KLB

KLB (β-Klotho) is an obligate transmembrane co-receptor that confers receptor selectivity for the endocrine fibroblast growth factors FGF19 and FGF21, enabling them to bind and activate FGFR complexes—FGF21 binds the FGFR1-KLB complex with high affinity while FGF19 engages both FGFR1-KLB and FGFR4-KLB, whereas FGF1 signaling is KLB-independent (PMID:22442730). At the plasma membrane KLB and FGFR1c assemble as a preformed 1:1 heterocomplex that transitions to a 1:2 KLB-FGFR1c signaling complex upon FGF21-induced FGFR1c dimerization, a process modulated by association with the galectin lattice (PMID:22523080). Ligand engagement of KLB is governed by conserved C-terminal determinants of FGF19/FGF21 that are sufficient for binding and can be engineered into antagonists or super-agonists (PMID:29789271). Through this co-receptor function KLB transduces FGF21/FGF19 signals across central and peripheral tissues: brain and NTS glutamatergic KLB-expressing neurons mediate FGF21 control of alcohol preference and adaptation to dietary protein restriction (PMID:27911795), hypothalamic KLB is required for FGF21 signaling to GnRH neurons with loss-of-function KLB mutations causing congenital hypogonadotropic hypothorism in patients (PMID:28754744), and liver and adipose KLB mediate FGF21/FGF19 regulation of lipid metabolism, hepatic steatosis, and inflammation (PMID:38609395, PMID:41810430). KLB also restrains myogenesis through inhibition of mTOR in muscle progenitor cells (PMID:35081669) and acts in hepatocellular carcinoma where it is deacetylated by HDAC3 and regulates β-catenin-driven EMT (PMID:37350415).

1. 2012 High

   Established the core molecular function of KLB: it is the indispensable co-receptor that determines which FGFRs FGF19 and FGF21 can bind and activate, resolving how these endocrine FGFs achieve receptor selectivity.

   Evidence Quantitative binding kinetics, signaling assays, and early-response gene measurements in mice with KLB/FGFR1 ablation

   PMID:22442730

   Open questions at the time

   • Did not resolve the stoichiometry or membrane organization of the receptor complex
   • Did not address tissue-specific downstream physiology
2. 2012 High

   Defined the membrane architecture and assembly dynamics of the receptor complex, showing a preformed 1:1 KLB-FGFR1c heterocomplex converts to a 1:2 signaling complex on ligand binding and that the galectin lattice tunes signaling.

   Evidence FRAP, number-and-brightness analysis, galectin-3 co-IP, and live-cell imaging of tagged KLB and FGFR1c

   PMID:22523080

   Open questions at the time

   • Structural basis of the transition not determined
   • Mechanism by which the galectin lattice restrains KLB mobility unresolved
3. 2016 High

   Demonstrated a physiological central role for KLB by showing brain KLB mediates FGF21's suppression of alcohol intake, defining a liver-brain endocrine axis.

   Evidence Brain-specific KLB knockout mice with alcohol preference testing plus exogenous FGF21 administration

   PMID:27911795

   Open questions at the time

   • Specific neuronal populations not identified
   • Downstream neural circuitry unresolved
4. 2017 High

   Linked KLB to human reproductive disease by showing it is required for FGF21 signaling to GnRH neurons and that loss-of-function mutations cause congenital hypogonadotropic hypogonadism.

   Evidence CHH patient genetic screening, Klb knockout mice, GnRH neuron functional assays, and FGF21 brain-delivery tracking

   PMID:28754744

   Open questions at the time

   • Mechanism by which FGF21 triggers GnRH release downstream of KLB not detailed
   • Route of peripheral FGF21 access fully via circumventricular organs not exhaustively mapped
5. 2018 High

   Mapped the ligand-side determinants of KLB binding, showing FGF C-terminal sequences are sufficient and engineerable, enabling rational agonist/antagonist design.

   Evidence C-terminal peptide functional assays, alanine-scanning mutagenesis, and in vivo metabolic outcomes in obese mice; complemented by site-directed mutagenesis identifying FGF21 Y207 as critical for FGFR1-KLB affinity

   PMID:29789271 PMID:30317562

   Open questions at the time

   • Co-crystal structure of KLB with the C-terminal peptides not reported
   • Generalizability of single-residue tuning across both FGFR1 and FGFR4 contexts limited
6. 2021 Medium

   Extended KLB's central function beyond FGF21, showing neuronal KLB contributes to glucagon-receptor-agonist-induced weight loss, and revealed KLB as a mediator of muscle development via mTOR.

   Evidence Neuronal Klb conditional KO and central pharmacological antagonism with GCGR agonism; separately, KLB siRNA knockdown and FGF21 treatment in pig and human fetal muscle progenitor cells

   PMID:33411693 PMID:35081669

   Open questions at the time

   • How a GCGR agonist engages the central KLB pathway mechanistically unclear
   • Direct demonstration that endogenous FGF21 drives the muscle phenotype in vivo lacking
7. 2024 Medium

   Established peripheral metabolic and cancer-associated roles, showing liver KLB is required for ketogenic-diet amelioration of fatty liver and that KLB acts cell-autonomously as anti-lipogenic/anti-inflammatory in hepatocytes; also implicated KLB in tumor biology and PTM regulation.

   Evidence Liver-specific AAV KLB knockdown with ketogenic diet and multi-omics; KLB overexpression in HepG2 with FFA loading and LPS/NF-κB readouts; KLB knockdown with HDAC3 acetylation co-IP and metastasis assays in HCC

   PMID:37350415 PMID:38609395 PMID:41810430

   Open questions at the time

   • Whether acetylation/deacetylation directly alters ligand binding versus downstream output not resolved
   • Connection between KLB co-receptor function and cell-autonomous anti-inflammatory effects unclear
8. 2025 Medium

   Refined the central circuit, identifying a discrete glutamatergic Klb-expressing NTS neuronal population that is necessary and sufficient for FGF21-driven metabolic adaptation to protein restriction.

   Evidence Klb-Flp mouse line, intersectional genetics, neuronal ablation, and chemogenetic activation during dietary protein restriction (preprint)

   Open questions at the time

   • Downstream projection targets of these NTS neurons not mapped
   • Preprint, not yet peer-reviewed

• How KLB acetylation status, galectin-lattice association, and tissue-specific receptor partner availability are integrated to control the magnitude and selectivity of FGF19/FGF21 signaling in vivo remains unresolved.
• No structural model of the full ligand-KLB-FGFR signaling complex
• Causal role of KLB acetylation in physiological signaling not established in vivo
• Mechanism connecting co-receptor activity to ferroptosis suppression unconfirmed