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An inorganic phosphate (Pi)-restricted diet is important for patients with chronic kidney disease and patients on hemodialysis. Phosphate binders are essential for preventing hyperphosphatemia and ectopic calcification. The sodium-dependent Pi (Na/Pi) transport system is involved in intestinal Pi absorption and is regulated by several factors. The type II sodium-dependent Pi transporter Npt2b is expressed in the brush-border membrane in intestinal epithelial cells and transports Pi. In the present study, we analyzed the phenotype of Npt2b−/− and hetero+/− mice. Npt2b−/− mice died in utero soon after implantation, indicating that Npt2b is essential for early embryonic development. At 4 wk of age, Npt2b+/− mice showed hypophosphatemia and low urinary Pi excretion. Plasma fibroblast growth factor 23 levels were significantly decreased and 1,25(OH)2D3 levels were significantly increased in Npt2b+/− mice compared with Npt2b+/+ mice. Npt2b mRNA levels were reduced to 50% that in Npt2b+/+ mice. In contrast, renal Npt2a and Npt2c transporter protein levels were significantly increased in Npt2b+/− mice. At 20 wk of age, Npt2b+/− mice showed hypophosphaturia and reduced Na/Pi cotransport activity in the distal intestine. Npt2b+/+ mice with adenine-induced renal failure had hyperphosphatemia and high plasma creatinine levels. Npt2b+/− mice treated with adenine had significantly reduced plasma Pi levels compared with Npt2b+/+ mice. Intestinal Npt2b protein and Na+/Pi transport activity levels were significantly lower in Npt2b+/− mice than in the Npt2b+/+ mice. The findings of the present studies suggest that Npt2b is an important target for the prevention of hyperphosphatemia.

Background/Aims: Hospitalized patients with Crohn’s disease (CD) can develop severe nutritional deficits. However, the nutritional screening tools with the most utility for such patients are still unknown. Methods: Nutritional status of 40 CD patients was assessed on admission using several screening tools and laboratory tests. Their validity was evaluated in relation to length of hospital stay (LOS) and intestinal resection. Receiver operating characteristic analysis was performed to predict prolonged LOS (≥28 days). Results: Prolonged LOS was correlated with each of the following screening parameters: Subjective Global Assessment, Nutrition Risk Screening 2002 (NRS 2002), Onodera’s Prognostic Nutritional Index (O-PNI), Controlling Nutritional Status, serum albumin level, and weight loss. These parameters were not correlated with intestinal resection. Evaluation of prognostic yield showed cutoff values of serum albumin 3.3 g/dL (AUC 0.797, sensitivity 57.1%, specificity 89.5%) and O-PNI 36.5 (0.749, 71.4%, 73.7%). By combining the serum albumin cutoff value and NRS 2002 score, patients were divided into 4 groups, with a prolonged LOS rate of 68.2% in the group with the worst prognosis. Conclusions: A combination of serum albumin (given the simplicity of testing) and NRS 2002 as nutritional screening tools may be useful for hospitalized CD patients.

ϩ -dependent phosphate cotransporter (NaPi-IIc) is specifically targeted to, and expressed on, the apical membrane of renal proximal tubular cells and mediates phosphate transport. In the present study, we investigated the signals that determine apical expression of NaPi-IIc with a focus on the role of the N-and the C-terminal tails of mouse NaPi-IIc in renal epithelial cells [opossum kidney (OK) and Madin-Darby canine kidney cells]. Wild-type NaPi-IIc, the cotransporter NaPi-IIa, as well as several IIa-IIc chimeras and deletion mutants, were fused to enhanced green fluorescent protein (EGFP), and their cellular localization was analyzed in polarized renal epithelial cells by confocal microscopy and by cell-surface biotinylation. Fluorescent EGFP-fused NaPi-IIc transporter proteins are correctly expressed in the apical membrane of OK cells. The apical expression of N-terminal deletion mutants (deletion of N-terminal 25, 50, or 69 amino acids) was not affected by truncation. In contrast, C-terminal deletion mutants (deletion of C-terminal 45, 50, or 62 amino acids) did not have correct apical expression. A more detailed mutational analysis indicated that a domain (amino acids WLHSL) in the cytoplasmic C terminus is required for apical expression of NaPi-IIc in renal epithelial cells. We conclude that targeting of NaPi-IIc to the apical cell surface is regulated by a unique amino acid motif in the cytoplasmic C-terminal domain.Npt2c; SLC34A3; localization; OK cell; hereditary hypophosphatemic rickets with hypercalciuria INORGANIC PHOSPHATE (P i ) reabsorption in renal proximal tubules is required for P i homeostasis in the body. Na ϩ -dependent P i transporters (NaPi) in the brush-border membrane (BBM) of proximal tubule cells mediate the rate-limiting step in the overall P i -reabsorption process (26,28). Type IIa (NaPiIIa) and type IIc (NaPi-IIc) NaPi cotransporters are expressed in the apical membrane of proximal tubule cells and mediate P i transport (27, 38). The extent of P i reabsorption in the proximal tubules is determined largely by the abundance of the NaPi-IIa cotransporter (11,28,29).Several groups have shown that hereditary hypophosphatemic rickets with hypercalciuria (HHRH), a Mendelian disorder of renal P i reabsorption (39, 42), results from the lack of a functional NaPi-IIc protein, leading to severe renal wasting with hypophosphatemia (5,16,19,22,25). These data suggest that the NaPi-IIc cotransporter plays an important role in renal P i reabsorption and may be a key determinant of plasma P i concentrations in humans. However, it is not clear why a loss of function of the less abundant and energetically less favorable electroneutral NaPi-IIc transporter causes hypophosphatemia/rickets and osteomalacia in humans, whereas mutations in the more abundant electrogenic NaPi-IIa elicits a mild skeletal phenotype (3).The plasma membrane of epithelial cells is divided into two separate membrane compartments, the apical and the basolateral domains (36,37). This polarity is maintained by intracellular mach...

Mutations in the apically located Na(+)-dependent phosphate (NaPi) cotransporter, SLC34A3 (NaPi-IIc), are a cause of hereditary hypophosphatemic rickets with hypercalciuria (HHRH). We have characterized the impact of several HHRH mutations on the processing and stability of human NaPi-IIc. Mutations S138F, G196R, R468W, R564C, and c.228delC in human NaPi-IIc significantly decreased the levels of NaPi cotransport activities in Xenopus oocytes. In S138F and R564C mutant proteins, this reduction is a result of a decrease in the V(max) for P(i), but not the K(m). G196R, R468W, and c.228delC mutants were not localized to oocyte membranes. In opossum kidney (OK) cells, cell surface labeling, microscopic confocal imaging, and pulse-chase experiments showed that G196R and R468W mutations resulted in an absence of cell surface expression owing to endoplasmic reticulum (ER) retention. G196R and R468W mutants could be partially stabilized by low temperature. In blue native-polyacrylamide gel electrophoresis analysis, G196R and R468W mutants were either denatured or present in an aggregation complex. In contrast, S138F and R564C mutants were trafficked to the cell surface, but more rapidly degraded than WT protein. The c.228delC mutant did not affect endogenous NaPi uptake in OK cells. Thus, G196R and R468W mutations cause ER retention, while S138F and R564C mutations stimulate degradation of human NaPi-IIc in renal epithelial cells. Together, these data suggest that the NaPi-IIc mutants in HHRH show defective processing and stability.