KLB

β-Klotho (KLB) is a single-pass transmembrane co-receptor that enables endocrine FGF signaling across metabolically active tissues including adipose, liver, muscle, and discrete neuronal populations. KLB forms a ligand-independent 1:1 heterocomplex with FGFR1c at the plasma membrane; FGF21 binding induces FGFR1 dimerization to generate a 1:2 KLB–FGFR1c signaling complex that activates p38 and mTOR cascades, while FGF19 binds KLB–FGFR1 and KLB–FGFR4 with comparable affinity, with receptor selectivity determined by ligand C-terminal residues (PMID:22442730, PMID:22523080, PMID:29789271). In the brain, KLB in GnRH neurons is required for FGF21-dependent reproductive function—loss-of-function KLB mutations cause congenital hypogonadotropic hypogonadism—and in the nucleus of the solitary tract and other central circuits, KLB mediates FGF21-dependent suppression of alcohol preference and metabolic adaptation to protein restriction (PMID:28754744, PMID:27911795). In the liver, KLB transduces FGF21 signals governing fatty acid oxidation, lipogenesis, and NF-κB–driven inflammation, while HDAC3-mediated deacetylation maintains KLB in its active state and acetylation inactivates it, linking KLB post-translational regulation to β-catenin signaling and hepatocellular carcinoma metastasis (PMID:38609395, PMID:37350415).

1. 2012 High

   Quantitative binding studies established that KLB is indispensable for FGF21 and FGF19 receptor engagement and revealed ligand-specific FGFR selectivity—FGF21 preferentially signals through FGFR1–KLB whereas FGF19 engages both FGFR1–KLB and FGFR4–KLB—resolving how two endocrine FGFs elicit distinct tissue-specific responses through a shared co-receptor.

   Evidence Quantitative binding kinetics, downstream signaling assays, and adipose-specific KLB/FGFR1 ablation

   PMID:22442730

   Open questions at the time

   • Structural basis of ligand-selective FGFR engagement not resolved
   • Contribution of heparan sulfate to endocrine FGF–KLB binding in vivo unclear
2. 2012 High

   Live-cell imaging demonstrated that KLB and FGFR1c pre-associate as a 1:1 complex before ligand binding, and that FGF21 induces FGFR1 dimerization without changing KLB stoichiometry, yielding a 1:2 KLB–FGFR1c signaling unit—establishing the assembly mechanism of the active receptor complex.

   Evidence FRAP, number-and-brightness analysis of fluorescently tagged KLB/FGFR1c, co-IP with galectin-3 in living cells

   PMID:22523080

   Open questions at the time

   • Role of the galectin lattice in modulating KLB signaling kinetics in vivo not defined
   • No atomic-resolution structure of the ternary complex at this time
3. 2016 High

   Brain-specific KLB knockout revealed a liver–brain endocrine axis in which hepatic FGF21 signals through central KLB to suppress alcohol preference, establishing the first neuronal function for KLB.

   Evidence Brain-specific Klb KO mice with behavioral alcohol-preference assays; FGF21 administration

   PMID:27911795

   Open questions at the time

   • Precise neuronal populations mediating alcohol preference not identified at this point
   • Downstream intracellular signaling in KLB-expressing neurons not characterized
4. 2017 High

   Human genetic screening and mouse Klb knockout demonstrated that KLB-mediated FGF21 signaling in GnRH neurons is essential for puberty onset and reproductive cyclicity, linking KLB loss-of-function to congenital hypogonadotropic hypogonadism.

   Evidence Genetic screening of CHH patients; Klb KO mice with reproductive phenotyping; FGF21-induced GnRH release assays

   PMID:28754744

   Open questions at the time

   • Mechanism by which FGF21–KLB signaling regulates GnRH release at the molecular level not defined
   • Number and penetrance of KLB variants in CHH remain limited
5. 2018 High

   Systematic mutagenesis showed that C-terminal residues of FGF19/FGF21 are the primary KLB-binding determinant and that a single residue governs FGFR1-vs-FGFR4 selectivity, enabling design of super-agonist FGF21 variants with enhanced metabolic efficacy.

   Evidence Alanine scanning mutagenesis of C-terminal peptides; competitive antagonism; in vivo metabolic studies in obese mice

   PMID:29789271

   Open questions at the time

   • KLB residues that receive C-terminal peptide not mapped
   • In vivo pharmacokinetics and safety of super-agonists not fully established
6. 2021 Medium

   Neuronal Klb-deficient mice showed partial resistance to glucagon-receptor-agonist–induced weight loss, demonstrating that central KLB signaling integrates with the glucagon axis for body weight regulation but is dispensable for hepatic lipid improvements.

   Evidence Neuronal Klb KO mice treated chronically with GCGR agonist IUB288; central KLB antagonist compound 1153

   PMID:33411693

   Open questions at the time

   • Specific neuronal populations and circuits downstream of KLB in GCGR-mediated weight loss undefined
   • Single-laboratory finding
   • Whether FGF21 is the sole KLB ligand mediating these effects not tested
7. 2021 Medium

   The KLB rs17618244 variant was shown to reduce KLB protein in hepatic stellate cells, driving a pro-fibrogenic myofibroblast-like phenotype and linking KLB loss-of-function to hepatic fibrosis progression.

   Evidence LX-2 stellate cells transfected with WT vs. mutant KLB; proliferation and fibrogenic gene assays; retrospective histological cohort

   PMID:33640795

   Open questions at the time

   • Mechanism of reduced KLB protein from the variant (degradation vs. folding) not determined
   • Causality in prospective human cohorts not established
8. 2022 Medium

   KLB knockdown in fetal muscle progenitor cells promoted myogenesis and mTOR activation, revealing KLB as a negative regulator of muscle differentiation through FGF21-dependent mTOR suppression.

   Evidence KLB siRNA KD in porcine and human fetal muscle progenitor cells; FGF21 dose–response; fusion index and mTOR activity assays

   PMID:35081669

   Open questions at the time

   • In vivo relevance in adult myogenesis or muscle regeneration not tested
   • Downstream mTOR substrates mediating myogenic suppression unidentified
9. 2023 Medium

   HDAC3 was identified as the deacetylase that maintains KLB in its active state; HDAC3 inhibition acetylates KLB, inactivating FGF21–KLB signaling and unleashing β-catenin–driven EMT and HCC metastasis, thereby placing KLB upstream of Wnt/β-catenin pathway regulation.

   Evidence Co-IP for KLB acetylation; HDAC3 inhibitor treatment; KLB KD in Huh7 with migration/invasion/metastasis assays

   PMID:37350415

   Open questions at the time

   • Specific acetylation sites on KLB not mapped
   • Mechanism linking KLB inactivation to β-catenin activation not fully resolved
   • Single-laboratory finding
10. 2024 Medium

    Liver-specific KLB knockdown demonstrated that hepatic FGF21–KLB–FGFR1 signaling is required for ketogenic-diet–mediated improvements in steatosis and insulin resistance, and that KLB overexpression suppresses NF-κB p65 phosphorylation and inflammatory gene activation.

    Evidence AAV-mediated liver-specific KLB KD in mice on ketogenic diet; KLB overexpression in HepG2 with FFA/LPS challenge; multi-omics analysis

    PMID:38609395

    Open questions at the time

    • Direct KLB mechanism for NF-κB suppression (direct vs. indirect) not resolved
    • Single-laboratory study

• The identity and functional logic of all KLB-expressing neuronal populations (including newly identified NTS-KLB neurons), the structural basis of KLB's FGFR selectivity at atomic resolution, the specific acetylation sites regulating KLB activity, and the in vivo relevance of KLB in adult muscle regeneration remain unresolved.
• Full atlas of KLB-expressing neuronal subtypes and their individual contributions to metabolic regulation
• Atomic-resolution structure of the FGF21–KLB–FGFR1 ternary complex
• Mapping of KLB acetylation sites and their individual functional consequences
• In vivo role of KLB in adult myogenesis and muscle repair