{"gene":"SLC2A8","run_date":"2026-06-10T07:46:33","timeline":{"discoveries":[{"year":2000,"finding":"GLUTX1 (SLC2A8) is a facilitative glucose transporter with a Km of ~2 mM for glucose; transport activity was inhibited by cytochalasin B and partially competed by D-fructose and D-galactose when expressed in Xenopus oocytes. The protein contains an N-terminal dileucine internalization motif that retains it intracellularly; mutation of this motif redirected the protein to the cell surface. In vitro translated GLUTX1 migrates as a 35-kDa protein that becomes glycosylated in the presence of microsomal membranes.","method":"Xenopus oocyte expression with radiolabeled transport assay, site-directed mutagenesis of dileucine motif, in vitro translation with microsomal glycosylation, immunofluorescence microscopy in HEK293T cells","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — reconstitution of transport activity in Xenopus oocytes, mutagenesis of targeting motif, glycosylation assay, multiple orthogonal methods in one rigorous study","pmids":["10671487"],"is_preprint":false},{"year":2002,"finding":"GLUTX1 (SLC2A8) protein localizes in the testis to differentiating spermatocytes (type 1 stage) but is undetectable in mature spermatozoa. In the brain, it localizes specifically to vasopressin neurons (not oxytocin neurons) of the supraoptic nucleus, and immunogold labeling identified it in dense core vesicles of synaptic nerve endings and secretory granules of vasopressin-positive neurons.","method":"Immunohistochemistry, double immunofluorescence microscopy, immunogold labeling of ultrathin cryosections","journal":"Endocrinology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct subcellular localization by immunogold on cryosections with cell-type specificity, single lab but multiple orthogonal imaging methods","pmids":["11751619"],"is_preprint":false},{"year":2001,"finding":"GLUTx1 (SLC2A8) protein in rat hippocampus is primarily localized to the intracellular compartment with limited association with the plasma membrane, as determined by immunoblot analysis of hippocampal membrane fractions. Immunohistochemistry showed expression restricted to neuronal cell bodies and most proximal dendrites. In streptozotocin diabetic rats, GLUTx1 mRNA levels increased in pyramidal and granule neurons without a concomitant increase in protein levels.","method":"Subcellular fractionation with immunoblot, immunohistochemistry, quantitative autoradiography, single-cell emulsion autoradiography","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — subcellular fractionation with immunoblot plus immunohistochemistry, single lab, two orthogonal methods","pmids":["11226324"],"is_preprint":false},{"year":2009,"finding":"GLUT8 (SLC2A8) constitutively associates with endosomes, lysosomes, and endoplasmic reticulum membranes via its N-terminal [DE]XXXL[LI] endosomal/lysosomal targeting motif. No conventional signal tested induced translocation of GLUT8 to the plasma membrane in heterologous expression systems. Knockout mice showed viable development but mild phenotypes including increased neuronal proliferation in dentate gyrus, hyperactivity, impaired atrial electrical conduction, and reduced sperm motility with decreased mitochondrial membrane potential and ATP levels in sperm.","method":"Heterologous overexpression with subcellular localization, Slc2a8 knockout mouse phenotyping (behavioral, cardiac, sperm analysis, mitochondrial membrane potential assay)","journal":"American journal of physiology. Endocrinology and metabolism","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — KO mouse with multiple defined cellular phenotypes plus localization data; review article synthesizing original work, single lab","pmids":["19176349"],"is_preprint":false},{"year":2014,"finding":"GLUT8 (SLC2A8) is a cell surface-localized transporter in hepatocytes that mediates fructose uptake. GLUT8 overexpression significantly increased radiolabeled fructose uptake, while shRNA-mediated GLUT8 knockdown blocked it. GLUT8-deficient mice fed a high-fructose diet showed diminished hepatic fructose uptake, reduced de novo lipogenesis, and attenuated hepatic triglyceride and cholesterol accumulation without changes in insulin-stimulated Akt phosphorylation.","method":"Radiolabeled fructose uptake assay, shRNA knockdown, GLUT8 overexpression, Slc2a8 knockout mouse with high-fructose diet challenge, hepatic lipid quantification","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1-2 / Moderate — radiolabeled transport assay with both gain- and loss-of-function, in vivo KO confirmation, multiple orthogonal methods in single rigorous study","pmids":["24519932"],"is_preprint":false},{"year":2016,"finding":"SLC2A8 (GLUT8) functions as a mammalian trehalose transporter required for trehalose-induced autophagy. GC/MS, fluorescence microscopy, and radiolabeled uptake studies demonstrated that trehalose traverses the plasma membrane via GLUT8. GLUT8-deficient hepatocytes and mice exposed to trehalose resisted trehalose-induced AMPK phosphorylation and autophagic induction. Although trehalose suppressed mTORC1 signaling, mTORC1 suppression alone was insufficient to activate autophagy in the absence of AMPK or GLUT8. Heterologous overexpression of Drosophila trehalose transporter-1 (Tret1) reconstituted autophagic flux and AMPK signaling defects in GLUT8-deficient hepatocytes.","method":"GC/MS trehalose quantification, fluorescence microscopy, radiolabeled uptake assay, GLUT8-deficient mouse and primary hepatocytes, AMPK/mTORC1 phosphorylation assays, Tret1 reconstitution experiment","journal":"Scientific reports","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — multiple orthogonal transport assays, genetic KO in vitro and in vivo, reconstitution with heterologous transporter, epistasis (AMPK required downstream of GLUT8)","pmids":["27922102"],"is_preprint":false},{"year":2012,"finding":"SLC2A8 deficiency in mice impairs oocyte metabolism and ATP production, and causes defective decidualization of endometrial stromal cells required for embryo implantation. Ovarian transplantation studies confirmed that SLC2A8 loss affects both embryo and implantation processes. Null mice display decreased litter size, reduced postnatal growth, decreased body fat, and resistance to high-fat/high-carbohydrate diet.","method":"Slc2a8 knockout mouse (exons 1–4 deletion, protein expression confirmed by Western blot), oocyte metabolic/ATP assays, decidualization assay, ovarian transplantation, MRI body composition","journal":"Biology of reproduction","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic KO with multiple defined cellular and physiological phenotypes, ovarian transplantation for epistasis; single lab","pmids":["22649075"],"is_preprint":false},{"year":2018,"finding":"Hepatocyte GLUT8 (SLC2A8) regulates the adaptive fasting response through the PPARα signaling cascade. Slc2a8-disrupted mice exhibited enhanced thermogenesis, ketogenesis, and peripheral lipid mobilization during fasting, with mildly impaired hepatic mitochondrial oxidative metabolism. Hepatic PPARα and its target FGF21 were cell-autonomously hyperactivated in GLUT8-deficient liver and primary hepatocytes during nutrient depletion. Hepatic PPARα knockdown in GLUT8-deficient mice normalized the enhanced ketogenic and FGF21 secretory responses.","method":"Slc2a8 knockout mouse fasting challenge, metabolic phenotyping (thermogenesis, ketogenesis, lipid mobilization), mitochondrial respiration assay, PPARα/FGF21 signaling assays in vivo and in primary hepatocytes, hepatic PPARα knockdown epistasis","journal":"Endocrinology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic KO plus rescue by PPARα knockdown (epistasis), cell-autonomous confirmation in primary hepatocytes, single lab","pmids":["29596655"],"is_preprint":false},{"year":2024,"finding":"SLC2A8 knockdown (79% mRNA reduction) in human first-trimester trophoblast cells (ACH-3P) caused an 11% reduction in glucose uptake. RNAseq identified 1525 differentially expressed transcripts, with enrichment in metabolic pathways associated with cellular respiration, oxidative phosphorylation, and ATP synthesis, indicating that SLC2A8's primary function in trophoblast cells is to support cellular respiration rather than net glucose delivery.","method":"Lentiviral RNAi knockdown in ACH-3P cells, glucose uptake assay, RNAseq transcriptome analysis, real-time qPCR validation","journal":"Cells","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — loss-of-function with direct glucose uptake measurement plus transcriptome; single lab, two orthogonal methods","pmids":["38474355"],"is_preprint":false}],"current_model":"SLC2A8 (GLUT8) is an intracellular-targeted facilitative hexose transporter retained in endosomes/lysosomes/ER by an N-terminal [DE]XXXL[LI] dileucine motif (removable by mutagenesis to allow cell-surface expression), with a Km of ~2 mM for glucose and additional capacity to transport fructose and trehalose; it mediates hepatocyte fructose uptake and fructose-induced lipogenesis, serves as the carrier required for cytoplasmic trehalose entry and subsequent AMPK-dependent autophagic induction, regulates the adaptive hepatic fasting response through a PPARα–FGF21 axis, and supports cellular respiration in trophoblast cells, while its loss in mice impairs oocyte ATP production, decidualization, and spermatogenesis."},"narrative":{"mechanistic_narrative":"SLC2A8 (GLUT8) is a facilitative hexose transporter that couples substrate uptake to cellular energy metabolism and nutrient-sensing across multiple tissues [PMID:10671487, PMID:24519932]. It transports glucose with a Km of ~2 mM and additionally carries fructose and trehalose [PMID:10671487, PMID:24519932, PMID:27922102]. The protein is normally retained on intracellular membranes—endosomes, lysosomes, and the endoplasmic reticulum—through an N-terminal [DE]XXXL[LI] dileucine targeting motif, and mutation of this motif redirects it to the plasma membrane [PMID:10671487, PMID:19176349]. In hepatocytes GLUT8 mediates fructose uptake that drives de novo lipogenesis, and its loss attenuates hepatic triglyceride and cholesterol accumulation on a high-fructose diet [PMID:24519932]. GLUT8 is also the carrier required for cytoplasmic entry of trehalose, which activates AMPK-dependent autophagy; this defect is rescued by heterologous expression of the Drosophila trehalose transporter Tret1 [PMID:27922102]. In liver it further governs the adaptive fasting response by restraining the cell-autonomous PPARα–FGF21 axis, such that GLUT8 deficiency hyperactivates ketogenesis and FGF21 secretion [PMID:29596655]. Consistent with a role in supporting cellular respiration, SLC2A8 loss impairs oocyte ATP production and decidualization, reduces sperm motility and mitochondrial membrane potential, and diminishes oxidative-phosphorylation transcriptional programs in trophoblast cells [PMID:19176349, PMID:22649075, PMID:38474355].","teleology":[{"year":2000,"claim":"Established that SLC2A8 is a bona fide facilitative hexose transporter and defined its intracellular-retention mechanism, answering what biochemical activity the gene encodes.","evidence":"Xenopus oocyte radiolabeled transport assay, dileucine-motif mutagenesis, in vitro translation/glycosylation, and immunofluorescence in HEK293T","pmids":["10671487"],"confidence":"High","gaps":["Did not establish the physiological substrate in mammalian tissues","Surface trafficking only achieved by motif mutation, not by a physiological signal"]},{"year":2001,"claim":"Showed that endogenous GLUT8 in brain is predominantly intracellular and neuron-restricted, addressing whether native localization matches the heterologous retention behavior.","evidence":"Subcellular fractionation/immunoblot and immunohistochemistry in rat hippocampus, including streptozotocin-diabetic rats","pmids":["11226324"],"confidence":"Medium","gaps":["mRNA/protein discordance in diabetes left unexplained","No functional transport assay in neurons"]},{"year":2002,"claim":"Mapped cell-type-specific expression to differentiating spermatocytes and vasopressin neurons with vesicular/secretory-granule localization, suggesting tissue-specific secretory roles.","evidence":"Immunohistochemistry, double immunofluorescence, and immunogold labeling of cryosections in testis and brain","pmids":["11751619"],"confidence":"Medium","gaps":["Functional consequence of dense-core-vesicle localization not tested","No substrate flux measured in these cell types"]},{"year":2009,"claim":"A knockout mouse linked GLUT8 loss to defined cellular phenotypes (sperm motility, mitochondrial potential/ATP, neuronal proliferation, cardiac conduction), connecting the transporter to energy metabolism in vivo.","evidence":"Slc2a8 knockout phenotyping (behavior, cardiac conduction, sperm analysis, mitochondrial membrane potential) plus heterologous localization","pmids":["19176349"],"confidence":"Medium","gaps":["Mechanistic link between transport activity and each phenotype not resolved","Substrate driving sperm energetics not identified"]},{"year":2012,"claim":"Demonstrated that GLUT8 is required for oocyte ATP production and endometrial decidualization, establishing a reproductive role tied to cellular energetics.","evidence":"Slc2a8 knockout mouse with oocyte metabolic/ATP assays, decidualization assay, ovarian transplantation, and MRI body composition","pmids":["22649075"],"confidence":"Medium","gaps":["Transported substrate underlying the oocyte/implantation defect not defined","Single lab"]},{"year":2014,"claim":"Identified hepatic fructose as a physiological substrate and linked GLUT8-mediated fructose uptake to de novo lipogenesis, providing a metabolic-disease-relevant function.","evidence":"Radiolabeled fructose uptake with overexpression and shRNA knockdown plus Slc2a8 KO mice on high-fructose diet with hepatic lipid quantification","pmids":["24519932"],"confidence":"High","gaps":["Surface vs. intracellular site of fructose transport in hepatocytes not fully reconciled with earlier retention data","Insulin signaling unchanged, leaving the lipogenic mechanism partly open"]},{"year":2016,"claim":"Showed GLUT8 is the mammalian trehalose carrier required for trehalose-induced, AMPK-dependent autophagy, defining a signaling pathway downstream of transport.","evidence":"GC/MS and radiolabeled trehalose uptake, fluorescence microscopy, GLUT8-deficient hepatocytes/mice, AMPK/mTORC1 phosphorylation assays, and Tret1 reconstitution","pmids":["27922102"],"confidence":"High","gaps":["How cytoplasmic trehalose activates AMPK not mechanistically resolved","Physiological source of trehalose in mammals unclear"]},{"year":2018,"claim":"Placed hepatic GLUT8 upstream of the PPARα–FGF21 fasting program, showing it restrains adaptive ketogenesis and lipid mobilization in a cell-autonomous manner.","evidence":"Slc2a8 KO fasting challenge with metabolic phenotyping, mitochondrial respiration, PPARα/FGF21 assays in vivo and in primary hepatocytes, and PPARα-knockdown epistasis","pmids":["29596655"],"confidence":"Medium","gaps":["Mechanism connecting transport activity to PPARα activation not defined","Identity of the metabolic signal sensed by hepatocytes unknown"]},{"year":2024,"claim":"Established that in human trophoblast cells GLUT8 supports cellular respiration/oxidative phosphorylation rather than bulk glucose delivery, refining its functional role in the placenta.","evidence":"Lentiviral RNAi knockdown in ACH-3P cells with glucose uptake assay and RNAseq transcriptome analysis","pmids":["38474355"],"confidence":"Medium","gaps":["Modest glucose-uptake reduction leaves the respiratory link correlative","Direct measurement of respiration not performed"]},{"year":null,"claim":"How GLUT8 trafficking is physiologically regulated to expose its transport activity at the relevant membrane, and how intracellular substrate flux is mechanistically transduced into AMPK and PPARα signaling, remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No physiological signal shown to relieve dileucine-motif retention","Molecular link between transport and downstream kinase/nuclear-receptor signaling undefined"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0005215","term_label":"transporter activity","supporting_discovery_ids":[0,4,5]}],"localization":[{"term_id":"GO:0005768","term_label":"endosome","supporting_discovery_ids":[3]},{"term_id":"GO:0005764","term_label":"lysosome","supporting_discovery_ids":[3]},{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[3]},{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,4]},{"term_id":"GO:0031410","term_label":"cytoplasmic vesicle","supporting_discovery_ids":[1]}],"pathway":[{"term_id":"R-HSA-382551","term_label":"Transport of small molecules","supporting_discovery_ids":[0,4,5]},{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[4,7]},{"term_id":"R-HSA-9612973","term_label":"Autophagy","supporting_discovery_ids":[5]},{"term_id":"R-HSA-1474165","term_label":"Reproduction","supporting_discovery_ids":[6]}],"complexes":[],"partners":[],"other_free_text":[]}},"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":"30335591","id":"PMC_30335591","title":"Trehalose induces autophagy via lysosomal-mediated TFEB activation in models of motoneuron degeneration.","date":"2018","source":"Autophagy","url":"https://pubmed.ncbi.nlm.nih.gov/30335591","citation_count":338,"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":"11237212","id":"PMC_11237212","title":"Intracellular organization of insulin signaling and GLUT4 translocation.","date":"2001","source":"Recent progress in hormone research","url":"https://pubmed.ncbi.nlm.nih.gov/11237212","citation_count":170,"is_preprint":false},{"pmid":"10970791","id":"PMC_10970791","title":"Activity and genomic organization of human glucose transporter 9 (GLUT9), a novel member of the family of sugar-transport facilitators predominantly expressed in brain and leucocytes.","date":"2000","source":"The Biochemical journal","url":"https://pubmed.ncbi.nlm.nih.gov/10970791","citation_count":127,"is_preprint":false},{"pmid":"33706671","id":"PMC_33706671","title":"Trehalose causes low-grade lysosomal stress to activate TFEB and the autophagy-lysosome biogenesis response.","date":"2021","source":"Autophagy","url":"https://pubmed.ncbi.nlm.nih.gov/33706671","citation_count":116,"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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mM for glucose; transport activity was inhibited by cytochalasin B and partially competed by D-fructose and D-galactose when expressed in Xenopus oocytes. The protein contains an N-terminal dileucine internalization motif that retains it intracellularly; mutation of this motif redirected the protein to the cell surface. In vitro translated GLUTX1 migrates as a 35-kDa protein that becomes glycosylated in the presence of microsomal membranes.\",\n      \"method\": \"Xenopus oocyte expression with radiolabeled transport assay, site-directed mutagenesis of dileucine motif, in vitro translation with microsomal glycosylation, immunofluorescence microscopy in HEK293T cells\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — reconstitution of transport activity in Xenopus oocytes, mutagenesis of targeting motif, glycosylation assay, multiple orthogonal methods in one rigorous study\",\n      \"pmids\": [\"10671487\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"GLUTX1 (SLC2A8) protein localizes in the testis to differentiating spermatocytes (type 1 stage) but is undetectable in mature spermatozoa. In the brain, it localizes specifically to vasopressin neurons (not oxytocin neurons) of the supraoptic nucleus, and immunogold labeling identified it in dense core vesicles of synaptic nerve endings and secretory granules of vasopressin-positive neurons.\",\n      \"method\": \"Immunohistochemistry, double immunofluorescence microscopy, immunogold labeling of ultrathin cryosections\",\n      \"journal\": \"Endocrinology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct subcellular localization by immunogold on cryosections with cell-type specificity, single lab but multiple orthogonal imaging methods\",\n      \"pmids\": [\"11751619\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"GLUTx1 (SLC2A8) protein in rat hippocampus is primarily localized to the intracellular compartment with limited association with the plasma membrane, as determined by immunoblot analysis of hippocampal membrane fractions. Immunohistochemistry showed expression restricted to neuronal cell bodies and most proximal dendrites. In streptozotocin diabetic rats, GLUTx1 mRNA levels increased in pyramidal and granule neurons without a concomitant increase in protein levels.\",\n      \"method\": \"Subcellular fractionation with immunoblot, immunohistochemistry, quantitative autoradiography, single-cell emulsion autoradiography\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — subcellular fractionation with immunoblot plus immunohistochemistry, single lab, two orthogonal methods\",\n      \"pmids\": [\"11226324\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"GLUT8 (SLC2A8) constitutively associates with endosomes, lysosomes, and endoplasmic reticulum membranes via its N-terminal [DE]XXXL[LI] endosomal/lysosomal targeting motif. No conventional signal tested induced translocation of GLUT8 to the plasma membrane in heterologous expression systems. Knockout mice showed viable development but mild phenotypes including increased neuronal proliferation in dentate gyrus, hyperactivity, impaired atrial electrical conduction, and reduced sperm motility with decreased mitochondrial membrane potential and ATP levels in sperm.\",\n      \"method\": \"Heterologous overexpression with subcellular localization, Slc2a8 knockout mouse phenotyping (behavioral, cardiac, sperm analysis, mitochondrial membrane potential assay)\",\n      \"journal\": \"American journal of physiology. Endocrinology and metabolism\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — KO mouse with multiple defined cellular phenotypes plus localization data; review article synthesizing original work, single lab\",\n      \"pmids\": [\"19176349\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"GLUT8 (SLC2A8) is a cell surface-localized transporter in hepatocytes that mediates fructose uptake. GLUT8 overexpression significantly increased radiolabeled fructose uptake, while shRNA-mediated GLUT8 knockdown blocked it. GLUT8-deficient mice fed a high-fructose diet showed diminished hepatic fructose uptake, reduced de novo lipogenesis, and attenuated hepatic triglyceride and cholesterol accumulation without changes in insulin-stimulated Akt phosphorylation.\",\n      \"method\": \"Radiolabeled fructose uptake assay, shRNA knockdown, GLUT8 overexpression, Slc2a8 knockout mouse with high-fructose diet challenge, hepatic lipid quantification\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — radiolabeled transport assay with both gain- and loss-of-function, in vivo KO confirmation, multiple orthogonal methods in single rigorous study\",\n      \"pmids\": [\"24519932\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"SLC2A8 (GLUT8) functions as a mammalian trehalose transporter required for trehalose-induced autophagy. GC/MS, fluorescence microscopy, and radiolabeled uptake studies demonstrated that trehalose traverses the plasma membrane via GLUT8. GLUT8-deficient hepatocytes and mice exposed to trehalose resisted trehalose-induced AMPK phosphorylation and autophagic induction. Although trehalose suppressed mTORC1 signaling, mTORC1 suppression alone was insufficient to activate autophagy in the absence of AMPK or GLUT8. Heterologous overexpression of Drosophila trehalose transporter-1 (Tret1) reconstituted autophagic flux and AMPK signaling defects in GLUT8-deficient hepatocytes.\",\n      \"method\": \"GC/MS trehalose quantification, fluorescence microscopy, radiolabeled uptake assay, GLUT8-deficient mouse and primary hepatocytes, AMPK/mTORC1 phosphorylation assays, Tret1 reconstitution experiment\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — multiple orthogonal transport assays, genetic KO in vitro and in vivo, reconstitution with heterologous transporter, epistasis (AMPK required downstream of GLUT8)\",\n      \"pmids\": [\"27922102\"],\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 required for embryo implantation. Ovarian transplantation studies confirmed that SLC2A8 loss affects both embryo and implantation processes. Null mice display decreased litter size, reduced postnatal growth, decreased body fat, and resistance to high-fat/high-carbohydrate diet.\",\n      \"method\": \"Slc2a8 knockout mouse (exons 1–4 deletion, protein expression confirmed by Western blot), oocyte metabolic/ATP assays, decidualization assay, ovarian transplantation, MRI body composition\",\n      \"journal\": \"Biology of reproduction\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic KO with multiple defined cellular and physiological phenotypes, ovarian transplantation for epistasis; single lab\",\n      \"pmids\": [\"22649075\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Hepatocyte GLUT8 (SLC2A8) regulates the adaptive fasting response through the PPARα signaling cascade. Slc2a8-disrupted mice exhibited enhanced thermogenesis, ketogenesis, and peripheral lipid mobilization during fasting, with mildly impaired hepatic mitochondrial oxidative metabolism. Hepatic PPARα and its target FGF21 were cell-autonomously hyperactivated in GLUT8-deficient liver and primary hepatocytes during nutrient depletion. Hepatic PPARα knockdown in GLUT8-deficient mice normalized the enhanced ketogenic and FGF21 secretory responses.\",\n      \"method\": \"Slc2a8 knockout mouse fasting challenge, metabolic phenotyping (thermogenesis, ketogenesis, lipid mobilization), mitochondrial respiration assay, PPARα/FGF21 signaling assays in vivo and in primary hepatocytes, hepatic PPARα knockdown epistasis\",\n      \"journal\": \"Endocrinology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic KO plus rescue by PPARα knockdown (epistasis), cell-autonomous confirmation in primary hepatocytes, single lab\",\n      \"pmids\": [\"29596655\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"SLC2A8 knockdown (79% mRNA reduction) in human first-trimester trophoblast cells (ACH-3P) caused an 11% reduction in glucose uptake. RNAseq identified 1525 differentially expressed transcripts, with enrichment in metabolic pathways associated with cellular respiration, oxidative phosphorylation, and ATP synthesis, indicating that SLC2A8's primary function in trophoblast cells is to support cellular respiration rather than net glucose delivery.\",\n      \"method\": \"Lentiviral RNAi knockdown in ACH-3P cells, glucose uptake assay, RNAseq transcriptome analysis, real-time qPCR validation\",\n      \"journal\": \"Cells\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — loss-of-function with direct glucose uptake measurement plus transcriptome; single lab, two orthogonal methods\",\n      \"pmids\": [\"38474355\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"SLC2A8 (GLUT8) is an intracellular-targeted facilitative hexose transporter retained in endosomes/lysosomes/ER by an N-terminal [DE]XXXL[LI] dileucine motif (removable by mutagenesis to allow cell-surface expression), with a Km of ~2 mM for glucose and additional capacity to transport fructose and trehalose; it mediates hepatocyte fructose uptake and fructose-induced lipogenesis, serves as the carrier required for cytoplasmic trehalose entry and subsequent AMPK-dependent autophagic induction, regulates the adaptive hepatic fasting response through a PPARα–FGF21 axis, and supports cellular respiration in trophoblast cells, while its loss in mice impairs oocyte ATP production, decidualization, and spermatogenesis.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"SLC2A8 (GLUT8) is a facilitative hexose transporter that couples substrate uptake to cellular energy metabolism and nutrient-sensing across multiple tissues [#0, #4]. It transports glucose with a Km of ~2 mM and additionally carries fructose and trehalose [#0, #4, #5]. The protein is normally retained on intracellular membranes—endosomes, lysosomes, and the endoplasmic reticulum—through an N-terminal [DE]XXXL[LI] dileucine targeting motif, and mutation of this motif redirects it to the plasma membrane [#0, #3]. In hepatocytes GLUT8 mediates fructose uptake that drives de novo lipogenesis, and its loss attenuates hepatic triglyceride and cholesterol accumulation on a high-fructose diet [#4]. GLUT8 is also the carrier required for cytoplasmic entry of trehalose, which activates AMPK-dependent autophagy; this defect is rescued by heterologous expression of the Drosophila trehalose transporter Tret1 [#5]. In liver it further governs the adaptive fasting response by restraining the cell-autonomous PPARα–FGF21 axis, such that GLUT8 deficiency hyperactivates ketogenesis and FGF21 secretion [#7]. Consistent with a role in supporting cellular respiration, SLC2A8 loss impairs oocyte ATP production and decidualization, reduces sperm motility and mitochondrial membrane potential, and diminishes oxidative-phosphorylation transcriptional programs in trophoblast cells [#3, #6, #8].\",\n  \"teleology\": [\n    {\n      \"year\": 2000,\n      \"claim\": \"Established that SLC2A8 is a bona fide facilitative hexose transporter and defined its intracellular-retention mechanism, answering what biochemical activity the gene encodes.\",\n      \"evidence\": \"Xenopus oocyte radiolabeled transport assay, dileucine-motif mutagenesis, in vitro translation/glycosylation, and immunofluorescence in HEK293T\",\n      \"pmids\": [\"10671487\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not establish the physiological substrate in mammalian tissues\", \"Surface trafficking only achieved by motif mutation, not by a physiological signal\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Showed that endogenous GLUT8 in brain is predominantly intracellular and neuron-restricted, addressing whether native localization matches the heterologous retention behavior.\",\n      \"evidence\": \"Subcellular fractionation/immunoblot and immunohistochemistry in rat hippocampus, including streptozotocin-diabetic rats\",\n      \"pmids\": [\"11226324\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"mRNA/protein discordance in diabetes left unexplained\", \"No functional transport assay in neurons\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Mapped cell-type-specific expression to differentiating spermatocytes and vasopressin neurons with vesicular/secretory-granule localization, suggesting tissue-specific secretory roles.\",\n      \"evidence\": \"Immunohistochemistry, double immunofluorescence, and immunogold labeling of cryosections in testis and brain\",\n      \"pmids\": [\"11751619\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional consequence of dense-core-vesicle localization not tested\", \"No substrate flux measured in these cell types\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"A knockout mouse linked GLUT8 loss to defined cellular phenotypes (sperm motility, mitochondrial potential/ATP, neuronal proliferation, cardiac conduction), connecting the transporter to energy metabolism in vivo.\",\n      \"evidence\": \"Slc2a8 knockout phenotyping (behavior, cardiac conduction, sperm analysis, mitochondrial membrane potential) plus heterologous localization\",\n      \"pmids\": [\"19176349\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanistic link between transport activity and each phenotype not resolved\", \"Substrate driving sperm energetics not identified\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Demonstrated that GLUT8 is required for oocyte ATP production and endometrial decidualization, establishing a reproductive role tied to cellular energetics.\",\n      \"evidence\": \"Slc2a8 knockout mouse with oocyte metabolic/ATP assays, decidualization assay, ovarian transplantation, and MRI body composition\",\n      \"pmids\": [\"22649075\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Transported substrate underlying the oocyte/implantation defect not defined\", \"Single lab\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Identified hepatic fructose as a physiological substrate and linked GLUT8-mediated fructose uptake to de novo lipogenesis, providing a metabolic-disease-relevant function.\",\n      \"evidence\": \"Radiolabeled fructose uptake with overexpression and shRNA knockdown plus Slc2a8 KO mice on high-fructose diet with hepatic lipid quantification\",\n      \"pmids\": [\"24519932\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Surface vs. intracellular site of fructose transport in hepatocytes not fully reconciled with earlier retention data\", \"Insulin signaling unchanged, leaving the lipogenic mechanism partly open\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Showed GLUT8 is the mammalian trehalose carrier required for trehalose-induced, AMPK-dependent autophagy, defining a signaling pathway downstream of transport.\",\n      \"evidence\": \"GC/MS and radiolabeled trehalose uptake, fluorescence microscopy, GLUT8-deficient hepatocytes/mice, AMPK/mTORC1 phosphorylation assays, and Tret1 reconstitution\",\n      \"pmids\": [\"27922102\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How cytoplasmic trehalose activates AMPK not mechanistically resolved\", \"Physiological source of trehalose in mammals unclear\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Placed hepatic GLUT8 upstream of the PPARα–FGF21 fasting program, showing it restrains adaptive ketogenesis and lipid mobilization in a cell-autonomous manner.\",\n      \"evidence\": \"Slc2a8 KO fasting challenge with metabolic phenotyping, mitochondrial respiration, PPARα/FGF21 assays in vivo and in primary hepatocytes, and PPARα-knockdown epistasis\",\n      \"pmids\": [\"29596655\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism connecting transport activity to PPARα activation not defined\", \"Identity of the metabolic signal sensed by hepatocytes unknown\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Established that in human trophoblast cells GLUT8 supports cellular respiration/oxidative phosphorylation rather than bulk glucose delivery, refining its functional role in the placenta.\",\n      \"evidence\": \"Lentiviral RNAi knockdown in ACH-3P cells with glucose uptake assay and RNAseq transcriptome analysis\",\n      \"pmids\": [\"38474355\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Modest glucose-uptake reduction leaves the respiratory link correlative\", \"Direct measurement of respiration not performed\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How GLUT8 trafficking is physiologically regulated to expose its transport activity at the relevant membrane, and how intracellular substrate flux is mechanistically transduced into AMPK and PPARα signaling, remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No physiological signal shown to relieve dileucine-motif retention\", \"Molecular link between transport and downstream kinase/nuclear-receptor signaling undefined\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005215\", \"supporting_discovery_ids\": [0, 4, 5]},\n      {\"term_id\": \"GO:0005351\", \"supporting_discovery_ids\": [0]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005768\", \"supporting_discovery_ids\": [3]},\n      {\"term_id\": \"GO:0005764\", \"supporting_discovery_ids\": [3]},\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [3]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 4]},\n      {\"term_id\": \"GO:0031410\", \"supporting_discovery_ids\": [1]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-382551\", \"supporting_discovery_ids\": [0, 4, 5]},\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [4, 7]},\n      {\"term_id\": \"R-HSA-9612973\", \"supporting_discovery_ids\": [5]},\n      {\"term_id\": \"R-HSA-1474165\", \"supporting_discovery_ids\": [6]}\n    ],\n    \"complexes\": [],\n    \"partners\": [],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":6,"faith_total":7,"faith_pct":85.71428571428571}}