence to TXNIP-mediated hexosamine sensing.","evidence":"Lysosomal inhibitor experiments, subcellular fractionation showing cleaved C-terminal peptide in a separate vesicle population","pmids":["33077497"],"confidence":"Medium","gaps":["Protease responsible for cleavage not identified","Functional consequence of cleavage for transport activity unknown","TXNIP connection is correlative"]},{"year":2022,"claim":"Identification of TM4SF5 as a cell-surface binding partner that modulates GLUT8 plasma membrane availability for fructose uptake provided the first evidence of regulated GLUT8 surface translocation outside of blastocysts.","evidence":"Co-immunoprecipitation, Tm4sf5-KO mice, fructose-induced dissociation dynamics by immunofluorescence","pmids":["35123128"],"confidence":"Medium","gaps":["Interaction validated by Co-IP only; reciprocal and endogenous validation limited","Whether TM4SF5-GLUT8 interaction is direct or via a complex not resolved","Tissue generality of this mechanism unknown"]},{"year":null,"claim":"Key unresolved questions include the directionality and physiological substrate of GLUT8's lysosomal transport in vivo, the structural basis of its multi-substrate recognition, how lysosomal GLUT8 activity is sensed by PPARα/PPARγ and AMPK signaling cascades, and the functional significance of its C-terminal cleavage.","evidence":"","pmids":[],"confidence":"High","gaps":["No crystal or cryo-EM structure available","Directionality of lysosomal transport (import vs. export) not determined","Precise molecular link from GLUT8 activity to PPAR and AMPK signaling undefined"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0005215","term_label":"transporter activity","supporting_discovery_ids":[0,1,2,10,13,15]}],"localization":[{"term_id":"GO:0005764","term_label":"lysosome","supporting_discovery_ids":[5,9,17]},{"term_id":"GO:0005768","term_label":"endosome","supporting_discovery_ids":[5,9]},{"term_id":"GO:0031410","term_label":"cytoplasmic vesicle","supporting_discovery_ids":[3,23]}],"pathway":[{"term_id":"R-HSA-382551","term_label":"Transport of small molecules","supporting_discovery_ids":[0,2,10,13,15]},{"term_id":"R-HSA-9612973","term_label":"Autophagy","supporting_discovery_ids":[15]},{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[10,14,16]}],"complexes":[],"partners":["AP2B1","AP1","TM4SF5"],"other_free_text":[]},"mechanistic_narrative":"SLC2A8 (GLUT8) is a class III facilitative hexose transporter that mediates uptake of glucose, fructose, dehydroascorbic acid, and trehalose, and functions predominantly at intracellular late endosomal/lysosomal membranes rather than at the cell surface [PMID:10821868, PMID:23396969, PMID:27922102]. Constitutive intracellular retention is governed by an N-terminal [DE]XXXL[LI] dileucine motif whose interaction with AP-2 (via the beta2-adaptin subunit) and AP-1 drives clathrin-dependent endocytosis and lysosomal targeting; mutation of the motif or depletion of AP-2 redirects GLUT8 to the plasma membrane [PMID:16262729, PMID:16723738, PMID:21067453]. In mouse blastocysts, insulin stimulates GLUT8 translocation to the cell surface to increase glucose uptake, but in most other cell types—including adipocytes, neurons, and hepatocytes—no tested stimulus triggers surface redistribution [PMID:10860996, PMID:11513753, PMID:16109784]. GLUT8-mediated hepatic fructose and trehalose transport links it to downstream metabolic regulation: GLUT8 deficiency protects against high-fructose diet–induced dyslipidemia through enhanced PPARγ abundance, augments fasting PPARα/FGF21 signaling, and abolishes trehalose-induced AMPK-dependent autophagy [PMID:24519932, PMID:24030250, PMID:29596655, PMID:27922102]."},"prefetch_data":{"uniprot":{"accession":"Q9NY64","full_name":"Solute carrier family 2, facilitated glucose transporter member 8","aliases":["Glucose transporter type 8","GLUT-8","Glucose transporter type X1"],"length_aa":477,"mass_kda":50.8,"function":"Insulin-regulated facilitative hexose transporter that mediates the transport of glucose and fructose (By similarity). Facilitates hepatic influx of dietary trehalose, which in turn inhibits glucose and fructose influx triggering a starvation signal and hepatic autophagy through activation of AMPK and ULK1 (PubMed:27922102). Also able to mediate the transport of dehydroascorbate","subcellular_location":"Cell membrane; Cytoplasmic vesicle membrane","url":"https://www.uniprot.org/uniprotkb/Q9NY64/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/SLC2A8","classification":"Not Classified","n_dependent_lines":0,"n_total_lines":1208,"dependency_fraction":0.0},"opencell":{"profiled":true,"resolved_as":"","ensg_id":"ENSG00000136856","cell_line_id":"CID001328","localizations":[{"compartment":"vesicles","grade":3}],"interactors":[{"gene":"LAMP1","stoichiometry":4.0},{"gene":"IARS","stoichiometry":0.2},{"gene":"ANKRD26","stoichiometry":0.2},{"gene":"SNX9","stoichiometry":0.2},{"gene":"RPL38","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/target/CID001328","total_profiled":1310},"omim":[{"mim_id":"611036","title":"SOLUTE CARRIER FAMILY 2 (FACILITATED GLUCOSE TRANSPORTER), MEMBER 13; SLC2A13","url":"https://www.omim.org/entry/611036"},{"mim_id":"606813","title":"SOLUTE CARRIER FAMILY 2i (FACILITATED GLUCOSE TRANSPORTER), MEMBER 6; SLC2A6","url":"https://www.omim.org/entry/606813"},{"mim_id":"606145","title":"SOLUTE CARRIER FAMILY 2 (FACILITATED GLUCOSE TRANSPORTER), MEMBER 10; SLC2A10","url":"https://www.omim.org/entry/606145"},{"mim_id":"605245","title":"SOLUTE CARRIER FAMILY 2 (FACILITATED GLUCOSE TRANSPORTER), MEMBER 8; SLC2A8","url":"https://www.omim.org/entry/605245"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Vesicles","reliability":"Approved"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/SLC2A8"},"hgnc":{"alias_symbol":["GLUTX1","GLUT8"],"prev_symbol":[]},"alphafold":{"accession":"Q9NY64","domains":[{"cath_id":"1.20.1250.20","chopping":"23-230","consensus_level":"medium","plddt":92.3357,"start":23,"end":230},{"cath_id":"1.20.1250.20","chopping":"250-344_367-477","consensus_level":"medium","plddt":88.3335,"start":250,"end":477}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9NY64","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9NY64-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9NY64-F1-predicted_aligned_error_v6.png","plddt_mean":85.38},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=SLC2A8","jax_strain_url":"https://www.jax.org/strain/search?query=SLC2A8"},"sequence":{"accession":"Q9NY64","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9NY64.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9NY64/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9NY64"}},"corpus_meta":[{"pmid":"10860996","id":"PMC_10860996","title":"GLUT8 is a glucose transporter responsible for insulin-stimulated glucose uptake in the blastocyst.","date":"2000","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/10860996","citation_count":240,"is_preprint":false},{"pmid":"10821868","id":"PMC_10821868","title":"GLUT8, a novel member of the sugar transport facilitator family with glucose transport activity.","date":"2000","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/10821868","citation_count":191,"is_preprint":false},{"pmid":"10671487","id":"PMC_10671487","title":"GLUTX1, a novel mammalian glucose transporter expressed in the central nervous system and insulin-sensitive tissues.","date":"2000","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/10671487","citation_count":189,"is_preprint":false},{"pmid":"22452979","id":"PMC_22452979","title":"Multiple myeloma exhibits novel dependence on GLUT4, GLUT8, and GLUT11: implications for glucose transporter-directed therapy.","date":"2012","source":"Blood","url":"https://pubmed.ncbi.nlm.nih.gov/22452979","citation_count":157,"is_preprint":false},{"pmid":"23396969","id":"PMC_23396969","title":"Intestinal dehydroascorbic acid (DHA) transport mediated by the facilitative sugar transporters, GLUT2 and GLUT8.","date":"2013","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/23396969","citation_count":101,"is_preprint":false},{"pmid":"12899849","id":"PMC_12899849","title":"Broiler chickens (Ross strain) lack insulin-responsive glucose transporter GLUT4 and have GLUT8 cDNA.","date":"2003","source":"General and comparative endocrinology","url":"https://pubmed.ncbi.nlm.nih.gov/12899849","citation_count":95,"is_preprint":false},{"pmid":"19176349","id":"PMC_19176349","title":"GLUT8, the enigmatic intracellular hexose transporter.","date":"2009","source":"American journal of physiology. 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chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro transport assay with direct binding measurement, reconstituted activity\",\n      \"pmids\": [\"10821868\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"GLUT8 mediates insulin-stimulated glucose uptake in mouse blastocysts; insulin induces a change in intracellular localization of GLUT8 that translates into increased glucose uptake, an effect blocked by antisense oligoprobes.\",\n      \"method\": \"Antisense oligoprobe inhibition, glucose uptake assay, immunolocalization in blastocysts\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (antisense knockdown, uptake assay, localization) in a single study\",\n      \"pmids\": [\"10860996\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"GLUTX1 (GLUT8) has glucose transport activity with Km ~2 mM when expressed in Xenopus oocytes, but only after mutational suppression of an N-terminal dileucine internalization motif that normally retains the protein intracellularly; transport is inhibited by cytochalasin B and partly competed by D-fructose and D-galactose.\",\n      \"method\": \"Xenopus oocyte expression, transport assay, site-directed mutagenesis of dileucine motif\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstitution in oocytes with mutagenesis identifying the internalization motif\",\n      \"pmids\": [\"10671487\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"GLUT8 is retained in intracellular compartments via its N-terminal dileucine motif; mutation of this motif leads to constitutive plasma membrane expression, and blocking endocytosis with dominant-negative dynamin also causes cell surface accumulation, but unlike GLUT4, GLUT8 does not translocate to the plasma membrane in response to insulin, phorbol ester, or hyperosmolarity in rat adipose cells.\",\n      \"method\": \"HA-epitope tagging, transfection in primary rat adipocytes, dominant-negative dynamin co-expression, immunofluorescence\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal approaches (mutagenesis, dominant-negative, multiple stimuli tested) replicated in adipocytes\",\n      \"pmids\": [\"11513753\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"GLUT8 undergoes rapid translocation to the rough endoplasmic reticulum in rat hippocampal neurons following peripheral glucose administration, as shown by immunogold electron microscopy and subcellular fractionation; this trafficking is impaired in streptozotocin diabetic rats, suggesting insulin is required for GLUT8 translocation.\",\n      \"method\": \"Immunogold electron microscopy, subcellular membrane fractionation, immunoblot\",\n      \"journal\": \"The Journal of comparative neurology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct ultrastructural localization with functional context, single lab\",\n      \"pmids\": [\"12271485\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"GLUT8 contains a [DE]XXXL[LI] late endosomal/lysosomal targeting motif; mutation of the glutamate to arginine (as in GLUT4) alters GLUT8 endocytosis and retains it at the plasma membrane; GLUT8 does not reside in a recycling vesicle pool and localizes to late endosomes/lysosomes.\",\n      \"method\": \"Site-directed mutagenesis, immunofluorescence, subcellular localization in 3T3L1, HEK293, CHO cells\",\n      \"journal\": \"Traffic (Copenhagen, Denmark)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — mutagenesis plus localization in multiple cell types identifying the targeting motif\",\n      \"pmids\": [\"16262729\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"GLUT8 translocation to the plasma membrane in neuronal N2A cells is not stimulated by insulin, IGF-1, KCl depolarization, or hypoxia; mutation of the N-terminal dileucine motif (L12,13→A12,13) constitutively localizes GLUT8 to the plasma membrane.\",\n      \"method\": \"GLUT8-GFP stable transfection, immunohistochemistry, subcellular fractionation, site-directed mutagenesis\",\n      \"journal\": \"Journal of neuroscience research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple stimuli tested with mutagenesis confirmation, single lab\",\n      \"pmids\": [\"14994344\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"GLUT8 endocytosis is mediated by direct interaction of its N-terminal dileucine motif with the beta2-adaptin subunit of the AP-2 adaptor complex, targeting GLUT8 to clathrin-coated vesicles; RNAi knockdown of AP-2 mu2 subunit causes GLUT8 accumulation at the plasma membrane comparable to dominant-negative dynamin.\",\n      \"method\": \"Yeast two-hybrid, GST pulldown, RNAi knockdown of AP-2, immunofluorescence\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — multiple orthogonal methods (Y2H, GST pulldown, RNAi) identifying the molecular endocytic mechanism\",\n      \"pmids\": [\"16723738\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Deletion of the Slc2a8 gene in mice results in increased hippocampal neuronal proliferation and increased P-wave duration in the heart, but does not impair normal embryonic or postnatal development or glucose homeostasis; GLUT8 is dispensable for embryonic development.\",\n      \"method\": \"Knockout mouse generation, cardiac electrophysiology, hippocampal BrdU incorporation assay\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with defined cellular phenotypes in multiple tissues\",\n      \"pmids\": [\"16705176\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Endogenous GLUT8 in spermatocytes and spermatids localizes to a late endosomal/lysosomal compartment; its N-terminal intracellular domain interacts with AP1 and AP2 (but not AP3 or AP4), and the GLUT8 N-terminal intracellular domain fused to the tailless IL-2 receptor alpha chain is sufficient to direct that chimera to intracellular membranes.\",\n      \"method\": \"Immunofluorescence of endogenous protein, AP complex interaction assays, chimeric protein targeting experiments\",\n      \"journal\": \"The FEBS journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — endogenous localization plus chimeric protein experiments identifying sufficient targeting signal, multiple methods\",\n      \"pmids\": [\"19523115\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"GLUT8 is required for hepatocyte fructose transport; GLUT8 overexpression or shRNA-mediated knockdown significantly increases or decreases radiolabeled fructose uptake in cultured hepatocytes; GLUT8-deficient mice show diminished fructose uptake, reduced de novo lipogenesis, and attenuated hepatic triglyceride/cholesterol accumulation on a high-fructose diet.\",\n      \"method\": \"shRNA knockdown, adenoviral overexpression, radiolabeled fructose uptake, GLUT8-deficient mouse model, hepatic lipid quantification\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods in vitro and in vivo confirming fructose transport function\",\n      \"pmids\": [\"24519932\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"GLUT8 regulates enterocyte fructose transport; shRNA-mediated GLUT8 knockdown in Caco2 cells stimulates fructose uptake; GLUT8-deficient mice exhibit greater jejunal fructose uptake and GLUT8 deficiency leads to compensatory upregulation of GLUT12 in enterocytes.\",\n      \"method\": \"shRNA knockdown in Caco2 cells, GLUT8-KO mouse model, 14C-fructose uptake assay, immunoblot\",\n      \"journal\": \"Endocrinology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vitro and in vivo KO with defined molecular phenotype (GLUT12 upregulation)\",\n      \"pmids\": [\"22822162\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Slc2a8 deficiency in mice impairs oocyte metabolism and ATP production, and causes defective decidualization of endometrial stromal cells, leading to reduced litter size and impaired implantation; ovarian transplantation studies confirm effects on both embryo and implantation.\",\n      \"method\": \"Slc2a8 knockout mice, oocyte metabolic assays, ATP measurement, decidualization assay, ovarian transplantation\",\n      \"journal\": \"Biology of reproduction\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — KO with multiple defined cellular phenotypes and transplantation rescue experiment\",\n      \"pmids\": [\"22649075\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"GLUT8 transports dehydroascorbic acid (DHA) in Xenopus oocytes with Km of 3.23 mM and Vmax of 10.1 pmol/min/oocyte; DHA transport by GLUT8 is inhibited by glucose, fructose, and flavonoids (phloretin, quercetin), and maximal transport rates for DHA are lower than for 2-deoxy-D-glucose or fructose.\",\n      \"method\": \"Xenopus oocyte expression system, radiolabeled DHA and glucose transport assays, competitive inhibition studies\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct transport reconstitution in oocytes with kinetic characterization\",\n      \"pmids\": [\"23396969\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"GLUT8 deficiency in male mice confers resistance to high-fructose diet-induced glucose intolerance and dyslipidemia, associated with enhanced hepatic PPARγ protein abundance; adenoviral GLUT8 overexpression in liver suppresses hepatic PPARγ expression, placing GLUT8 upstream of PPARγ in fructose-induced metabolic regulation.\",\n      \"method\": \"GLUT8-KO mice, high-fructose diet challenge, adenoviral overexpression, immunoblot for PPARγ\",\n      \"journal\": \"Molecular endocrinology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — KO and overexpression in vivo establishing pathway position, single lab\",\n      \"pmids\": [\"24030250\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"SLC2A8 (GLUT8) is a mammalian trehalose transporter; trehalose enters hepatocytes via GLUT8 (demonstrated by GC/MS, fluorescence microscopy, and radiolabeled uptake); GLUT8-deficient hepatocytes and mice resist trehalose-induced AMPK phosphorylation and autophagic induction; heterologous overexpression of the Drosophila trehalose transporter Tret1 rescues autophagic flux in GLUT8-deficient hepatocytes.\",\n      \"method\": \"GC/MS, fluorescence microscopy, radiolabeled trehalose uptake, GLUT8-KO mouse/hepatocytes, Tret1 rescue experiment, AMPK phosphorylation immunoblot, autophagy flux assay\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — multiple orthogonal methods including functional rescue identifying GLUT8 as trehalose transporter\",\n      \"pmids\": [\"27922102\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"GLUT8 deficiency in mice enhances hepatic PPARα activity and FGF21 secretion during fasting; hepatic PPARα knockdown in GLUT8-deficient mice normalizes the enhanced ketogenic and FGF21 secretory responses, placing GLUT8 upstream of PPARα in the adaptive fasting response.\",\n      \"method\": \"GLUT8-KO mice, fasting metabolic phenotyping, adenoviral PPARα knockdown, FGF21 ELISA, mitochondrial respiratory function assays\",\n      \"journal\": \"Endocrinology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — epistasis via in vivo PPARα knockdown rescue, single lab\",\n      \"pmids\": [\"29596655\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"GLUT8 undergoes a lysosome-dependent cleavage reaction that releases the carboxy-terminal peptide to a separate vesicle population; GLUT8 does not transport glucose to the cell surface but is localized at the late endosomal/lysosomal interface where it may function as a sensory component of TXNIP-mediated hexosamine homeostasis.\",\n      \"method\": \"Splice variant cataloging, lysosomal inhibitor experiments, subcellular fractionation, immunofluorescence\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — novel cleavage mechanism identified with lysosomal inhibitor experiments, single lab\",\n      \"pmids\": [\"33077497\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"TM4SF5 binds GLUT8 at the cell surface and modulates its translocation to the plasma membrane; fructose treatment transiently decreases TM4SF5-GLUT8 binding, allowing GLUT8 to separate and become active for fructose uptake; Tm4sf5 suppression or knockout reduces fructose uptake, de novo lipogenesis, and steatosis.\",\n      \"method\": \"Co-immunoprecipitation, Tm4sf5-KO mice, in vitro fructose uptake assays, immunofluorescence of TM4SF5-GLUT8 interaction dynamics\",\n      \"journal\": \"Molecular metabolism\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — Co-IP identifies binding partner and KO confirms functional consequence, single lab\",\n      \"pmids\": [\"35123128\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"GLUT8 in PC12 cells and primary hippocampal neurons localizes to a perinuclear compartment partially overlapping with ER markers but not trans-Golgi, early endosomes, lysosomes, or synaptic vesicles; no stimulus tested (depolarization, PKA/PKC activation, tyrosine kinase signaling, glucose deprivation, AMPK stimulation, osmotic shock) induces GLUT8 surface translocation, and no constitutive recycling through the plasma membrane was detected.\",\n      \"method\": \"Recombinant adenoviral GLUT8 with extracellular myc tag, immunofluorescence, dominant-negative dynamin co-expression, anti-myc antibody internalization assay\",\n      \"journal\": \"Endocrinology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — systematic testing of multiple stimuli with multiple detection methods, single lab\",\n      \"pmids\": [\"16109784\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"The proline at position -2 from the dileucine residues in the [DE]XXXL[LI] motif of GLUT8 influences the affinity of AP1 and AP2 for GLUT8 and is critical for intracellular sorting to lysosomes; replacing the XXX (TQP) residues in GLUT8 with those from GLUT12 (GPN) causes dramatic missorting of GLUT8 to the cell surface.\",\n      \"method\": \"Site-directed mutagenesis of dileucine motif flanking residues, immunofluorescence, subcellular localization in CHO/HEK293 cells\",\n      \"journal\": \"Molecular membrane biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — systematic mutagenesis identifying specific residues governing sorting, single lab\",\n      \"pmids\": [\"21067453\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"SLC2A8 RNAi knockdown (79% mRNA reduction) in human first-trimester trophoblast ACH-3P cells reduces glucose uptake by 11% and differentially expresses genes involved in cellular respiration, oxidative phosphorylation, and ATP synthesis, suggesting GLUT8's primary function in trophoblasts is supporting cellular respiration rather than glucose supply.\",\n      \"method\": \"Lentiviral RNAi knockdown, glucose uptake assay, RNA-seq transcriptomics\",\n      \"journal\": \"Cells\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — direct KD with uptake measurement and transcriptomic pathway analysis, single lab\",\n      \"pmids\": [\"38474355\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Endogenous GLUT8 partially co-localizes with cis-Golgi markers (58K protein, GM130) and with α-lactalbumin (a component of lactose synthase) in mammary epithelial cells, suggesting GLUT8 supplies glucose to the Golgi to support lactose synthesis.\",\n      \"method\": \"Immunohistochemistry, immunofluorescence co-localization with Golgi markers and lactose synthase component\",\n      \"journal\": \"Journal of physiology and biochemistry\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — co-localization only, no direct functional transport assay in Golgi\",\n      \"pmids\": [\"31020623\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"GLUT8 protein is localized to dense core vesicles of synaptic nerve endings in supraoptic nucleus and secretory granules of vasopressin-positive neurons, specifically in vasopressin (but not oxytocin) neurons, as demonstrated by immunogold electron microscopy and double immunofluorescence.\",\n      \"method\": \"Immunogold labeling of ultrathin cryosections, double immunofluorescence microscopy\",\n      \"journal\": \"Endocrinology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — ultrastructural localization with cell-type specificity, single lab\",\n      \"pmids\": [\"11751619\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"GLUT8 in spermatozoa is predominantly associated with the acrosomal region in both mouse and human sperm, with immunoreactivity at both the plasma membrane and intracellularly; expression onset in mouse testis coincides with appearance of mature spermatozoa.\",\n      \"method\": \"Immunohistochemistry with C-terminus-specific antiserum, developmental timing analysis\",\n      \"journal\": \"Cell and tissue research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — immunolocalization with developmental correlation, single lab\",\n      \"pmids\": [\"11845330\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"SLC2A8/GLUT8 is a class III facilitative hexose transporter (transporting glucose, fructose, dehydroascorbic acid, and trehalose) that is constitutively retained in late endosomal/lysosomal intracellular compartments via a [DE]XXXL[LI] dileucine motif that mediates endocytosis through direct interaction with the beta2-adaptin subunit of the AP-2 complex and with AP-1; in blastocysts, insulin induces its translocation to the plasma membrane to drive glucose uptake, but in most other cell types (adipocytes, neurons, hepatocytes) no stimulus induces surface translocation; GLUT8 is required for hepatocyte fructose and trehalose uptake (the latter linking it to AMPK-dependent autophagy induction), regulates enterocyte fructose transport, and its loss in mice causes impaired oocyte metabolism, decidualization defects, altered hippocampal neurogenesis, cardiac conduction changes, and enhanced PPARα-driven fasting responses.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"SLC2A8 (GLUT8) is a class III facilitative hexose transporter that mediates uptake of glucose, fructose, dehydroascorbic acid, and trehalose, and functions predominantly at intracellular late endosomal/lysosomal membranes rather than at the cell surface [PMID:10821868, PMID:23396969, PMID:27922102]. Constitutive intracellular retention is governed by an N-terminal [DE]XXXL[LI] dileucine motif whose interaction with AP-2 (via the beta2-adaptin subunit) and AP-1 drives clathrin-dependent endocytosis and lysosomal targeting; mutation of the motif or depletion of AP-2 redirects GLUT8 to the plasma membrane [PMID:16262729, PMID:16723738, PMID:21067453]. In mouse blastocysts, insulin stimulates GLUT8 translocation to the cell surface to increase glucose uptake, but in most other cell types—including adipocytes, neurons, and hepatocytes—no tested stimulus triggers surface redistribution [PMID:10860996, PMID:11513753, PMID:16109784]. GLUT8-mediated hepatic fructose and trehalose transport links it to downstream metabolic regulation: GLUT8 deficiency protects against high-fructose diet–induced dyslipidemia through enhanced PPARγ abundance, augments fasting PPARα/FGF21 signaling, and abolishes trehalose-induced AMPK-dependent autophagy [PMID:24519932, PMID:24030250, PMID:29596655, PMID:27922102].\",\n  \"teleology\": [\n    {\n      \"year\": 2000,\n      \"claim\": \"Establishing that GLUT8 is a bona fide facilitative glucose transporter resolved the question of whether this orphan SLC2 family member had intrinsic sugar transport activity.\",\n      \"evidence\": \"Reconstituted glucose transport and cytochalasin B binding in COS-7 cells; parallel Km determination (~2 mM) and dileucine motif identification in Xenopus oocytes\",\n      \"pmids\": [\"10821868\", \"10671487\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Substrate selectivity beyond glucose not yet characterized\", \"Physiological site of action (intracellular vs. plasma membrane) unclear\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Demonstrating insulin-stimulated GLUT8 translocation and glucose uptake in blastocysts established the first physiological context for GLUT8 function and raised the question of whether insulin-regulated trafficking was a general feature.\",\n      \"evidence\": \"Antisense knockdown, glucose uptake assay, and immunolocalization in mouse blastocysts\",\n      \"pmids\": [\"10860996\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of insulin-regulated translocation in blastocysts not defined\", \"Whether this occurs in somatic tissues was untested\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Systematic testing in adipocytes showed GLUT8 does not undergo insulin-stimulated surface translocation like GLUT4, establishing that stimulus-regulated trafficking is cell-type specific and that intracellular retention is the default state.\",\n      \"evidence\": \"HA-tagged GLUT8, dominant-negative dynamin, and dileucine mutagenesis in primary rat adipocytes\",\n      \"pmids\": [\"11513753\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Why blastocysts but not adipocytes support translocation remained unexplained\", \"Nature of the intracellular compartment not resolved\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Identifying the [DE]XXXL[LI] motif as a late endosomal/lysosomal targeting signal—distinct from GLUT4's recycling endosome pathway—resolved the compartment identity and explained constitutive intracellular retention.\",\n      \"evidence\": \"Site-directed mutagenesis of the dileucine motif glutamate residue combined with immunofluorescence in 3T3-L1, HEK293, and CHO cells\",\n      \"pmids\": [\"16262729\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Adaptor proteins mediating the sorting not yet identified\", \"Functional role of lysosomal localization unknown\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Comprehensive stimulus screens in neurons confirmed that no known signaling pathway induces GLUT8 surface translocation in neuronal cells, reinforcing its constitutive intracellular residence and distinguishing it mechanistically from GLUT4.\",\n      \"evidence\": \"Myc-tagged GLUT8 adenovirus in PC12 cells and primary hippocampal neurons tested with depolarization, PKA/PKC activators, tyrosine kinase signaling, glucose deprivation, AMPK activation, and osmotic shock\",\n      \"pmids\": [\"16109784\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Subcellular compartment in neurons (partially ER-overlapping) differs from late endosome assignment in other cells\", \"Functional role of neuronal GLUT8 unresolved\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Identification of AP-2 beta2-adaptin as the direct binding partner for the GLUT8 dileucine motif defined the molecular mechanism of clathrin-mediated endocytosis that enforces intracellular retention.\",\n      \"evidence\": \"Yeast two-hybrid, GST pulldown, and AP-2 mu2 RNAi knockdown causing surface accumulation\",\n      \"pmids\": [\"16723738\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Role of AP-1 interaction in biosynthetic trafficking not fully dissected\", \"Whether AP-2-mediated endocytosis is regulated in any tissue not tested\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Generation of Slc2a8-knockout mice showed the gene is dispensable for embryonic development and glucose homeostasis but revealed unexpected phenotypes—increased hippocampal neurogenesis and prolonged cardiac P-wave duration—pointing to specialized tissue functions.\",\n      \"evidence\": \"Slc2a8 knockout mice with BrdU incorporation in hippocampus and cardiac electrophysiology\",\n      \"pmids\": [\"16705176\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanistic basis for hippocampal proliferation and cardiac conduction phenotypes unknown\", \"Metabolic challenge conditions not yet tested\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Demonstrating that the GLUT8 N-terminal domain interacts with both AP-1 and AP-2 and is sufficient to redirect a heterologous reporter to intracellular membranes established the minimal sorting determinant and extended the adaptor interaction map.\",\n      \"evidence\": \"Chimeric IL-2Rα/GLUT8 constructs and AP complex interaction assays in spermatocytes/spermatids\",\n      \"pmids\": [\"19523115\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Relative contributions of AP-1 versus AP-2 to steady-state localization not resolved\", \"Functional role in spermatogenesis not defined\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Fine-mapping the dileucine motif flanking residues showed that the proline at position −2 tunes AP-1/AP-2 binding affinity and is critical for lysosomal versus surface sorting, explaining differences between GLUT8 and GLUT12 trafficking.\",\n      \"evidence\": \"Systematic mutagenesis of XXX residues in [DE]XXXL[LI] motif with subcellular localization in CHO/HEK293 cells\",\n      \"pmids\": [\"21067453\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether these residues are post-translationally modified to regulate sorting is unknown\", \"In vivo relevance of motif variants not tested\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Establishing GLUT8 as a hepatic fructose transporter and showing that its loss protects against fructose-induced lipogenesis and steatosis redefined GLUT8's primary physiological substrate in the liver as fructose rather than glucose.\",\n      \"evidence\": \"shRNA knockdown and adenoviral overexpression with radiolabeled fructose uptake in hepatocytes; GLUT8-KO mice on high-fructose diet\",\n      \"pmids\": [\"24519932\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Intracellular compartment where fructose transport occurs not specified\", \"Relationship to known fructolysis enzymes not explored\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Finding that GLUT8 deficiency increases enterocyte fructose uptake (with compensatory GLUT12 upregulation) revealed tissue-specific directionality: GLUT8 restricts fructose absorption in the gut but promotes it in the liver.\",\n      \"evidence\": \"GLUT8 shRNA in Caco2 cells and GLUT8-KO mice with jejunal 14C-fructose uptake\",\n      \"pmids\": [\"22822162\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which GLUT8 suppresses enterocyte fructose uptake (competition vs. regulation) unclear\", \"Whether GLUT8 functions on the apical or intracellular membranes in enterocytes not resolved\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Demonstrating that GLUT8 loss impairs oocyte ATP production and endometrial decidualization established a non-redundant metabolic role in female reproduction.\",\n      \"evidence\": \"Slc2a8 knockout mice with oocyte metabolic assays, ATP measurement, decidualization assay, and ovarian transplantation\",\n      \"pmids\": [\"22649075\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Substrate transported in oocytes (glucose, fructose, or trehalose) not determined\", \"Signaling link between GLUT8 and decidualization not identified\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Kinetic characterization of dehydroascorbic acid transport expanded the substrate repertoire beyond hexoses, showing GLUT8 is a multi-substrate transporter.\",\n      \"evidence\": \"Xenopus oocyte reconstitution with radiolabeled DHA, kinetic analysis (Km 3.23 mM), competitive inhibition with glucose/fructose/flavonoids\",\n      \"pmids\": [\"23396969\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological relevance of DHA transport through a lysosomal transporter not established\", \"Whether DHA and hexose transport use the same binding site not structurally resolved\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Linking GLUT8 deficiency to enhanced hepatic PPARγ abundance and protection from fructose-induced metabolic syndrome positioned GLUT8 upstream of lipogenic transcriptional programs.\",\n      \"evidence\": \"GLUT8-KO mice on high-fructose diet and adenoviral GLUT8 overexpression suppressing PPARγ\",\n      \"pmids\": [\"24030250\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct versus indirect regulation of PPARγ by GLUT8-mediated fructose flux not distinguished\", \"Mechanism of PPARγ regulation not identified\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Identifying GLUT8 as the mammalian trehalose transporter and showing that trehalose-induced AMPK activation and autophagy require GLUT8 provided a mechanistic explanation for how a lysosomal transporter connects sugar sensing to autophagic signaling.\",\n      \"evidence\": \"GC/MS, radiolabeled trehalose uptake, GLUT8-KO hepatocytes, rescue by Drosophila Tret1, AMPK phosphorylation and autophagy flux assays\",\n      \"pmids\": [\"27922102\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether trehalose is transported into or out of the lysosome not resolved\", \"Molecular link from trehalose to AMPK activation unknown\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Epistasis experiments placing GLUT8 upstream of PPARα/FGF21 during fasting revealed that GLUT8 normally suppresses the fasting ketogenic response, integrating its lysosomal transport function with systemic metabolic adaptation.\",\n      \"evidence\": \"GLUT8-KO mice with fasting metabolic phenotyping; adenoviral PPARα knockdown normalizing enhanced ketogenesis and FGF21\",\n      \"pmids\": [\"29596655\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Substrate whose lysosomal transport by GLUT8 controls PPARα activity not identified\", \"Whether this operates through mTORC1 or another nutrient-sensing pathway unknown\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Discovery of lysosome-dependent C-terminal cleavage of GLUT8 suggested it undergoes regulated proteolytic processing, potentially linking its lysosomal residence to TXNIP-mediated hexosamine sensing.\",\n      \"evidence\": \"Lysosomal inhibitor experiments, subcellular fractionation showing cleaved C-terminal peptide in a separate vesicle population\",\n      \"pmids\": [\"33077497\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Protease responsible for cleavage not identified\", \"Functional consequence of cleavage for transport activity unknown\", \"TXNIP connection is correlative\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Identification of TM4SF5 as a cell-surface binding partner that modulates GLUT8 plasma membrane availability for fructose uptake provided the first evidence of regulated GLUT8 surface translocation outside of blastocysts.\",\n      \"evidence\": \"Co-immunoprecipitation, Tm4sf5-KO mice, fructose-induced dissociation dynamics by immunofluorescence\",\n      \"pmids\": [\"35123128\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Interaction validated by Co-IP only; reciprocal and endogenous validation limited\", \"Whether TM4SF5-GLUT8 interaction is direct or via a complex not resolved\", \"Tissue generality of this mechanism unknown\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the directionality and physiological substrate of GLUT8's lysosomal transport in vivo, the structural basis of its multi-substrate recognition, how lysosomal GLUT8 activity is sensed by PPARα/PPARγ and AMPK signaling cascades, and the functional significance of its C-terminal cleavage.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No crystal or cryo-EM structure available\", \"Directionality of lysosomal transport (import vs. export) not determined\", \"Precise molecular link from GLUT8 activity to PPAR and AMPK signaling undefined\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005215\", \"supporting_discovery_ids\": [0, 1, 2, 10, 13, 15]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005764\", \"supporting_discovery_ids\": [5, 9, 17]},\n      {\"term_id\": \"GO:0005768\", \"supporting_discovery_ids\": [5, 9]},\n      {\"term_id\": \"GO:0031410\", \"supporting_discovery_ids\": [3, 23]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-382551\", \"supporting_discovery_ids\": [0, 2, 10, 13, 15]},\n      {\"term_id\": \"R-HSA-9612973\", \"supporting_discovery_ids\": [15]},\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [10, 14, 16]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"AP2B1\", \"AP1\", \"TM4SF5\"],\n    \"other_free_text\": []\n  }\n}\n```"}