Genetic diseases of renal phosphate handling

リンは，普遍的に存在するミネラルである。ヒトにおける生理学的重要性にもかかわらず，リン代謝に関する多くの見解が明らかになってきたのは比較的最近である。特に，FGF23 はリン恒常性の調節に関係する重要なホルモンであることがわかってきた。遺伝性リン代謝異常症の研究は，全体的なリン調節の複雑性を浮き彫りにし，これからの研究における新しい領域を開拓してきた。 これらは，低リン血症性くる病/骨軟化症の病態のみならず，慢性腎臓病にみられるリン代謝異常と心血管障害との関係の理解に新しい知見を提示することが期待される

Control of phosphate appetite in young rats

In the present study, we investigated whether a diet deficient in inorganic phosphate (Pi) stimulates an ingestive behavior to seek sources of Pi. Male Wistar rats were placed in individual cages with unrestricted access to tap water and a low (LPD, 0.02% Pi) or normal (NPD, 0.6% Pi) Pi diet for 6 days. On day 7, LDP rats were given unlimited access to a solution of 25 mM potassium phosphate water (Pi-water) for 9 additional days. Rats fed LPD consumed 70-100% more Pi-water then those fed NPD. The increase in Pi-water intake resulted in a marked rise in the growth rate of rats fed LPD during day 9. A similar intake of Pi was induced after only 2 days of LPD and was associated with significant reductions in both plasma and cerebrospinal fluid (CSF) levels of Pi ; these levels remained low throughout Pi restriction, despite a significant intake of Pi-water. Replenishment with a high-Pi diet rapidly quenched the appetite for Pi-water and was associated with restoration of both plasma and CSF Pi levels. These findings suggest that an appetite for Pi can be induced in rats, perhaps through lowered plasma and CSF Pi levels

Na依存性PiトランスポーターNpt2cは、KlothoノックアウトマウスPi恒常性において成長期と成熟期では異なる作用を有する

SLC34A3/NPT2c/NaPi-2c/Npt2c is a growth-related NaPi cotransporter that mediates the uptake of renal sodium-dependent phosphate (Pi). Mutation of human NPT2c causes hereditary hypophosphatemic rickets with hypercalciuria. Mice with Npt2c knockout, however, exhibit normal Pi metabolism. To investigate the role of Npt2c in Pi homeostasis, we generated α-klotho−/−/Npt2c−/− (KL2cDKO) mice and analyzed Pi homeostasis. α-Klotho−/− (KLKO) mice exhibit hyperphosphatemia and markedly increased kidney Npt2c protein levels. Genetic disruption of Npt2c extended the lifespan of KLKO mice similar to that of α-Klotho−/−/Npt2a−/− mice. Adult KL2cDKO mice had hyperphosphatemia, but analysis of Pi metabolism revealed significantly decreased intestinal and renal Pi (re)absorption compared with KLKO mice. The 1,25-dihydroxy vitamin D3 concentration was not reduced in KL2cDKO mice compared with that in KLKO mice. The KL2cDKO mice had less severe soft tissue and vascular calcification compared with KLKO mice. Juvenile KL2cDKO mice had significantly reduced plasma Pi levels, but Pi metabolism was not changed. In Npt2cKO mice, plasma Pi levels began to decrease around the age of 15 days and significant hypophosphatemia developed within 21 days. The findings of the present study suggest that Npt2c contributes to regulating plasma Pi levels in the juvenile stage and affects Pi retention in the soft and vascular tissues in KLKO mice

NaPi-IIc induces vacuole formation in OK cells

NaPi-IIc/SLC34A3 is a sodium-dependent inorganic phosphate (Pi) transporter in the renal proximal tubules and its mutations cause hereditary hypophosphatemic rickets with hypercalciuria (HHRH). In the present study, we created a specific antibody for opossum SLC34A3, NaPi-IIc (oNaPi-IIc), and analyzed its localization and regulation in opossum kidney cells (a tissue culture model of proximal tubular cells). Immunoreactive oNaPi-IIc protein levels increased during the proliferative phase and decreased during differentiation. Moreover, stimulating cell growth upregulated oNaPi-IIc protein levels, whereas suppressing cell proliferation downregulated oNaPi-IIc protein levels. Immunocytochemistry revealed that endogenous and exogenous oNaPi-IIc proteins localized at the protrusion of the plasma membrane, which is a phosphatidylinositol 4,5-bisphosphate (PIP2) rich-membrane, and at the intracellular vacuolar membrane. Exogenous NaPi-IIc also induced cellular vacuoles and localized in the plasma membrane. The ability to form vacuoles is specific to electroneutral NaPi-IIc, and not electrogenic NaPi-IIa or NaPi-IIb. In addition, mutations of NaPi-IIc (S138F and R468W) in HHRH did not cause cellular PIP2-rich vacuoles. In conclusion, our data anticipate that NaPi-IIc may regulate PIP2 production at the plasma membrane and cellular vesicle formation

Fibroblast growth factor 23 mediates the phosphaturic actions of cadmium

Phosphaturia has been documented following cadmium (Cd) exposure in both humans and experimental animals. The fibroblast growth factor 23 (FGF23)/klotho axis serves as an essential phosphate homeostasis pathway in the bone-kidney axis. In the present study, we investigated the effects of Cd on phosphate (Pi) homeostasis in mice. Following Cd injection into WT mice, plasma FGF23 concentration was significantly increased. Urinary Pi excretion levels were significantly higher in Cd-injected WT mice than in control group. Plasma Pi concentration decreased only slightly compared with control group. No change was observed in plasma parathyroid hormone and 1,25-dihydroxy vitamin D3 in both group of mice. We observed a decrease in phosphate transport activity and also decrease in expression of renal phosphate transporter SLC34A3 [NaPi-IIc/NPT2c], but not SLC34A1 [NaPi-IIa/NPT2a]. Furthermore, we examined the effect of Cd on Npt2c in Npt2a-knockout (KO) mice which expresses Npt2c as a major NaPi co-transporter. Injecting Cd to Npt2aKO mice induced significant increase in plasma FGF23 concentration and urinary Pi excretion levels. Furthermore, we observed a decrease in phosphate transport activity and renal Npt2c expression in Cd-injected Npt2a KO mice. The present study suggests that hypophosphatemia induced by Cd may be closely associated with the FGF23/klotho axis

Effect of dietary components on renal inorganic phosphate (Pi) excretion induced by a Pi-depleted diet

Dietary inorganic phosphate (Pi) is the most important factor in the regulation of renal Pi excretion. Recent studies suggest the presence of an enteric-renal signaling axis for dietary Pi as well as the existence of a mechanism by which the intestine detects changes in luminal Pi concentrations. The mechanisms of intestinal Pi sensing, however, are unknown. In the present study, we focused on Pi depletion signals and investigated the effects of dietary components on intestinal Pi sensing. After feeding rats experimental diets for 3 days, we investigated urinary Pi excretion and plasma biochemical parameters. Renal Pi excretion was suppressed in rats fed a low-Pi diet (0.02% Pi). Elimination of dietary calcium (Ca) completely blocked the suppression of Pi excretion, suggesting that the presence of Ca is essential for the Pi depletion signal. Furthermore, a minimum Ca content of more than 0.02% was necessary for the Pi depletion signal. Magnesium, lanthanum, and strontium, which are agonists of calcium sensing receptor, instead of Ca, reduced Pi excretion.Therefore, dietary Ca appears to be important for the Pi depletion-sensing mechanism in the gastrointestinal tract. In addition, the calcium sensing receptor may be involved in the Pi depletion signal

Phosphate Homeostasis and Osteocyte Ablation

In response to kidney damage, osteocytes increase the production of several hormones critically involved in mineral metabolism. Recent studies suggest that osteocyte function is altered very early in the course of chronic kidney disease. In the present study, to clarify the role of osteocytes and the canalicular network in mineral homeostasis, we performed four experiments. In Experiment 1, we investigated renal and intestinal Pi handling in osteocyte-less (OCL) model mice [transgenic mice with the dentin matrix protein-1 promoter-driven diphtheria toxin (DT)-receptor that were injected with DT]. In Experiment 2, we administered granulocyte colony-stimulating factor to mice to disrupt the osteocyte canalicular network. In Experiment 3, we investigated the role of osteocytes in dietary Pi signaling. In Experiment 4, we analyzed gene expression level fluctuations in the intestine and liver by comparing mice fed a high Pi diet and OCL mice. Together, the findings of these experiments indicate that osteocyte ablation caused rapid renal Pi excretion (P < 0.01) before the plasma fibroblast growth factor 23 (FGF23) and parathyroid hormone (PTH) levels increased. At the same time, we observed a rapid suppression of renal Klotho (P < 0.01), type II sodium phosphate transporters Npt2a (P < 0.01) and Npt2c (P < 0.05), and an increase in intestinal Npt2b (P < 0.01) protein. In OCL mice, Pi excretion in feces was markedly reduced (P < 0.01). Together, these effects of osteocyte ablation are predicted to markedly increase intestinal Pi absorption (P < 0.01), thus suggesting that increased intestinal Pi absorption stimulates renal Pi excretion in OCL mice. In addition, the ablation of osteocytes and feeding of a high Pi diet affected FGF15/bile acid metabolism and controlled Npt2b expression. In conclusion, OCL mice exhibited increased renal Pi excretion due to enhanced intestinal Pi absorption. We discuss the role of FGF23–Klotho on renal and intestinal Pi metabolism in OCL mice