In 1997, Makoto Kuro-o identified a gene that, when disrupted in mice, produced a syndrome mimicking accelerated aging: shortened lifespan, arteriosclerosis, skin atrophy, osteoporosis, emphysema. He named it after the Greek goddess Klotho, who spins the thread of life. Kurosu et al. (2005) then showed the opposite: mice overexpressing klotho lived 20 to 30% longer. The protein has two forms. A transmembrane version acts as a co-receptor for FGF23 in mineral metabolism. A secreted ectodomain (α-klotho) circulates in blood and cerebrospinal fluid.

Dena Dubal's lab at UCSF has focused on what klotho does for the brain. Dubal et al. (2014) showed in Cell Reports that lifelong klotho elevation in mice enhanced long-term potentiation (LTP) in the hippocampus and improved spatial and working memory. The cognitive boost worked at every age, young and old, and correlated with increased NMDA receptor GluN2B subunit expression at postsynaptic densities.

The therapeutic leap came when researchers found that a single injection could replicate what genetic overexpression achieved. Leon et al. (2024) gave aged mice a single peripheral injection of recombinant α-klotho ectodomain and saw enhanced synaptic plasticity and rescued cognitive deficits within hours, with effects lasting at least two weeks. The protein doesn't cross the blood-brain barrier. It acts through peripheral signaling cascades, likely involving FGF receptor activation, that boost central glutamatergic neurotransmission.

The primate study by Castner et al. (2023), published in Nature Aging, takes this closer to the clinic. A single low-dose subcutaneous injection of α-klotho (10 μg/kg) in aged rhesus macaques (18 to 25 years, equivalent to 55 to 75 human years) improved performance on a prefrontal cortex-dependent working memory task and a hippocampus-dependent spatial memory task. The cognitive improvement amounted to turning back the clock by about 15 years.

Biopsies from prefrontal cortex collected at a later time point showed increased dendritic spine density and elevated synaptic plasticity genes: BDNF, Arc, and CREB. GluN2B receptor levels were up in synaptosomal fractions, matching the mouse findings. Amyloid-β and phosphorylated tau levels didn't change, which suggests klotho enhances normal synaptic function rather than targeting Alzheimer's pathology per se.

Human klotho levels decline about 50% between ages 30 and 70. Epidemiological studies link higher levels to better cognition, lower cardiovascular risk, and longer lifespan. Semba et al. (2014) found that low plasma klotho predicted all-cause mortality in community-dwelling older adults. Yokoyama et al. (2015) identified a common functional variant (KL-VS) that raises klotho levels and is associated with better cognition and larger prefrontal cortical volume.

A Phase I trial of recombinant α-klotho for age-related cognitive decline is enrolling at UCSF, with initial results expected in late 2026. The primate data gives reason to expect human translation. If a single injection can reverse years of cognitive decline in a species close to ours, this could be the first therapy that targets normal age-related cognitive loss rather than a specific neurodegenerative disease.