Differential modulation of the molecular dynamics of the type IIa and IIc sodium phosphate cotransporters by parathyroid hormone

Abstract

The kidney is a key regulator of phosphate homeostasis. There are two predominant renal sodium phosphate cotransporters, NaPi2a and NaPi2c. Both are regulated by parathyroid hormone (PTH), which decreases the abundance of the NaPi cotransporters in the apical membrane of renal proximal tubule cells. The time course of PTH-induced removal of the two cotransporters from the apical membrane, however, is markedly different for NaPi2a compared with NaPi2c. In animals and in cell culture, PTH treatment results in almost complete removal of NaPi2a from the brush border (BB) within 1 h whereas for NaPi2c this process in not complete until 4 to 8 h after PTH treatment. The reason for this is poorly understood. We have previously shown that the unconventional myosin motor myosin VI is required for PTH-induced removal of NaPi2a from the proximal tubule BB. Here we demonstrate that myosin VI is also necessary for PTH-induced removal of NaPi2c from the apical membrane. In addition, we show that, while at baseline the two cotransporters have similar diffusion coefficients within the membrane, after PTH addition the diffusion coefficient for NaPi2a initially exceeds that for NaPi2c. Thus NaPi2c appears to remain "tethered" in the apical membrane for longer periods of time after PTH treatment, accounting, at least in part, for the difference in response times to PTH of NaPi2a versus NaPi2c.

Renal Phosphate Wasting in the Absence of Adenylyl Cyclase 6

“…In contrast to studies localizing Npt2c mainly to the S1 and weaker in the S2 segment, 6,17 our antibody 16,42,43 identified Npt2c along the entire length of proximal tubules, with clear transitions from a labeled S3 segment to an unlabeled thin descending limb of Henle's loop. Our antibody previously gave a strong signal in the outer stripe of the outer medulla in WT mice, consistent with Npt2c expression in S3 segments.…”

“…Npt2c retrieval in response to PTH is not complete until 4-8 hours after treatment. 17,42 Consistent with these studies, we did not see a difference in Npt2c abundance or distribution in either genotype after PTH administration.…”

“…4,6,52 Our immunogold electron microscopy shows that, 1 hour after PTH treatment (a timeframe shown to result in almost complete retrieval of Npt2a from the brush border membrane 6,42 ), there is a shift of Npt2a from the microvilli to intracellular compartments and lysosomes; however, this acute PTH-induced Npt2a trafficking is absent in AC6 2/2 mice. Moreover, 1 hour after PTH application, both genotypes had a comparable amount of Npt2a gold particles in the microvilli, which may explain why fractional urinary P i excretion was not significantly different at the highest PTH doses used in our clearance studies.…”

“…26,27 Proteins were transferred to nitrocellulose membranes and immunoblotted with Npt2a (1:1000) and Npt2c (1:2500). 16,42 Chemiluminescent detection was performed with ECL Plus (Amersham, Piscataway, NJ). Densitometric analysis was performed by ImageJ Software (National Institutes of Health).…”

The phosphate transporter NaPi-IIa determines the rapid renal adaptation to dietary phosphate intake in mouse irrespective of persistently high FGF23 levels

“…This regulation occurs essentially by trafficking of the protein, lysosomal degradation, recruitment of newly synthesized transporters [5,26,[29][30] and may also include transcriptional regulation [34]. Much less is known about the regulation of NaPi-IIc being regulated also by PTH, FGF23, and Pi intake with a slower change in brush border membrane abundance upon high Pi intake or PTH application [8,28,[43][44]. Similarly, Pit-2 is regulated by PTH and dietary Pi or potassium intake but no further regulators have been identified to date [15,44,59].…”