Phosphate transport: from microperfusion to molecular cloning

Abstract

Inorganic phosphate (P i ) is a constituent of important biological molecules (e.g., nucleic acids and phospholipids) and is essential for cellular energetics and signaling, and protein synthesis as well as skeletal development in all mammalian organisms. Inadequate P i supply causes bone malformations such as rickets and spinal deformations, whereas an excess in P i is linked to vascular calcification or ectopic CaP i deposits. In general, whole-body P i homeostasis is maintained by transepithelial transport mechanisms in the small intestine and kidney where P i is absorbed from the diet and reabsorbed from the glomerular filtrate, respectively. The renal proximal tubule is the main locus of P i regulation so that under Bsteady-state^physiological conditions, renal P i -excretion corresponds approximately to dietary P i intake.Membrane transport proteins belonging to the SLC34 solute carrier family 1 lie at the heart of maintaining P i homeostasis. In the small intestine, NaPi-IIb (SLC34A2) mediates luminal P i uptake together with a paracellular component, whereas the renal isoforms NaPi-IIa (SLC34A1) and NaPi-IIc (SLC34A3) are responsible for P i reabsorption in the proximal tubule. This Special Issue focusing on phosphate transport mediated by SLC34 proteins was conceived to coincide with an important milestone in renal physiology: the expression cloning of the first member of the SLC34 family (NaPi-IIa) just over a quarter of a century ago [33]. This, together with the subsequent identification of the other two SLC34 isoforms [24,42], has paved the way for a deeper understanding of the molecular basis of P i homeostasis, many aspects of which are the subject of dedicated reviews in this issue. Whereas the pivotal role of SLC34 cotransporters in maintaining P i homeostasis is undisputed, other carriers such as PIT-1 and PIT-2 (SLC20 family) may also contribute to epithelial transport of P i in the intestine and the kidney. However, their respective contributions to 1 Historically, the nomenclature of the cloned, mammalian P i transporters followed a strictly chronological convention, beginning with the type 1 (NaPi-I) [10,53] (since shown to be related to an anion conductance and not directly mediate Na-dependent P i transport [8]); the type II (NaPi-II [33] or npt2) and the type III (NaPi-III [28,29], or Pit-1,2). The widely used solute carrier (SLC) nomenclature [22] assigns the type I transporters to the SLC17 gene family; type II Na-P i transporters to SLC34 and type III transporters to SLC20, respectively. The SLC system is increasingly used, i.e., SLC34A1, SLC34A2, and SLC34A3 for the human type II transporters NaPi-IIa, NaPi-IIb, and NaPi-IIc, respectively. The structurally similar isoforms are grouped into the same sub-family (SLC34A) with individual indices (1, 2, 3). This nomenclature is easily applicable to most vertebrates; it only becomes less clear in the case of fish that have undergone genome duplications followed by lineage specific gene losses.

Dye inhibition of phosphate transport in everted duodenal sacs of mice

“…the basolateral membrane of enterocytes. [5] It is difficult to pin point the exact site of action of the dyes. Nicotinamide a vitamin, when injected into the rat peritoneal cavity, has been shown to enter the enterocyte and prevent the intestinal absorption of phosphate by reducing the number of NaPi IIb type of phosphate transporters on the brush border membrane.…”

The Intricacies of Renal Phosphate Reabsorption—An Overview

“…Phosphate reabsorption is mediated by at least three sodium-linked transporters belonging to solute carrier families SLC34 and SLC20, NaPi-2a (NPT2A, Npt2a, SLC34A1), NaPi-2c (NaPi-2c, SLC34A3), and Pit-2 (Npt3, Ram-1, SLC20A2) sited in the apical brush border membrane (BBM) [12,24,43]. They actively transport phosphate into the cells.…”

“…The driving force is the inwardly directed electrochemical gradient of Na + ions established by Na + /K + -ATPase located in the cell basal membranes. There is uncertainty about which proteins transport phosphate out of the cells across the basolateral membrane [4,15,24,[44][45][46][47][48]. mRNA abundance of Slc34a1 was about ten times higher than Slc34a3 in mouse kidney [49], and in mouse and rat kidneys was >50-fold higher than slc20a2 mRNA [50].…”

“…In the proximal tubule, the pH can fall from approximately 7.4 in the glomerular filtrate to around 6.6, decreasing the ratio of HPO4 2-/ H2PO4 -from 4.0 to 1.6 [50]. Phosphate is transported with a 3:1 ratio of Na + to HPO4 2-and hence each transport cycle carries net positive charge across the membrane [12,24,44,52,53,57,58]. In a proposed model, phosphate transport by NaPi-2a is viewed as a kinetic cyclic process comprising a sequence of transitions between conformationally distinct states of the protein [44,52,60].…”

“…These have been researched intensively for decades. With the very powerful analytical tools now available, the complexity, speed 2 and amazing co-ordination of the processes involved have become increasingly apparent [11,12,23,24]. Already, findings have shown new avenues for pharmacological intervention to regulate phosphate in common conditions, including chronic renal failure and osteoporosis, as well as rare inherited biochemical disorders [12,19,23,[25][26][27][28][29][30][31][32][33].…”