Klotho's Kidney Roles Redefined: Proximal and Distal Tubules Exhibit Distinct Functions in Mineral Metabolism and Aging

The protein Klotho was initially identified as an aging-suppressor, with its function intrinsically linked to the kidney. Its soluble ectodomain, sKlotho, circulates in blood and urine, acting as an obligate co-receptor for FGF23. This places Klotho at a critical intersection of mineral metabolism and chronic kidney disease (CKD). While Klotho expression varies along the nephron, its functional significance has been unclear, leading to a need for a refined understanding of its segment-specific roles and how this heterogeneity impacts health and disease.

This review synthesizes recent findings from nephron segment-specific Klotho knockout mouse models and other studies to re-evaluate the long-standing paradigm of renal Klotho biology. The authors systematically analyzed emerging evidence for functional specialization of tubular Klotho along the nephron. They critically assessed existing literature to propose a revised model that clarifies the distinct contributions of proximal and distal nephron Klotho to mineral metabolism, bone remodeling, aging, and CKD progression, moving beyond a generalized view of its kidney functions and highlighting the importance of tubular heterogeneity.

The review establishes a revised model demonstrating functional specialization of Klotho along the nephron. It highlights that distal nephron Klotho primarily regulates calcium reabsorption, bone remodeling, and urinary sKlotho levels. In contrast, proximal tubular Klotho is identified as the likely principal source of circulating sKlotho and is crucial for regulating phosphate and vitamin D metabolism.

Loss of proximal tubular Klotho recapitulates hyperphosphatemia, FGF23 resistance, and various aging-like phenotypes, underscoring its central role in systemic mineral homeostasis and age-related processes. This functional heterogeneity challenges previous monolithic views, emphasizing the kidney's intricate role in integrating mineral metabolism and aging through segment-specific Klotho actions, thereby placing tubular Klotho heterogeneity at the center of these complex physiological processes.