Phosphate transport: from microperfusion to molecular cloning
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Inorganic phosphate (Pi) 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 Pi supply causes bone malformations such as rickets and spinal deformations, whereas an excess in Pi is linked to vascular calcification or ectopic CaPi deposits. In general, whole-body Pi homeostasis is maintained by transepithelial transport mechanisms in the small intestine and kidney where Pi is absorbed from the diet and reabsorbed from the glomerular filtrate, respectively. The renal proximal tubule is the main locus of Pi regulation so that under “steady-state” physiological conditions, renal Pi-excretion corresponds approximately to dietary Pi intake.
Membrane transport proteins belonging to the SLC34 solute carrier family1 lie at the heart of maintaining Pi homeostasis. In the small intestine, NaPi-IIb (SLC34A2) mediates luminal Pi uptake together with a paracellular component, whereas the renal isoforms NaPi-IIa (SLC34A1) and NaPi-IIc (SLC34A3) are responsible for Pi 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 . 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 Pi homeostasis, many aspects of which are the subject of dedicated reviews in this issue. Whereas the pivotal role of SLC34 cotransporters in maintaining Pi homeostasis is undisputed, other carriers such as PIT-1 and PIT-2 (SLC20 family) may also contribute to epithelial transport of Pi in the intestine and the kidney. However, their respective contributions to overall Pi balance remain to be clarified and will not be the main focus of the present Special Issue.
Following the cloning of NaPi-IIa , the phosphate physiology field has progressed rapidly, benefitting from advances in molecular and cell biology, imaging, and biophysical assays, as exemplified by the invited articles. Therefore, it is easy to overlook how key aspects of our present knowledge of Pi handling had already been established prior to 1993. A brief reflection on the main findings of these earlier studies seems appropriate to set the scene for the developments that have taken place over the past 25 years.
The molecular identity of the proteins involved in Pi transport was a pressing question and attempts were undertaken to identify the transporter by biochemical means in the pre-cloning era. For example, given that PTH affects renal Pi reabsorption and Na+/Pi-cotransport in BBMVs in a cAMP-dependent manner, phosphorylation studies were performed with isolated BBMVs. These studies demonstrated cAMP-dependent changes of BBMV protein-phosphorylation, but the identity of those phospho-proteins remained unknown .
The cloning of NaPi-IIa [32, 33] heralded a new era in the field by enabling the study of the transport mechanisms/characteristics in isolation after injection of cRNA into oocytes of Xenopus laevis . By knowing the amino acid sequence, specific antibodies could be raised to enable specific in situ protein detection of each isoform. Studies using these antibodies allowed the detailed characterization of the cellular mechanisms that are involved in the physiological regulation by an increasing number of hormones. It was also possible to answer questions regarding the interaction with other proteins that are involved in the cellular regulation of Na+/Pi-cotransport.
The collection of invited reviews in this Special Issue gives a comprehensive summary of the impressive progress that has been made in the Pi field since the early cloning days. It is our hope that these articles will provide a useful overview for experts and non-experts alike and stimulate continued research in the Pi field. The reviews have been grouped according to five themes: Molecular and Mechanistic Aspects, including interacting proteins [12, 13, 23, 43], Physiological Regulation [26, 27, 48], Human Mutations/Clinical Aspects and Diseases [4, 30], Non-renal Roles for SLC34 Proteins [3, 34], and Comparative Aspects [39, 52].
Historically, the nomenclature of the cloned, mammalian Pi 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 Pi transport ); the type II (NaPi-II  or npt2) and the type III (NaPi-III [28, 29], or Pit-1,2). The widely used solute carrier (SLC) nomenclature  assigns the type I transporters to the SLC17 gene family; type II Na-Pi 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.
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