Targeted Deletion of Klotho in Kidney Distal Tubule Disrupts Mineral Metabolism

Abstract

Renal Klotho controls mineral metabolism by directly modulating tubular reabsorption of phosphate and calcium and by acting as a co-receptor for the phosphaturic and vitamin D-regulating hormone fibroblast growth factor-23 (FGF23). Klotho null mice have a markedly abnormal phenotype. We sought to determine effects of renal-specific and partial deletion of Klotho to facilitate investigation of its roles in health and disease. We generated a mouse model with partial deletion of Klotho in distal tubular segments (Ksp-KL 2/2 ).In contrast to Klotho null mice, Ksp-KL 2/2 mice were fertile, had a normal gross phenotype, and did not have vascular or tubular calcification on renal histology. However, Ksp-KL 2/2 mice were hyperphosphatemic with elevated FGF23 levels and abundant expression of the sodium-phosphate cotransporter Npt2a at the brush border membrane. Serum calcium and 1,25-dihydroxyvitamin D 3 levels were normal but parathyroid hormone levels were decreased. TRPV5 protein was reduced with a parallel mild increase in urinary calcium excretion. Renal expression of vitamin D regulatory enzymes and vitamin D receptor was higher in Ksp-KL 2/2 mice than controls, suggesting increased turnover of vitamin D metabolites and a functional increase in vitamin D signaling. There was a threshold effect of residual renal Klotho expression on FGF23: deletion of .70% of Klotho resulted in FGF23 levels 30-250 times higher than in wild-type mice. A subgroup of Ksp-KL 2/2 mice with normal phosphate levels had elevated FGF23, suggesting a Klothoderived renal-bone feedback loop. Taken together, renal FGF23-Klotho signaling, which is disrupted in CKD, is essential for homeostatic control of mineral metabolism.

Fractional Excretion of Phosphorus Modifies the Association between Fibroblast Growth Factor-23 and Outcomes

“…18,19 Prior studies in mice show that klotho haplo insufficiency (approximately 50% reduction in klotho) is accompanied by an approximately threefold higher FGF23 concentration, suggesting that FGF23 may be increased in a compensatory fashion to induce its biologic effects when klotho is decreased. 14,15 In this context, our findings could suggest that concurrent high FGF23 and low FePi may be serving as a noninvasive marker of low kidney klotho expression or activity. Future studies with kidney pathology specimens suitable for klotho assays are required to confirm this hypothesis.…”

“…In individuals with relative renal tubule resistance to FGF23, serum FGF23 concentrations may be secondarily increased in a feedback loop in an attempt to further increase phosphorus excretion. 14,15 To our knowledge, no prior study has examined the joint relationships of FGF23 concentrations and urine FePi with adverse outcomes. To that end, we evaluated the strength of the associations of serum FGF23 concentrations with mortality and CVD events and determined the extent to which these associations were modified by the degree of renal phosphorus excretion.…”

Competitive Interaction Between Fibroblast Growth Factor 23 And Asymmetric Dimethylarginine in Patients With CKD

“…34 FGF-23 and the associated co-receptor Klotho impinge on the same system. Indeed, Klotho knockout mice 35,36 (i.e., animal models characterized by high plasma levels of FGF-23 37 ) do not express NO synthase in the vascular system and exhibit almost abolished response to acetylcholine, the strongest stimulant of NO activity. 36 Our findings show that ADMA has a variable influence on renal outcomes across the spectrum of FGF-23 values because it is associated with a higher risk for CKD progression only when FGF-23 levels are in the low-normal range (i.e., when the expression of NO synthase is not suppressed by FGF-23-Klotho).…”

Post-transcriptional regulation of the Type IIA sodium-phosphate cotransporter by parathyroid hormone.

“…This knockout mouse displays increased protein expression of Npt2a in the proximal tubule as a result of the Klotho deletion, and yet decreased levels of the Npt2a transcript, perhaps in response to the accompanying hyperphosphatemia 56 . Glucocorticoids, known downregulators of sodium-phosphate cotransport in the proximal tubule, have been shown to reduce both Npt2a protein and mRNA expression.…”

“…Conversely, mice with FGF23 gene ablation display increased serum phosphate, increased apical expression of Npt2a, and an accompanying increase in the renal phosphate transport maximum 53 . Distal-tubule-specific deletion of Klotho also produces a phenotype of hyperphosphatemia with abundant Npt2a BBM expression 56 .…”