Calcium phosphate microcrystals in the renal tubular fluid accelerate chronic kidney disease progression
Kazuhiro Shiizaki, … , Masayuki Murata, Makoto Kuro-o
Kazuhiro Shiizaki, … , Masayuki Murata, Makoto Kuro-o
Published June 29, 2021
Citation Information: J Clin Invest. 2021;131(16):e145693. https://doi.org/10.1172/JCI145693.
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Research Article Nephrology

Calcium phosphate microcrystals in the renal tubular fluid accelerate chronic kidney disease progression

Abstract

The Western pattern diet is rich not only in fat and calories but also in phosphate. The negative effects of excessive fat and calorie intake on health are widely known, but the potential harms of excessive phosphate intake are poorly recognized. Here, we show the mechanism by which dietary phosphate damages the kidney. When phosphate intake was excessive relative to the number of functioning nephrons, circulating levels of FGF23, a hormone that increases the excretion of phosphate per nephron, were increased to maintain phosphate homeostasis. FGF23 suppressed phosphate reabsorption in renal tubules and thus raised the phosphate concentration in the tubule fluid. Once it exceeded a threshold, microscopic particles containing calcium phosphate crystals appeared in the tubule lumen, which damaged tubule cells through binding to the TLR4 expressed on them. Persistent tubule damage induced interstitial fibrosis, reduced the number of nephrons, and further boosted FGF23 to trigger a deterioration spiral leading to progressive nephron loss. In humans, the progression of chronic kidney disease (CKD) ensued when serum FGF23 levels exceeded 53 pg/mL. The present study identified calcium phosphate particles in the renal tubular fluid as an effective therapeutic target to decelerate nephron loss during the course of aging and CKD progression.

Authors

Kazuhiro Shiizaki, Asako Tsubouchi, Yutaka Miura, Kinya Seo, Takahiro Kuchimaru, Hirosaka Hayashi, Yoshitaka Iwazu, Marina Miura, Batpurev Battulga, Nobuhiko Ohno, Toru Hara, Rina Kunishige, Mamiko Masutani, Keita Negishi, Kazuomi Kario, Kazuhiko Kotani, Toshiyuki Yamada, Daisuke Nagata, Issei Komuro, Hiroshi Itoh, Hiroshi Kurosu, Masayuki Murata, Makoto Kuro-o

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Figure 1

Calcium phosphate particles induce renal tubular cell damage.

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Calcium phosphate particles induce renal tubular cell damage. (A) Human ...
(A) Human HK-2 renal proximal tubule cells were cultured in control medium (DMEM containing 0.1% FBS, 3 mM calcium, and 1 mM phosphate) or in high-phosphate media (DMEM containing 0.1% FBS, 3 mM calcium, and 3, 5, or 7 mM phosphate). Cell viability was quantified 24 hours later by MTT assay and is expressed as the percentage of viable cells in the control medium. The same experiment was repeated in the presence of alendronate at the indicated concentrations. Data represent the mean ± SD. n = 4 for each culture condition. **P < 0.0001 versus the control in each culture condition, by 1-way ANOVA with Tukey’s multiple-comparison test. (B) Transmission electron microscopic observation of the medium containing 7 mM phosphate. Scale bar: 500 nm. (C) Particle size distribution of calcium phosphate particles. DMEM containing 3 mM calcium and 7 mM phosphate was incubated at room temperature for 24 hours and then subjected to nanoparticle tracking analysis before (black, CaPi particles) or after centrifugation at 16,000g for 30 minutes (CFG) or treatment with HCl (at 100 mM for 30 minutes) or EDTA (at 50 mM for 30 minutes). (D) The relative viability of HK-2 cells cultured in the control medium and in the supernatant (sup) of the high-phosphate medium (7 mM phosphate) after centrifugation at 16,000g for 2 hours was determined by MTT assay. Data represent the mean ± SD. n = 9 for each culture condition. No significant difference was observed (NS) by Student’s t test. (E) Relative viability of HK-2 cells cultured in the control medium inoculated with the indicated doses of synthesized calcium phosphate particles. Data represent the mean ± SD. n = 8 for each culture condition. *P < 0.05 and **P < 0.0001 versus control, by 1-way ANOVA with Dunnett’s multiple-comparison test.