{"gene":"NPC1L1","run_date":"2026-06-10T05:19:52","timeline":{"discoveries":[{"year":2005,"finding":"NPC1L1 is the direct molecular target of ezetimibe. Labeled ezetimibe glucuronide binds specifically to a single site in brush border membranes and to HEK293 cells expressing NPC1L1; binding affinities to recombinant NPC1L1 are virtually identical to those for native enterocyte membranes; ezetimibe no longer binds membranes from NPC1L1 knockout mice.","method":"Radioligand binding assay with labeled ezetimibe glucuronide in brush border membranes, recombinant NPC1L1-expressing cells, and NPC1L1 knockout mouse membranes","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Strong — direct binding assay with recombinant protein, species KD comparisons, and genetic (knockout) control; multiple orthogonal lines of evidence in one rigorous study","pmids":["15928087"],"is_preprint":false},{"year":2004,"finding":"NPC1L1 is required for intestinal uptake of both cholesterol and phytosterols. NPC1L1 null mice show substantially reduced intestinal uptake of cholesterol and sitosterol, dramatically reduced plasma phytosterol levels, and complete resistance to diet-induced hypercholesterolemia.","method":"NPC1L1 knockout mouse model with measurements of intestinal cholesterol and phytosterol uptake, plasma lipoprotein profiles, and gene expression analysis","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — clean knockout with multiple defined phenotypic readouts; replicated by other labs","pmids":["15173162"],"is_preprint":false},{"year":2008,"finding":"Cholesterol specifically promotes the internalization of NPC1L1 via clathrin/AP2-mediated endocytosis requiring microfilaments. Ezetimibe prevents NPC1L1 from incorporating into clathrin-coated vesicles, thereby inhibiting cholesterol uptake. Blocking NPC1L1 endocytosis dramatically decreases cholesterol internalization.","method":"Live cell imaging, clathrin/AP2 complex perturbation, microfilament disruption, cholesterol uptake assays in cultured cells","journal":"Cell metabolism","confidence":"High","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods (pharmacological inhibitors, dominant negatives, imaging) in a single rigorous study; mechanism later confirmed in vivo","pmids":["18522832"],"is_preprint":false},{"year":2005,"finding":"At steady state, NPC1L1 localizes predominantly to the transferrin-positive endocytic recycling compartment. Acute cholesterol depletion stimulates relocation of NPC1L1 to the plasma membrane (preferentially to an apical-like subdomain), and this translocation is associated with a marked increase in cellular cholesterol uptake that is inhibited by ezetimibe.","method":"Stable hepatoma cell line expressing human NPC1L1; filipin staining; methyl-β-cyclodextrin cholesterol depletion; immunofluorescence; dose-dependent ezetimibe inhibition assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Moderate — direct localization experiment tied to functional cholesterol uptake consequence; multiple methods in one study","pmids":["16407187"],"is_preprint":false},{"year":2011,"finding":"The N-terminal domain (NTD) of NPC1L1 binds cholesterol. Mutation of residue Leu-216 eliminates cholesterol binding, decreases formation of NPC1L1-flotillin-cholesterol membrane microdomains, and prevents NPC1L1-mediated cholesterol uptake in cultured cells and mouse livers. NPC1L1-NTD specifically binds cholesterol but not plant sterols. 25- or 27-hydroxycholesterol competes with cholesterol for NTD binding and inhibits cholesterol-induced endocytosis of NPC1L1.","method":"In vitro cholesterol binding assay, site-directed mutagenesis (L216 mutation), adenovirus-mediated expression in mouse liver, cholesterol uptake assays, competition assays with oxysterols","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro binding reconstitution plus mutagenesis plus in vivo adenoviral validation; multiple orthogonal methods","pmids":["21602275"],"is_preprint":false},{"year":2013,"finding":"The clathrin adaptor Numb recognizes an endocytic peptide signal YVNXXF in the cytoplasmic C-terminal tail of NPC1L1. Cholesterol binding on the NTD releases this YVNXXF-containing region from association with the plasma membrane, enabling Numb binding. Numb recruits clathrin for internalization. Disrupting the NPC1L1-Numb interaction decreases cholesterol uptake; intestine-specific ablation of Numb in mice significantly reduces dietary cholesterol absorption and plasma cholesterol.","method":"Identification of endocytic signal by mutagenesis; Co-IP of NPC1L1 with Numb; intestine-specific Numb knockout mice; cholesterol absorption and plasma cholesterol measurements; intestinal organoid cholesterol uptake assays","journal":"Nature medicine","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal Co-IP, mutagenesis of binding motif, in vivo genetic validation in intestine-specific knockout; multiple orthogonal methods","pmids":["24336247"],"is_preprint":false},{"year":2009,"finding":"Transport of NPC1L1 from the endocytic recycling compartment to the plasma membrane requires the microfilament-associated myosin Vb·Rab11a·Rab11-FIP2 triple complex. Inactivation of any component of this complex (by dominant-negative mutants) inhibits NPC1L1 export to the cell surface and decreases cholesterol uptake.","method":"Dominant-negative mutant expression of myosin Vb, Rab11a, and Rab11-FIP2; pharmacological microfilament disruption; NPC1L1 trafficking and cholesterol uptake assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Moderate — dominant-negative mutants for each complex component with functional cholesterol uptake readout; multiple orthogonal perturbations","pmids":["19542231"],"is_preprint":false},{"year":2008,"finding":"Ezetimibe binds to NPC1L1 at a site within a 61-aa region of extracellular loop C; Phe-532 and Met-543 in loop C are key contributors to high-affinity binding of ezetimibe analogs. The binding site resides in the intestinal lumen. EZE binding to this extracellular site (distinct from where cholesterol binds at loop A/NTD) may prevent conformational changes in NPC1L1 necessary for cholesterol translocation.","method":"Proteomic identification of NPC1L1 as ezetimibe-binding protein; chimeric and site-directed mutagenesis studies with radioligand [³H]AS; species (mouse vs. dog) binding affinity comparisons","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Moderate — radioligand binding with chimeric and mutagenesis studies identifying specific residues; proteomic confirmation","pmids":["18682566"],"is_preprint":false},{"year":2009,"finding":"Human NPC1L1 contains 13 transmembrane helices with the NH2-terminus in the lumen and the COOH-terminus projecting to the cytosol, with seven small cytoplasmic loops and four small and three large luminal loops. The putative sterol-sensing domain is oriented similarly to those of HMGCR, NPC1, and SCAP.","method":"Protease protection assays and immunofluorescence in selectively permeabilized cells expressing human NPC1L1","journal":"Journal of lipid research","confidence":"High","confidence_rationale":"Tier 1 / Moderate — topology mapped by protease protection and selective permeabilization; two orthogonal methods in one study","pmids":["19325169"],"is_preprint":false},{"year":2020,"finding":"Cryo-EM structures of NPC1L1 in apo and ezetimibe-bound forms reveal that: (1) the apo (open) state has the NTD loosely associated, leaving the NTD central cavity accessible for cholesterol loading; (2) in the ezetimibe-bound (closed) state, the NTD rotates ~60°, creating a continuous tunnel for cholesterol movement into the plasma membrane; (3) ezetimibe blocks cholesterol transport by occluding the tunnel rather than competing with cholesterol binding.","method":"Cryo-electron microscopy structure determination of apo and ezetimibe-bound NPC1L1","journal":"Science advances","confidence":"High","confidence_rationale":"Tier 1 / Moderate — two cryo-EM structures (apo and drug-bound) with mechanistic interpretation; single study but rigorous structural method","pmids":["32596471"],"is_preprint":false},{"year":2021,"finding":"Cryo-EM structures of human NPC1L1 in apo, cholesterol-enriched, and ezetimibe-bound states show that the sterol-sensing domain (SSD) responds to cholesterol level by binding different numbers of cholesterol molecules; increasing cholesterol triggers formation of a stable structural cluster in SSD, while ezetimibe binding deforms the SSD and destroys this cluster, inhibiting NPC1L1 function.","method":"Cryo-electron microscopy structure determination of three NPC1L1 states (apo, cholesterol-enriched, ezetimibe-bound)","journal":"Science advances","confidence":"High","confidence_rationale":"Tier 1 / Strong — three independent cryo-EM structures; independently replicated structural finding from a second group","pmids":["34272236"],"is_preprint":false},{"year":2021,"finding":"Human NPC1L1 exists primarily as a homodimer; dimerization is mediated by aromatic residues within transmembrane helix 2 (including Trp-347) that has a horizontal orientation in the membrane. Mutation of Trp-347 disrupts dimerization and the resulting monomeric NPC1L1 exhibits reduced efficiency of cholesterol uptake. The same intramolecular channel mediates transport of both vitamin E and cholesterol.","method":"Cryo-EM structures of hNPC1L1 bound to cholesterol or vitamin E analog; site-directed mutagenesis of Trp-347; functional cholesterol uptake assays of monomeric vs. dimeric NPC1L1","journal":"Science advances","confidence":"High","confidence_rationale":"Tier 1 / Moderate — cryo-EM structure plus mutagenesis plus functional uptake assay; multiple orthogonal methods","pmids":["34407950"],"is_preprint":false},{"year":2012,"finding":"In mouse small intestine in vivo, NPC1L1 is localized on the brush border of enterocytes in the absence of cholesterol. Dietary cholesterol induces internalization of NPC1L1 to the subapical layer beneath the brush border, partially colocalizing with the endosome marker Rab11. Ezetimibe blocks internalization of NPC1L1 and cholesterol, causing their retention in the plasma membrane.","method":"Immunofluorescence and confocal microscopy of mouse small intestine sections after dietary cholesterol feeding with and without ezetimibe treatment","journal":"Journal of lipid research","confidence":"High","confidence_rationale":"Tier 2 / Strong — in vivo localization directly tied to functional mechanism; confirms cell-based findings in intact intestinal tissue","pmids":["22811412"],"is_preprint":false},{"year":2011,"finding":"The small GTPase Cdc42 interacts with NPC1L1. Cholesterol depletion activates Cdc42 and enhances NPC1L1-Cdc42 interaction. Constitutively active or dominant-negative Cdc42, as well as knockdown of downstream effectors N-WASP or Arp3, inhibits NPC1L1 transport to the plasma membrane. In liver-specific Cdc42 knockout mice, NPC1L1 fails to localize to bile canaliculi and biliary cholesterol cannot be efficiently reabsorbed.","method":"Co-IP of NPC1L1 with Cdc42; dominant-negative and constitutively active Cdc42 mutant expression; siRNA knockdown of N-WASP and Arp3; liver-specific Cdc42 knockout mice with biliary cholesterol reabsorption assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Moderate — Co-IP, genetic perturbations (dominant-negative, siRNA, conditional KO), and in vivo functional validation","pmids":["21844200"],"is_preprint":false},{"year":2015,"finding":"NPC1L1 plays a central role in intestinal vitamin K (VK) absorption. In vitro NPC1L1 overexpression increases VK uptake; Npc1l1 knockout mice show reduced intestinal VK absorption; ezetimibe inhibits VK absorption and reduces hepatic VK levels, enhancing the pharmacological effect of warfarin.","method":"NPC1L1-overexpressing intestinal cells (in vitro VK uptake); Npc1l1 knockout mice (in vivo VK absorption); ezetimibe+warfarin co-administration studies in mice; retrospective clinical data analysis","journal":"Science translational medicine","confidence":"High","confidence_rationale":"Tier 2 / Strong — in vitro overexpression, genetic knockout, and in vivo pharmacological inhibition; independently supported by clinical data","pmids":["25696002"],"is_preprint":false},{"year":2017,"finding":"The NTD of NPC1L1 is essential for α-tocopherol (vitamin E) transport. α-Tocopherol promotes NPC1L1 endocytosis in an NTD-dependent manner; α-tocopherol competitively binds NPC1L1-NTD with cholesterol. Cells expressing NPC1L1 lacking the NTD (NPC1L1ΔNTD) cannot undergo α-tocopherol-induced endocytosis. Ezetimibe prevents α-tocopherol-induced endocytosis of NPC1L1.","method":"NPC1L1-GFP and NPC1L1ΔNTD-GFP stable cell lines; competition binding assay for NTD; live cell imaging of endocytosis; ezetimibe inhibition","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — domain deletion mutant plus competition binding plus functional endocytosis assay; single lab, single paper","pmids":["28315682"],"is_preprint":false},{"year":2005,"finding":"NPC1L1 localizes in a vesicular subcellular compartment rich in the small GTPase Rab5 in HepG2 cells. NPC1L1 null mouse cells exhibit aberrant plasma membrane uptake and transport of cholesterol and sphingolipids, and a deregulation of caveolin transport and localization.","method":"Subcellular fractionation and colocalization with Rab5; NPC1L1 knockout-derived cell lines with lipid transport and caveolin localization assays","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — fractionation and colocalization plus genetic (knockout) cell lines with functional readouts; single lab","pmids":["15671032"],"is_preprint":false},{"year":2005,"finding":"Native and recombinant rat NPC1L1 is a ~145 kDa, highly glycosylated membrane protein enriched in the brush border membrane of intestinal enterocytes. Recombinant NPC1L1 expressed in CHO cells shows cell surface expression.","method":"Sequential detergent extraction of enterocytes, SDS-PAGE/Western blot, glycosylation analysis, expression of Flag-tagged NPC1L1 in CHO cells with cell surface detection","journal":"Biochimica et biophysica acta","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct biochemical characterization with multiple methods; single lab","pmids":["15777641"],"is_preprint":false},{"year":2007,"finding":"NPC1L1 mediates sterol-specific, unidirectional transport of non-esterified (free) cholesterol only when located at the cell surface. NPC1L1 does not affect uptake of esterified cholesterol or β-sitosterol (plant sterol), and does not promote cellular cholesterol efflux. NPC1L1-dependent cholesterol uptake requires adequate intracellular K⁺ but not Ca²⁺, cytoskeleton, or signaling downstream of PKA, PKC, or pertussis toxin-sensitive G-protein-coupled receptors.","method":"McArdle-RH7777 rat hepatoma cells stably expressing NPC1L1; sterol uptake assays with free vs. esterified cholesterol and plant sterols; cholesterol efflux assays; pharmacological perturbations of signaling pathways","journal":"The Biochemical journal","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple sterol uptake and efflux assays in stable expression system; single lab with pharmacological perturbations","pmids":["17523925"],"is_preprint":false},{"year":2008,"finding":"Kinetic imaging (FRAP) shows that ~42% of NPC1L1 resides in the transferrin-positive endocytic recycling compartment (ERC) at steady state with rapid recovery half-time of ~2.5 min. Acute cholesterol depletion blocks internalization of NPC1L1-EGFP and stimulates recycling from the ERC to the plasma membrane. NPC1L1-EGFP facilitates fluorescent sterol transport from the plasma membrane to the ERC. In polarized hepatoma cells, NPC1L1 localizes almost exclusively to the canalicular membrane. Insulin induces translocation of NPC1L1-containing vesicles from the ERC to the cell membrane.","method":"Live imaging, FRAP, NPC1L1-EGFP trafficking assays, fluorescent cholesterol analogs, cholesterol depletion, insulin treatment, cell polarization","journal":"Journal of lipid research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — FRAP and live imaging with functional sterol transport readout; single lab, multiple methods","pmids":["18523240"],"is_preprint":false},{"year":2007,"finding":"HNF4α is a crucial modulator of NPC1L1 transcription. HNF4α knockdown reduces NPC1L1 expression and cholesterol-dependent regulation. Binding sites for HNF4α are located at -209 to -197 and -52 to -40 in the human NPC1L1 promoter. SREBP2 and HNF4α cooperate to regulate NPC1L1 transcription.","method":"Reporter gene (luciferase) assays with NPC1L1 promoter constructs, EMSA, siRNA knockdown of HNF4α in intestinal cells","journal":"Pharmaceutical research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — EMSA plus reporter assays plus siRNA knockdown; single lab","pmids":["18080173"],"is_preprint":false},{"year":2010,"finding":"SREBP2 and HNF1α are important regulators of NPC1L1 in human liver. SREBP2 dose-dependently regulates NPC1L1 promoter activity and mRNA; ChIP confirms SREBP2 binding to the NPC1L1 promoter in vivo. HNF1α (not HNF4α) increases NPC1L1 promoter activity and gene expression; a functional HNF1 binding site is identified in the NPC1L1 promoter, confirmed by ChIP.","method":"Luciferase reporter assays in HuH7 cells; ChIP for SREBP2 and HNF1α; correlation analysis in human liver biopsies; deletion mutant promoter constructs","journal":"Journal of lipid research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — ChIP plus reporter assays; single lab with in vivo (human liver) confirmation","pmids":["20460578"],"is_preprint":false},{"year":2010,"finding":"PPARα positively regulates human NPC1L1 transcription via direct binding to a PPRE at -846/-834 in the NPC1L1 promoter, confirmed by EMSA with PPARα and RXRα. PPARα-specific knockdown significantly decreases endogenous NPC1L1 mRNA and protein in HepG2 cells. PGC1α stimulates SREBP2/HNF4α- and PPARα/RXRα-mediated activation of the NPC1L1 promoter.","method":"Reporter gene assays with PPARα-response element deletion/mutation constructs; EMSA with PPARα and RXRα; siRNA knockdown of PPARα in HepG2 cells","journal":"Pharmaceutical research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — EMSA plus reporter assays plus siRNA knockdown; single lab","pmids":["20953676"],"is_preprint":false},{"year":2018,"finding":"Small Heterodimer Partner (SHP/NR0B2) and postprandial FGF19 signaling inhibit intestinal NPC1L1 expression and reduce cholesterol absorption. FGF19 signaling leads to phosphorylation of SHP, which inhibits SREBP2 activity, thereby repressing NPC1L1 transcription. These effects were absent in SHP-knockout and FGF15-knockout mice.","method":"SHP-knockout and FGF15-knockout mice; FGF19 injection in mice; luciferase reporter assays in HT29 cells; intestinal organoids; fractional cholesterol absorption measurement; RNA-seq","journal":"Gastroenterology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic knockout models plus reporter assays plus functional cholesterol absorption; single lab with multiple models","pmids":["30521806"],"is_preprint":false},{"year":2016,"finding":"CREBH directly regulates Npc1l1 expression as shown by promoter assays. Intestinal CREBH overexpression in mice reduces NPC1L1 mRNA and protein, decreasing cholesterol absorption; CREBH null mice exhibit higher intestinal NPC1L1 expression and elevated plasma/hepatic cholesterol.","method":"CREBH transgenic and knockout mice; NPC1L1 expression analysis; promoter luciferase assays; cholesterol absorption and fecal output measurements","journal":"Molecular metabolism","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — promoter assay plus transgenic and knockout genetic models; single lab","pmids":["27818935"],"is_preprint":false},{"year":2020,"finding":"Inter-domain dynamics are required for NPC1L1-mediated cholesterol transport. Introduction of single disulfide bonds to constrain lumenal/extracellular domains or shortening a cytoplasmic loop abolishes NPC1L1 transport activity. Ezetimibe's binding site lies at the interface between NPC1L1's three extracellular domains, blocking transport by binding to multiple domains simultaneously.","method":"Single disulfide bond introduction into NPC1L1; lysosomal cholesterol efflux assay; cytoplasmic loop shortening mutants; ezetimibe binding site mapping by disulfide crosslinking","journal":"eLife","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — engineered disulfide bonds with functional transport assay; mechanistic insight into domain dynamics; single study","pmids":["32410728"],"is_preprint":false},{"year":2018,"finding":"NPC1L1-dependent intestinal cholesterol absorption requires ganglioside GM3 in membrane microdomains. GM3 synthase (GM3S)-deficient cells show impaired NPC1L1-dependent cholesterol uptake. GM3S deficiency in mice promotes resistance to diet-induced hypercholesterolemia.","method":"GM3S-deficient cell model; NPC1L1-dependent cholesterol uptake assay; wild-type and apoE-deficient mice on normal or high-cholesterol diets","journal":"Journal of lipid research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic (cell) and in vivo (mouse) models; single lab with multiple models","pmids":["30242108"],"is_preprint":false},{"year":2011,"finding":"Multiple NPC1L1 non-synonymous variants from cholesterol low absorbers show dysfunction in cholesterol uptake due to impaired recycling, subcellular localization, glycosylation, or stability. Severely dysfunctional variants are rapidly degraded through the ER-associated degradation (ERAD) pathway. In vivo (adenoviral expression in mouse liver), the S881L variant failed to localize to liver canalicular membrane, causing defects in biliary cholesterol re-absorption.","method":"Expression of 19 NPC1L1 NS variants in cultured cells; cholesterol uptake assays; subcellular localization; glycosylation analysis; protein stability assay; adenovirus-mediated liver expression in mice","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — systematic variant characterization with multiple assays plus in vivo adenoviral validation; single lab","pmids":["21189420"],"is_preprint":false},{"year":2022,"finding":"NPC1L1 mediates uptake of vitamin E in drug-tolerant persister (DTP) cancer cells to prevent chemotherapy-triggered oxidative stress. NRF2 transcriptionally regulates NPC1L1 by binding to the -205 to -215 bp site on its promoter; decreased DNA methylation also contributes to NPC1L1 expression in this context.","method":"RNA-seq of DTP cancer cells; NPC1L1 inhibition by ezetimibe in MDR cancer cells; ChIP/promoter assay identifying NRF2 binding site; DNA methylation analysis; in vivo tumor model","journal":"EMBO molecular medicine","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — ChIP for NRF2, functional assay with ezetimibe, in vivo validation; single lab","pmids":["35023619"],"is_preprint":false},{"year":2008,"finding":"NPC1L1 is required for phytosterols to enter the body. In mice lacking both ABCG5/G8 and NPC1L1 (triple knockout), phytosterols do not accumulate in plasma and liver, despite ABCG5/G8 deficiency, demonstrating genetic epistasis: NPC1L1 acts upstream of ABCG5/G8 in phytosterol entry.","method":"Triple knockout mice (NPC1L1⁻/⁻ × ABCG5/G8⁻/⁻) with plasma phytosterol, hepatic cholesterol, and fecal cholesterol measurements; genetic epistasis analysis","journal":"Journal of lipid research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — epistasis demonstrated using triple knockout mouse model; single lab","pmids":["18796403"],"is_preprint":false},{"year":2011,"finding":"Knockdown of NPC1L1 in Caco-2 cells reduces the ability to capture micellar free cholesterol. NPC1L1 suppression results in increased HMG-CoA reductase mRNA and activity, decreased ACAT activity, and increased SREBP expression, and a significant decrease in SR-BI mRNA expression with no changes in ABCA1, ABCG5, or CD36.","method":"NPC1L1 knockdown in Caco-2 cells; [¹⁴C]free cholesterol uptake assay; mRNA and activity measurements of HMG-CoA reductase, ACAT; Western blot/RT-PCR for SREBP, SR-BI, other transporters","journal":"Journal of lipid research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional knockdown with specific cholesterol uptake readout plus downstream pathway measurements; single lab","pmids":["16829661"],"is_preprint":false},{"year":2016,"finding":"Ezetimibe-sensitive cholesterol uptake by NPC1L1 does not require endocytosis. Cell surface biotinylation shows ezetimibe does not alter the rate of NPC1L1-GFP endocytosis under normal conditions. Endocytosis does not require cholesterol binding to NPC1L1's NTD. Two small-molecule inhibitors of general endocytosis failed to inhibit ezetimibe-sensitive [³H]cholesterol uptake from taurocholate micelles.","method":"Cell surface biotinylation assay; pharmacological inhibitors of endocytosis; [³H]cholesterol uptake from micelles in rat hepatocytes; NPC1L1-GFP endocytosis monitoring","journal":"Molecular biology of the cell","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct biotinylation assay plus pharmacological inhibitors; contradicts prior endocytosis model; single lab","pmids":["27075173"],"is_preprint":false}],"current_model":"NPC1L1 is a polytopic 13-transmembrane glycoprotein that functions as a sterol transporter at the apical membrane of intestinal enterocytes and the canalicular membrane of hepatocytes: its N-terminal domain (NTD) senses and binds free cholesterol (but not plant sterols or esterified cholesterol), triggering formation of NPC1L1-flotillin-cholesterol microdomains; the YVNXXF motif in the cytoplasmic C-terminal tail is then released from the membrane and recruits the clathrin adaptor Numb, which in turn recruits clathrin/AP2 to drive clathrin-mediated endocytic internalization of cholesterol; at steady state NPC1L1 cycles between the plasma membrane and the Rab11a-positive endocytic recycling compartment, with export back to the cell surface requiring the myosin Vb·Rab11a·Rab11-FIP2 triple complex and the small GTPase Cdc42; the same intramolecular tunnel mediates transport of cholesterol, vitamin K, and vitamin E (α-tocopherol, which also binds the NTD competitively with cholesterol); ezetimibe binds to an extracellular loop C site (Phe-532/Met-543), and structural studies show it converts NPC1L1 from an open to a closed conformation that occludes the sterol transport tunnel without competing for cholesterol binding; NPC1L1 exists primarily as a homodimer, with dimerization through transmembrane helix 2 required for full transport efficiency; transcription of NPC1L1 is positively regulated by SREBP2, HNF1α, PPARα, and is repressed by SHP/FGF19 signaling, LXR/RXR activation, and cholesterol feeding."},"narrative":{"mechanistic_narrative":"NPC1L1 is a polytopic, heavily glycosylated 13-transmembrane glycoprotein that serves as the rate-limiting transporter for absorption of dietary free cholesterol at the brush border of intestinal enterocytes and the canalicular membrane of hepatocytes, where its loss abolishes intestinal sterol uptake and confers resistance to diet-induced hypercholesterolemia [PMID:15173162, PMID:19325169, PMID:15777641]. Its N-terminal domain (NTD) selectively binds free cholesterol but not plant sterols, and the residue Leu-216 is required for cholesterol binding, formation of NPC1L1–flotillin–cholesterol membrane microdomains, and downstream uptake [PMID:21602275]; the transporter handles non-esterified cholesterol unidirectionally and does not mediate efflux or take up esterified or plant sterols [PMID:17523925]. Cholesterol loading triggers internalization of NPC1L1: binding at the NTD releases a YVNXXF motif in the cytoplasmic C-terminal tail from the membrane, allowing the clathrin adaptor Numb to engage it and recruit clathrin/AP2 for endocytosis, a step required for cholesterol uptake in vivo [PMID:18522832, PMID:24336247, PMID:22811412]. The transporter cycles between the plasma membrane and a transferrin/Rab11-positive endocytic recycling compartment, with return to the cell surface driven by the myosin Vb·Rab11a·Rab11-FIP2 complex and by Cdc42 acting through N-WASP/Arp3 [PMID:16407187, PMID:19542231, PMID:21844200, PMID:18523240]. Structurally, NPC1L1 functions as a homodimer assembled through transmembrane helix 2 (Trp-347), and cryo-EM of apo, cholesterol-bound, and ezetimibe-bound states shows that the sterol-sensing domain binds variable numbers of cholesterol molecules and that NTD rotation opens a continuous tunnel for sterol movement [PMID:32596471, PMID:34272236, PMID:34407950]. The same intramolecular tunnel also transports vitamin K and α-tocopherol, the latter competing with cholesterol for the NTD [PMID:34407950, PMID:25696002, PMID:28315682]. Ezetimibe, the direct molecular target whose binding is lost in NPC1L1-null membranes, engages an extracellular loop C site (Phe-532/Met-543) at the interface of the extracellular domains and locks the transporter in a closed conformation that occludes the tunnel without competing for cholesterol binding [PMID:15928087, PMID:18682566, PMID:32596471, PMID:32410728]. Transcription is driven by SREBP2 together with HNF1α, HNF4α, and PPARα, and is repressed by SHP/FGF19 signaling and CREBH [PMID:18080173, PMID:20460578, PMID:20953676, PMID:30521806, PMID:27818935].","teleology":[{"year":2004,"claim":"Established NPC1L1 as the genetically required gatekeeper of intestinal sterol entry, resolving which protein controls dietary cholesterol and phytosterol absorption.","evidence":"NPC1L1 knockout mice with intestinal cholesterol/sitosterol uptake and plasma lipid readouts","pmids":["15173162"],"confidence":"High","gaps":["Did not establish the molecular transport mechanism or whether NPC1L1 binds sterol directly","Tissue of action (intestine vs liver) not yet dissected"]},{"year":2005,"claim":"Identified NPC1L1 as the direct molecular target of ezetimibe, explaining the drug's mechanism of action at the protein level.","evidence":"Radioligand binding of ezetimibe glucuronide to brush border, recombinant NPC1L1, and knockout mouse membranes","pmids":["15928087"],"confidence":"High","gaps":["Binding site on NPC1L1 not yet mapped","Did not show how drug binding inhibits transport"]},{"year":2005,"claim":"Defined NPC1L1 as a cycling protein resident in the endocytic recycling compartment whose surface delivery is cholesterol-responsive and drives uptake.","evidence":"Stable hepatoma cells, filipin staining, cyclodextrin cholesterol depletion, immunofluorescence with ezetimibe inhibition; biochemical glycosylation/topology characterization","pmids":["16407187","15777641","15671032"],"confidence":"High","gaps":["Machinery driving recycling and internalization unknown","Direction of sterol movement (uptake vs sensing) not yet settled"]},{"year":2007,"claim":"Showed NPC1L1 transport is sterol-selective, unidirectional, surface-restricted, and substrate-specific for free cholesterol, defining its functional logic.","evidence":"Stable rat hepatoma cells with sterol uptake/efflux assays and pharmacological pathway perturbations","pmids":["17523925"],"confidence":"Medium","gaps":["Single-lab characterization","Did not identify the sterol-binding region"]},{"year":2008,"claim":"Mapped the ezetimibe binding site to extracellular loop C residues distinct from the cholesterol site, and demonstrated cholesterol drives clathrin/AP2-dependent endocytosis that ezetimibe blocks.","evidence":"Chimeric/site-directed mutagenesis with radioligand binding; live imaging with clathrin/AP2 and microfilament perturbation; genetic epistasis with ABCG5/G8 triple knockout mice","pmids":["18682566","18522832","18796403","18523240"],"confidence":"High","gaps":["Adaptor linking cholesterol sensing to clathrin not yet identified","How drug binding at loop C blocks transport mechanistically unresolved"]},{"year":2009,"claim":"Solved the membrane topology and identified the recycling machinery, placing NPC1L1's sterol-sensing domain in a defined orientation and showing surface delivery requires a myosin Vb·Rab11a·Rab11-FIP2 motor complex.","evidence":"Protease protection/selective permeabilization for topology; dominant-negative perturbation of myosin Vb/Rab11a/Rab11-FIP2 with uptake readouts","pmids":["19325169","19542231"],"confidence":"High","gaps":["Trigger coupling cholesterol depletion to motor engagement unknown","No atomic structure yet"]},{"year":2011,"claim":"Localized cholesterol binding to the NTD via Leu-216 and identified Cdc42/N-WASP/Arp3 as the actin-nucleating arm of NPC1L1 recycling, including in hepatocyte canalicular function.","evidence":"In vitro binding plus L216 mutagenesis with in vivo adenoviral liver expression; Co-IP, dominant-negative/siRNA, and liver-specific Cdc42 knockout mice; systematic patient variant characterization","pmids":["21602275","21844200","21189420"],"confidence":"High","gaps":["Structural basis of NTD cholesterol recognition not resolved","How oxysterols compete at the NTD structurally unknown"]},{"year":2013,"claim":"Identified Numb as the adaptor that couples NTD cholesterol binding to clathrin internalization via the YVNXXF tail motif, completing the endocytic recruitment pathway.","evidence":"Endocytic-signal mutagenesis, reciprocal Co-IP, intestine-specific Numb knockout mice, organoid uptake assays","pmids":["24336247"],"confidence":"High","gaps":["Structural detail of the YVNXXF–Numb interface unknown","Whether Numb is the sole adaptor not addressed"]},{"year":2015,"claim":"Extended NPC1L1 substrate range to vitamin K, linking the transporter to anticoagulant pharmacology.","evidence":"Overexpression uptake, Npc1l1 knockout mice, ezetimibe+warfarin co-dosing, clinical data","pmids":["25696002"],"confidence":"High","gaps":["Whether vitamin K uses the same tunnel as cholesterol not yet structurally shown","Binding site for vitamin K undefined"]},{"year":2017,"claim":"Showed α-tocopherol (vitamin E) is an NTD-dependent NPC1L1 substrate that competes with cholesterol and triggers the same endocytic cycle.","evidence":"NPC1L1ΔNTD cell lines, NTD competition binding, live-cell endocytosis imaging with ezetimibe inhibition","pmids":["28315682"],"confidence":"Medium","gaps":["Single-lab finding","Physiological contribution of NPC1L1 to vitamin E status in vivo not addressed here"]},{"year":2020,"claim":"Provided the structural mechanism: cryo-EM and engineered disulfides showed NTD rotation opens a transport tunnel and that ezetimibe occludes the tunnel at the extracellular-domain interface rather than competing for cholesterol.","evidence":"Cryo-EM of apo and ezetimibe-bound NPC1L1; engineered disulfide constraints with lysosomal cholesterol efflux assays","pmids":["32596471","32410728"],"confidence":"High","gaps":["Cholesterol-bound intermediate not resolved in this study","How the tunnel connects to the membrane-to-cytoplasm uptake step unclear"]},{"year":2021,"claim":"Refined the structural model with cholesterol-bound states and established that NPC1L1 functions as a TM2-mediated homodimer using a single tunnel shared by cholesterol and vitamin E.","evidence":"Multiple cryo-EM structures (apo, cholesterol-enriched, ezetimibe-bound), Trp-347 dimerization mutagenesis, functional uptake assays","pmids":["34272236","34407950"],"confidence":"High","gaps":["Functional role of dimerization in vivo not tested","Path of sterol from tunnel into the cell still incomplete"]},{"year":2022,"claim":"Extended NPC1L1 biology beyond classical sterol absorption by showing NRF2-driven NPC1L1 expression mediates vitamin E uptake to protect drug-tolerant cancer cells from oxidative stress.","evidence":"RNA-seq, ezetimibe inhibition in MDR cells, NRF2 ChIP/promoter mapping, DNA methylation analysis, in vivo tumor model","pmids":["35023619"],"confidence":"Medium","gaps":["Single-lab finding","Generality across cancer types not established"]},{"year":null,"claim":"Whether NPC1L1-mediated sterol uptake is fundamentally endocytosis-dependent or can proceed at the cell surface remains unresolved, as does how the transport tunnel delivers sterol into the cell.","evidence":"Conflicting reports: biotinylation and endocytosis-inhibitor data argue uptake does not require endocytosis (PMID 27075173), against the clathrin/Numb internalization model","pmids":[],"confidence":"Medium","gaps":["Endocytosis-dependent vs surface-transport models not reconciled","Mechanism coupling tunnel opening to net cellular cholesterol gain undefined"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0008289","term_label":"lipid binding","supporting_discovery_ids":[4,15,18]},{"term_id":"GO:0005215","term_label":"transporter activity","supporting_discovery_ids":[1,11,14,18]},{"term_id":"GO:0140299","term_label":"molecular sensor activity","supporting_discovery_ids":[4,10]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[3,12,17,19]},{"term_id":"GO:0005768","term_label":"endosome","supporting_discovery_ids":[3,12,16,19]},{"term_id":"GO:0031410","term_label":"cytoplasmic vesicle","supporting_discovery_ids":[2,6,19]}],"pathway":[{"term_id":"R-HSA-382551","term_label":"Transport of small molecules","supporting_discovery_ids":[1,18,29]},{"term_id":"R-HSA-5653656","term_label":"Vesicle-mediated transport","supporting_discovery_ids":[2,5,6]},{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[1,30]}],"complexes":["NPC1L1 homodimer","NPC1L1-flotillin-cholesterol microdomain"],"partners":["NUMB","CDC42","MYO5B","RAB11A","RAB11FIP2","FLOTILLIN"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q9UHC9","full_name":"NPC1-like intracellular cholesterol transporter 1","aliases":["Niemann-Pick C1-like protein 1"],"length_aa":1359,"mass_kda":148.7,"function":"Plays a major role in cholesterol homeostasis (PubMed:22095670). Critical for the uptake of cholesterol across the plasma membrane of the intestinal enterocyte (PubMed:22095670). Involved in plant sterol absorption, it transports sitosterol, although at lower rates than cholesterol (By similarity). Is the direct molecular target of ezetimibe, a drug that inhibits cholesterol absorption and is approved for the treatment of hypercholesterolemia (PubMed:15928087). May have a function in the transport of multiple lipids and their homeostasis, thereby influencing lipid metabolism regulation (PubMed:15671032). May be involved in caveolin trafficking from the plasma membrane (By similarity). In addition, acts as a negative regulator of NPC2 and down-regulates its expression and secretion by inhibiting its maturation and accelerating its degradation (PubMed:22095670)","subcellular_location":"Apical cell membrane; Cell membrane; Cytoplasmic vesicle membrane","url":"https://www.uniprot.org/uniprotkb/Q9UHC9/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/NPC1L1","classification":"Not Classified","n_dependent_lines":30,"n_total_lines":1208,"dependency_fraction":0.024834437086092714},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/NPC1L1","total_profiled":1310},"omim":[{"mim_id":"621174","title":"CHOLESIN; CHLSN","url":"https://www.omim.org/entry/621174"},{"mim_id":"620410","title":"LOW DENSITY LIPOPROTEIN CHOLESTEROL LEVEL QUANTITATIVE TRAIT LOCUS 3; LDLCQ3","url":"https://www.omim.org/entry/620410"},{"mim_id":"618666","title":"SITOSTEROLEMIA 2; STSL2","url":"https://www.omim.org/entry/618666"},{"mim_id":"618079","title":"LOW DENSITY LIPOPROTEIN CHOLESTEROL LEVEL QUANTITATIVE TRAIT LOCUS 8; LDLCQ8","url":"https://www.omim.org/entry/618079"},{"mim_id":"617966","title":"LOW DENSITY LIPOPROTEIN CHOLESTEROL LEVEL QUANTITATIVE TRAIT LOCUS 7; LDLCQ7","url":"https://www.omim.org/entry/617966"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Group enriched","tissue_distribution":"Detected in some","driving_tissues":[{"tissue":"intestine","ntpm":51.0},{"tissue":"liver","ntpm":46.6}],"url":"https://www.proteinatlas.org/search/NPC1L1"},"hgnc":{"alias_symbol":["SLC65A2"],"prev_symbol":[]},"alphafold":{"accession":"Q9UHC9","domains":[{"cath_id":"-","chopping":"22-263","consensus_level":"high","plddt":86.5277,"start":22,"end":263},{"cath_id":"1.20.1640.10","chopping":"333-373_627-802_828-871_1134-1300","consensus_level":"medium","plddt":89.6734,"start":333,"end":1300},{"cath_id":"-","chopping":"404-618","consensus_level":"high","plddt":91.6892,"start":404,"end":618},{"cath_id":"-","chopping":"884-1045_1070-1124","consensus_level":"high","plddt":87.1028,"start":884,"end":1124}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9UHC9","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9UHC9-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9UHC9-F1-predicted_aligned_error_v6.png","plddt_mean":84.19},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=NPC1L1","jax_strain_url":"https://www.jax.org/strain/search?query=NPC1L1"},"sequence":{"accession":"Q9UHC9","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9UHC9.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9UHC9/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9UHC9"}},"corpus_meta":[{"pmid":"15928087","id":"PMC_15928087","title":"The 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Labeled ezetimibe glucuronide binds specifically to a single site in brush border membranes and to HEK293 cells expressing NPC1L1; binding affinities to recombinant NPC1L1 are virtually identical to those for native enterocyte membranes; ezetimibe no longer binds membranes from NPC1L1 knockout mice.\",\n      \"method\": \"Radioligand binding assay with labeled ezetimibe glucuronide in brush border membranes, recombinant NPC1L1-expressing cells, and NPC1L1 knockout mouse membranes\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — direct binding assay with recombinant protein, species KD comparisons, and genetic (knockout) control; multiple orthogonal lines of evidence in one rigorous study\",\n      \"pmids\": [\"15928087\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"NPC1L1 is required for intestinal uptake of both cholesterol and phytosterols. NPC1L1 null mice show substantially reduced intestinal uptake of cholesterol and sitosterol, dramatically reduced plasma phytosterol levels, and complete resistance to diet-induced hypercholesterolemia.\",\n      \"method\": \"NPC1L1 knockout mouse model with measurements of intestinal cholesterol and phytosterol uptake, plasma lipoprotein profiles, and gene expression analysis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — clean knockout with multiple defined phenotypic readouts; replicated by other labs\",\n      \"pmids\": [\"15173162\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Cholesterol specifically promotes the internalization of NPC1L1 via clathrin/AP2-mediated endocytosis requiring microfilaments. Ezetimibe prevents NPC1L1 from incorporating into clathrin-coated vesicles, thereby inhibiting cholesterol uptake. Blocking NPC1L1 endocytosis dramatically decreases cholesterol internalization.\",\n      \"method\": \"Live cell imaging, clathrin/AP2 complex perturbation, microfilament disruption, cholesterol uptake assays in cultured cells\",\n      \"journal\": \"Cell metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods (pharmacological inhibitors, dominant negatives, imaging) in a single rigorous study; mechanism later confirmed in vivo\",\n      \"pmids\": [\"18522832\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"At steady state, NPC1L1 localizes predominantly to the transferrin-positive endocytic recycling compartment. Acute cholesterol depletion stimulates relocation of NPC1L1 to the plasma membrane (preferentially to an apical-like subdomain), and this translocation is associated with a marked increase in cellular cholesterol uptake that is inhibited by ezetimibe.\",\n      \"method\": \"Stable hepatoma cell line expressing human NPC1L1; filipin staining; methyl-β-cyclodextrin cholesterol depletion; immunofluorescence; dose-dependent ezetimibe inhibition assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct localization experiment tied to functional cholesterol uptake consequence; multiple methods in one study\",\n      \"pmids\": [\"16407187\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"The N-terminal domain (NTD) of NPC1L1 binds cholesterol. Mutation of residue Leu-216 eliminates cholesterol binding, decreases formation of NPC1L1-flotillin-cholesterol membrane microdomains, and prevents NPC1L1-mediated cholesterol uptake in cultured cells and mouse livers. NPC1L1-NTD specifically binds cholesterol but not plant sterols. 25- or 27-hydroxycholesterol competes with cholesterol for NTD binding and inhibits cholesterol-induced endocytosis of NPC1L1.\",\n      \"method\": \"In vitro cholesterol binding assay, site-directed mutagenesis (L216 mutation), adenovirus-mediated expression in mouse liver, cholesterol uptake assays, competition assays with oxysterols\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro binding reconstitution plus mutagenesis plus in vivo adenoviral validation; multiple orthogonal methods\",\n      \"pmids\": [\"21602275\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"The clathrin adaptor Numb recognizes an endocytic peptide signal YVNXXF in the cytoplasmic C-terminal tail of NPC1L1. Cholesterol binding on the NTD releases this YVNXXF-containing region from association with the plasma membrane, enabling Numb binding. Numb recruits clathrin for internalization. Disrupting the NPC1L1-Numb interaction decreases cholesterol uptake; intestine-specific ablation of Numb in mice significantly reduces dietary cholesterol absorption and plasma cholesterol.\",\n      \"method\": \"Identification of endocytic signal by mutagenesis; Co-IP of NPC1L1 with Numb; intestine-specific Numb knockout mice; cholesterol absorption and plasma cholesterol measurements; intestinal organoid cholesterol uptake assays\",\n      \"journal\": \"Nature medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal Co-IP, mutagenesis of binding motif, in vivo genetic validation in intestine-specific knockout; multiple orthogonal methods\",\n      \"pmids\": [\"24336247\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Transport of NPC1L1 from the endocytic recycling compartment to the plasma membrane requires the microfilament-associated myosin Vb·Rab11a·Rab11-FIP2 triple complex. Inactivation of any component of this complex (by dominant-negative mutants) inhibits NPC1L1 export to the cell surface and decreases cholesterol uptake.\",\n      \"method\": \"Dominant-negative mutant expression of myosin Vb, Rab11a, and Rab11-FIP2; pharmacological microfilament disruption; NPC1L1 trafficking and cholesterol uptake assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — dominant-negative mutants for each complex component with functional cholesterol uptake readout; multiple orthogonal perturbations\",\n      \"pmids\": [\"19542231\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Ezetimibe binds to NPC1L1 at a site within a 61-aa region of extracellular loop C; Phe-532 and Met-543 in loop C are key contributors to high-affinity binding of ezetimibe analogs. The binding site resides in the intestinal lumen. EZE binding to this extracellular site (distinct from where cholesterol binds at loop A/NTD) may prevent conformational changes in NPC1L1 necessary for cholesterol translocation.\",\n      \"method\": \"Proteomic identification of NPC1L1 as ezetimibe-binding protein; chimeric and site-directed mutagenesis studies with radioligand [³H]AS; species (mouse vs. dog) binding affinity comparisons\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — radioligand binding with chimeric and mutagenesis studies identifying specific residues; proteomic confirmation\",\n      \"pmids\": [\"18682566\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Human NPC1L1 contains 13 transmembrane helices with the NH2-terminus in the lumen and the COOH-terminus projecting to the cytosol, with seven small cytoplasmic loops and four small and three large luminal loops. The putative sterol-sensing domain is oriented similarly to those of HMGCR, NPC1, and SCAP.\",\n      \"method\": \"Protease protection assays and immunofluorescence in selectively permeabilized cells expressing human NPC1L1\",\n      \"journal\": \"Journal of lipid research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — topology mapped by protease protection and selective permeabilization; two orthogonal methods in one study\",\n      \"pmids\": [\"19325169\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Cryo-EM structures of NPC1L1 in apo and ezetimibe-bound forms reveal that: (1) the apo (open) state has the NTD loosely associated, leaving the NTD central cavity accessible for cholesterol loading; (2) in the ezetimibe-bound (closed) state, the NTD rotates ~60°, creating a continuous tunnel for cholesterol movement into the plasma membrane; (3) ezetimibe blocks cholesterol transport by occluding the tunnel rather than competing with cholesterol binding.\",\n      \"method\": \"Cryo-electron microscopy structure determination of apo and ezetimibe-bound NPC1L1\",\n      \"journal\": \"Science advances\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — two cryo-EM structures (apo and drug-bound) with mechanistic interpretation; single study but rigorous structural method\",\n      \"pmids\": [\"32596471\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Cryo-EM structures of human NPC1L1 in apo, cholesterol-enriched, and ezetimibe-bound states show that the sterol-sensing domain (SSD) responds to cholesterol level by binding different numbers of cholesterol molecules; increasing cholesterol triggers formation of a stable structural cluster in SSD, while ezetimibe binding deforms the SSD and destroys this cluster, inhibiting NPC1L1 function.\",\n      \"method\": \"Cryo-electron microscopy structure determination of three NPC1L1 states (apo, cholesterol-enriched, ezetimibe-bound)\",\n      \"journal\": \"Science advances\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — three independent cryo-EM structures; independently replicated structural finding from a second group\",\n      \"pmids\": [\"34272236\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Human NPC1L1 exists primarily as a homodimer; dimerization is mediated by aromatic residues within transmembrane helix 2 (including Trp-347) that has a horizontal orientation in the membrane. Mutation of Trp-347 disrupts dimerization and the resulting monomeric NPC1L1 exhibits reduced efficiency of cholesterol uptake. The same intramolecular channel mediates transport of both vitamin E and cholesterol.\",\n      \"method\": \"Cryo-EM structures of hNPC1L1 bound to cholesterol or vitamin E analog; site-directed mutagenesis of Trp-347; functional cholesterol uptake assays of monomeric vs. dimeric NPC1L1\",\n      \"journal\": \"Science advances\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — cryo-EM structure plus mutagenesis plus functional uptake assay; multiple orthogonal methods\",\n      \"pmids\": [\"34407950\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"In mouse small intestine in vivo, NPC1L1 is localized on the brush border of enterocytes in the absence of cholesterol. Dietary cholesterol induces internalization of NPC1L1 to the subapical layer beneath the brush border, partially colocalizing with the endosome marker Rab11. Ezetimibe blocks internalization of NPC1L1 and cholesterol, causing their retention in the plasma membrane.\",\n      \"method\": \"Immunofluorescence and confocal microscopy of mouse small intestine sections after dietary cholesterol feeding with and without ezetimibe treatment\",\n      \"journal\": \"Journal of lipid research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — in vivo localization directly tied to functional mechanism; confirms cell-based findings in intact intestinal tissue\",\n      \"pmids\": [\"22811412\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"The small GTPase Cdc42 interacts with NPC1L1. Cholesterol depletion activates Cdc42 and enhances NPC1L1-Cdc42 interaction. Constitutively active or dominant-negative Cdc42, as well as knockdown of downstream effectors N-WASP or Arp3, inhibits NPC1L1 transport to the plasma membrane. In liver-specific Cdc42 knockout mice, NPC1L1 fails to localize to bile canaliculi and biliary cholesterol cannot be efficiently reabsorbed.\",\n      \"method\": \"Co-IP of NPC1L1 with Cdc42; dominant-negative and constitutively active Cdc42 mutant expression; siRNA knockdown of N-WASP and Arp3; liver-specific Cdc42 knockout mice with biliary cholesterol reabsorption assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP, genetic perturbations (dominant-negative, siRNA, conditional KO), and in vivo functional validation\",\n      \"pmids\": [\"21844200\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"NPC1L1 plays a central role in intestinal vitamin K (VK) absorption. In vitro NPC1L1 overexpression increases VK uptake; Npc1l1 knockout mice show reduced intestinal VK absorption; ezetimibe inhibits VK absorption and reduces hepatic VK levels, enhancing the pharmacological effect of warfarin.\",\n      \"method\": \"NPC1L1-overexpressing intestinal cells (in vitro VK uptake); Npc1l1 knockout mice (in vivo VK absorption); ezetimibe+warfarin co-administration studies in mice; retrospective clinical data analysis\",\n      \"journal\": \"Science translational medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — in vitro overexpression, genetic knockout, and in vivo pharmacological inhibition; independently supported by clinical data\",\n      \"pmids\": [\"25696002\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"The NTD of NPC1L1 is essential for α-tocopherol (vitamin E) transport. α-Tocopherol promotes NPC1L1 endocytosis in an NTD-dependent manner; α-tocopherol competitively binds NPC1L1-NTD with cholesterol. Cells expressing NPC1L1 lacking the NTD (NPC1L1ΔNTD) cannot undergo α-tocopherol-induced endocytosis. Ezetimibe prevents α-tocopherol-induced endocytosis of NPC1L1.\",\n      \"method\": \"NPC1L1-GFP and NPC1L1ΔNTD-GFP stable cell lines; competition binding assay for NTD; live cell imaging of endocytosis; ezetimibe inhibition\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — domain deletion mutant plus competition binding plus functional endocytosis assay; single lab, single paper\",\n      \"pmids\": [\"28315682\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"NPC1L1 localizes in a vesicular subcellular compartment rich in the small GTPase Rab5 in HepG2 cells. NPC1L1 null mouse cells exhibit aberrant plasma membrane uptake and transport of cholesterol and sphingolipids, and a deregulation of caveolin transport and localization.\",\n      \"method\": \"Subcellular fractionation and colocalization with Rab5; NPC1L1 knockout-derived cell lines with lipid transport and caveolin localization assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — fractionation and colocalization plus genetic (knockout) cell lines with functional readouts; single lab\",\n      \"pmids\": [\"15671032\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Native and recombinant rat NPC1L1 is a ~145 kDa, highly glycosylated membrane protein enriched in the brush border membrane of intestinal enterocytes. Recombinant NPC1L1 expressed in CHO cells shows cell surface expression.\",\n      \"method\": \"Sequential detergent extraction of enterocytes, SDS-PAGE/Western blot, glycosylation analysis, expression of Flag-tagged NPC1L1 in CHO cells with cell surface detection\",\n      \"journal\": \"Biochimica et biophysica acta\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct biochemical characterization with multiple methods; single lab\",\n      \"pmids\": [\"15777641\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"NPC1L1 mediates sterol-specific, unidirectional transport of non-esterified (free) cholesterol only when located at the cell surface. NPC1L1 does not affect uptake of esterified cholesterol or β-sitosterol (plant sterol), and does not promote cellular cholesterol efflux. NPC1L1-dependent cholesterol uptake requires adequate intracellular K⁺ but not Ca²⁺, cytoskeleton, or signaling downstream of PKA, PKC, or pertussis toxin-sensitive G-protein-coupled receptors.\",\n      \"method\": \"McArdle-RH7777 rat hepatoma cells stably expressing NPC1L1; sterol uptake assays with free vs. esterified cholesterol and plant sterols; cholesterol efflux assays; pharmacological perturbations of signaling pathways\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple sterol uptake and efflux assays in stable expression system; single lab with pharmacological perturbations\",\n      \"pmids\": [\"17523925\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Kinetic imaging (FRAP) shows that ~42% of NPC1L1 resides in the transferrin-positive endocytic recycling compartment (ERC) at steady state with rapid recovery half-time of ~2.5 min. Acute cholesterol depletion blocks internalization of NPC1L1-EGFP and stimulates recycling from the ERC to the plasma membrane. NPC1L1-EGFP facilitates fluorescent sterol transport from the plasma membrane to the ERC. In polarized hepatoma cells, NPC1L1 localizes almost exclusively to the canalicular membrane. Insulin induces translocation of NPC1L1-containing vesicles from the ERC to the cell membrane.\",\n      \"method\": \"Live imaging, FRAP, NPC1L1-EGFP trafficking assays, fluorescent cholesterol analogs, cholesterol depletion, insulin treatment, cell polarization\",\n      \"journal\": \"Journal of lipid research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — FRAP and live imaging with functional sterol transport readout; single lab, multiple methods\",\n      \"pmids\": [\"18523240\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"HNF4α is a crucial modulator of NPC1L1 transcription. HNF4α knockdown reduces NPC1L1 expression and cholesterol-dependent regulation. Binding sites for HNF4α are located at -209 to -197 and -52 to -40 in the human NPC1L1 promoter. SREBP2 and HNF4α cooperate to regulate NPC1L1 transcription.\",\n      \"method\": \"Reporter gene (luciferase) assays with NPC1L1 promoter constructs, EMSA, siRNA knockdown of HNF4α in intestinal cells\",\n      \"journal\": \"Pharmaceutical research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — EMSA plus reporter assays plus siRNA knockdown; single lab\",\n      \"pmids\": [\"18080173\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"SREBP2 and HNF1α are important regulators of NPC1L1 in human liver. SREBP2 dose-dependently regulates NPC1L1 promoter activity and mRNA; ChIP confirms SREBP2 binding to the NPC1L1 promoter in vivo. HNF1α (not HNF4α) increases NPC1L1 promoter activity and gene expression; a functional HNF1 binding site is identified in the NPC1L1 promoter, confirmed by ChIP.\",\n      \"method\": \"Luciferase reporter assays in HuH7 cells; ChIP for SREBP2 and HNF1α; correlation analysis in human liver biopsies; deletion mutant promoter constructs\",\n      \"journal\": \"Journal of lipid research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — ChIP plus reporter assays; single lab with in vivo (human liver) confirmation\",\n      \"pmids\": [\"20460578\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"PPARα positively regulates human NPC1L1 transcription via direct binding to a PPRE at -846/-834 in the NPC1L1 promoter, confirmed by EMSA with PPARα and RXRα. PPARα-specific knockdown significantly decreases endogenous NPC1L1 mRNA and protein in HepG2 cells. PGC1α stimulates SREBP2/HNF4α- and PPARα/RXRα-mediated activation of the NPC1L1 promoter.\",\n      \"method\": \"Reporter gene assays with PPARα-response element deletion/mutation constructs; EMSA with PPARα and RXRα; siRNA knockdown of PPARα in HepG2 cells\",\n      \"journal\": \"Pharmaceutical research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — EMSA plus reporter assays plus siRNA knockdown; single lab\",\n      \"pmids\": [\"20953676\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Small Heterodimer Partner (SHP/NR0B2) and postprandial FGF19 signaling inhibit intestinal NPC1L1 expression and reduce cholesterol absorption. FGF19 signaling leads to phosphorylation of SHP, which inhibits SREBP2 activity, thereby repressing NPC1L1 transcription. These effects were absent in SHP-knockout and FGF15-knockout mice.\",\n      \"method\": \"SHP-knockout and FGF15-knockout mice; FGF19 injection in mice; luciferase reporter assays in HT29 cells; intestinal organoids; fractional cholesterol absorption measurement; RNA-seq\",\n      \"journal\": \"Gastroenterology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic knockout models plus reporter assays plus functional cholesterol absorption; single lab with multiple models\",\n      \"pmids\": [\"30521806\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"CREBH directly regulates Npc1l1 expression as shown by promoter assays. Intestinal CREBH overexpression in mice reduces NPC1L1 mRNA and protein, decreasing cholesterol absorption; CREBH null mice exhibit higher intestinal NPC1L1 expression and elevated plasma/hepatic cholesterol.\",\n      \"method\": \"CREBH transgenic and knockout mice; NPC1L1 expression analysis; promoter luciferase assays; cholesterol absorption and fecal output measurements\",\n      \"journal\": \"Molecular metabolism\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — promoter assay plus transgenic and knockout genetic models; single lab\",\n      \"pmids\": [\"27818935\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Inter-domain dynamics are required for NPC1L1-mediated cholesterol transport. Introduction of single disulfide bonds to constrain lumenal/extracellular domains or shortening a cytoplasmic loop abolishes NPC1L1 transport activity. Ezetimibe's binding site lies at the interface between NPC1L1's three extracellular domains, blocking transport by binding to multiple domains simultaneously.\",\n      \"method\": \"Single disulfide bond introduction into NPC1L1; lysosomal cholesterol efflux assay; cytoplasmic loop shortening mutants; ezetimibe binding site mapping by disulfide crosslinking\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — engineered disulfide bonds with functional transport assay; mechanistic insight into domain dynamics; single study\",\n      \"pmids\": [\"32410728\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"NPC1L1-dependent intestinal cholesterol absorption requires ganglioside GM3 in membrane microdomains. GM3 synthase (GM3S)-deficient cells show impaired NPC1L1-dependent cholesterol uptake. GM3S deficiency in mice promotes resistance to diet-induced hypercholesterolemia.\",\n      \"method\": \"GM3S-deficient cell model; NPC1L1-dependent cholesterol uptake assay; wild-type and apoE-deficient mice on normal or high-cholesterol diets\",\n      \"journal\": \"Journal of lipid research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic (cell) and in vivo (mouse) models; single lab with multiple models\",\n      \"pmids\": [\"30242108\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Multiple NPC1L1 non-synonymous variants from cholesterol low absorbers show dysfunction in cholesterol uptake due to impaired recycling, subcellular localization, glycosylation, or stability. Severely dysfunctional variants are rapidly degraded through the ER-associated degradation (ERAD) pathway. In vivo (adenoviral expression in mouse liver), the S881L variant failed to localize to liver canalicular membrane, causing defects in biliary cholesterol re-absorption.\",\n      \"method\": \"Expression of 19 NPC1L1 NS variants in cultured cells; cholesterol uptake assays; subcellular localization; glycosylation analysis; protein stability assay; adenovirus-mediated liver expression in mice\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — systematic variant characterization with multiple assays plus in vivo adenoviral validation; single lab\",\n      \"pmids\": [\"21189420\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"NPC1L1 mediates uptake of vitamin E in drug-tolerant persister (DTP) cancer cells to prevent chemotherapy-triggered oxidative stress. NRF2 transcriptionally regulates NPC1L1 by binding to the -205 to -215 bp site on its promoter; decreased DNA methylation also contributes to NPC1L1 expression in this context.\",\n      \"method\": \"RNA-seq of DTP cancer cells; NPC1L1 inhibition by ezetimibe in MDR cancer cells; ChIP/promoter assay identifying NRF2 binding site; DNA methylation analysis; in vivo tumor model\",\n      \"journal\": \"EMBO molecular medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — ChIP for NRF2, functional assay with ezetimibe, in vivo validation; single lab\",\n      \"pmids\": [\"35023619\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"NPC1L1 is required for phytosterols to enter the body. In mice lacking both ABCG5/G8 and NPC1L1 (triple knockout), phytosterols do not accumulate in plasma and liver, despite ABCG5/G8 deficiency, demonstrating genetic epistasis: NPC1L1 acts upstream of ABCG5/G8 in phytosterol entry.\",\n      \"method\": \"Triple knockout mice (NPC1L1⁻/⁻ × ABCG5/G8⁻/⁻) with plasma phytosterol, hepatic cholesterol, and fecal cholesterol measurements; genetic epistasis analysis\",\n      \"journal\": \"Journal of lipid research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — epistasis demonstrated using triple knockout mouse model; single lab\",\n      \"pmids\": [\"18796403\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Knockdown of NPC1L1 in Caco-2 cells reduces the ability to capture micellar free cholesterol. NPC1L1 suppression results in increased HMG-CoA reductase mRNA and activity, decreased ACAT activity, and increased SREBP expression, and a significant decrease in SR-BI mRNA expression with no changes in ABCA1, ABCG5, or CD36.\",\n      \"method\": \"NPC1L1 knockdown in Caco-2 cells; [¹⁴C]free cholesterol uptake assay; mRNA and activity measurements of HMG-CoA reductase, ACAT; Western blot/RT-PCR for SREBP, SR-BI, other transporters\",\n      \"journal\": \"Journal of lipid research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional knockdown with specific cholesterol uptake readout plus downstream pathway measurements; single lab\",\n      \"pmids\": [\"16829661\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Ezetimibe-sensitive cholesterol uptake by NPC1L1 does not require endocytosis. Cell surface biotinylation shows ezetimibe does not alter the rate of NPC1L1-GFP endocytosis under normal conditions. Endocytosis does not require cholesterol binding to NPC1L1's NTD. Two small-molecule inhibitors of general endocytosis failed to inhibit ezetimibe-sensitive [³H]cholesterol uptake from taurocholate micelles.\",\n      \"method\": \"Cell surface biotinylation assay; pharmacological inhibitors of endocytosis; [³H]cholesterol uptake from micelles in rat hepatocytes; NPC1L1-GFP endocytosis monitoring\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct biotinylation assay plus pharmacological inhibitors; contradicts prior endocytosis model; single lab\",\n      \"pmids\": [\"27075173\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"NPC1L1 is a polytopic 13-transmembrane glycoprotein that functions as a sterol transporter at the apical membrane of intestinal enterocytes and the canalicular membrane of hepatocytes: its N-terminal domain (NTD) senses and binds free cholesterol (but not plant sterols or esterified cholesterol), triggering formation of NPC1L1-flotillin-cholesterol microdomains; the YVNXXF motif in the cytoplasmic C-terminal tail is then released from the membrane and recruits the clathrin adaptor Numb, which in turn recruits clathrin/AP2 to drive clathrin-mediated endocytic internalization of cholesterol; at steady state NPC1L1 cycles between the plasma membrane and the Rab11a-positive endocytic recycling compartment, with export back to the cell surface requiring the myosin Vb·Rab11a·Rab11-FIP2 triple complex and the small GTPase Cdc42; the same intramolecular tunnel mediates transport of cholesterol, vitamin K, and vitamin E (α-tocopherol, which also binds the NTD competitively with cholesterol); ezetimibe binds to an extracellular loop C site (Phe-532/Met-543), and structural studies show it converts NPC1L1 from an open to a closed conformation that occludes the sterol transport tunnel without competing for cholesterol binding; NPC1L1 exists primarily as a homodimer, with dimerization through transmembrane helix 2 required for full transport efficiency; transcription of NPC1L1 is positively regulated by SREBP2, HNF1α, PPARα, and is repressed by SHP/FGF19 signaling, LXR/RXR activation, and cholesterol feeding.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"NPC1L1 is a polytopic, heavily glycosylated 13-transmembrane glycoprotein that serves as the rate-limiting transporter for absorption of dietary free cholesterol at the brush border of intestinal enterocytes and the canalicular membrane of hepatocytes, where its loss abolishes intestinal sterol uptake and confers resistance to diet-induced hypercholesterolemia [#1, #8, #17]. Its N-terminal domain (NTD) selectively binds free cholesterol but not plant sterols, and the residue Leu-216 is required for cholesterol binding, formation of NPC1L1–flotillin–cholesterol membrane microdomains, and downstream uptake [#4]; the transporter handles non-esterified cholesterol unidirectionally and does not mediate efflux or take up esterified or plant sterols [#18]. Cholesterol loading triggers internalization of NPC1L1: binding at the NTD releases a YVNXXF motif in the cytoplasmic C-terminal tail from the membrane, allowing the clathrin adaptor Numb to engage it and recruit clathrin/AP2 for endocytosis, a step required for cholesterol uptake in vivo [#2, #5, #12]. The transporter cycles between the plasma membrane and a transferrin/Rab11-positive endocytic recycling compartment, with return to the cell surface driven by the myosin Vb·Rab11a·Rab11-FIP2 complex and by Cdc42 acting through N-WASP/Arp3 [#3, #6, #13, #19]. Structurally, NPC1L1 functions as a homodimer assembled through transmembrane helix 2 (Trp-347), and cryo-EM of apo, cholesterol-bound, and ezetimibe-bound states shows that the sterol-sensing domain binds variable numbers of cholesterol molecules and that NTD rotation opens a continuous tunnel for sterol movement [#9, #10, #11]. The same intramolecular tunnel also transports vitamin K and α-tocopherol, the latter competing with cholesterol for the NTD [#11, #14, #15]. Ezetimibe, the direct molecular target whose binding is lost in NPC1L1-null membranes, engages an extracellular loop C site (Phe-532/Met-543) at the interface of the extracellular domains and locks the transporter in a closed conformation that occludes the tunnel without competing for cholesterol binding [#0, #7, #9, #25]. Transcription is driven by SREBP2 together with HNF1α, HNF4α, and PPARα, and is repressed by SHP/FGF19 signaling and CREBH [#20, #21, #22, #23, #24].\",\n  \"teleology\": [\n    {\n      \"year\": 2004,\n      \"claim\": \"Established NPC1L1 as the genetically required gatekeeper of intestinal sterol entry, resolving which protein controls dietary cholesterol and phytosterol absorption.\",\n      \"evidence\": \"NPC1L1 knockout mice with intestinal cholesterol/sitosterol uptake and plasma lipid readouts\",\n      \"pmids\": [\"15173162\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not establish the molecular transport mechanism or whether NPC1L1 binds sterol directly\", \"Tissue of action (intestine vs liver) not yet dissected\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Identified NPC1L1 as the direct molecular target of ezetimibe, explaining the drug's mechanism of action at the protein level.\",\n      \"evidence\": \"Radioligand binding of ezetimibe glucuronide to brush border, recombinant NPC1L1, and knockout mouse membranes\",\n      \"pmids\": [\"15928087\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Binding site on NPC1L1 not yet mapped\", \"Did not show how drug binding inhibits transport\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Defined NPC1L1 as a cycling protein resident in the endocytic recycling compartment whose surface delivery is cholesterol-responsive and drives uptake.\",\n      \"evidence\": \"Stable hepatoma cells, filipin staining, cyclodextrin cholesterol depletion, immunofluorescence with ezetimibe inhibition; biochemical glycosylation/topology characterization\",\n      \"pmids\": [\"16407187\", \"15777641\", \"15671032\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Machinery driving recycling and internalization unknown\", \"Direction of sterol movement (uptake vs sensing) not yet settled\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Showed NPC1L1 transport is sterol-selective, unidirectional, surface-restricted, and substrate-specific for free cholesterol, defining its functional logic.\",\n      \"evidence\": \"Stable rat hepatoma cells with sterol uptake/efflux assays and pharmacological pathway perturbations\",\n      \"pmids\": [\"17523925\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab characterization\", \"Did not identify the sterol-binding region\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Mapped the ezetimibe binding site to extracellular loop C residues distinct from the cholesterol site, and demonstrated cholesterol drives clathrin/AP2-dependent endocytosis that ezetimibe blocks.\",\n      \"evidence\": \"Chimeric/site-directed mutagenesis with radioligand binding; live imaging with clathrin/AP2 and microfilament perturbation; genetic epistasis with ABCG5/G8 triple knockout mice\",\n      \"pmids\": [\"18682566\", \"18522832\", \"18796403\", \"18523240\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Adaptor linking cholesterol sensing to clathrin not yet identified\", \"How drug binding at loop C blocks transport mechanistically unresolved\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Solved the membrane topology and identified the recycling machinery, placing NPC1L1's sterol-sensing domain in a defined orientation and showing surface delivery requires a myosin Vb·Rab11a·Rab11-FIP2 motor complex.\",\n      \"evidence\": \"Protease protection/selective permeabilization for topology; dominant-negative perturbation of myosin Vb/Rab11a/Rab11-FIP2 with uptake readouts\",\n      \"pmids\": [\"19325169\", \"19542231\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Trigger coupling cholesterol depletion to motor engagement unknown\", \"No atomic structure yet\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Localized cholesterol binding to the NTD via Leu-216 and identified Cdc42/N-WASP/Arp3 as the actin-nucleating arm of NPC1L1 recycling, including in hepatocyte canalicular function.\",\n      \"evidence\": \"In vitro binding plus L216 mutagenesis with in vivo adenoviral liver expression; Co-IP, dominant-negative/siRNA, and liver-specific Cdc42 knockout mice; systematic patient variant characterization\",\n      \"pmids\": [\"21602275\", \"21844200\", \"21189420\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of NTD cholesterol recognition not resolved\", \"How oxysterols compete at the NTD structurally unknown\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Identified Numb as the adaptor that couples NTD cholesterol binding to clathrin internalization via the YVNXXF tail motif, completing the endocytic recruitment pathway.\",\n      \"evidence\": \"Endocytic-signal mutagenesis, reciprocal Co-IP, intestine-specific Numb knockout mice, organoid uptake assays\",\n      \"pmids\": [\"24336247\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural detail of the YVNXXF–Numb interface unknown\", \"Whether Numb is the sole adaptor not addressed\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Extended NPC1L1 substrate range to vitamin K, linking the transporter to anticoagulant pharmacology.\",\n      \"evidence\": \"Overexpression uptake, Npc1l1 knockout mice, ezetimibe+warfarin co-dosing, clinical data\",\n      \"pmids\": [\"25696002\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether vitamin K uses the same tunnel as cholesterol not yet structurally shown\", \"Binding site for vitamin K undefined\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Showed α-tocopherol (vitamin E) is an NTD-dependent NPC1L1 substrate that competes with cholesterol and triggers the same endocytic cycle.\",\n      \"evidence\": \"NPC1L1ΔNTD cell lines, NTD competition binding, live-cell endocytosis imaging with ezetimibe inhibition\",\n      \"pmids\": [\"28315682\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab finding\", \"Physiological contribution of NPC1L1 to vitamin E status in vivo not addressed here\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Provided the structural mechanism: cryo-EM and engineered disulfides showed NTD rotation opens a transport tunnel and that ezetimibe occludes the tunnel at the extracellular-domain interface rather than competing for cholesterol.\",\n      \"evidence\": \"Cryo-EM of apo and ezetimibe-bound NPC1L1; engineered disulfide constraints with lysosomal cholesterol efflux assays\",\n      \"pmids\": [\"32596471\", \"32410728\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Cholesterol-bound intermediate not resolved in this study\", \"How the tunnel connects to the membrane-to-cytoplasm uptake step unclear\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Refined the structural model with cholesterol-bound states and established that NPC1L1 functions as a TM2-mediated homodimer using a single tunnel shared by cholesterol and vitamin E.\",\n      \"evidence\": \"Multiple cryo-EM structures (apo, cholesterol-enriched, ezetimibe-bound), Trp-347 dimerization mutagenesis, functional uptake assays\",\n      \"pmids\": [\"34272236\", \"34407950\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional role of dimerization in vivo not tested\", \"Path of sterol from tunnel into the cell still incomplete\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Extended NPC1L1 biology beyond classical sterol absorption by showing NRF2-driven NPC1L1 expression mediates vitamin E uptake to protect drug-tolerant cancer cells from oxidative stress.\",\n      \"evidence\": \"RNA-seq, ezetimibe inhibition in MDR cells, NRF2 ChIP/promoter mapping, DNA methylation analysis, in vivo tumor model\",\n      \"pmids\": [\"35023619\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab finding\", \"Generality across cancer types not established\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Whether NPC1L1-mediated sterol uptake is fundamentally endocytosis-dependent or can proceed at the cell surface remains unresolved, as does how the transport tunnel delivers sterol into the cell.\",\n      \"evidence\": \"Conflicting reports: biotinylation and endocytosis-inhibitor data argue uptake does not require endocytosis (PMID 27075173), against the clathrin/Numb internalization model\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Endocytosis-dependent vs surface-transport models not reconciled\", \"Mechanism coupling tunnel opening to net cellular cholesterol gain undefined\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0008289\", \"supporting_discovery_ids\": [4, 15, 18]},\n      {\"term_id\": \"GO:0005215\", \"supporting_discovery_ids\": [1, 11, 14, 18]},\n      {\"term_id\": \"GO:0140299\", \"supporting_discovery_ids\": [4, 10]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [3, 12, 17, 19]},\n      {\"term_id\": \"GO:0005768\", \"supporting_discovery_ids\": [3, 12, 16, 19]},\n      {\"term_id\": \"GO:0031410\", \"supporting_discovery_ids\": [2, 6, 19]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-382551\", \"supporting_discovery_ids\": [1, 18, 29]},\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [2, 5, 6]},\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [1, 30]}\n    ],\n    \"complexes\": [\n      \"NPC1L1 homodimer\",\n      \"NPC1L1-flotillin-cholesterol microdomain\"\n    ],\n    \"partners\": [\n      \"Numb\",\n      \"Cdc42\",\n      \"MYO5B\",\n      \"RAB11A\",\n      \"RAB11FIP2\",\n      \"flotillin\"\n    ],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":8,"faith_total":8,"faith_pct":100.0}}