Characterization of a murine type II sodium-phosphate cotransporter expressed in mammalian small intestine

Abstract

An isoform of the mammalian renal type II Na͞P i -cotransporter is described. Homology of this isoform to described mammalian and nonmammalian type II cotransporters is between 57 and 75%. Based on major diversities at the C terminus, the new isoform is designed as type IIb Na͞P i -cotransporter. Na͞P i -cotransport mediated by the type IIb cotransporter was studied in oocytes of Xenopus laevis. The results indicate that type IIb Na͞P i -cotransport is electrogenic and in contrast to the renal type II isoform of opposite pH dependence. Expression of type IIb mRNA was detected in various tissues, including small intestine. The type IIb protein was detected as a 108-kDa protein by Western blots using isolated small intestinal brush border membranes and by immunohistochemistry was localized at the luminal membrane of mouse enterocytes. Expression of the type IIb protein in the brush borders of enterocytes and transport characteristics suggest that the described type IIb Na͞P i -cotransporter represents a candidate for small intestinal apical Na͞P icotransport.

Intestinal and renal adaptation to a low-Pi diet of type II NaPi cotransporters in vitamin D receptor- and 1αOHase-deficient mice

“…The following primary antibodies were used: anti NaPi-IIa and anti NaPi-IIb, as described previously (5,14), anti-calbindin D9k (SWANT, Bellinzona, Switzerland), and anti ␤-actin (no. 5316; Sigma).…”

“…For Western blot analysis, 50 g of total protein were separated by SDS-PAGE and analyzed for the abundance of NaPi-IIa, NaPi-IIb, calbindin D9k, and ␤-actin after transfer onto nitrocellulose membranes according to standard procedures using 5% fat-free milk powder as a blocking reagent (5,14). The following primary antibodies were used: anti NaPi-IIa and anti NaPi-IIb, as described previously (5,14), anti-calbindin D9k (SWANT, Bellinzona, Switzerland), and anti ␤-actin (no.…”

“…In adult mice, renal reabsorption of P i is determined largely by the abundance of NaP i type IIa (NaP i -IIa) in the brush-border membrane vesicles (BBMV) of proximal tubular cells and in small intestine by NaP i type IIb (NaP i -IIb), the only apical NaP i cotransporter known to be involved in the absorption of P i (14,19). Thus regulation of intestinal absorption and renal reabsorption of P i is mainly explained by alterations in the apical abundances of NaP i -IIb and NaP i -IIa, respectively (12,15,19,28).…”

Phosphate transporters of the SLC20 and SLC34 families

“…NaPi-IIb (SLC34A2) was identified from a mouse embryo EST clone and was specifically localized to enterocytes of the small intestine (Hilfiker et al, 1998). In humans mRNA is detected in lungs, testis, salivary gland, thyroid gland, small intestine, liver, mammary gland, uterus but not in renal tissue (Fig 1B) (Nishimura and Naito, 2008).…”

“…NaPi-IIb has a wider pattern of expression (Hilfiker et al, 1998;Nishimura and Naito, 2008) and its constitutive depletion in mice is lethal (Shibasaki et al, 2009). It is the only SLC34 member present in intestinal microvilli.…”