Klotho

A large (~130 kDa) glycosylated transmembrane and circulating protein encoded by the KL gene, known for mouse lifespan genetics and a single-dose rhesus macaque cognition study, with no human trial of the protein itself.

Also referenced as: Alpha-Klotho, α-Klotho, Soluble Klotho, s-Klotho, AlphaKlotho LR

Also appears in: Longevity

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What is Klotho?

Klotho is not a peptide. Alpha-klotho — the form meant when the research market says “klotho” — is a 1,012-amino-acid single-pass transmembrane glycoprotein encoded by the KL gene, with an extracellular region built from two homologous domains, KL1 and KL2, each roughly 450 residues long and structurally related to family-1 β-glycosidases. Its bare polypeptide backbone is about 116 kDa; N-glycosylation across seven sites carries the observed mass to the ~130 kDa the literature usually quotes. That extracellular region is cleaved from the cell surface by the α-secretases ADAM10 and ADAM17 and released into blood, cerebrospinal fluid, and urine as soluble klotho, which is the circulating form measured in human studies.

The size matters for everything that follows. A typical research peptide is 5 to 40 residues and is built by solid-phase synthesis; klotho is a folded, N-glycosylated protein one to two orders of magnitude larger, and it can only be produced in mammalian expression systems such as CHO or HEK293 cells. It cannot be synthesized on a peptide synthesizer, and the reversed-phase HPLC and mass-spectrometry methods that establish identity and purity for a short peptide do not carry the same meaning for a heterogeneously glycosylated 130 kDa protein.

Klotho reached the research market on the strength of its reputation rather than its clinical file. The gene was discovered as an aging suppressor in mice in 1997, overexpression was shown to extend mouse lifespan in 2005, and a 2023 study in aged monkeys reported that a single injection improved memory — a sequence of high-profile results that produced one of the widest hype-to-evidence gaps in the longevity space.

How it works

• Obligate co-receptor for FGF23. Alpha-klotho’s established physiological job is to convert a canonical fibroblast growth factor receptor into a high-affinity receptor for FGF23, the bone-derived hormone that drives renal phosphate excretion and suppresses 1,25-dihydroxyvitamin D (Urakawa et al., Nature, 2006;444(7120):770–774). Klotho is therefore a node in mineral metabolism first and a “longevity protein” second.
• A scaffold, not an enzyme. Despite its glycosidase-like fold, the crystal structure of the klotho–FGF23–FGFR complex showed that α-klotho has no catalytic activity and functions as a non-enzymatic molecular scaffold that binds FGF23 and presents it to the receptor (Chen et al., Nature, 2018;553(7689):461–466). This finding retired a large body of earlier mechanism claims that described klotho as a circulating sialidase or glycosidase acting on cell-surface channels — those explanations still circulate in secondary sources but predate the structure.
• Suppression of insulin/IGF-1 signaling. The lifespan-extension paper attributed the effect to klotho acting as a circulating hormone that represses intracellular insulin and IGF-1 signaling, an evolutionarily conserved longevity mechanism shared with other long-lived mutants (Kurosu et al., Science, 2005;309(5742):1829–1833). The same paper noted that the long-lived transgenic mice were mildly insulin and IGF-1 resistant — the benefit and the metabolic cost are the same signal.
• NMDA-receptor-dependent synaptic effects, without entering the brain. Elevating klotho in mice enhanced learning, memory, and long-term potentiation, and enriched the NMDA receptor subunit GluN2B at synapses; blocking GluN2B abolished the effect (Dubal et al., Cell Reports, 2014;7(4):1065–1076). A follow-up showed that peripheral administration of a klotho fragment enhanced cognition and neural resilience in young, aging, and α-synuclein transgenic mice despite the fragment not crossing the blood-brain barrier (Leon et al., Cell Reports, 2017;20(6):1360–1371). How a peripherally restricted protein changes synaptic physiology remains unexplained.

Research status

The founding mouse work. Kuro-o et al. described mice carrying a hypomorphic klotho allele that developed a syndrome resembling human aging — short lifespan, infertility, arteriosclerosis, skin atrophy, osteoporosis, and emphysema (Nature, 1997;390(6655):45–51). These animals live roughly eight weeks and also show hyperphosphatemia, hypercalcemia, elevated 1,25-dihydroxyvitamin D, and vascular calcification; a low-phosphate diet substantially rescues the phenotype, which argues that much of the “accelerated aging” is disordered mineral metabolism rather than aging as such. The mirror-image experiment came eight years later, when overexpression extended lifespan in male and female mice (Kurosu et al., Science, 2005, above).

The primate study. The result driving current interest is Castner et al., Nature Aging, 2023;3(8):931–937. Aged rhesus macaques — 18 animals aged 15 to 28 years, mean about 21.8, which the authors put at a human equivalent of roughly 65 — received a single subcutaneous dose of vehicle or rhesus klotho protein and were tested on a spatial delayed-response memory task. The low dose, 10 µg/kg, was given to nine of the animals and was chosen because it raised serum klotho about fivefold, into the range humans carry at birth. It improved performance by four hours after injection, and the benefit persisted across two weeks of testing. The two higher doses, 20 and 30 µg/kg, did not, and the authors state the low-dose-but-not-high-dose result directly in the abstract. This is one acute experiment in a small cohort with a non-monotonic dose response, which is a caution flag, not a confirmation.

The human genetics did not hold. The human case for klotho rests substantially on KL-VS, a functional haplotype whose heterozygous carriers were reported to have higher serum klotho and better cognition (Dubal et al., 2014, above). Replication has been poor. A population-based sample of adults aged 55–87 found no cognitive advantage in KL-VS heterozygotes and, if anything, slightly lower scores across every task (Müller et al., Scientific Reports, 2021;11:13852). A study of the Newcastle 85+ cohort and UK Biobank found no evidence that genetic variation at the klotho locus is associated with longevity (Amin et al., Journal of Gerontology: Series A, 2022;77(3):457–461). What survives more consistently is the observational finding that lower plasma klotho tracks with worse outcomes — for example, higher mortality risk in older community-dwelling adults (Semba et al., Journal of Gerontology: Series A, 2011;66(7):794–800) — which is an association in the wrong direction to establish that giving klotho helps.

No human trial has ever administered klotho protein. As of this writing the ClinicalTrials.gov registry returns more than 120 klotho entries, and the overwhelming majority measure klotho as a biomarker of exercise, kidney function, or vitamin D status rather than administering anything. Three registered interventional studies deliver klotho, and none of them delivers the protein:

• NCT07216781 (sponsor Minicircle, Phase 1, recruiting, started October 2025) is an open-label, non-placebo-controlled pilot of a klotho plasmid gene therapy given by subcutaneous injection into abdominal fat in 24 healthy adults aged 23–90. Primary outcomes are serum α-klotho by ELISA, adverse events, and a panel of mineral-metabolism markers.
• NCT07285629 (sponsor Minicircle, Early Phase 1, recruiting, started December 2025) is the same design applied to a combination klotho and follistatin plasmid, in 30 healthy adults aged 50–80. Because the two genes are delivered together, this study cannot attribute any result to klotho alone.
• NCT07544420 (sponsor Klothea Bio, Phase 1b, not yet recruiting) is a randomized, placebo-controlled, quadruple-masked safety and pharmacokinetics study of AKL003, an mRNA-in-lipid-nanoparticle therapeutic designed to make the body produce α-klotho. About 21 healthy volunteers aged 50–75 are to be randomized 2:1, so roughly 14 would receive two intravenous 0.5 mg doses four weeks apart and roughly 7 would receive buffer. Its estimated start date of May 2026 has passed with the record still listed as not yet recruiting.

All three dose participants at the GARM clinic in Roatán, Honduras. The two Minicircle records also list a US site — the Apeiron Center in Austin, Texas — and state explicitly that the investigational product is administered outside US FDA jurisdiction, with only non-treatment assessments such as cognitive testing and blood draws occurring in the United States.

No klotho product is approved by any regulator for any indication, and there is no compounded klotho. Half-life-extended recombinant α-klotho constructs — fusions to albumin, Fc, or XTEN-type partners intended to slow clearance — are described in patent filings from Klotho Therapeutics (for example WO2017210607A1, published December 2017), but no such construct has a published primate or human dataset.

Common dosage forms

• Lyophilized powder in microgram vials, commonly labeled 10 mcg or 20 mcg. This is a far smaller unit than the milligram vials used across the rest of the research market, and it reflects the cost of mammalian-cell protein expression rather than any dosing convention.
• “LR”-suffixed constructs, sold under names such as alphaKlotho LR. The suffix denotes a long-acting recombinant fusion — α-klotho joined to a half-life-extending partner — which is a different molecule from the native rhesus klotho protein used in the macaque study, and from the soluble klotho measured in human observational work.
• Laboratory reagent presentations, supplied by life-science suppliers as CHO- or HEK293-expressed protein in PBS with glycerol and stabilizers, shipped and stored cold. These are catalog reagents for cell-culture and assay use.
• Multi-vial kits, where two or more microgram vials are packaged together.

For scale, and as arithmetic rather than guidance: the only primate study used 10 µg per kilogram of body weight per dose, so a single 10 mcg vial holds the quantity that study allotted to one kilogram of monkey. This section describes formats only and is not dosing guidance.

Key considerations

• The evidence gap is the story. Klotho has a genuinely strong preclinical pedigree in mice, one acute positive result in monkeys, and zero human administration data for the protein. Marketing that cites “the longevity protein” is describing 1997 mouse genetics; the three human trials that exist deliver a gene or an mRNA rather than the protein, are early-phase safety studies, and dose participants at an offshore clinic explicitly outside FDA jurisdiction. None has posted results.
• You cannot easily verify what is in the vial. Certificate-of-analysis conventions built for short synthetic peptides do not transfer to a 130 kDa glycoprotein: purity by HPLC, molecular weight by mass spectrometry, and sequence confirmation all mean something different for a glycosylated protein whose apparent mass on SDS-PAGE shifts with glycosylation. Independent confirmation would require SDS-PAGE, a validated immunoassay, and ideally an FGF23 co-receptor bioactivity assay, none of which appear on standard research-market testing documents.
• Serum klotho is itself hard to measure. In a head-to-head on kidney-disease serum, a commercial ELISA recovered spiked exogenous klotho at 60–81% against 81–115% for an immunoprecipitation-immunoblot assay, and barely tracked kidney function at all (R = 0.18, versus 0.80 for the immunoblot method); a freeze-thaw cycle was enough to dissipate the correlation entirely (Neyra et al., Clinical Kidney Journal, 2020;13(2):235–244; assay development in Yamazaki et al., Biochemical and Biophysical Research Communications, 2010;398(3):513–518). Anyone treating a klotho blood test as a clean readout should know the assay literature is unsettled.
• The known safety questions are metabolic and mineral, not exotic. Alpha-klotho’s established function sits on top of the FGF23 axis governing phosphate, PTH, and vitamin D — the gene-therapy trial’s own primary outcome measures include FGF23, PTH, 1,25-dihydroxyvitamin D, serum and urinary phosphate, and cystatin C, which is the sponsor watching that axis. Separately, the lifespan-extending mice in Kurosu et al. were insulin and IGF-1 resistant. A large recombinant human protein also carries immunogenicity questions that no published study has addressed in humans.
• “Klotho” is ambiguous, and the family members are unrelated in function. Alpha-klotho (KL) is the FGF23 co-receptor described here. Beta-klotho (KLB) is a different gene and the obligate co-receptor for FGF19 and FGF21, and it is the target of a separate class of metabolic drug programs. Gamma-klotho (LCTL) is a third member. Literature searches, supplement labels, and vendor listings that say only “klotho” frequently conflate them.