Intestinal Npt2b Plays a Major Role in Phosphate Absorption and Homeostasis

Intestinal phosphate absorption occurs through both a paracellular mechanism involving tight junctions and an active transcellular mechanism involving the type II sodium-dependent phosphate cotransporter NPT2b (SLC34a2). To define the contribution of NPT2b to total intestinal phosphate absorption, we generated an inducible conditional knockout mouse, Npt2b^−/− (Npt2b^fl/fl:Cre^+/−). Npt2b^−/− animals had increased fecal phosphate excretion and hypophosphaturia, but serum phosphate remained unchanged. Decreased urinary phosphate excretion correlated with reduced serum levels of the phosphaturic hormone FGF23 and increased protein expression of the renal phosphate transporter Npt2a. These results demonstrate that the absence of Npt2b triggers compensatory renal mechanisms to maintain phosphate homeostasis. In animals fed a low phosphate diet followed by acute administration of a phosphate bolus, Npt2b^−/− animals absorbed approximately 50% less phosphate than wild-type animals, confirming a major role of this transporter in phosphate regulation. In vitro analysis of active phosphate transport in ileum segments isolated from wild-type or Npt2b^−/− mice demonstrated that Npt2b contributes to >90% of total active phosphate absorption. In summary, Npt2b is largely responsible for intestinal phosphate absorption and contributes to the maintenance of systemic phosphate homeostasis.Inorganic phosphate is an essential mineral critical for cellular processes and bone mineralization. Severe disruptions in serum phosphate have pathologic consequences.^1,2 Hypophosphatemic disorders are associated with rickets, osteomalacia, and a host of secondary dysfunctions.³ In contrast, hyperphosphatemia associated with chronic kidney disease (CKD) is linked tightly to increased risk of cardiovascular morbidity and mortality.^4–6 Recent studies show that elevated phosphate concentrations within the high normal range in individuals with functional kidneys also are correlated with increased cardiovascular risk and mortality.^7,8 Thus, an elevated serum phosphate level is an emerging health risk.Despite the importance of maintaining a relatively narrow serum phosphate range, nearly 70% of dietary phosphate is absorbed, resulting in transient postprandial increases in serum phosphate concentrations.⁹ Normalization of serum phosphate appears to be managed primarily within the renal proximal tubule by the type II sodium-dependent phosphate cotransporters NPT2a (SLC34a1) and NPT2c (SLC34a3). Genetic knockout mouse models demonstrate that 80% and 20% of total urinary phosphorus are managed by the Npt2a and Npt2c transporters, respectively.^10,11 Chronic and acute regulation of these renal transporters is modulated by changes in dietary and serum phosphate levels and by three major hormones: parathyroid hormone (PTH), 1,25-dihydroxy vitamin D₃ (1,25(OH)₂D₃), and fibroblast growth factor 23 (FGF23).¹The emergence of data demonstrating that regulation of the renal phosphate transporters can adequately maintain systemic phosphate levels has reduced contemporary interest in intestinal phosphate regulation. Furthermore, early studies of intestinal transport suggest that paracellular transport driven by a passive diffusional process predominates under standard dietary conditions.^12,13 An alternative transcellular mechanism in the small intestine is dependent on active transport through the sodium-dependent phosphate cotransporter NPT2b (SLC34a2).¹⁴ Npt2b has a relatively low K_m for phosphate, suggesting that transport would be readily saturated under standard conditions and therefore may be important only under conditions of hypophosphatemia or low dietary phosphate intake.¹⁵ Furthermore, individuals with inactivating NPT2b mutations have pulmonary alveolar microlithiasis (PAM) but do not have serum or urinary phosphate abnormalities.^16,17 Despite this cumulative evidence downplaying the relative importance of NPT2b, its expression is increased by either 1,25(OH)₂D₃ or low dietary phosphate and decreased by nicotinamide. Interestingly, nicotinamide treatment in late stage hyperphosphatemic CKD patients has been shown to lower serum phosphate concentrations,¹⁸ raising the possibility that phosphate transport in the intestine by Npt2b may be important under the pathologic conditions associated with loss of renal function.To assess Npt2b''s relative importance under defined conditions, we have generated a tamoxifen-inducible ubiquitous Npt2b deletion. Surprisingly, deletion of the intestinal transporter leads to altered compensatory hormonal responses that maintain serum phosphate levels within normal limits. These data demonstrate that Npt2b plays an active role in systemic phosphate regulation.