Klotho: The Anti-Aging Protein That Took 30 Years to Reach Human Trials

In 1997, a Japanese researcher named Makoto Kuro-o was breeding transgenic mice in a laboratory at the National Institute of Diabetes and Digestive and Kidney Diseases when an accident produced one of the most consequential discoveries in modern aging biology. A mutation his lab had introduced for an unrelated experiment disrupted a previously unknown gene, and the affected mice began to age in fast forward. By six weeks they showed atherosclerosis, osteoporosis, skin thinning, infertility, emphysema, and cognitive decline. They were dead by two months. Kuro-o named the gene Klotho, after the youngest of the three Fates of Greek mythology, the goddess who spins the thread of life. The paper appeared in Nature on November 6, 1997. Almost thirty years later, the protein it encoded is finally entering human trials as one of the most promising candidate therapeutics in longevity medicine.

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The Klotho story matters because it is the rare case in aging research where the same molecule explains aging at three different scales at once. It maps the molecular biology of the kidney filtration unit, the cognitive resilience of the human cortex, and the genetic variation that separates centenarians from their less fortunate siblings. The therapy pipeline that has emerged in 2025 and 2026 reflects that breadth, with programs targeting cognitive decline, chronic kidney disease, sarcopenia, and cardiovascular aging all running in parallel.

The Discovery That Named a Hormone After a Goddess of Fate

Kuro-o’s original Nature paper documented something biologists had not seen before. The mutant mice were not simply sick. They displayed a coordinated, premature aging syndrome that resembled what gerontologists call segmental progeria, only across more tissues than any previously known mutation. The Klotho gene encoded a single transmembrane protein expressed most strongly in the kidney, the parathyroid, and the choroid plexus of the brain. A shorter, soluble form circulated in blood and cerebrospinal fluid.

The mirror image experiment came in 2005, when Hiroshi Kurosu, Kuro-o, and colleagues published in Science what may still be the cleanest demonstration of a single gene extending lifespan in a mammal. Transgenic mice that overexpressed Klotho lived between 19 and 31 percent longer than littermates. They also resisted insulin signaling in a way that mimicked the longevity phenotype of caloric restriction. The Science paper was titled, with unusual confidence for the genre, "Suppression of Aging in Mice by the Hormone Klotho."

What made Klotho different from the dozens of other longevity genes catalogued in worms, flies, and mice was that the soluble form could be measured in human blood. It was a hormone, not just a gene. That meant it was, in principle, drug-like. You could envision a future where Klotho deficiency could be diagnosed with a blood test and corrected with a recombinant infusion. Three decades on, parts of that future have become real.

What Klotho Actually Does in the Body

Klotho is, in the simplest terms, a counter regulatory hormone that opposes accelerated aging in three coupled systems. In the kidney, alpha Klotho serves as an obligate co receptor for fibroblast growth factor 23, the bone derived hormone that controls phosphate excretion. When Klotho declines, FGF23 signaling fails, phosphate rises, vascular calcification accelerates, and the kidney itself ages faster. This was the axis that earned Klotho its first sustained attention from nephrologists, who in the early 2010s began documenting how soluble Klotho levels fell sharply in chronic kidney disease and predicted patient outcomes more accurately than creatinine.

The second axis is metabolic. Klotho modulates insulin and IGF-1 signaling, two pathways that have been linked to longevity in essentially every model organism studied. Reduced signaling through these pathways generally extends lifespan, and Klotho overexpression appears to dial them down without producing the side effects of diabetes drugs or strict caloric restriction.

The third axis is neurological, and it is where the field has been most surprised. Soluble alpha Klotho crosses into the cerebrospinal fluid, where it interacts with NMDA receptors on neurons and modulates synaptic plasticity. Dena Dubal’s laboratory at the University of California San Francisco has spent the better part of two decades demonstrating that Klotho can improve cognition in young, old, and disease model mice, including animals that carry the human APP mutation that causes familial Alzheimer’s disease. The cognitive benefits appear within hours of a single injection of Klotho protein in some experiments, an effect that is unusual in aging biology and suggests Klotho is acting on circuits that are still functional rather than reversing structural damage.

The Brain Story: Dena Dubal’s UCSF Lab

In May 2014, Dubal and colleagues published a paper in Cell Reports that has shaped the cognitive Klotho field ever since. They injected old mice with the soluble alpha Klotho protein and tested them on the Morris water maze. The treated mice navigated the maze as if they were young again. Dubal’s group then turned to a transgenic Alzheimer’s mouse line and showed that boosting Klotho protected against amyloid related cognitive deficits.

The discovery that propelled the field was the identification of a human Klotho variant called KL-VS. Roughly one in five people of European ancestry carry one copy of this variant. People with one copy live longer on average, perform better on cognitive tests, and have larger volumes of the right dorsolateral prefrontal cortex in MRI imaging. People with two copies, paradoxically, have shorter lifespans, a phenomenon called heterozygote advantage that recurs throughout evolutionary genetics.

Several independent cohorts have replicated the KL-VS heterozygote cognitive benefit. The Health and Retirement Study, the Wisconsin Registry for Alzheimer’s Prevention, the Religious Orders Study, and the Memory and Aging Project have all identified the same association. Even more compellingly, a 2020 paper from Dubal’s group reported that KL-VS heterozygosity reduces the risk of conversion to dementia among carriers of the APOE4 allele, the strongest common genetic risk factor for late onset Alzheimer’s disease. In other words, a high Klotho genotype appears to buffer against a high Alzheimer’s risk genotype.

Carmela Abraham at Boston University has run a parallel research program for two decades, focused on the choroid plexus as the primary source of brain Klotho and on the decline of Klotho expression in Alzheimer’s disease tissue. Her work showed that Klotho levels drop by roughly 50 percent in the aged human brain and drop further still in Alzheimer’s pathology, providing both a biomarker and a therapeutic rationale.

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The Kidney Story: FGF23, CKD, and the Phosphate Axis

For nephrologists, Klotho has been the most clinically actionable longevity hormone for more than a decade. Orson Moe at UT Southwestern, Makoto Kuro-o himself, and Myles Wolf at Duke have built a deep evidence base showing that soluble alpha Klotho measured in serum or urine predicts chronic kidney disease progression, cardiovascular events in dialysis patients, and all cause mortality in elderly cohorts. The 2011 paper from Moe and colleagues in the Journal of the American Society of Nephrology showed that Klotho protein declines early in chronic kidney disease, before glomerular filtration rate begins to fall. That kinetic insight reframed Klotho as not just a marker of damaged kidneys but a potential cause of kidney aging itself.

The therapeutic implication is that restoring Klotho, either by infusing recombinant protein or by inducing endogenous Klotho production with small molecules, could slow the progression of kidney disease independent of blood pressure or glucose control. Several preclinical programs have demonstrated this in mouse models. Recombinant Klotho protein given to mice with acute kidney injury improves recovery and reduces fibrosis. Klotho gene therapy delivered by adeno associated virus also slows progression in models of polycystic kidney disease.

The Cardiovascular and Vascular Aging Story

The phosphate axis links Klotho to vascular aging in a way that has gained increasing attention from cardiologists. When Klotho falls and FGF23 fails to suppress phosphate as effectively, calcium phosphate crystals begin to deposit in the medial layer of arteries. This is the Monckeberg type calcification that drives stiffness and pulse pressure elevation in older adults and in patients with chronic kidney disease.

Several large observational studies, including work from the Cardiovascular Health Study and the Heart and Soul Study, have shown that low circulating Klotho levels predict incident heart failure, atrial fibrillation, and cardiovascular mortality. A 2017 paper in the European Heart Journal from a group at the University of Tokyo showed that Klotho declines mirrored vascular stiffness measures across the human lifespan. The interpretation that has emerged is that Klotho is, in part, an antifibrotic and anti calcification hormone. Restoring it may be a strategy not just for kidney patients but for the much larger population of older adults with arterial stiffness and isolated systolic hypertension.

Klotho in Centenarians: The Human Genetics

The KL-VS variant has been a recurring signal in the genetics of human exceptional longevity. Studies of centenarian populations in Ashkenazi Jewish, Italian, Chinese, and US populations have found higher than expected rates of KL-VS heterozygosity. The original observation came from Nir Barzilai’s Longevity Genes Project at Albert Einstein College of Medicine, where the variant was associated with reaching age 95 in good health. Subsequent work from the New England Centenarian Study and the Long Life Family Study added independent confirmation.

What the genetics suggests is that small differences in lifelong Klotho exposure, of the size that a single genetic variant produces, are enough to shift cardiovascular, cognitive, and metabolic aging trajectories. That is the kind of biological signal that makes pharmacology compelling. If a modest, sustained Klotho boost over decades shifts lifespan, then a larger therapeutic boost given to people with already declining Klotho might shift it more.

The Therapeutic Pipeline in 2026

The translational landscape for Klotho has evolved meaningfully in the last 24 months. Several companies and academic groups are now developing Klotho based therapeutics through three broad strategies.

The first is recombinant alpha Klotho protein. Klotho Neurosciences, a publicly listed biotechnology company built around technology originally licensed from Carmela Abraham’s Boston University laboratory, has been advancing a recombinant Klotho program intended for neurodegeneration and amyotrophic lateral sclerosis. The challenge with a recombinant protein is half life and brain penetration, and the program has focused on engineering variants that bypass these constraints.

The second strategy is gene therapy delivered by adeno associated virus. Several preclinical programs, including work from Mayo Clinic groups, have tested AAV Klotho in models of acute kidney injury, polycystic kidney disease, and Alzheimer’s disease. AAV approaches face their own constraints, particularly around dose, immunogenicity, and durability, but the field has seen sufficient progress with AAV in neurology and ophthalmology that a Klotho gene therapy is no longer a far horizon proposition.

The third strategy is small molecule Klotho inducers. Several screens have identified compounds, including some repurposed drugs, that increase endogenous Klotho expression in kidney and brain tissue. The argument for this approach is pharmacological convenience. An oral pill that raises a longevity hormone could enter primary care in a way that an IV protein infusion or a one shot gene therapy cannot.

Phase 1 safety studies of Klotho based therapies are now underway in 2026, and the next 18 to 36 months will deliver the first human pharmacodynamic and pharmacokinetic data that any honest assessment of the field has been waiting for since Kuro-o’s 1997 paper.

The Open Questions

Klotho is one of the cleaner longevity stories in modern biology, but it is not without unresolved questions. The first is whether circulating Klotho measured in serum is a faithful proxy for tissue Klotho activity. The kidney is the dominant source of soluble Klotho, but most of the biology, particularly in the brain, depends on local production. The biomarker problem is not trivial, and the field will need better assays before Klotho can be used as a clinical readout the way LDL cholesterol or HbA1c are now.

The second question is about the dose response curve. The KL-VS data hint at a U shaped relationship, where moderate Klotho elevation extends life and large elevation may not. That kind of hormetic shape is common in biological systems but is poorly understood for Klotho specifically. Trials will need careful dose finding.

The third question is whether Klotho works in already damaged tissue or only in tissue that retains the capacity to respond. Cognitive benefits in mice have appeared rapidly even in old animals, suggesting Klotho is acting on circuits that are still salvageable. Whether the same is true for advanced Alzheimer’s disease, advanced chronic kidney disease, or end stage heart failure remains an open question.

The fourth and most important question for the longevity field is whether any individual Klotho intervention will be sufficient on its own. Aging is multi causal. Klotho touches several axes, but it does not address mitochondrial dysfunction, telomere attrition, stem cell exhaustion, or the immune aging that drives much of late life morbidity. A future longevity practice may use Klotho the way modern cardiology uses statins, as one of several pillars of risk reduction rather than as a single bullet.

What This Means For You

Klotho is not yet a clinical product. There is no FDA approved Klotho therapy for any indication, and no licensed direct to consumer test measures it in a way that would let an individual track and modify their levels with confidence. That will likely change in the next several years, and the choices you make now will determine how prepared you are when it does.

There are three actionable items worth knowing today. The first is that the modifiable lifestyle factors most strongly associated with higher endogenous Klotho expression are the same ones that show up everywhere else in longevity medicine. Regular aerobic exercise raises circulating Klotho in human studies. Resistance training does as well. Vitamin D sufficiency is associated with higher Klotho expression in kidney tissue. Adequate sleep and chronic stress reduction also appear to protect Klotho production, although the mechanistic data here are less mature.

The second is that the upstream drivers of Klotho loss are the same conditions that drive the chronic diseases of aging. Hypertension, hyperphosphatemia, smoking, sedentary lifestyle, and uncontrolled diabetes all suppress Klotho expression. Treating these conditions well, with the tools you and your clinician already have, protects your endogenous Klotho more reliably than any speculative supplement.

The third is that the next 36 months are going to be unusually informative for this field. If you or a family member has a strong family history of Alzheimer’s disease, especially with an APOE4 background, or has progressive chronic kidney disease, the emerging Klotho trials may be worth tracking with your clinician. The first signals of human efficacy, when they arrive, will help clarify whether Klotho is one of the great long bets of longevity medicine or one of the great cautionary tales. For now, it is the rare hormone that has accumulated thirty years of biology, three converging organ stories, and a genetics signal from human centenarians, and is finally in the clinic.

That is enough to take seriously, and not yet enough to act on alone. Watch the trials. Treat the kidneys, the heart, and the brain you already have. The therapy will come.

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