Hypothesis

Early CKD triggers microglial activation in the choroid plexus, which sheds membrane‑bound α‑Klotho via ADAM10/17 mediated cleavage. Loss of local Klotho converts FGF23 signaling from the canonical Klotho/FGFR1c/4 MAPK route to a Klotho‑independent FGFR4/PLCγ/calcineurin/NFAT cascade in choroid plexus epithelial cells. This shift drives excess phosphate transport into the cerebrospinal fluid (CSF) and stimulates pro‑inflammatory cytokine release, thereby suppressing neuronal Klotho expression and accelerating cognitive decline. The loop can be broken by pharmacological inhibition of FGFR4 or by replenishing soluble Klotho, which should normalize CSF phosphate and mitigate neuroinflammation.

Mechanistic Rationale

1. Microglial‑derived TNFα and IL‑1β upregulate ADAM proteases in choroid plexus epithelium, a mechanism shown in neurodegenerative models [2].
2. Membrane Klotho serves as an obligate co‑receptor that biases FGF23 toward FGFR1c/4 MAPK signaling; without it, FGF23 favors FGFR4 and activates PLCγ/calcineurin/NFAT, a pathway linked to cardiac hypertrophy [1].
3. Choroid plexus epithelium expresses FGFR4 and Klotho; loss of Klotho redirects FGF23 to FGFR4, increasing intracellular calcium and NFAT nuclear translocation, which drives transcription of NaPi‑2c and inflammatory mediators [3].
4. Elevated CSF phosphate directly inhibits neuronal Klotho transcription via Pi‑dependent HIF‑1α stabilization, as observed in cortical neurons [4].
5. Inflammatory cytokines (IL‑6, CCL2) further suppress Klotho expression in neurons and oligodendrocytes, creating a feed‑forward loop [5].

Testable Predictions

• In early‑stage CKD patients (eGFR 45‑60 mL/min/1.73 m²), CSF Klotho will be significantly lower than serum Klotho, while CSF FGF23 will be unchanged or modestly elevated.
• CSF phosphate concentration will correlate inversely with CSF Klotho and positively with CSF markers of microglial activation (e.g., sTREM2, YKL‑40).
• Experimental activation of microglia in the choroid plexus of mice (via intracereventricular LPS) will reduce choroidal Klotho protein, increase p‑NFAT in epithelial cells, raise CSF phosphate, and impair performance in the Morris water maze.
• Co‑administration of an FGFR4 inhibitor (e.g., BLU‑9933) or exogenous sKlotho will rescue CSF phosphate levels, reduce NFAT signaling, and improve cognitive scores without altering serum FGF23.
• In CKD mouse models, conditional knockout of ADAM10 in choroid plexus epithelium will preserve Klotho, prevent CSF phosphate rise, and protect against cognitive decline despite systemic kidney injury.

Falsifiability

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

1. CSF Klotho levels remain proportional to serum Klotho across CKD stages.
2. Microglial activation markers in CSF do not correlate with CSF phosphate or Klotho.
3. FGFR4 inhibition fails to modify CSF phosphate or NFAT signaling in cholinergic choroid plexus cultures.
4. Exogenous sKlotho does not ameliorate cognitive deficits in CKD mice despite normalizing serum phosphate.

These outcomes can be assessed with existing clinical CSF assays, phospho‑specific antibodies, and behavioral testing, making the hypothesis experimentally addressable within a 2‑year grant cycle.