Growth is critically dependent on the retention of a variety of nutrients. The kidney contributes to this positive external balance. In the present study, we isolated a cDNA from the human and rat kidney that encodes a growth-related Na ؉ -dependent inorganic phosphate (P i ) cotransporter (type IIc). Microinjection of type IIc cRNA into Xenopus oocytes demonstrated sodium-dependent P i cotransport activity. Affinity for P i was 0.07 mM in 100 mM Na ؉ . The transport activity was dependent on extracellular pH. In electrophysiological studies, type IIc Na/P i cotransport was electroneutral, whereas type IIa was highly electrogenic. In Northern blotting analysis, the type IIc transcript was only expressed in the kidney and highly in weaning animals. In immunohistochemical analysis, the type IIc protein was shown to be localized at the apical membrane of the proximal tubular cells in superficial and midcortical nephrons of weaning rat kidney. Hybrid depletion experiments suggested that type IIc could function as a Na/P i cotransporter in weaning animals, but its role is reduced in adults. The finding of the present study suggest that the type IIc is a growth-related renal Na/P i cotransporter, which has a high affinity for P i and is electroneutral.

Notch (N) signaling is an evolutionarily conserved mechanism that regulates many cell-fate decisions. deltex (dx) encodes an E3-ubiquitin ligase that binds to the intracellular domain of N and positively regulates N signaling. However, the precise mechanism of Dx action is unknown. Here, we found that Dx was required and sufficient to activate the expression of gene targets of the canonical Su(H)-dependent N signaling pathway. Although Dx required N and a cis-acting element that overlaps with the Su(H)-binding site, Dx activated a target enhancer of N signaling, the dorsoventral compartment boundary enhancer of vestigial (vgBE), in a manner that was independent of the Delta (Dl)/Serrate (Ser) ligands- or Su(H). Dx caused N to be moved from the apical cell surface into the late-endosome, where it accumulated stably and co-localized with Dx. Consistent with this, the dx gene was required for the presence of N in the endocytic vesicles. Finally, blocking the N transportation from the plasma membrane to the late-endosome by a dominant-negative form of Rab5 inhibited the Dx-mediated activation of N signaling, suggesting that the accumulation of N in the late-endosome was required for the Dx-mediated Su(H)-independent N signaling.

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Notch signalling, which is highly conserved from nematodes to mammals,plays crucial roles in many developmental processes. In the Drosophila embryo, deficiency in Notch signalling results in neural hyperplasia, commonly referred to as the neurogenic phenotype. We identify a novel maternal neurogenic gene, neurotic, and show that it is essential for Notch signalling. neurotic encodes a Drosophila homolog of mammalian GDP-fucose protein O-fucosyltransferase, which adds fucose sugar to epidermal growth factor-like repeats and is known to play a crucial role in Notch signalling. neurotic functions in a cell-autonomous manner, and genetic epistasis tests reveal that Neurotic is required for the activity of the full-length but not an activated form of Notch. Further, we show that neurotic is required for Fringe activity, which encodes a fucose-specific β1, 3 N-acetylglucosaminyltransferase, previously shown to modulate Notch receptor activity. Finally, Neurotic is essential for the physical interaction of Notch with its ligand Delta, and for the ability of Fringe to modulate this interaction in Drosophila cultured cells. We present an unprecedented example of an absolute requirement of a protein glycosylation event for a ligand-receptor interaction. Our results suggest that O-fucosylation catalysed by Neurotic is also involved in the Fringe-independent activities of Notch and may provide a novel on-off mechanism that regulates ligand-receptor interactions.

Recent studies suggest that vitamin D may play a role in intestinal Na(+)-dependent phosphate transport adaptation to variable levels of dietary P(i). Therefore, the goal of the current study was to assess Na(+)-dependent P(i) cotransport activity in transgenic mice to determine whether vitamin D is an essential mediator of this process. Intestinal brush-border membrane (BBM), Na(+)-dependent P(i) cotransport activity was significantly decreased in vitamin D receptor (VDR) null [VDR (-/-)] mice compared with wild-type (VDR+/+) mice. While intestinal Na-P(i) cotransporter (type IIb) mRNA levels were similar in VDR (-/-) and VDR (+/+) mice, type IIb Na-P(i) cotransporter protein expression was markedly suppressed in VDR (-/-) mice compared with VDR (+/+) mice. Furthermore, Na-P(i) cotransport activity in renal BBM was similar in VDR (-/-) and VDR (+/+) mice, but type IIa Na-P(i) cotransporter protein expression was decreased in VDR (-/-) mice. After administration of a low-P(i) diet, type IIb protein expression was significantly increased in VDR (+/+) and VDR (-/-) mice, and type IIb protein expression was present in the intestinal BBM of VDR (-/-) mice. These data demonstrate that intestinal Na-P(i) cotransport adaptation to a low-P(i) diet occurs independently of vitamin D.