Klotho Isoforms: Molecular Functions and Clinical Implications

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Overview of Klotho

The Klotho gene consists of five exons and encodes a type I single-pass transmembrane glycoprotein, with 1014 amino acids in mice and 1012 amino acids in humans. The protein is primarily localized to the plasma membrane and the Golgi apparatus. Its intracellular region is minimal, comprising only about 10 amino acids, and lacks recognizable functional motifs.

The extracellular portion contains two internal repeats, designated KL1 and KL2, which share sequence similarity with family 1 glycosidases. These enzymes typically hydrolyze β-glycosidic linkages in saccharides, glycoproteins, and glycolipids. However, despite this homology, recombinant Klotho protein demonstrates no classical glycosidase enzymatic activity, likely due to divergence in key catalytic residues. In vitro, Klotho exhibits weak β-glucuronidase activity, and its biological actions are thought to involve β-glucuronidase and/or sialidase-like mechanisms.

A critical feature is a proteolytic cleavage site within the linker between KL1 and KL2, where metalloproteases such as ADAM10 and ADAM17 release the extracellular domain. This soluble form circulates in blood, urine, and cerebrospinal fluid, functioning in endocrine, paracrine, and autocrine pathways. Additionally, an alternatively spliced transcript lacking exons 4 and 5 produces a secreted variant in mice.

Tissue Distribution

Klotho is most prominently expressed in the kidney, particularly in the distal convoluted tubules, but is also found in the choroid plexus of the brain, the renal proximal tubule, parathyroid glands, and reproductive tissues including ovary, testis, and placenta. Recent studies also demonstrate local expression in the adventitial layer of the aorta, suggesting vascular protective effects. Tissue-specific expression patterns continue to be updated.

Within this family of proteins, three isoforms are recognized:

• α-Klotho: Encoded by five exons, with a large extracellular domain, a transmembrane segment, and a short cytoplasmic tail of 11 amino acids. The KL1 and KL2 regions can be released into circulation following proteolytic cleavage, forming soluble Klotho, the predominant circulating form.
• β-Klotho: Expressed mainly in the liver, but also present in kidney, gut, and spleen. It serves as a co-receptor regulating fibroblast growth factor (FGF) signaling, especially FGF19 and FGF21.
• γ-Klotho: Expressed in skin and kidney; functional roles remain under investigation.

Mechanistic Insights

Membrane-bound α-Klotho interacts with fibroblast growth factor receptors (FGFRs) to form complexes required for FGF23 signaling. This pathway is central to phosphate and vitamin D homeostasis in the kidney.

Proteolytic cleavage by ADAM10 and ADAM17 releases the extracellular domain into circulation. Soluble Klotho exhibits sialidase-like activity, modifying glycans on ion channels such as TRPV5, thereby regulating calcium transport. It also suppresses signaling from growth factors including insulin, IGF-1, and Wnt. The inhibitory effect on IGF-1 signaling is thought to contribute to antioxidant and potential anti-tumor properties.

Evidence From Research

Cellular and Physiological Roles

Experimental studies have identified multiple functions of Klotho, including:

• Reduction of oxidative stress
• Improvement of mitochondrial performance
• Attenuation of renal fibrosis
• Modulation of inflammatory pathways
• Mitigation of premature aging processes
• Enhancement of cognitive performance
• Regulation of autophagy and lysosomal activity via TFEB activation
• Clearance of β-amyloid and phosphorylated tau proteins

Klotho and Disease

Aging

Deficiency of Klotho or FGF23 in mice results in phosphate retention and a premature aging phenotype, linking phosphate metabolism with aging biology. The soluble form also regulates multiple growth factors and protects tissues from oxidative stress, reinforcing its role as an anti-aging factor.

Coronary Artery Disease

Variants in the Klotho gene, particularly the KL-VS allele, are associated with increased risk of early-onset coronary artery disease (CAD). Around 25% of individuals carry this allele, highlighting its clinical relevance for risk stratification and precision medicine.

Chronic Kidney Disease

Klotho deficiency plays a central role in CKD progression and complications. Supplementation with soluble Klotho has been shown to reduce renal fibrosis, suppress epithelial-mesenchymal transition, and inhibit pro-fibrotic signaling pathways such as TGF-β and Wnt. These findings position Klotho as a potential therapeutic target in CKD.

Cancer

First identified as a tumor suppressor in breast cancer, Klotho has since been shown to be downregulated in several malignancies including cervical, pancreatic, and digestive cancers, as well as melanoma. Its tumor-suppressive role involves modulation of FGF, IGF-1R, and Wnt signaling. Elevated expression is associated with improved outcomes in diffuse large B-cell lymphoma (DLBCL) and enhanced sensitivity to chemotherapy.

Atherosclerosis

Low serum Klotho levels correlate with increased carotid intima-media thickness, epicardial fat, and impaired vascular dilation. These findings suggest that reduced circulating Klotho is an early biomarker for atherosclerotic risk.

Cardiovascular Disease

Deficiency of vascular-derived Klotho promotes vascular calcification, endothelial dysfunction, and oxidative stress. Genetic variations in Klotho are linked to myocardial infarction, left ventricular hypertrophy, and other cardiac pathologies. Experimental supplementation demonstrates protective effects, including reduced fibrosis, improved mitochondrial function, and restoration of cardiac performance.

Conclusion

Klotho is a multifunctional protein with significant implications for aging, kidney disease, cardiovascular health, and cancer biology. Its dual roles as a membrane-bound co-receptor and a soluble endocrine factor allow it to influence diverse signaling pathways. Restoring or enhancing Klotho activity represents a promising therapeutic avenue across a spectrum of age-related and chronic diseases.

REFERENCES

1. Olejnik, A., Franczak, A., Krzywonos-Zawadzka, A., Kałużna-Oleksy, M., & Bil-Lula, I. (2018). The Biological Role of Klotho Protein in the Development of Cardiovascular Diseases. BioMed research international, 2018, 5171945. https://doi.org/10.1155/2018/5171945
2. Zhou, X., Fang, X., Jiang, Y. et al. Klotho, an anti-aging gene, acts as a tumor suppressor and inhibitor of IGF-1R signaling in diffuse large B cell lymphoma. J Hematol Oncol 10, 37 (2017). https://doi.org/10.1186/s13045-017-0391-5
3. KLOTHO Allele Status and the Risk of Early-Onset Occult Coronary Artery Disease
4. Kuro-o, M. The Klotho proteins in health and disease. Nat Rev Nephrol 15, 27–44 (2019). https://doi.org/10.1038/s41581-018-0078-3