Hypothesis

Soluble Klotho does not merely suppress oxidative stress; it actively remodels the endpoint of proteostatic failure by steering stressed neurons toward the formation of ordered, amyloid‑like aggregates that are biologically inert. Soluble Klotho, primarily from the kidney, exerts systemic anti‑aging effects including neuroprotection [3]. When Klotho signaling declines, lysosomal capacity falters and misfolded proteins accumulate as disordered, toxic oligomers. Restoring Klotho re‑engages a lysosomal‑dependent conversion pathway that compacts these oligomers into tightly packed, β‑sheet‑rich fibrils—structurally analogous to the protective inclusions seen in yeast prion stress responses. This conversion reduces membrane permeabilization and caspase activation while preserving aggregate mass, explaining why Klotho improves outcomes without lowering total amyloid load.

Testable Predictions

1. Aggregate morphology shift – In primary cortical neurons treated with oligomeric Aβ, Klotho overexpression will increase the proportion of Thioflavin‑S positive, Congo‑red birefringent fibrils relative to amorphous deposits, detectable by electron microscopy and filter‑trap assay.
2. Lysosome dependence – The Klotho‑induced shift will be blocked by lysosomal inhibitors (e.g., bafilomycin A1) or by knock‑down of V‑ATPase subunits, indicating that lysosomal acidification is required for the conversion.
3. Functional inertness – Isolated aggregates from Klotho‑overexpressing cells will show reduced seeding capacity in biosensor assays and lower cytotoxicity (LDH release, caspase‑3 activation) compared with aggregates from Klotho‑deficient cells, despite similar total protein content.
4. In vivo correlation – Aged Klotho‑heterozygous mice will exhibit a higher ratio of oligomeric to fibrillar tau (measured by TOMA‑2/TOMA‑1 antibodies) in hippocampus; Klotho supplementation will reverse this ratio without changing total tau burden.

Mechanistic Rationale

Klotho’s known actions on mitochondrial integrity lower ROS production, decreasing ongoing misfolding. Klotho deficiency leads to kidney dysfunction, hyperphosphatemia, vascular calcification, and brain atrophy [4]. Simultaneously, Klotho enhances lysosomal cathepsin activity via FGF‑independent signaling (see [2]), raising the proteolytic capacity needed to restructure aggregated substrates. Under high substrate load, the lysosome may act as a “chaperone‑like” vesicle where partially unfolded polypeptides are annealed into β‑stacked filaments—a process akin to controlled amyloidosis observed in bacterial inclusion bodies. This reframes aggregates not as end‑point garbage but as a Klotho‑tuned depot that sequesters dangerous species into a thermodynamically stable, non‑propagating lattice.

If Klotho’s neuroprotection operates through aggregate quality rather than quantity, therapeutic strategies that dissolve fibrils could be counterproductive, whereas those that promote ordered sequestration may preserve neuronal function.