LongeCityNews Last Updated: 03 April 2025 - 09:31 AM

When people talk about the klotho gene, they usually mean α-klotho, one of the better documented longevity-associated genes. It encodes a transmembrane protein, is expressed in a number of organs that sheds a portion of its structure to circulate in blood and tissues, interacting with other cells. In animal models artificially increased klotho expression improves late life health and life span while artificially diminished klotho expression worsens late life health and reduces life span. Interestingly, increased levels of klotho can improve cognitive function even in young animals. In humans, data shows the same correlation between circulating levels of klotho and age-related health and longevity.

The mechanisms by which klotho affects health on an organ by organ basis are far from fully understood, particularly when it comes to effects on the brain. It is best understood in the kidney, where it is protective against damage and diminished function with age. One hypothesis is that its body-wide effects are secondary to to kidney function, that loss of kidney function is an important contribution to age-related issues throughout the body. It does seem likely that it has direct effects on other organs as well, however.

The usual challenge in mechanisms relating to aging is that many processes are underway at the same time, interacting with one another. It is somewhere between hard and impossible to determine the relative size of each contribution to the end result of pathology and disease. The fastest path to that goal is to produce a therapy that only affects one contribution and observe the outcomes, but that is not always possible or practical.

Klotho antiaging protein: molecular mechanisms and therapeutic potential in diseases

Aging is not only a compilation of ailments that occur in the later stages of life; it is a dynamic process that unfolds throughout the lifespan. The escalating issue of an aging population is a significant economic, social, and medical concern of modern society. Over time, aging causes a segmental and gradual loss of strength and biological function, which leads to a decline in resistance and increasing physiological weakness. Multiple biochemical pathways actively control aging. It is distinguished by a number of molecular and cellular features, including abnormal nutrient sensing, mitochondrial dysfunction, cellular senescence, epigenetic imbalance, and loss of connectivity between cells. Globally, chronic illnesses tend to be more common in the aging population. Chronic illnesses need lengthy therapy, which alters the character of healthcare facilities and raises demand for them.

On the other hand, Klotho is an anti-aging protein with diverse therapeutic roles in the pathophysiology of different organs, such as the kidneys and skeletal muscle. Numerous pathways implicated in aging processes are regulated by Klotho, including Wnt signaling, insulin signaling, and phosphate homeostasis. It also impacts intracellular signaling pathways, including TGF-β, p53/p21, cAMP, and protein kinase C (PKC). Klotho expression and circulation levels decrease with increasing age. Klotho-deficient mice have excessive phosphate levels because of phosphate excretion imbalance in the urine. However, they also exhibit a complicated phenotype that includes stunted development, atrophy of several organs, hypercalcemia, kidney fibrosis, cardiac hypertrophy, and reduced lifespans. Given that supplementation or Klotho gene expression has been shown to suppress and repair Klotho-deficient phenotypes, it is likely that Klotho might have a protective impact against aging illness.

Recent cross-sectional cohort research with 346 healthy individuals aged 18 to 85 years showed that serum Klotho levels are negatively correlated with age and that older individuals (ages 55 to 85) exhibited the lowest serum α-Klotho levels. Another observational cohort research, which had around 804 adults over 65 years old, was conducted in Italy and found a negative correlation between serum Klotho levels and all-cause mortality. Furthermore, those with decreased serum Klotho levels had a comparable increased risk for all-cause death, according to a meta-analysis of six cohort studies that included adult chronic kidney disease (CKD) patients. Additionally, preclinical research has demonstrated that overexpressing the Klotho gene in transgenic mice can postpone or reverse aging. Therefore, increasing Klotho levels emerges as a promising strategy in diabetic kidney disease, CKD, and aging disorders.

Using a mouse model, researchers from UCSF have found that the genes that become activated on the silent X chromosome might explain some sex-dependent differences in cognitive abilities during aging [1].

XX and XY

It is widely known that women live longer than men [2]. Women also show differences in cognitive aging [3]. “In typical aging, women have a brain that looks younger, with fewer cognitive deficits compared to men,” said the senior author of this study, Dena Dubal, MD, PhD, a professor of neurology and the David A. Coulter Endowed Chair in Aging and Neurodegenerative Disease at UCSF.

This study’s authors link the genetic differences between sexes, specifically the X chromosome, of which women have two and men have one, to cognitive aging differences. “Cognition is one of our biggest biomedical problems, but things are changeable in the aging brain, and the X chromosome clearly can teach us what’s possible,” Dubal said.

Controlled inactivation

Even though women have two X chromosomes, they don’t simply express twice as many X-linked genes; instead, one X chromosome in each female cell is kept inactive or silent. However, some genes escape the inactivation of the second X chromosome. Such escapees from the silent X chromosome might be a part of sex-dependent differences, possibly those affecting cognition.

The inactivation of the X chromosome is a random process, and some cells in the same body might have an inactive maternal chromosome, while in others, the paternal X chromosome is inactive.

In experiments, it is challenging to distinguish whether a gene was expressed from the maternal or paternal chromosome. Therefore, the researchers crossed two mouse strains– Mus musculus and Mus castaneus. M. musculus is genetically modified to make the X chromosome from this strain always active, while the M. castaneus-derived X chromosome is inactive in every cell. Any gene expression that comes from M. castaneus, which can be assessed based on genetic differences, must be from the silent chromosome.

Changes on the X chromosome

The researchers used single-nucleus RNA sequencing to analyze the gene expression in 40,000 nuclei derived from different cell types in hippocampi taken from four young and four old female mice. The hippocampus is the the brain structure responsible for learning and memory.

An analysis of X chromosome-linked aging-impacted gene expression across hippocampal cell types uncovered that aging remodeled this expression from both X chromosomes in a cell-type-specific manner, suggesting differential responses to aging in different cells. Among the notable changes were several genes whose expression was activated from the silent X chromosome only in aged animals. “These results show that the silent X in females actually reawakens late in life, probably helping to slow cognitive decline,” said Dubal.

Many of these activated genes from the silent X chromosomes had neural-related functions. Additionally, nearly half of those genes are related to human X-linked conditions of intellectual disability, typically in males, who do not have the second X. In females, the second X can compensate for the mutation in a single X chromosome, suggesting the importance of those genes in cognitive functioning.

Female biology helping everyone

Among the identified genes, the researchers focused on the gene Plp1, which is activated on the silent X chromosome, and its expression increases with aging in a few cell types. Its protein, PLP1, is a component of myelin, the neuronal protective sheath essential for transmitting signals, and it is linked to Pelizaeus-Merzbacher disease, which results in intellectual disability.

A comparison of young and old mice’s hippocampi showed that Plp1 levels increased in aged female mice but not in the male parahippocampus, a region that surrounds the hippocampus and is involved in spatial memory, information, and context. These results were confirmed by measuring Plp1 expression in the aging human parahippocampus. Specifically, PLP1 levels were higher in older women than men in the parahippocampus but not in other tested brain regions.

To further test Plp1‘s effect on cognition in aging, the researchers created a genetically engineered virus that overexpressed Plp1 in oligodendrocytes. The focus on oligodendrocytes, cells that produce myelin, stems from the observation that “the highest overall expression and most robust aging-induced increase in Plp1” was in oligodendrocytes.

The researchers injected the dentate gyrus, one of the hippocampus regions, of mice of both sexes with the engineered virus expressing Plp1 or a control virus, which was identical except for the lack of Plp1 expression. The researchers chose the dentate gyrus because this brain region is essential for cognitive functions like spatial memory, and it exhibited the most differentially expressed genes in their analysis, suggesting sensitivity to aging.

Overexpression of Plp1 in oligodendrocytes of the hippocampi of aging mice didn’t change anxiety-like behaviors and total activity, but it improved learning and memory in both sexes.

These positive changes were observed in mice that received the treatment in relatively old age, showing that long-term treatment is not necessary. This is promising for developing future therapies for neurodegenerative diseases that occur later in life.

Understanding the biology

The authors discuss that the aging-induced activation of the silent X chromosome increases the dose of genes activated in the female hippocampus. They speculate that since the X chromosome is enriched in cognition-related genes, the increased dose of those genes might benefit cognitive abilities. “We immediately thought this might explain how women’s brains remain resilient in typical aging, because men wouldn’t have this extra X,” said Margaret Gadek, a graduate student in the MD PhD program at UCSF and the first author of the paper.

The authors suggest that aging-related epigenetic alterations, specifically methylation, might be responsible for making the chromatin more accessible, thus allowing some genes to be active on the silent X chromosome, but this still needs to be tested.

This research adds to a better understanding of sex-dependent differences in aging and what pathways and molecular processes are responsible for those differences. Understanding those differences might help find targets for interventions to increase both sexes’ healthspan and lifespan.

Literature

[1] Gadek, M., Shaw, C. K., Abdulai-Saiku, S., Saloner, R., Marino, F., Wang, D., Bonham, L. W., Yokoyama, J. S., Panning, B., Benayoun, B. A., Casaletto, K. B., Ramani, V., & Dubal, D. B. (2025). Aging activates escape of the silent X chromosome in the female mouse hippocampus. Science advances, 11(10), eads8169.

[2] Zarulli, V., Barthold Jones, J. A., Oksuzyan, A., Lindahl-Jacobsen, R., Christensen, K., & Vaupel, J. W. (2018). Women live longer than men even during severe famines and epidemics. Proceedings of the National Academy of Sciences of the United States of America, 115(4), E832–E840.

[3] McCarrey, A. C., An, Y., Kitner-Triolo, M. H., Ferrucci, L., & Resnick, S. M. (2016). Sex differences in cognitive trajectories in clinically normal older adults. Psychology and aging, 31(2), 166–175.

The aging of the gut microbiome involves a shift in the relative numbers of different microbial species. As a result the production of some beneficial metabolites declines while inflammatory microbial activities increase. At present there are few practical ways to permanently adjust the gut microbiome, one of which is the transplantation of fecal matter from a donor. Animal studies have demonstrated that fecal microbiota transplantation from a young animal to an old animal rejuvenates the gut microbiome, improves health, and extends life. Human studies are relatively limited, but this approach to treatment is established for patients with C. difficile infection. It remains to be seen as to whether it will find broader use, versus the more challenging approach of developing the means to culture a full or close to full gut microbiome artificially.

The gut microbiota has become a potential therapeutic target in several diseases, including cardiovascular diseases. Animal models of fecal microbiota transplantation (FMT) were established in elderly and young rats. 16S rRNA sequencing revealed that the gut microbiota of the recipients shifted toward the profile of the donors, with concomitant cardiac structure and diastolic function changes detected via ultrasound and positron emission tomography-computed tomography (PET-CT). The elderly recipient rats that received young fecal bacteria presented an overall reduction in aging characteristics, whereas young rats that received reverse transplantation presented an overall increase in aging characteristics.

After FMT, the structure and function of the hearts of the recipient rats changed correspondingly. The age-related thickening of the left ventricular wall and interventricular septum at the organ level, along with the disordered arrangement of cardiomyocytes and increased interstitial volume at the tissue level, decreased following FMT in young rats. These structural modifications are accompanied by alterations in cardiac function; however, systolic function did not significantly change, whereas diastolic function notably improved. The young rats that received reverse transplantation presented the opposite results as the aged rats did; that is, the structure and function of the heart were lower in the reverse-transplanted rats than in the same-aged control rats.

A group of significantly enriched myocardial metabolites detected by liquid chromatography-mass spectrometry (LC/MS) were involved in the fatty acid β-oxidation process. Together with altered glucose uptake, as revealed by PET-CT, changes in ATP content and mitochondrial structure further verified a metabolic difference related to energy among rats transplanted with the gut microbiota from donors of different ages. This study demonstrated that gut microbes may participate in the physiological aging process of the rat heart by regulating oxidative stress and autophagy. The gut microbiota has been shown to be involved in the natural aging of the heart at multiple levels, from the organ level to the metabolically plastic myocardiocytes and associated molecules.

Link: https://doi.org/10.1016/j.exger.2025.112734

While much of the focus on cellular senescence in aging remains to find ways to selectively destroy these problem cells, there are also efforts to instead change their behavior. The reason why a growing burden of senescent cells in aged tissues is harmful, even when these cells make up only a tiny fraction of the overall cell population, is that they energetically secrete pro-inflammatory factors. This activity is disruptive to tissue structure and function when sustained over time. If senescent cells could be blocked from producing inflammatory secretions, their harms would be much reduced.

Accumulation of DNA damage can accelerate aging through cellular senescence. Previously, we established a Drosophila model to investigate the effects of radiation-induced DNA damage on the intestine. In this model, we examined irradiation-responsive senescence in the fly intestine. Through an unbiased genome-wide association study (GWAS) utilizing 156 strains from the Drosophila Genetic Reference Panel (DGRP), we identified meltrin (the drosophila orthologue of mammalian ADAM19) as a potential modulator of the senescence-associated secretory phenotype (SASP).

Knockdown of meltrin resulted in reduced gut permeability, DNA damage, and expression of the senescence marker β-galactosidase (SA-β-gal) in the fly gut following irradiation. Additionally, inhibition of ADAM19 in mice using batimastat-94 reduced gut permeability and inflammation in the gut. Our findings extend to human primary fibroblasts, where ADAM19 knockdown or pharmacological inhibition decreased expression of specific SASP factors and SA-β-gal. Furthermore, proteomics analysis of the secretory factor of senescent cells revealed a significant decrease in SASP factors associated with the ADAM19 cleavage site. These data suggest that ADAM19 inhibition could represent a novel senomorphic strategy.

Link: https://doi.org/10.18632/aging.206224