Differential expression, abundance, and regulation of Na⁺-phosphate cotransporter genes in murine kidney

Abstract

Three classes of high-affinity Na+-Picotransporters are expressed in mammalian kidney. These include Npt1 (type I), Npt2 (type II), and the cellular receptors for gibbon ape leukemia virus (Glvr-1) and amphotropic murine retrovirus (Ram-1) (type III). We defined the tissue distribution as well as the relative renal abundance of Npt1, Npt2, Glvr-1, and Ram-1 mRNAs and examined the effects of low-Pi diet, the Hyp mutation, and growth hormone (GH) on their renal expression by ribonuclease protection assay. In normal mouse kidney, Npt1, Npt2, Glvr-1, and Ram-1 accounted for 15 ± 1.0, 84 ± 1.0, 0.5 ± 0.2, and 0.5 ± 0.3% of total Na+-Picotransporter mRNAs, respectively. Evidence was obtained for low-abundance Npt1 mRNA expression in liver and Npt2 mRNA expression in intestine, whereas Glvr-1 and Ram-1 mRNAs were also detected in bone, intestine, heart, and liver. Npt2 mRNA was localized to proximal tubules in the renal outer cortex, whereas Glvr-1 transcripts were detected throughout the kidney by in situ hybridization. The Hyp mutation elicited a significant reduction in renal Npt1 and Npt2 mRNAs (78 ± 8 and 57 ± 3% of normal, respectively), whereas neither low-Pi diet nor GH influenced the renal abundance of Npt1 and Npt2 transcripts. Renal Glvr-1 mRNA expression was significantly increased in Hyp mice and GH-treated mice (145 ± 6 and 165 ± 5% of control, respectively), whereas the renal abundance of Ram-1 transcript was unaffected by either the Hyp mutation, low-Pi diet, or GH treatment. In summary, we demonstrate that Npt2 is the predominant Na+-Picotransporter in mouse kidney, that Npt2 and Glvr-1 have distinct patterns of renal expression, and that the Hyp mutation modulates the renal expression of Npt1, Npt2, and Glvr-1 mRNAs. Our results suggest that increased renal Glvr-1 mRNA may contribute to GH stimulation of renal Na+-Picotransport.

Effects of Npt2 gene ablation and low-phosphate diet on renal Na+/phosphate cotransport and cotransporter gene expression

“…In contrast, both acute and chronic Pi deprivation elicit an adaptive increase in BBM Na + /Pi cotransport that can be ascribed to microtubule-dependent recruitment of Npt2 protein to the apical surface (25). The increase in Npt2 protein in acute Pi restriction is not associated with an increase in Npt2 mRNA, whereas in chronic Pi deprivation, a corresponding augmentation in Npt2 transcript abundance might (26)(27)(28) or might not (22,25) be apparent.…”

“…polymerases and [α-32 P]UTP (800 Ci/mmol; ICN Biomedicals Inc., Mississauga, Ontario, Canada) as described previously (22). For the size and position of the subcloned Na + /Pi cotransporter cDNA fragments, see Table 1 in ref.…”

“…We recently demonstrated that Npt1, Npt2, Glvr-1, and Ram-1 account for approximately 15%, 84%, 0.5%, and 0.5%, respectively, of total Na + /Pi cotransporter mRNA in mouse kidney (22). Not only is Npt2 the most abundant of the renal Na + /Pi cotransporters, it is also the major target for regulation by parathyroid hormone (PTH) and dietary Pi.…”

Fibroblast Growth Factor (FGF)-23 and Hypophosphatemic Rickets/Osteomalacia.

“…We could easily find the expression of type III co-transporter while that of type ha was barely detectable. Similar lack of expression of sodium-phosphate cotransporter type IIa in primary cultured proximal tubular cells and renal cell lines has also been described [11]. Therefore, because cultured kidney cells do not necessary express type IIa co-transporter, it is difficult to establish an in vitro assay that accurately evaluates physiological proximal tubular phosphate reabsorption.…”

Disorders of Renal Tubular Phosphate Transport

“…Type III mRNA expression is detected in all nephron segments (19). To date, precise information on membrane localization of type III protein (e.g., apical versus basolateral) is not available.…”