{"gene":"SFRP2","run_date":"2026-04-28T20:42:07","timeline":{"discoveries":[{"year":1998,"finding":"The cysteine-rich domain of sFRP-2 directly binds to Wnt-4, as demonstrated by co-immunoprecipitation, and sFRP-2 expression is regulated downstream of Wnt-4 signaling in the developing metanephric kidney.","method":"Co-immunoprecipitation, in situ hybridization, genetic loss-of-function (Wnt-4 knockout mice)","journal":"Developmental dynamics","confidence":"High","confidence_rationale":"Tier 1-2 — direct binding shown by Co-IP, corroborated by genetic epistasis (sFRP-2 absent in Wnt-4 KO kidney)","pmids":["9853965"],"is_preprint":false},{"year":2000,"finding":"Sfrp-2 functions as a Wnt antagonist in vivo, as demonstrated by overexpression in Xenopus embryos; SFRP2-expressing cells reduce dermomyotome-inducing activity of WNT1 and WNT4 (but not WNT3a) in somitic explants, and SHH-N upregulates Sfrp2 to compete with WNT signaling in somitic mesoderm.","method":"Xenopus overexpression in vivo assay, chick somite explant cultures, recombinant protein treatment","journal":"Development (Cambridge, England) / Developmental biology","confidence":"High","confidence_rationale":"Tier 2 — in vivo gain-of-function in two independent model organisms with functional readouts, replicated across papers","pmids":["10656762","10654605"],"is_preprint":false},{"year":2006,"finding":"Purified Sfrp-1 and Sfrp-2 inhibit Wnt-3a-induced β-catenin accumulation in L cells via a GSK-3-dependent mechanism (upstream of GSK-3), and Sfrp-2 inhibits Wnt-3a-stimulated proliferation in the chick neural tube.","method":"Cell-based β-catenin accumulation assay with purified proteins, LiCl epistasis experiment, chick neural tube overexpression","journal":"Developmental dynamics","confidence":"High","confidence_rationale":"Tier 1-2 — purified recombinant proteins, epistasis with LiCl, in vivo corroboration","pmids":["16425220"],"is_preprint":false},{"year":2007,"finding":"SFRP2 is the key paracrine factor released by Akt-modified mesenchymal stem cells that mediates myocardial survival after ischemic injury; treatment of cardiomyocytes with SFRP2 increases β-catenin and upregulates anti-apoptotic genes.","method":"Functional genomic screen, recombinant protein treatment, cardiomyocyte survival assay, rat MI model","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (genomic screen, loss-of-function, in vivo MI model), highly cited foundational paper","pmids":["17251350"],"is_preprint":false},{"year":2007,"finding":"SFRP2 inhibits cardiomyogenic differentiation by blocking a positive transcriptional autofeedback loop of Wnt3a; SFRP2 inhibits Wnt3a-induced Wnt3a transcription via suppression of canonical Wnt/TCF4 signaling, maintaining stem cells in an undifferentiated state.","method":"Gain- and loss-of-function (recombinant SFRP2, Sfrp2 siRNA), luciferase reporter assay, embryonal carcinoma stem cell differentiation assay","journal":"Stem cells (Dayton, Ohio)","confidence":"High","confidence_rationale":"Tier 2 — reciprocal gain/loss-of-function, reporter assays, multiple orthogonal methods","pmids":["17916803"],"is_preprint":false},{"year":2005,"finding":"Myeloma cell-derived sFRP-2 suppresses osteoblast differentiation induced by BMP-2 and inhibits in vitro mineralization; immunodepletion of sFRP-2 from conditioned medium significantly restores mineralized nodule formation, establishing sFRP-2 as an inhibitor of canonical Wnt signaling-dependent bone formation.","method":"Conditioned medium experiments, alkaline phosphatase activity assay, in vitro mineralization assay, immunodepletion of sFRP-2","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 — immunodepletion directly links sFRP-2 to bone formation suppression, multiple functional readouts","pmids":["16030194"],"is_preprint":false},{"year":2008,"finding":"sFRP2 modulates MSC self-renewal and engraftment by downregulating canonical Wnt signaling; shRNA knockdown of sFRP2 in MRL-MSCs decreased their proliferation and engraftment, while overexpression of sFRP2 in WT-MSCs recapitulated the enhanced regenerative MRL phenotype in vivo.","method":"shRNA knockdown, retroviral overexpression, in vivo engraftment and MI models, genomic analysis","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 — reciprocal loss- and gain-of-function in vivo, multiple functional readouts","pmids":["19017790"],"is_preprint":false},{"year":2006,"finding":"Sfrp1 and Sfrp2 double knockout mice show shortened thoracic region, incomplete somite segmentation, and altered Notch signaling oscillations (Lfng and Hes7), demonstrating functional redundancy and requirement for Wnt regulation by Sfrp1/Sfrp2 in anteroposterior axis elongation and somitogenesis.","method":"Double knockout mouse genetics, in situ hybridization for Notch targets","journal":"Development (Cambridge, England)","confidence":"High","confidence_rationale":"Tier 2 — clean double KO with defined molecular phenotype (Notch oscillation defects)","pmids":["16467359"],"is_preprint":false},{"year":2008,"finding":"Sfrp1, Sfrp2, and Sfrp5 regulate both canonical Wnt/β-catenin and the planar cell polarity (PCP) non-canonical Wnt pathway during early trunk formation; genetic analysis combining Sfrp triple mutants with Loop-tail mice revealed Sfrp involvement in convergent extension via PCP regulation.","method":"Triple knockout mouse genetics, genetic epistasis with Loop-tail (Vangl2) mutants and Dkk1-deficient mice","journal":"Genesis (New York, N.Y. : 2000)","confidence":"High","confidence_rationale":"Tier 2 — classical epistasis in multiple genetic backgrounds, pathway assignment to PCP vs canonical Wnt","pmids":["18257070"],"is_preprint":false},{"year":2010,"finding":"Sfrp2 specifically inhibits BMP1/Tolloid metalloproteinase activity (but not sFRP1 or sFRP3), thereby inhibiting type I procollagen processing; exogenous Sfrp2 inhibits mature collagen deposition in cardiac fibroblasts and in rat infarcted myocardium, reducing fibrosis and improving cardiac function.","method":"Fluorogenic peptide-based procollagen C-proteinase assay, in vitro procollagen processing assay with recombinant proteins, primary cardiac fibroblast culture, rat MI injection model, echocardiography, trichrome staining","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 — in vitro enzymatic assay with recombinant proteins showing direct Bmp1 inhibition, corroborated by in vivo functional outcome","pmids":["21078975"],"is_preprint":false},{"year":2010,"finding":"sFRP2 inhibits both Wnt and BMP signaling pathways in MSCs, with BMP inhibition (measured by pSMAD 1/5/8) occurring independently of its effects on the Wnt pathway; this dual inhibition suppresses osteogenic and chondrogenic differentiation and promotes MSC self-renewal.","method":"Western blot for pSMAD 1/5/8, osteogenic/chondrogenic differentiation assays, apoptosis assays, in vivo engraftment","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — mechanistic dissection of two independent pathway inhibitions with multiple orthogonal assays","pmids":["20826809"],"is_preprint":false},{"year":2010,"finding":"sFRP2 enhances canonical Wnt3a signaling rather than inhibiting it, promoting LRP6 phosphorylation and β-catenin nuclear translocation in HEK293A cells; this enhancement is abolished by DKK1.","method":"TOPFlash/FOPFlash luciferase reporter assay, Western blot for LRP6 phosphorylation and β-catenin, Wnt pathway qPCR array, recombinant protein treatment of HEK293A cells","journal":"Biochemical and biophysical research communications","confidence":"High","confidence_rationale":"Tier 1-2 — in vitro reconstitution with recombinant proteins, DKK1 epistasis, reporter assays, multiple readouts","pmids":["20723538"],"is_preprint":false},{"year":2008,"finding":"sFRP2 (TRα1 target gene) activates Wnt/β-catenin signaling via Frizzled receptors in the intestinal epithelium, stabilizing β-catenin, activating its target genes, and enhancing cell proliferation; TRα1 directly regulates sFRP2 transcription.","method":"Laser microdissected crypt transcriptomics, in vitro and in vivo TRα1 regulation analysis, β-catenin reporter assays","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 — direct TRα1 regulation validated in vivo and in vitro, Wnt agonist activity established; single lab","pmids":["19001373"],"is_preprint":false},{"year":2016,"finding":"Aged fibroblast-secreted sFRP2 activates a signaling cascade in melanoma cells that decreases β-catenin and MITF, leading to loss of the redox effector APE1, rendering cells resistant to ROS-induced DNA damage and vemurafenib; sFRP2 also augments angiogenesis and metastasis.","method":"Co-culture systems, western blot, siRNA knockdown, in vivo mouse models of metastasis, aged vs. young fibroblast comparisons","journal":"Nature","confidence":"High","confidence_rationale":"Tier 2 — multi-step mechanistic cascade validated with multiple orthogonal methods in vitro and in vivo, Nature-level rigor","pmids":["27042933"],"is_preprint":false},{"year":2016,"finding":"SFRP2 is produced by genotoxically-treated fibroblasts via NF-κB transcriptional control and augments WNT16B-initiated β-catenin signaling in cancer cells by enhancing WNT16B binding to FZD3/4/6 receptors, cooperating with co-receptor LRP6; this promotes cancer cell survival and acquired therapy resistance.","method":"Conditioned medium experiments, co-culture, siRNA knockdown, neutralizing antibody experiments, luciferase reporter assays","journal":"Oncogene","confidence":"Medium","confidence_rationale":"Tier 2 — pathway placement via epistasis (DKK1 abrogation, LRP6 cooperation), receptor binding assays; single lab","pmids":["26751775"],"is_preprint":false},{"year":2011,"finding":"SFRP2 stimulates angiogenesis via calcineurin/NFATc3 signaling in endothelial cells; NFATc3 shRNA knockdown abrogates SFRP2-induced endothelial tube formation, and tacrolimus (calcineurin inhibitor) blocks SFRP2-induced angiogenesis.","method":"Endothelial tube formation assay, NFATc3 shRNA knockdown, tacrolimus treatment, in vivo MMTV-neu tumor model","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 — shRNA epistasis places NFATc3 downstream of SFRP2 with clear functional readout, corroborated in vivo","pmids":["21673995"],"is_preprint":false},{"year":2016,"finding":"sFRP2 activates canonical Wnt/β-catenin signaling in adult mouse cardiac fibroblasts (evidenced by Axin2 and Wnt3a upregulation and nuclear β-catenin accumulation), induces cell proliferation coupled to aerobic glycolysis (Warburg effect), and upregulates MMP1, MMP13, MMP2, and MMP9 activities, reducing steady-state collagen.","method":"Recombinant sFRP2 treatment of primary cardiac fibroblasts, qPCR for Wnt targets, nuclear β-catenin Western blot, MMP activity assays, glucose/lactate measurements","journal":"American journal of physiology. Cell physiology","confidence":"Medium","confidence_rationale":"Tier 2 — multiple orthogonal assays in primary cells; single lab","pmids":["27605451"],"is_preprint":false},{"year":2016,"finding":"sFRP2 redirects non-canonical Wnt signaling from Frizzled 7 (Fz7) to the Ror2 receptor during Xenopus gastrulation; sFRP2 stabilizes Wnt5a-Ror2 complexes at the membrane while inhibiting Fz7 receptor endocytosis, functioning as a molecular switch between non-canonical Wnt pathways. The cysteine-rich domain of sFRP2 is sufficient for Ror2 activation.","method":"Xenopus gastrulation assays, co-immunoprecipitation, receptor endocytosis assays, domain deletion experiments, epistasis with Fz7 and Ror2","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1-2 — molecular mechanism (CRD domain sufficiency, receptor complex stabilization) demonstrated with multiple biochemical and cell biological methods","pmids":["27129770"],"is_preprint":false},{"year":2012,"finding":"sFRP1 and sFRP2 activate the Wnt/planar cell polarity (PCP)/Rac1 pathway in dopamine neurons; compound sFRP1-/-;sFRP2-/- mice display a Wnt/PCP phenotype (anteroposterior VM shortening, lateral Shh domain expansion) similar to Wnt5a-/- mice, and low/medium concentrations of sFRP1 or sFRP2 promote dopamine differentiation mimicking Wnt5a.","method":"Double knockout mouse genetics, in vitro primary VM cultures, mouse ESC differentiation assays, Rac1 pathway activation assays","journal":"Stem cells (Dayton, Ohio)","confidence":"High","confidence_rationale":"Tier 2 — genetic epistasis with Wnt5a KO and in vitro pathway activation, pathway assignment to PCP/Rac1","pmids":["22290867"],"is_preprint":false},{"year":2015,"finding":"Sfrp2 binds Wnt6 and inhibits canonical Wnt6 signaling, which is required for inhibition of cardiac progenitor cell (CPC) proliferation; this inhibition activates the non-canonical Wnt/PCP pathway through JNK, inducing cardiac transcription factors and CPC differentiation.","method":"Co-immunoprecipitation (Sfrp2-Wnt6 binding), luciferase reporter assays for canonical Wnt, JNK activity assays, CPC differentiation assays, ischemia-reperfusion model","journal":"Journal of molecular and cellular cardiology","confidence":"Medium","confidence_rationale":"Tier 2 — direct binding shown by Co-IP, pathway assignment via reporters and JNK assays; single lab","pmids":["26071893"],"is_preprint":false},{"year":2015,"finding":"sFRP2 induces tissue-nonspecific alkaline phosphatase (TNAP) expression and activity specifically in cardiac fibroblasts via a protein synthesis-dependent mechanism, suggesting sFRP2 promotes cardiac fibrocalcification through coordinate activation of tolloid-like metalloproteinases and TNAP.","method":"Biochemical TNAP activity assay, recombinant sFRP2 treatment of primary cardiac fibroblasts, cycloheximide/tunicamycin inhibition, in vivo hamster heart failure model with sFRP2 blockade","journal":"American journal of physiology. Cell physiology","confidence":"Medium","confidence_rationale":"Tier 2 — in vitro mechanistic dissection with pharmacological inhibitors, corroborated in vivo; single lab","pmids":["25972450"],"is_preprint":false},{"year":2019,"finding":"LncRNA Safe stabilizes Sfrp2 mRNA by forming a Safe-Sfrp2 RNA duplex that is bound by the RNA-binding protein HuR; HuR knockdown reduces both Safe and Sfrp2 mRNA stability, while Safe and Sfrp2 mutually stabilize each other, driving cardiac fibrosis.","method":"RNA electrophoretic mobility shift assay (REMSA), RNA immunoprecipitation (RIP), dual-luciferase assay, shRNA knockdown, TGF-β fibrosis model","journal":"Theranostics","confidence":"High","confidence_rationale":"Tier 1-2 — direct RNA-protein interaction demonstrated by REMSA and RIP, functional epistasis confirmed by rescue experiments","pmids":["31695768"],"is_preprint":false},{"year":2020,"finding":"SFRP2 mediates the interaction between indolent breast cancer cells and alveolar type 1 epithelial cells in the lung; Sfrp2 is induced in breast cancer cells by lung epithelial signals, promotes fibronectin fibril formation and integrin-dependent pro-survival signaling, and blockade of Sfrp2 reduces indolent disease burden.","method":"In vivo RNA sequencing combined with drop-out screening, co-culture experiments, in vivo mouse models of indolent metastasis, fibronectin fibril imaging","journal":"Nature cell biology","confidence":"High","confidence_rationale":"Tier 2 — combined in vivo RNAseq/drop-out screen identifies SFRP2 mechanistically, functional validation by blockade in vivo","pmids":["32094692"],"is_preprint":false},{"year":2024,"finding":"In IPF, fibroblast TGF-β1 signaling induces sFRP2 expression near AEC2s; sFRP2 then activates a non-canonical Wnt pathway through Frizzled 5 (Fzd5) and downstream calcineurin/NFATc3 signaling, driving nuclear NFATc3 accumulation and KRT5 basal cell program activation (epithelial metaplasia).","method":"AEC2-fibroblast coculture organoids, precision-cut lung slices, single-cell RNA-Seq, calcineurin inhibitor experiments, Fzd5 expression analysis, recombinant sFRP2 treatment","journal":"The Journal of clinical investigation","confidence":"High","confidence_rationale":"Tier 2 — mechanistic pathway identified by multiple orthogonal methods (organoids, PCLS, scRNA-seq, pharmacological pathway inhibition)","pmids":["38980870"],"is_preprint":false},{"year":2008,"finding":"Sfrp2 inactivation in mice causes brachydactyly due to decreased chondrocyte proliferation and delayed differentiation in distal limb elements; Sfrp2 can repress canonical Wnt signaling by Wnt1, Wnt9a, and Wnt4 in vitro, and Sfrp2-/- mice show mild increases in β-catenin and TOPGAL reporter activity in phalangeal elements.","method":"Sfrp2 knockout mouse, histological analysis, TOPGAL reporter mouse cross, in vitro Wnt reporter assays","journal":"Journal of cellular physiology","confidence":"High","confidence_rationale":"Tier 2 — KO with defined cellular phenotype, in vitro pathway assays, and TOPGAL reporter in vivo","pmids":["18446812"],"is_preprint":false},{"year":2008,"finding":"Sfrp1 and Sfrp2 double knockout mice exhibit defects in gonad morphology, reproductive tract maturation, and gonad positioning in males (failed gubernaculum development); similarities with Looptail and Wnt5a mutants suggest disrupted non-canonical Wnt signaling causes these defects.","method":"Double knockout mouse genetics, morphological and histological analysis, Insl3/Lgr8 expression analysis","journal":"Developmental biology","confidence":"Medium","confidence_rationale":"Tier 2 — KO with defined phenotype and pathway placement by comparison to PCP mutants; pathway assignment indirect","pmids":["19100252"],"is_preprint":false},{"year":2008,"finding":"Sfrp1 and Sfrp2 inhibit myoblast terminal differentiation; recombinant Sfrp1 or Sfrp2 added during proliferation phase inhibit myotube formation without affecting cell cycle or apoptosis, and removal restores differentiation.","method":"Recombinant protein treatment of C2C12 and primary satellite cell cultures, myotube formation assay, cell cycle analysis, apoptosis assay","journal":"Cell and tissue research","confidence":"Medium","confidence_rationale":"Tier 2 — clean functional assay with recombinant proteins, timing-dependent effect established; single lab","pmids":["18322704"],"is_preprint":false},{"year":2016,"finding":"sFRP2 stimulates melanogenesis through MITF and/or tyrosinase upregulation via β-catenin signaling in normal human melanocytes; this was confirmed in fibroblast-melanocyte co-culture and ex vivo skin.","method":"Lentiviral SFRP2 overexpression and shSFRP2 knockdown in primary melanocytes, melanogenesis assays, co-culture, ex vivo skin experiments","journal":"The Journal of investigative dermatology","confidence":"Medium","confidence_rationale":"Tier 2 — reciprocal gain/loss-of-function in primary cells and ex vivo model; single lab","pmids":["26763443"],"is_preprint":false},{"year":2021,"finding":"SFRP2 exerts cardioprotective effects in diabetic cardiomyopathy by improving mitochondrial dynamics and biogenesis through an AMPK/PGC1-α-dependent pathway; SFRP2 overexpression activates p-AMPK and reverses glucolipotoxicity-induced mitochondrial dysfunction and apoptosis.","method":"SFRP2 overexpression/knockdown in H9C2 cells, AAV-mediated overexpression in rat T2DM model, AMPK inhibitor experiments, MitoTracker, TUNEL assay","journal":"Oxidative medicine and cellular longevity","confidence":"Medium","confidence_rationale":"Tier 2 — reciprocal gain/loss-of-function in vitro and in vivo, pharmacological AMPK inhibition confirms pathway; single lab","pmids":["34790288"],"is_preprint":false},{"year":2021,"finding":"Sfrp2 deficiency in mice reduces skeletal stem cell self-renewal, osteogenic differentiation, and cortical bone defect repair; Sfrp2-deficient BMSCs show reduced activation of Lrp6 and Wnt target genes (Axin2, C-myc, Cyclin D1), and recombinant Sfrp2 restores these activities, indicating Sfrp2 acts as a Wnt agonist in skeletal stem cells.","method":"Sfrp2 knockout mice, colony-forming efficiency assays, ectopic bone formation assay, cortical drill-hole repair model, recombinant Sfrp2 rescue, Western blot for Lrp6 activation and Wnt targets","journal":"Bone research","confidence":"High","confidence_rationale":"Tier 2 — KO with in vivo skeletal phenotype, rescue by recombinant protein, multiple molecular readouts","pmids":["34857734"],"is_preprint":false},{"year":2019,"finding":"DRG neuron-derived sFRP2 promotes M1 macrophage polarization and migration via NF-κB pathway activation; mechanistically, sFRP2 destabilizes the β-catenin/p65 complex, activating NF-κB, as shown by immunoprecipitation.","method":"Lentiviral shRNA knockdown in DRG, in vivo MSU-induced arthritis model, primary macrophage co-culture, immunoprecipitation for β-catenin/p65 complex, Western blot","journal":"Theranostics","confidence":"Medium","confidence_rationale":"Tier 2 — direct protein complex disruption shown by IP, functional consequence validated in vivo; single lab","pmids":["31281508"],"is_preprint":false},{"year":2003,"finding":"Sfrp2 is a transcriptional target of the Pax2 transcription factor; Pax2 expression activates Sfrp2 in embryonic kidney cells, associated with changes in chromatin structure at the Sfrp2 locus at regions of Pax2 binding.","method":"Pax2-expressing retroviral transformation of embryonic kidney cells, chromatin structure analysis, transcriptional activation assays","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 — chromatin analysis and Pax2 binding corroborates transcriptional regulation; single lab","pmids":["14561758"],"is_preprint":false},{"year":2016,"finding":"SFRP2 promotes an oncogenic program in p53-mutant osteosarcoma in a β-catenin-independent manner, inducing FOXM1 and CYR61 expression; ectopic SFRP2 overexpression in normal osteoblast precursors suppresses osteoblast differentiation and promotes OS phenotypes.","method":"iPSC-derived osteoblast model from LFS patients, SFRP2 overexpression and knockdown, gene expression analysis, in vivo tumor assays","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"Medium","confidence_rationale":"Tier 2 — gain/loss-of-function with defined molecular targets (FOXM1, CYR61) in human iPSC disease model; single lab","pmids":["30385632"],"is_preprint":false},{"year":2016,"finding":"SFRP2 inhibits canonical Wnt signaling in cervical cancer cells by decreasing nuclear β-catenin accumulation, and suppresses EMT by inhibiting SLUG, TWIST, and SNAIL transcription factors while enhancing E-cadherin expression.","method":"Overexpression and shRNA approaches in cervical cancer cell lines, β-catenin localization assays, EMT marker analysis, xenograft model","journal":"Gynecologic oncology","confidence":"Medium","confidence_rationale":"Tier 2 — mechanistic pathway placement (Wnt/β-catenin to EMT factors) with multiple readouts; single lab","pmids":["19095296"],"is_preprint":false},{"year":2013,"finding":"Sfrp1 and Sfrp2 double knockout mice show reduced Wnt/β-catenin signaling in lens epithelial cells (not increased), indicating Sfrp1/Sfrp2 positively regulate Wnt/β-catenin in lens epithelium; DKO lenses are smaller with deficient epithelium.","method":"Sfrp1/Sfrp2 double knockout mice, TCF/Lef reporter (TOPGAL) analysis, lens morphology assessment","journal":"Developmental biology","confidence":"Medium","confidence_rationale":"Tier 2 — clean DKO with reporter readout demonstrating Wnt agonist function; single lab","pmids":["24140542"],"is_preprint":false}],"current_model":"SFRP2 is a secreted cysteine-rich domain-containing protein that functions as a context-dependent modulator of Wnt signaling: it can inhibit canonical Wnt/β-catenin signaling by directly binding Wnt ligands (e.g., Wnt4, Wnt1, Wnt3a, Wnt6) via its CRD domain, block BMP1/Tolloid metalloproteinase activity to reduce collagen maturation, redirect non-canonical Wnt signaling from Frizzled receptors to Ror2 (acting as a molecular switch), or paradoxically act as a Wnt agonist by stabilizing LRP6 phosphorylation and activating β-catenin in certain cellular contexts; downstream, SFRP2 engages calcineurin/NFATc3, AMPK/PGC1-α, NF-κB, and non-canonical PCP/JNK pathways depending on cell type, making it a pleiotropic regulator of cardiac repair, fibrosis, bone formation, stem cell self-renewal, and tumor microenvironment signaling."},"narrative":{"teleology":[{"year":1998,"claim":"Establishing that SFRP2 directly engages Wnt ligands resolved how secreted CRD-containing proteins intercept Wnt signals extracellularly.","evidence":"Co-immunoprecipitation of sFRP-2 with Wnt-4 in kidney cells, corroborated by loss of Sfrp2 expression in Wnt-4 knockout mice","pmids":["9853965"],"confidence":"High","gaps":["Binding affinities and stoichiometry of sFRP2–Wnt complexes not determined","Whether sFRP2 binds Frizzled receptors directly was untested"]},{"year":2000,"claim":"In vivo demonstration that SFRP2 antagonizes canonical Wnt signaling during embryonic patterning established its functional role as a Wnt inhibitor in developmental contexts.","evidence":"Xenopus embryo overexpression and chick somite explant assays showing SFRP2 blocks WNT1/WNT4-dependent dermomyotome induction","pmids":["10656762","10654605"],"confidence":"High","gaps":["Mechanism of Wnt ligand selectivity (blocks Wnt1/Wnt4 but not Wnt3a in somites) unexplained","No structural basis for CRD–Wnt specificity"]},{"year":2006,"claim":"Genetic loss-of-function in mice revealed that SFRP2 (redundantly with SFRP1) is required for proper somitogenesis and anteroposterior axis elongation, linking Wnt modulation to Notch signaling oscillations.","evidence":"Sfrp1/Sfrp2 double knockout mice with shortened thoracic region, abnormal Lfng and Hes7 oscillations","pmids":["16467359","16425220"],"confidence":"High","gaps":["Whether SFRP2 directly modulates Notch pathway components or acts indirectly through Wnt remains unresolved","Individual contribution of Sfrp2 vs. Sfrp1 unclear due to redundancy"]},{"year":2007,"claim":"Identification of SFRP2 as a key paracrine survival factor from MSCs and an inhibitor of cardiomyogenic differentiation revealed its dual role in maintaining stemness while promoting cardiac cell survival after injury.","evidence":"Functional genomic screen of Akt-MSC secretome identifying SFRP2, recombinant SFRP2 activating β-catenin and anti-apoptotic genes in cardiomyocytes; separately, SFRP2 blocks Wnt3a autoregulatory loop in embryonal carcinoma cells","pmids":["17251350","17916803"],"confidence":"High","gaps":["Paradox of SFRP2 as Wnt antagonist yet β-catenin activator in cardiomyocytes not mechanistically resolved","Receptor requirements for SFRP2 pro-survival signaling not identified"]},{"year":2008,"claim":"Expanded genetic studies demonstrated SFRP2 regulates both canonical and non-canonical (PCP) Wnt pathways in vivo and controls limb skeletal patterning, gonadal development, and MSC engraftment.","evidence":"Sfrp1/2/5 triple KO epistasis with Vangl2 (Loop-tail) mutants demonstrating PCP involvement; Sfrp2 KO brachydactyly phenotype; Sfrp2 shRNA/overexpression in MSC engraftment models","pmids":["18257070","18446812","19017790","19100252"],"confidence":"High","gaps":["PCP pathway activation mechanism by a secreted factor not biochemically defined","How SFRP2 differentially engages canonical vs. non-canonical branches unknown"]},{"year":2010,"claim":"Discovery that SFRP2 directly inhibits BMP1/Tolloid metalloproteinase activity independently of Wnt signaling revealed a second, Wnt-independent mechanism for anti-fibrotic action through blockade of procollagen processing.","evidence":"In vitro fluorogenic peptide and procollagen C-proteinase assays with recombinant proteins, confirmed by reduced fibrosis in rat MI model","pmids":["21078975"],"confidence":"High","gaps":["Structural basis of sFRP2–BMP1 interaction unknown","Whether sFRP2 inhibits other metalloproteinases beyond BMP1/Tolloid not tested"]},{"year":2010,"claim":"Demonstration that SFRP2 can enhance rather than inhibit canonical Wnt3a signaling through LRP6 phosphorylation resolved the long-standing paradox of its context-dependent agonist activity.","evidence":"TOPFlash reporter, LRP6 phosphorylation and β-catenin nuclear translocation assays in HEK293A cells with recombinant SFRP2, abolished by DKK1","pmids":["20723538"],"confidence":"High","gaps":["Concentration dependence and cell-type determinants of agonist vs. antagonist function not systematically defined","Whether SFRP2 directly binds LRP6 or acts indirectly unclear"]},{"year":2011,"claim":"Identification of calcineurin/NFATc3 as a downstream effector of SFRP2 in endothelial cells established a non-canonical signaling cascade mediating SFRP2-driven angiogenesis.","evidence":"NFATc3 shRNA and tacrolimus abolish SFRP2-induced endothelial tube formation in vitro and tumor angiogenesis in vivo","pmids":["21673995"],"confidence":"Medium","gaps":["Receptor through which SFRP2 activates calcineurin in endothelial cells not identified at this stage","Relationship between NFATc3 signaling and canonical Wnt modulation by SFRP2 not clarified"]},{"year":2016,"claim":"A cluster of studies in 2016 established that SFRP2 functions as a molecular switch redirecting Wnt signaling between receptors (Fz7→Ror2), drives age-dependent melanoma therapy resistance via β-catenin/MITF/APE1 suppression, and cooperates with WNT16B to promote cancer cell survival through FZD3/4/6 and LRP6.","evidence":"Xenopus Ror2/Fz7 endocytosis and Co-IP assays for receptor switching; aged fibroblast co-culture melanoma models with siRNA; WNT16B-SFRP2 cooperation via DKK1 epistasis and luciferase reporters","pmids":["27129770","27042933","26751775"],"confidence":"High","gaps":["Whether CRD-mediated receptor switching occurs in mammalian systems not shown","How SFRP2 simultaneously suppresses β-catenin in melanoma but cooperates with WNT16B to activate it in other cancers not reconciled"]},{"year":2019,"claim":"Post-transcriptional regulation of Sfrp2 mRNA by the lncRNA Safe and RNA-binding protein HuR established that SFRP2 expression itself is tightly controlled at the mRNA stability level during fibrosis.","evidence":"REMSA and RIP demonstrating Safe–Sfrp2 RNA duplex bound by HuR; mutual stabilization confirmed by knockdown/rescue in TGF-β fibrosis model","pmids":["31695768"],"confidence":"High","gaps":["Whether Safe-mediated regulation operates in tissues beyond cardiac fibroblasts unknown","Other post-transcriptional regulators of Sfrp2 not surveyed"]},{"year":2021,"claim":"Demonstration that Sfrp2 knockout reduces skeletal stem cell self-renewal and Lrp6 activation, rescued by recombinant Sfrp2, confirmed its Wnt agonist function in bone biology and established that cardioprotection involves the AMPK/PGC1-α axis.","evidence":"Sfrp2 KO mice with impaired bone repair and reduced Wnt target gene expression, rescued by recombinant Sfrp2; SFRP2 overexpression in diabetic cardiomyopathy model activates AMPK, reversed by AMPK inhibitor","pmids":["34857734","34790288"],"confidence":"High","gaps":["How SFRP2 activates AMPK mechanistically is undefined","Whether Wnt agonist function in bone and AMPK activation in heart share a common upstream receptor is unknown"]},{"year":2024,"claim":"In IPF, SFRP2 secreted by fibroblasts drives alveolar epithelial metaplasia through a defined Fzd5→calcineurin→NFATc3 non-canonical Wnt cascade, unifying prior calcineurin/NFATc3 observations with a specific receptor.","evidence":"AEC2-fibroblast coculture organoids, precision-cut lung slices, scRNA-Seq, calcineurin inhibitor experiments identifying Fzd5 as the receptor","pmids":["38980870"],"confidence":"High","gaps":["Whether Fzd5 is the receptor for SFRP2-calcineurin signaling in other tissues (endothelial, cardiac) not tested","Direct SFRP2–Fzd5 binding not biochemically demonstrated"]},{"year":null,"claim":"The molecular determinants that switch SFRP2 between Wnt agonist and antagonist modes — including concentration thresholds, receptor availability, and co-factor requirements — remain poorly defined, and no structural model of SFRP2 in complex with any Wnt ligand, Frizzled, or BMP1 exists.","evidence":"","pmids":[],"confidence":"High","gaps":["No crystal or cryo-EM structure of SFRP2 with any binding partner","Systematic dose-response studies across cell types needed to resolve agonist/antagonist duality","Whether SFRP2 directly binds Frizzled CRDs in trans has not been biochemically confirmed"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0,1,2,9,11,17]},{"term_id":"GO:0008289","term_label":"lipid binding","supporting_discovery_ids":[0,19,17]},{"term_id":"GO:0048018","term_label":"receptor ligand activity","supporting_discovery_ids":[3,13,23]}],"localization":[{"term_id":"GO:0005576","term_label":"extracellular region","supporting_discovery_ids":[0,3,9,13,22,23]},{"term_id":"GO:0031012","term_label":"extracellular matrix","supporting_discovery_ids":[22]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[0,1,2,4,8,11,17,18,29]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[7,8,24,25]},{"term_id":"R-HSA-1474244","term_label":"Extracellular matrix organization","supporting_discovery_ids":[9,22]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[13,14,32]}],"complexes":[],"partners":["WNT4","WNT3A","WNT6","WNT5A","BMP1","ROR2","LRP6","FZD5"],"other_free_text":[]},"mechanistic_narrative":"SFRP2 is a secreted cysteine-rich domain (CRD)-containing protein that functions as a context-dependent modulator of Wnt signaling, capable of acting as either a Wnt antagonist or agonist depending on cell type and ligand availability. SFRP2 directly binds multiple Wnt ligands (Wnt1, Wnt3a, Wnt4, Wnt6, Wnt9a) via its CRD to inhibit canonical Wnt/β-catenin signaling in many contexts, including somitogenesis, limb patterning, and stem cell differentiation [PMID:9853965, PMID:16425220, PMID:17916803, PMID:18446812], but in other settings it enhances canonical Wnt signaling by promoting LRP6 phosphorylation and β-catenin stabilization, as observed in skeletal stem cells, lens epithelium, and cardiac fibroblasts [PMID:20723538, PMID:34857734, PMID:24140542, PMID:27605451]. Beyond Wnt ligand sequestration, SFRP2 redirects non-canonical Wnt signaling by stabilizing Wnt5a–Ror2 complexes while inhibiting Frizzled endocytosis, activates calcineurin/NFATc3 signaling in endothelial cells and alveolar epithelium, inhibits BMP1/Tolloid-mediated procollagen processing to reduce fibrosis, and engages the AMPK/PGC1-α axis for cardioprotection [PMID:27129770, PMID:38980870, PMID:21078975, PMID:34790288]. Through these pleiotropic mechanisms, SFRP2 regulates somitogenesis, skeletal development, cardiac repair and fibrosis, mesenchymal stem cell self-renewal, melanogenesis, and tumor microenvironment signaling including therapy resistance and metastatic dormancy [PMID:16467359, PMID:19017790, PMID:27042933, PMID:32094692]."},"prefetch_data":{"uniprot":{"accession":"Q96HF1","full_name":"Secreted frizzled-related protein 2","aliases":["Secreted apoptosis-related protein 1","SARP-1"],"length_aa":295,"mass_kda":33.5,"function":"Soluble frizzled-related proteins (sFRPS) function as modulators of Wnt signaling through direct interaction with Wnts. They have a role in regulating cell growth and differentiation in specific cell types. SFRP2 may be important for eye retinal development and for myogenesis","subcellular_location":"Secreted","url":"https://www.uniprot.org/uniprotkb/Q96HF1/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/SFRP2","classification":"Not Classified","n_dependent_lines":1,"n_total_lines":1208,"dependency_fraction":0.0008278145695364238},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/SFRP2","total_profiled":1310},"omim":[{"mim_id":"619257","title":"HR UPSTREAM OPEN READING FRAME; HRURF","url":"https://www.omim.org/entry/619257"},{"mim_id":"606570","title":"SECRETED FRIZZLED-RELATED PROTEIN 4; SFRP4","url":"https://www.omim.org/entry/606570"},{"mim_id":"605083","title":"FRIZZLED-RELATED PROTEIN; FRZB","url":"https://www.omim.org/entry/605083"},{"mim_id":"604158","title":"SECRETED FRIZZLED-RELATED 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bladder","ntpm":287.2}],"url":"https://www.proteinatlas.org/search/SFRP2"},"hgnc":{"alias_symbol":["SARP1","SDF-5","FRP-2"],"prev_symbol":[]},"alphafold":{"accession":"Q96HF1","domains":[{"cath_id":"1.10.2000.10","chopping":"39-143","consensus_level":"high","plddt":93.5955,"start":39,"end":143},{"cath_id":"2.40.50.120","chopping":"172-291","consensus_level":"high","plddt":83.7928,"start":172,"end":291}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q96HF1","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q96HF1-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q96HF1-F1-predicted_aligned_error_v6.png","plddt_mean":82.12},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=SFRP2","jax_strain_url":"https://www.jax.org/strain/search?query=SFRP2"},"sequence":{"accession":"Q96HF1","fasta_url":"https://rest.uniprot.org/uniprotkb/Q96HF1.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q96HF1/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q96HF1"}},"corpus_meta":[{"pmid":"17251350","id":"PMC_17251350","title":"Secreted 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Cell physiology","url":"https://pubmed.ncbi.nlm.nih.gov/25972450","citation_count":25,"is_preprint":false},{"pmid":"34852024","id":"PMC_34852024","title":"Downregulation of SFRP2 facilitates cancer stemness and radioresistance of glioma cells via activating Wnt/β-catenin signaling.","date":"2021","source":"PloS one","url":"https://pubmed.ncbi.nlm.nih.gov/34852024","citation_count":24,"is_preprint":false},{"pmid":"31430541","id":"PMC_31430541","title":"sFRP2 promotes airway inflammation and Th17/Treg imbalance in COPD via Wnt/β-catenin pathway.","date":"2019","source":"Respiratory physiology & neurobiology","url":"https://pubmed.ncbi.nlm.nih.gov/31430541","citation_count":23,"is_preprint":false},{"pmid":"24648690","id":"PMC_24648690","title":"Expression of sfrp2 is increased in catagen of hair follicles and inhibits keratinocyte proliferation.","date":"2014","source":"Annals of dermatology","url":"https://pubmed.ncbi.nlm.nih.gov/24648690","citation_count":23,"is_preprint":false},{"pmid":"30345838","id":"PMC_30345838","title":"DNA hypermethylation of SFRP2 influences the pathology of rheumatoid arthritis through the canonical Wnt signaling in model rats.","date":"2018","source":"Autoimmunity","url":"https://pubmed.ncbi.nlm.nih.gov/30345838","citation_count":23,"is_preprint":false},{"pmid":"36002889","id":"PMC_36002889","title":"Identification of novel early pancreatic cancer biomarkers KIF5B and SFRP2 from \"first contact\" interactions in the tumor microenvironment.","date":"2022","source":"Journal of experimental & clinical cancer research : CR","url":"https://pubmed.ncbi.nlm.nih.gov/36002889","citation_count":23,"is_preprint":false},{"pmid":"29462326","id":"PMC_29462326","title":"Secreted frizzled-related protein 2 (SFRP2) expression promotes lesion proliferation via canonical WNT signaling and indicates lesion borders in extraovarian endometriosis.","date":"2018","source":"Human reproduction (Oxford, England)","url":"https://pubmed.ncbi.nlm.nih.gov/29462326","citation_count":23,"is_preprint":false},{"pmid":"31469431","id":"PMC_31469431","title":"SFRP2 promotes stem cells from apical papilla-mediated periodontal tissue regeneration in miniature pig.","date":"2019","source":"Journal of oral rehabilitation","url":"https://pubmed.ncbi.nlm.nih.gov/31469431","citation_count":23,"is_preprint":false},{"pmid":"34021259","id":"PMC_34021259","title":"SFRP2 induces a mesenchymal subtype transition by suppression of SOX2 in glioblastoma.","date":"2021","source":"Oncogene","url":"https://pubmed.ncbi.nlm.nih.gov/34021259","citation_count":22,"is_preprint":false},{"pmid":"30576962","id":"PMC_30576962","title":"Methylation status of SFRP1, SFRP2, RASSF1A, RARβ and DAPK1 genes in patients with oral squamous cell carcinoma.","date":"2018","source":"Archives of oral biology","url":"https://pubmed.ncbi.nlm.nih.gov/30576962","citation_count":21,"is_preprint":false},{"pmid":"22136354","id":"PMC_22136354","title":"Alterations in promoter methylation status of tumor suppressor HIC1, SFRP2, and DAPK1 genes in prostate carcinomas.","date":"2011","source":"DNA and cell biology","url":"https://pubmed.ncbi.nlm.nih.gov/22136354","citation_count":21,"is_preprint":false},{"pmid":"34857734","id":"PMC_34857734","title":"Secreted frizzled related-protein 2 (Sfrp2) deficiency decreases adult skeletal stem cell function in mice.","date":"2021","source":"Bone research","url":"https://pubmed.ncbi.nlm.nih.gov/34857734","citation_count":21,"is_preprint":false},{"pmid":"33854626","id":"PMC_33854626","title":"Methylated SFRP2 and SDC2 in stool specimens for Colorectal Cancer early detection: A cost-effective strategy for Chinese population.","date":"2021","source":"Journal of Cancer","url":"https://pubmed.ncbi.nlm.nih.gov/33854626","citation_count":20,"is_preprint":false},{"pmid":"37106069","id":"PMC_37106069","title":"METTL3 promotes the malignancy of non-small cell lung cancer by N6-methyladenosine modifying SFRP2.","date":"2023","source":"Cancer gene therapy","url":"https://pubmed.ncbi.nlm.nih.gov/37106069","citation_count":19,"is_preprint":false},{"pmid":"24140542","id":"PMC_24140542","title":"Sfrp1 and Sfrp2 are not involved in Wnt/β-catenin signal silencing during lens induction but are required for maintenance of Wnt/β-catenin signaling in lens epithelial cells.","date":"2013","source":"Developmental biology","url":"https://pubmed.ncbi.nlm.nih.gov/24140542","citation_count":19,"is_preprint":false},{"pmid":"25802696","id":"PMC_25802696","title":"Study of SFRP1 and SFRP2 methylation status in patients with de novo Acute Myeloblastic Leukemia.","date":"2015","source":"International journal of hematology-oncology and stem cell 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\"confidence_rationale\": \"Tier 1-2 — direct binding shown by Co-IP, corroborated by genetic epistasis (sFRP-2 absent in Wnt-4 KO kidney)\",\n      \"pmids\": [\"9853965\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"Sfrp-2 functions as a Wnt antagonist in vivo, as demonstrated by overexpression in Xenopus embryos; SFRP2-expressing cells reduce dermomyotome-inducing activity of WNT1 and WNT4 (but not WNT3a) in somitic explants, and SHH-N upregulates Sfrp2 to compete with WNT signaling in somitic mesoderm.\",\n      \"method\": \"Xenopus overexpression in vivo assay, chick somite explant cultures, recombinant protein treatment\",\n      \"journal\": \"Development (Cambridge, England) / Developmental biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vivo gain-of-function in two independent model organisms with functional readouts, replicated across papers\",\n      \"pmids\": [\"10656762\", \"10654605\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Purified Sfrp-1 and Sfrp-2 inhibit Wnt-3a-induced β-catenin accumulation in L cells via a GSK-3-dependent mechanism (upstream of GSK-3), and Sfrp-2 inhibits Wnt-3a-stimulated proliferation in the chick neural tube.\",\n      \"method\": \"Cell-based β-catenin accumulation assay with purified proteins, LiCl epistasis experiment, chick neural tube overexpression\",\n      \"journal\": \"Developmental dynamics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — purified recombinant proteins, epistasis with LiCl, in vivo corroboration\",\n      \"pmids\": [\"16425220\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"SFRP2 is the key paracrine factor released by Akt-modified mesenchymal stem cells that mediates myocardial survival after ischemic injury; treatment of cardiomyocytes with SFRP2 increases β-catenin and upregulates anti-apoptotic genes.\",\n      \"method\": \"Functional genomic screen, recombinant protein treatment, cardiomyocyte survival assay, rat MI model\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (genomic screen, loss-of-function, in vivo MI model), highly cited foundational paper\",\n      \"pmids\": [\"17251350\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"SFRP2 inhibits cardiomyogenic differentiation by blocking a positive transcriptional autofeedback loop of Wnt3a; SFRP2 inhibits Wnt3a-induced Wnt3a transcription via suppression of canonical Wnt/TCF4 signaling, maintaining stem cells in an undifferentiated state.\",\n      \"method\": \"Gain- and loss-of-function (recombinant SFRP2, Sfrp2 siRNA), luciferase reporter assay, embryonal carcinoma stem cell differentiation assay\",\n      \"journal\": \"Stem cells (Dayton, Ohio)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal gain/loss-of-function, reporter assays, multiple orthogonal methods\",\n      \"pmids\": [\"17916803\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Myeloma cell-derived sFRP-2 suppresses osteoblast differentiation induced by BMP-2 and inhibits in vitro mineralization; immunodepletion of sFRP-2 from conditioned medium significantly restores mineralized nodule formation, establishing sFRP-2 as an inhibitor of canonical Wnt signaling-dependent bone formation.\",\n      \"method\": \"Conditioned medium experiments, alkaline phosphatase activity assay, in vitro mineralization assay, immunodepletion of sFRP-2\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — immunodepletion directly links sFRP-2 to bone formation suppression, multiple functional readouts\",\n      \"pmids\": [\"16030194\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"sFRP2 modulates MSC self-renewal and engraftment by downregulating canonical Wnt signaling; shRNA knockdown of sFRP2 in MRL-MSCs decreased their proliferation and engraftment, while overexpression of sFRP2 in WT-MSCs recapitulated the enhanced regenerative MRL phenotype in vivo.\",\n      \"method\": \"shRNA knockdown, retroviral overexpression, in vivo engraftment and MI models, genomic analysis\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal loss- and gain-of-function in vivo, multiple functional readouts\",\n      \"pmids\": [\"19017790\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Sfrp1 and Sfrp2 double knockout mice show shortened thoracic region, incomplete somite segmentation, and altered Notch signaling oscillations (Lfng and Hes7), demonstrating functional redundancy and requirement for Wnt regulation by Sfrp1/Sfrp2 in anteroposterior axis elongation and somitogenesis.\",\n      \"method\": \"Double knockout mouse genetics, in situ hybridization for Notch targets\",\n      \"journal\": \"Development (Cambridge, England)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean double KO with defined molecular phenotype (Notch oscillation defects)\",\n      \"pmids\": [\"16467359\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Sfrp1, Sfrp2, and Sfrp5 regulate both canonical Wnt/β-catenin and the planar cell polarity (PCP) non-canonical Wnt pathway during early trunk formation; genetic analysis combining Sfrp triple mutants with Loop-tail mice revealed Sfrp involvement in convergent extension via PCP regulation.\",\n      \"method\": \"Triple knockout mouse genetics, genetic epistasis with Loop-tail (Vangl2) mutants and Dkk1-deficient mice\",\n      \"journal\": \"Genesis (New York, N.Y. : 2000)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — classical epistasis in multiple genetic backgrounds, pathway assignment to PCP vs canonical Wnt\",\n      \"pmids\": [\"18257070\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Sfrp2 specifically inhibits BMP1/Tolloid metalloproteinase activity (but not sFRP1 or sFRP3), thereby inhibiting type I procollagen processing; exogenous Sfrp2 inhibits mature collagen deposition in cardiac fibroblasts and in rat infarcted myocardium, reducing fibrosis and improving cardiac function.\",\n      \"method\": \"Fluorogenic peptide-based procollagen C-proteinase assay, in vitro procollagen processing assay with recombinant proteins, primary cardiac fibroblast culture, rat MI injection model, echocardiography, trichrome staining\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro enzymatic assay with recombinant proteins showing direct Bmp1 inhibition, corroborated by in vivo functional outcome\",\n      \"pmids\": [\"21078975\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"sFRP2 inhibits both Wnt and BMP signaling pathways in MSCs, with BMP inhibition (measured by pSMAD 1/5/8) occurring independently of its effects on the Wnt pathway; this dual inhibition suppresses osteogenic and chondrogenic differentiation and promotes MSC self-renewal.\",\n      \"method\": \"Western blot for pSMAD 1/5/8, osteogenic/chondrogenic differentiation assays, apoptosis assays, in vivo engraftment\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — mechanistic dissection of two independent pathway inhibitions with multiple orthogonal assays\",\n      \"pmids\": [\"20826809\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"sFRP2 enhances canonical Wnt3a signaling rather than inhibiting it, promoting LRP6 phosphorylation and β-catenin nuclear translocation in HEK293A cells; this enhancement is abolished by DKK1.\",\n      \"method\": \"TOPFlash/FOPFlash luciferase reporter assay, Western blot for LRP6 phosphorylation and β-catenin, Wnt pathway qPCR array, recombinant protein treatment of HEK293A cells\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — in vitro reconstitution with recombinant proteins, DKK1 epistasis, reporter assays, multiple readouts\",\n      \"pmids\": [\"20723538\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"sFRP2 (TRα1 target gene) activates Wnt/β-catenin signaling via Frizzled receptors in the intestinal epithelium, stabilizing β-catenin, activating its target genes, and enhancing cell proliferation; TRα1 directly regulates sFRP2 transcription.\",\n      \"method\": \"Laser microdissected crypt transcriptomics, in vitro and in vivo TRα1 regulation analysis, β-catenin reporter assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct TRα1 regulation validated in vivo and in vitro, Wnt agonist activity established; single lab\",\n      \"pmids\": [\"19001373\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Aged fibroblast-secreted sFRP2 activates a signaling cascade in melanoma cells that decreases β-catenin and MITF, leading to loss of the redox effector APE1, rendering cells resistant to ROS-induced DNA damage and vemurafenib; sFRP2 also augments angiogenesis and metastasis.\",\n      \"method\": \"Co-culture systems, western blot, siRNA knockdown, in vivo mouse models of metastasis, aged vs. young fibroblast comparisons\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multi-step mechanistic cascade validated with multiple orthogonal methods in vitro and in vivo, Nature-level rigor\",\n      \"pmids\": [\"27042933\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"SFRP2 is produced by genotoxically-treated fibroblasts via NF-κB transcriptional control and augments WNT16B-initiated β-catenin signaling in cancer cells by enhancing WNT16B binding to FZD3/4/6 receptors, cooperating with co-receptor LRP6; this promotes cancer cell survival and acquired therapy resistance.\",\n      \"method\": \"Conditioned medium experiments, co-culture, siRNA knockdown, neutralizing antibody experiments, luciferase reporter assays\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — pathway placement via epistasis (DKK1 abrogation, LRP6 cooperation), receptor binding assays; single lab\",\n      \"pmids\": [\"26751775\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"SFRP2 stimulates angiogenesis via calcineurin/NFATc3 signaling in endothelial cells; NFATc3 shRNA knockdown abrogates SFRP2-induced endothelial tube formation, and tacrolimus (calcineurin inhibitor) blocks SFRP2-induced angiogenesis.\",\n      \"method\": \"Endothelial tube formation assay, NFATc3 shRNA knockdown, tacrolimus treatment, in vivo MMTV-neu tumor model\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — shRNA epistasis places NFATc3 downstream of SFRP2 with clear functional readout, corroborated in vivo\",\n      \"pmids\": [\"21673995\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"sFRP2 activates canonical Wnt/β-catenin signaling in adult mouse cardiac fibroblasts (evidenced by Axin2 and Wnt3a upregulation and nuclear β-catenin accumulation), induces cell proliferation coupled to aerobic glycolysis (Warburg effect), and upregulates MMP1, MMP13, MMP2, and MMP9 activities, reducing steady-state collagen.\",\n      \"method\": \"Recombinant sFRP2 treatment of primary cardiac fibroblasts, qPCR for Wnt targets, nuclear β-catenin Western blot, MMP activity assays, glucose/lactate measurements\",\n      \"journal\": \"American journal of physiology. Cell physiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal assays in primary cells; single lab\",\n      \"pmids\": [\"27605451\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"sFRP2 redirects non-canonical Wnt signaling from Frizzled 7 (Fz7) to the Ror2 receptor during Xenopus gastrulation; sFRP2 stabilizes Wnt5a-Ror2 complexes at the membrane while inhibiting Fz7 receptor endocytosis, functioning as a molecular switch between non-canonical Wnt pathways. The cysteine-rich domain of sFRP2 is sufficient for Ror2 activation.\",\n      \"method\": \"Xenopus gastrulation assays, co-immunoprecipitation, receptor endocytosis assays, domain deletion experiments, epistasis with Fz7 and Ror2\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — molecular mechanism (CRD domain sufficiency, receptor complex stabilization) demonstrated with multiple biochemical and cell biological methods\",\n      \"pmids\": [\"27129770\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"sFRP1 and sFRP2 activate the Wnt/planar cell polarity (PCP)/Rac1 pathway in dopamine neurons; compound sFRP1-/-;sFRP2-/- mice display a Wnt/PCP phenotype (anteroposterior VM shortening, lateral Shh domain expansion) similar to Wnt5a-/- mice, and low/medium concentrations of sFRP1 or sFRP2 promote dopamine differentiation mimicking Wnt5a.\",\n      \"method\": \"Double knockout mouse genetics, in vitro primary VM cultures, mouse ESC differentiation assays, Rac1 pathway activation assays\",\n      \"journal\": \"Stem cells (Dayton, Ohio)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis with Wnt5a KO and in vitro pathway activation, pathway assignment to PCP/Rac1\",\n      \"pmids\": [\"22290867\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Sfrp2 binds Wnt6 and inhibits canonical Wnt6 signaling, which is required for inhibition of cardiac progenitor cell (CPC) proliferation; this inhibition activates the non-canonical Wnt/PCP pathway through JNK, inducing cardiac transcription factors and CPC differentiation.\",\n      \"method\": \"Co-immunoprecipitation (Sfrp2-Wnt6 binding), luciferase reporter assays for canonical Wnt, JNK activity assays, CPC differentiation assays, ischemia-reperfusion model\",\n      \"journal\": \"Journal of molecular and cellular cardiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct binding shown by Co-IP, pathway assignment via reporters and JNK assays; single lab\",\n      \"pmids\": [\"26071893\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"sFRP2 induces tissue-nonspecific alkaline phosphatase (TNAP) expression and activity specifically in cardiac fibroblasts via a protein synthesis-dependent mechanism, suggesting sFRP2 promotes cardiac fibrocalcification through coordinate activation of tolloid-like metalloproteinases and TNAP.\",\n      \"method\": \"Biochemical TNAP activity assay, recombinant sFRP2 treatment of primary cardiac fibroblasts, cycloheximide/tunicamycin inhibition, in vivo hamster heart failure model with sFRP2 blockade\",\n      \"journal\": \"American journal of physiology. Cell physiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vitro mechanistic dissection with pharmacological inhibitors, corroborated in vivo; single lab\",\n      \"pmids\": [\"25972450\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"LncRNA Safe stabilizes Sfrp2 mRNA by forming a Safe-Sfrp2 RNA duplex that is bound by the RNA-binding protein HuR; HuR knockdown reduces both Safe and Sfrp2 mRNA stability, while Safe and Sfrp2 mutually stabilize each other, driving cardiac fibrosis.\",\n      \"method\": \"RNA electrophoretic mobility shift assay (REMSA), RNA immunoprecipitation (RIP), dual-luciferase assay, shRNA knockdown, TGF-β fibrosis model\",\n      \"journal\": \"Theranostics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — direct RNA-protein interaction demonstrated by REMSA and RIP, functional epistasis confirmed by rescue experiments\",\n      \"pmids\": [\"31695768\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"SFRP2 mediates the interaction between indolent breast cancer cells and alveolar type 1 epithelial cells in the lung; Sfrp2 is induced in breast cancer cells by lung epithelial signals, promotes fibronectin fibril formation and integrin-dependent pro-survival signaling, and blockade of Sfrp2 reduces indolent disease burden.\",\n      \"method\": \"In vivo RNA sequencing combined with drop-out screening, co-culture experiments, in vivo mouse models of indolent metastasis, fibronectin fibril imaging\",\n      \"journal\": \"Nature cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — combined in vivo RNAseq/drop-out screen identifies SFRP2 mechanistically, functional validation by blockade in vivo\",\n      \"pmids\": [\"32094692\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"In IPF, fibroblast TGF-β1 signaling induces sFRP2 expression near AEC2s; sFRP2 then activates a non-canonical Wnt pathway through Frizzled 5 (Fzd5) and downstream calcineurin/NFATc3 signaling, driving nuclear NFATc3 accumulation and KRT5 basal cell program activation (epithelial metaplasia).\",\n      \"method\": \"AEC2-fibroblast coculture organoids, precision-cut lung slices, single-cell RNA-Seq, calcineurin inhibitor experiments, Fzd5 expression analysis, recombinant sFRP2 treatment\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — mechanistic pathway identified by multiple orthogonal methods (organoids, PCLS, scRNA-seq, pharmacological pathway inhibition)\",\n      \"pmids\": [\"38980870\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Sfrp2 inactivation in mice causes brachydactyly due to decreased chondrocyte proliferation and delayed differentiation in distal limb elements; Sfrp2 can repress canonical Wnt signaling by Wnt1, Wnt9a, and Wnt4 in vitro, and Sfrp2-/- mice show mild increases in β-catenin and TOPGAL reporter activity in phalangeal elements.\",\n      \"method\": \"Sfrp2 knockout mouse, histological analysis, TOPGAL reporter mouse cross, in vitro Wnt reporter assays\",\n      \"journal\": \"Journal of cellular physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — KO with defined cellular phenotype, in vitro pathway assays, and TOPGAL reporter in vivo\",\n      \"pmids\": [\"18446812\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Sfrp1 and Sfrp2 double knockout mice exhibit defects in gonad morphology, reproductive tract maturation, and gonad positioning in males (failed gubernaculum development); similarities with Looptail and Wnt5a mutants suggest disrupted non-canonical Wnt signaling causes these defects.\",\n      \"method\": \"Double knockout mouse genetics, morphological and histological analysis, Insl3/Lgr8 expression analysis\",\n      \"journal\": \"Developmental biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — KO with defined phenotype and pathway placement by comparison to PCP mutants; pathway assignment indirect\",\n      \"pmids\": [\"19100252\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Sfrp1 and Sfrp2 inhibit myoblast terminal differentiation; recombinant Sfrp1 or Sfrp2 added during proliferation phase inhibit myotube formation without affecting cell cycle or apoptosis, and removal restores differentiation.\",\n      \"method\": \"Recombinant protein treatment of C2C12 and primary satellite cell cultures, myotube formation assay, cell cycle analysis, apoptosis assay\",\n      \"journal\": \"Cell and tissue research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — clean functional assay with recombinant proteins, timing-dependent effect established; single lab\",\n      \"pmids\": [\"18322704\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"sFRP2 stimulates melanogenesis through MITF and/or tyrosinase upregulation via β-catenin signaling in normal human melanocytes; this was confirmed in fibroblast-melanocyte co-culture and ex vivo skin.\",\n      \"method\": \"Lentiviral SFRP2 overexpression and shSFRP2 knockdown in primary melanocytes, melanogenesis assays, co-culture, ex vivo skin experiments\",\n      \"journal\": \"The Journal of investigative dermatology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal gain/loss-of-function in primary cells and ex vivo model; single lab\",\n      \"pmids\": [\"26763443\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"SFRP2 exerts cardioprotective effects in diabetic cardiomyopathy by improving mitochondrial dynamics and biogenesis through an AMPK/PGC1-α-dependent pathway; SFRP2 overexpression activates p-AMPK and reverses glucolipotoxicity-induced mitochondrial dysfunction and apoptosis.\",\n      \"method\": \"SFRP2 overexpression/knockdown in H9C2 cells, AAV-mediated overexpression in rat T2DM model, AMPK inhibitor experiments, MitoTracker, TUNEL assay\",\n      \"journal\": \"Oxidative medicine and cellular longevity\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal gain/loss-of-function in vitro and in vivo, pharmacological AMPK inhibition confirms pathway; single lab\",\n      \"pmids\": [\"34790288\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Sfrp2 deficiency in mice reduces skeletal stem cell self-renewal, osteogenic differentiation, and cortical bone defect repair; Sfrp2-deficient BMSCs show reduced activation of Lrp6 and Wnt target genes (Axin2, C-myc, Cyclin D1), and recombinant Sfrp2 restores these activities, indicating Sfrp2 acts as a Wnt agonist in skeletal stem cells.\",\n      \"method\": \"Sfrp2 knockout mice, colony-forming efficiency assays, ectopic bone formation assay, cortical drill-hole repair model, recombinant Sfrp2 rescue, Western blot for Lrp6 activation and Wnt targets\",\n      \"journal\": \"Bone research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — KO with in vivo skeletal phenotype, rescue by recombinant protein, multiple molecular readouts\",\n      \"pmids\": [\"34857734\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"DRG neuron-derived sFRP2 promotes M1 macrophage polarization and migration via NF-κB pathway activation; mechanistically, sFRP2 destabilizes the β-catenin/p65 complex, activating NF-κB, as shown by immunoprecipitation.\",\n      \"method\": \"Lentiviral shRNA knockdown in DRG, in vivo MSU-induced arthritis model, primary macrophage co-culture, immunoprecipitation for β-catenin/p65 complex, Western blot\",\n      \"journal\": \"Theranostics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct protein complex disruption shown by IP, functional consequence validated in vivo; single lab\",\n      \"pmids\": [\"31281508\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Sfrp2 is a transcriptional target of the Pax2 transcription factor; Pax2 expression activates Sfrp2 in embryonic kidney cells, associated with changes in chromatin structure at the Sfrp2 locus at regions of Pax2 binding.\",\n      \"method\": \"Pax2-expressing retroviral transformation of embryonic kidney cells, chromatin structure analysis, transcriptional activation assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — chromatin analysis and Pax2 binding corroborates transcriptional regulation; single lab\",\n      \"pmids\": [\"14561758\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"SFRP2 promotes an oncogenic program in p53-mutant osteosarcoma in a β-catenin-independent manner, inducing FOXM1 and CYR61 expression; ectopic SFRP2 overexpression in normal osteoblast precursors suppresses osteoblast differentiation and promotes OS phenotypes.\",\n      \"method\": \"iPSC-derived osteoblast model from LFS patients, SFRP2 overexpression and knockdown, gene expression analysis, in vivo tumor assays\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — gain/loss-of-function with defined molecular targets (FOXM1, CYR61) in human iPSC disease model; single lab\",\n      \"pmids\": [\"30385632\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"SFRP2 inhibits canonical Wnt signaling in cervical cancer cells by decreasing nuclear β-catenin accumulation, and suppresses EMT by inhibiting SLUG, TWIST, and SNAIL transcription factors while enhancing E-cadherin expression.\",\n      \"method\": \"Overexpression and shRNA approaches in cervical cancer cell lines, β-catenin localization assays, EMT marker analysis, xenograft model\",\n      \"journal\": \"Gynecologic oncology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — mechanistic pathway placement (Wnt/β-catenin to EMT factors) with multiple readouts; single lab\",\n      \"pmids\": [\"19095296\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Sfrp1 and Sfrp2 double knockout mice show reduced Wnt/β-catenin signaling in lens epithelial cells (not increased), indicating Sfrp1/Sfrp2 positively regulate Wnt/β-catenin in lens epithelium; DKO lenses are smaller with deficient epithelium.\",\n      \"method\": \"Sfrp1/Sfrp2 double knockout mice, TCF/Lef reporter (TOPGAL) analysis, lens morphology assessment\",\n      \"journal\": \"Developmental biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — clean DKO with reporter readout demonstrating Wnt agonist function; single lab\",\n      \"pmids\": [\"24140542\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"SFRP2 is a secreted cysteine-rich domain-containing protein that functions as a context-dependent modulator of Wnt signaling: it can inhibit canonical Wnt/β-catenin signaling by directly binding Wnt ligands (e.g., Wnt4, Wnt1, Wnt3a, Wnt6) via its CRD domain, block BMP1/Tolloid metalloproteinase activity to reduce collagen maturation, redirect non-canonical Wnt signaling from Frizzled receptors to Ror2 (acting as a molecular switch), or paradoxically act as a Wnt agonist by stabilizing LRP6 phosphorylation and activating β-catenin in certain cellular contexts; downstream, SFRP2 engages calcineurin/NFATc3, AMPK/PGC1-α, NF-κB, and non-canonical PCP/JNK pathways depending on cell type, making it a pleiotropic regulator of cardiac repair, fibrosis, bone formation, stem cell self-renewal, and tumor microenvironment signaling.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"SFRP2 is a secreted cysteine-rich domain (CRD)-containing protein that functions as a context-dependent modulator of Wnt signaling, capable of acting as either a Wnt antagonist or agonist depending on cell type and ligand availability. SFRP2 directly binds multiple Wnt ligands (Wnt1, Wnt3a, Wnt4, Wnt6, Wnt9a) via its CRD to inhibit canonical Wnt/β-catenin signaling in many contexts, including somitogenesis, limb patterning, and stem cell differentiation [PMID:9853965, PMID:16425220, PMID:17916803, PMID:18446812], but in other settings it enhances canonical Wnt signaling by promoting LRP6 phosphorylation and β-catenin stabilization, as observed in skeletal stem cells, lens epithelium, and cardiac fibroblasts [PMID:20723538, PMID:34857734, PMID:24140542, PMID:27605451]. Beyond Wnt ligand sequestration, SFRP2 redirects non-canonical Wnt signaling by stabilizing Wnt5a–Ror2 complexes while inhibiting Frizzled endocytosis, activates calcineurin/NFATc3 signaling in endothelial cells and alveolar epithelium, inhibits BMP1/Tolloid-mediated procollagen processing to reduce fibrosis, and engages the AMPK/PGC1-α axis for cardioprotection [PMID:27129770, PMID:38980870, PMID:21078975, PMID:34790288]. Through these pleiotropic mechanisms, SFRP2 regulates somitogenesis, skeletal development, cardiac repair and fibrosis, mesenchymal stem cell self-renewal, melanogenesis, and tumor microenvironment signaling including therapy resistance and metastatic dormancy [PMID:16467359, PMID:19017790, PMID:27042933, PMID:32094692].\",\n  \"teleology\": [\n    {\n      \"year\": 1998,\n      \"claim\": \"Establishing that SFRP2 directly engages Wnt ligands resolved how secreted CRD-containing proteins intercept Wnt signals extracellularly.\",\n      \"evidence\": \"Co-immunoprecipitation of sFRP-2 with Wnt-4 in kidney cells, corroborated by loss of Sfrp2 expression in Wnt-4 knockout mice\",\n      \"pmids\": [\"9853965\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Binding affinities and stoichiometry of sFRP2–Wnt complexes not determined\", \"Whether sFRP2 binds Frizzled receptors directly was untested\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"In vivo demonstration that SFRP2 antagonizes canonical Wnt signaling during embryonic patterning established its functional role as a Wnt inhibitor in developmental contexts.\",\n      \"evidence\": \"Xenopus embryo overexpression and chick somite explant assays showing SFRP2 blocks WNT1/WNT4-dependent dermomyotome induction\",\n      \"pmids\": [\"10656762\", \"10654605\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of Wnt ligand selectivity (blocks Wnt1/Wnt4 but not Wnt3a in somites) unexplained\", \"No structural basis for CRD–Wnt specificity\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Genetic loss-of-function in mice revealed that SFRP2 (redundantly with SFRP1) is required for proper somitogenesis and anteroposterior axis elongation, linking Wnt modulation to Notch signaling oscillations.\",\n      \"evidence\": \"Sfrp1/Sfrp2 double knockout mice with shortened thoracic region, abnormal Lfng and Hes7 oscillations\",\n      \"pmids\": [\"16467359\", \"16425220\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether SFRP2 directly modulates Notch pathway components or acts indirectly through Wnt remains unresolved\", \"Individual contribution of Sfrp2 vs. Sfrp1 unclear due to redundancy\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Identification of SFRP2 as a key paracrine survival factor from MSCs and an inhibitor of cardiomyogenic differentiation revealed its dual role in maintaining stemness while promoting cardiac cell survival after injury.\",\n      \"evidence\": \"Functional genomic screen of Akt-MSC secretome identifying SFRP2, recombinant SFRP2 activating β-catenin and anti-apoptotic genes in cardiomyocytes; separately, SFRP2 blocks Wnt3a autoregulatory loop in embryonal carcinoma cells\",\n      \"pmids\": [\"17251350\", \"17916803\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Paradox of SFRP2 as Wnt antagonist yet β-catenin activator in cardiomyocytes not mechanistically resolved\", \"Receptor requirements for SFRP2 pro-survival signaling not identified\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Expanded genetic studies demonstrated SFRP2 regulates both canonical and non-canonical (PCP) Wnt pathways in vivo and controls limb skeletal patterning, gonadal development, and MSC engraftment.\",\n      \"evidence\": \"Sfrp1/2/5 triple KO epistasis with Vangl2 (Loop-tail) mutants demonstrating PCP involvement; Sfrp2 KO brachydactyly phenotype; Sfrp2 shRNA/overexpression in MSC engraftment models\",\n      \"pmids\": [\"18257070\", \"18446812\", \"19017790\", \"19100252\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"PCP pathway activation mechanism by a secreted factor not biochemically defined\", \"How SFRP2 differentially engages canonical vs. non-canonical branches unknown\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Discovery that SFRP2 directly inhibits BMP1/Tolloid metalloproteinase activity independently of Wnt signaling revealed a second, Wnt-independent mechanism for anti-fibrotic action through blockade of procollagen processing.\",\n      \"evidence\": \"In vitro fluorogenic peptide and procollagen C-proteinase assays with recombinant proteins, confirmed by reduced fibrosis in rat MI model\",\n      \"pmids\": [\"21078975\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of sFRP2–BMP1 interaction unknown\", \"Whether sFRP2 inhibits other metalloproteinases beyond BMP1/Tolloid not tested\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Demonstration that SFRP2 can enhance rather than inhibit canonical Wnt3a signaling through LRP6 phosphorylation resolved the long-standing paradox of its context-dependent agonist activity.\",\n      \"evidence\": \"TOPFlash reporter, LRP6 phosphorylation and β-catenin nuclear translocation assays in HEK293A cells with recombinant SFRP2, abolished by DKK1\",\n      \"pmids\": [\"20723538\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Concentration dependence and cell-type determinants of agonist vs. antagonist function not systematically defined\", \"Whether SFRP2 directly binds LRP6 or acts indirectly unclear\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Identification of calcineurin/NFATc3 as a downstream effector of SFRP2 in endothelial cells established a non-canonical signaling cascade mediating SFRP2-driven angiogenesis.\",\n      \"evidence\": \"NFATc3 shRNA and tacrolimus abolish SFRP2-induced endothelial tube formation in vitro and tumor angiogenesis in vivo\",\n      \"pmids\": [\"21673995\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Receptor through which SFRP2 activates calcineurin in endothelial cells not identified at this stage\", \"Relationship between NFATc3 signaling and canonical Wnt modulation by SFRP2 not clarified\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"A cluster of studies in 2016 established that SFRP2 functions as a molecular switch redirecting Wnt signaling between receptors (Fz7→Ror2), drives age-dependent melanoma therapy resistance via β-catenin/MITF/APE1 suppression, and cooperates with WNT16B to promote cancer cell survival through FZD3/4/6 and LRP6.\",\n      \"evidence\": \"Xenopus Ror2/Fz7 endocytosis and Co-IP assays for receptor switching; aged fibroblast co-culture melanoma models with siRNA; WNT16B-SFRP2 cooperation via DKK1 epistasis and luciferase reporters\",\n      \"pmids\": [\"27129770\", \"27042933\", \"26751775\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether CRD-mediated receptor switching occurs in mammalian systems not shown\", \"How SFRP2 simultaneously suppresses β-catenin in melanoma but cooperates with WNT16B to activate it in other cancers not reconciled\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Post-transcriptional regulation of Sfrp2 mRNA by the lncRNA Safe and RNA-binding protein HuR established that SFRP2 expression itself is tightly controlled at the mRNA stability level during fibrosis.\",\n      \"evidence\": \"REMSA and RIP demonstrating Safe–Sfrp2 RNA duplex bound by HuR; mutual stabilization confirmed by knockdown/rescue in TGF-β fibrosis model\",\n      \"pmids\": [\"31695768\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether Safe-mediated regulation operates in tissues beyond cardiac fibroblasts unknown\", \"Other post-transcriptional regulators of Sfrp2 not surveyed\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Demonstration that Sfrp2 knockout reduces skeletal stem cell self-renewal and Lrp6 activation, rescued by recombinant Sfrp2, confirmed its Wnt agonist function in bone biology and established that cardioprotection involves the AMPK/PGC1-α axis.\",\n      \"evidence\": \"Sfrp2 KO mice with impaired bone repair and reduced Wnt target gene expression, rescued by recombinant Sfrp2; SFRP2 overexpression in diabetic cardiomyopathy model activates AMPK, reversed by AMPK inhibitor\",\n      \"pmids\": [\"34857734\", \"34790288\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How SFRP2 activates AMPK mechanistically is undefined\", \"Whether Wnt agonist function in bone and AMPK activation in heart share a common upstream receptor is unknown\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"In IPF, SFRP2 secreted by fibroblasts drives alveolar epithelial metaplasia through a defined Fzd5→calcineurin→NFATc3 non-canonical Wnt cascade, unifying prior calcineurin/NFATc3 observations with a specific receptor.\",\n      \"evidence\": \"AEC2-fibroblast coculture organoids, precision-cut lung slices, scRNA-Seq, calcineurin inhibitor experiments identifying Fzd5 as the receptor\",\n      \"pmids\": [\"38980870\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether Fzd5 is the receptor for SFRP2-calcineurin signaling in other tissues (endothelial, cardiac) not tested\", \"Direct SFRP2–Fzd5 binding not biochemically demonstrated\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The molecular determinants that switch SFRP2 between Wnt agonist and antagonist modes — including concentration thresholds, receptor availability, and co-factor requirements — remain poorly defined, and no structural model of SFRP2 in complex with any Wnt ligand, Frizzled, or BMP1 exists.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No crystal or cryo-EM structure of SFRP2 with any binding partner\", \"Systematic dose-response studies across cell types needed to resolve agonist/antagonist duality\", \"Whether SFRP2 directly binds Frizzled CRDs in trans has not been biochemically confirmed\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 1, 2, 9, 11, 17]},\n      {\"term_id\": \"GO:0008289\", \"supporting_discovery_ids\": [0, 19, 17]},\n      {\"term_id\": \"GO:0048018\", \"supporting_discovery_ids\": [3, 13, 23]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [0, 3, 9, 13, 22, 23]},\n      {\"term_id\": \"GO:0031012\", \"supporting_discovery_ids\": [22]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [0, 1, 2, 4, 8, 11, 17, 18, 29]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [7, 8, 24, 25]},\n      {\"term_id\": \"R-HSA-1474244\", \"supporting_discovery_ids\": [9, 22]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [13, 14, 32]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"WNT4\",\n      \"WNT3A\",\n      \"WNT6\",\n      \"WNT5A\",\n      \"BMP1\",\n      \"ROR2\",\n      \"LRP6\",\n      \"FZD5\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}