Investigating the surface expression of the renal type IIa Na+/Pi-cotransporter in Xenopus laevis oocytes

Molecular cloning and functional characterization of swine sodium dependent phosphate cotransporter type II b (NaPi-IIb) gene

A sodium-dependent phosphate transporter gene, NaPi-IIb, was isolated from swine small intestine using cDNA library screening method. Sequencing analysis revealed that the NaPi-IIb cDNA sequences was 2,016 bp in length and encoded an open-reading frame consisting of 671 amino acids. The cDNA showed 83.1 % sequences identity to the human NaPi-IIb and 78.7 % sequences identity to the chicken NaPi-IIb. Prediction of membrane spanning domains based on the hydrophilic and hydrophobic properties of the amino acids suggested that a putative protein had nine transmembrane domains, with both the NH(2) and COOH terminal being intracellular. By northern blot, a ~4.2 kb transcript was found to be abundantly expressed in mall intestine, lung, ovary, mammary glands, liver, kidney, salivary glands, placenta and thymus. Microinjection of swine NaPi-IIb cRNA into Xenopus oocytes demonstrated that the NaPi-IIb showed sodium-dependent Pi cotransport activity, and an approximate 31-fold increase of Pi uptake was seen in cRNA injected oocytes. The swine NaPi-IIb transporter expressed in Xenopus oocytes had a Km for Pi of ~79.35 ± 7.2 μM. Furthermore, the pH dependency characterization of swine NaPi-IIb transporter showed activation at extracellular alkaline-pH.

Molecular regulation of NKCC2 in the thick ascending limb

The kidney plays an essential role in blood pressure regulation by controlling short-term and long-term NaCl and water balance. The thick ascending limb of the loop of Henle (TAL) reabsorbs 25-30% of the NaCl filtered by the glomeruli in a process mediated by the apical Na(+)-K(+)-2Cl(-) cotransporter NKCC2, which allows Na(+) and Cl(-) entry from the tubule lumen into TAL cells. In humans, mutations in the gene coding for NKCC2 result in decreased or absent activity characterized by severe salt and volume loss and decreased blood pressure (Bartter syndrome type 1). Opposite to Bartter's syndrome, enhanced NaCl absorption by the TAL is associated with human hypertension and animal models of salt-sensitive hypertension. TAL NaCl reabsorption is subject to exquisite control by hormones like vasopressin, parathyroid, glucagon, and adrenergic agonists (epinephrine and norepinephrine) that stimulate NaCl reabsorption. Atrial natriuretic peptides or autacoids like nitric oxide and prostaglandins inhibit NaCl reabsorption, promoting salt excretion. In general, the mechanism by which hormones control NaCl reabsorption is mediated directly or indirectly by altering the activity of NKCC2 in the TAL. Despite the importance of NKCC2 in renal physiology, the molecular mechanisms by which hormones, autacoids, physical factors, and intracellular ions regulate NKCC2 activity are largely unknown. During the last 5 years, it has become apparent that at least three molecular mechanisms determine NKCC2 activity. As such, membrane trafficking, phosphorylation, and protein-protein interactions have recently been described in TALs and heterologous expression systems as mechanisms that modulate NKCC2 activity. The focus of this review is to summarize recent data regarding NKCC2 regulation and discuss their potential implications in physiological control of TAL function, renal physiology, and blood pressure regulation.

Characterization of the Chicken Small Intestine Type IIb Sodium Phosphate Cotransporter

Intestinal absorption and renal resorption play a critical role in overall phosphorus homeostasis in chickens. Using RNase-ligase-mediated rapid amplification of cDNA ends PCR, we obtained a cDNA from the broiler small intestine that encodes a type IIb Na-dependent phosphate transporter. The cDNA has an open reading frame of 2,022 bp and predicts a 674-amino acid protein with a molecular mass of approximately 74 kDa. Prediction of membrane spanning domains based on the hydrophilic and hydrophobic properties of the amino acids suggests 8 transmembrane domains, with both the NH(2) and COOH termini being intracellular. The Na-inorganic phosphate (Pi) IIb cotransporter has relative high homology with other type II Na-Pi cotransporters but low homology with the type I or type III Na-Pi cotransporters. Northern blot analysis demonstrated the presence of a single mRNA transcript present predominantly in the small intestine, with the highest expression in the duodenum, followed by the jejunum and ileum. In situ hybridization indicated that the Na-Pi cotransporter mRNA is expressed throughout the vertical cryptvillus axis of the small intestine. Reduction of P in the diet of chicks from hatch to 4 d of age resulted in a significant induction of Na-Pi cotransporter mRNA expression in the small intestine. Further study is needed to elucidate its physiological role in intestinal phosphate absorption in chickens.

cAMP increases surface expression of NKCC2 in rat thick ascending limbs: role of VAMP

NaCl absorption by the thick ascending limb of Henle's loop (TAL) is mediated by the apical Na-K-2Cl cotransporter NKCC2. cAMP increases NaCl absorption in the TAL by stimulating NKCC2. In oocytes, cAMP increases NKCC2 activity by regulating its trafficking. However, the mechanism by which cAMP stimulates NKCC2 in TALs is not clear. We hypothesized that cAMP increases surface expression of NKCC2 and NaCl absorption in TALs and that vesicle-associated membrane protein (VAMP) is involved in this mechanism. We used surface biotinylation of rat medullary TALs (mTAL) to examine surface and total NKCC2 levels. When mTAL suspensions were treated with dibutyryl cAMP (db-cAMP) or forskolin plus IBMX for 20 min, surface NKCC2 expression increased by 126 +/- 23 and 92 +/- 17% above basal, respectively (P < 0.03). No changes in total NKCC2 expression were observed, suggesting that cAMP increased translocation of NKCC2. We studied the role of VAMP in NKCC2 translocation and found that incubating mTALs with tetanus toxin (30 nM), which inhibits vesicle trafficking by inactivating VAMP-2 and -3, completely blocked the stimulatory effect of db-cAMP on surface NKCC2 expression (tetanus toxin = 100% vs. tetanus toxin + db-cAMP = 102 +/- 21% of control; not significant). We studied VAMP-2 and -3 expression and localization in isolated perfused TALs by confocal microscopy and found that both of them were located in the subapical space of the TAL. Finally, in isolated perfused mTALs, db-cAMP increased net Cl absorption by 95.0 +/- 34.8% (P < 0.03), and pretreatment of TALs with tetanus toxin blocked the stimulation of Cl absorption (from 110.9 +/- 15.9 to 109.7 +/- 15.6 pmol.min(-1).mm(-1); not significant). We concluded that cAMP increases NKCC2 surface expression by a mechanism involving VAMP and that NKCC2 trafficking to the apical membrane is involved in the stimulation of TAL NaCl absorption by cAMP.