Hypothesis

Soluble Klotho (sKl) functions as a zinc‑dependent competitive antagonist of FGF23 binding to FGFR4 in cardiomyocytes, thereby blocking the Klotho‑independent calcineurin‑NFAT pathway that drives cardiac hypertrophy. This activity is distinct from sKl’s classical hormonal actions on Wnt, IGF‑1, NF‑κB, and TGF‑β signaling and depends on the preservation of its short extracellular domain that retains the FGF‑binding site but lacks the transmembrane region required for obligate co‑receptor function. We further propose that local zinc concentration, modulated by dietary intake and renal handling, determines the affinity of sKl for FGF23, such that hypozincemia converts sKl from an inhibitor to a passive scaffold, permitting FGF23‑FGFR4 signaling despite high circulating sKl.

Mechanistic Rationale

1. Structural basis – The extracellular region of Klotho contains two homologous KL domains (KL1 and KL2). KL1 binds FGF23 with high affinity only when Klotho is membrane‑anchored and stabilized by Zn²⁺ coordinated at the KL1–FGF23 interface [1]. Soluble Klotho retains KL1‑KL2 but loses the membrane‑proximal stalk that positions KL1 for optimal orientation; nevertheless, in the presence of physiological Zn²⁺ (≥15 µM), sKl can still dock FGF23, forming a ternary complex that sterically hinders FGFR4 engagement.

2. Enzymatic modulation – Klotho possesses β‑glucuronidase‑like activity that removes glucuronic acid moieties from heparan sulfate proteoglycans (HSPGs) on the cardiomyocyte surface, increasing the local diffusivity of FGF23. When Zn²⁺ is scarce, this activity wanes, leading to HSPG‑mediated sequestration of FGF23 near FGFR4 and amplified signaling.

3. Physiological switch – In CKD, dietary phosphate restriction often coincides with reduced zinc absorption, creating a hypozincemic milieu that diminishes sKl’s antagonistic capacity while FGF23 levels rise, explaining the dissociation between circulating sKl and cardiac outcomes observed in epidemiologic studies.

Testable Predictions

• Prediction 1: In vitro, addition of recombinant sKl to cultured rat cardiomyocytes will reduce FGF23‑induced phosphorylation of NFATc3 only when the medium is supplemented with Zn²⁺ (10‑30 µM); chelation with TPEN will abolish this effect.
• Prediction 2: Ex vivo, heart slices from Zn‑deficient mice will show higher p‑NFAT levels after FGF23 exposure compared with Zn‑adequate slices, despite identical sKl concentrations.
• Prediction 3: In a human CKD cohort, stratifying participants by serum zinc will reveal an inverse correlation between sKl and left‑ventricular mass index only in the zinc‑sufficient tertile (r ≈ ‑0.35, p<0.01), whereas no correlation will be evident in the zinc‑deficient group.
• Prediction 4: A 12‑week randomized trial of zinc supplementation (30 mg/day) combined with low‑dose recombinant sKl will produce a greater reduction in serum NT‑proBNP and echocardiographic LV wall thickness than sKl monotherapy.

Falsifiability

If any of the following observations occur, the hypothesis is refuted:

• sKl fails to inhibit FGF23‑NFAT signaling in cardiomyocytes regardless of zinc status.
• Zinc manipulation does not alter the binding affinity of sKl for FGF23 measured by surface plasmon resonance.
• Clinical data show no interaction between serum zinc and the sKl‑FGF23 ratio on cardiac hypertrophy endpoints.

References

[1] https://www.jgerontology-geriatrics.com/article/view/819 [2] https://pmc.ncbi.nlm.nih.gov/articles/PMC12713327/