SNPs of metabolism, not stones.

DICARBOXYLATES ARE THE SUBSTRATES for oxidative phosphorylation, aka the Krebs cycle or citric acid cycle. Some of these substrates are synthesized within cells. However, epithelia with high metabolic needs (e.g., the small intestine and kidney) bring these substrates directly into cells. The plasma membrane of cells separates the outside world from the inside due to the hydrophobic core of this lipid bilayer. This latter property means that ions and solutes (for metabolism or otherwise) must be carried by a transporter protein through the environment. These transporters use either the energy of ATP hydrolysis or of the electrochemical gradient to accomplish this task. The Na -coupled dicarboxylate cotransporter (NaDC1, SLC13A2) is localized at the apical membrane of these epithelia (15, 19) and is responsible for renal and gut dicarboxylate as well as citrate (tricarboxylate) uptake. As expected, knocking out NaDC1 in mice causes elevated urinary citrate and other metabolites (7). In the average 70-kg human, cellular metabolism generates 70 mmol of acid (H ). Urinary buffers such as citrate are also known as titratable acids because they can take on this H from metabolism. Ironically, this means that H acceptors are considered “bases.” The renal proximal tubule absorbs 70– 90% of urinary citrate, which enters the nephron from the filtered blood. Proximal tubule metabolism of citrate results in intracellular HCO3 , which either buffers intracellular pH or is absorbed into the blood via the electrogenic Na -HCO3 cotransporter (NBCe1-A) (4, 8, 18), leading to transepithelial NaHCO3 absorption. Acidosis stimulates HCO3 absorption but is also known to increase the amount of NaDC1 mRNA and protein (2) as well as NaDC1 activity (1, 6). Since citrate metabolism leads to HCO3 production and increases blood HCO3 concentration, acidosis decreases urinary citrate (increased NaDC1 activity ¡ a decrease in citrate in the ultrafiltrate). Alkalosis reduces transepithelial HCO3 absorption by reducing basolateral HCO3 exit (via NBCe1-A). Alkalosis causes a pH increase of the proximal ultrafiltrate, and this elevated luminal pH decreases citrate absorption and enhances urinary citrate excretion (3, 9, 11, 20). Alkalosis (in opossum kidney cells) does seem to decrease NaDC1 cotransport of citrate but not succinate (1). This observation seems consistent: a decreasing H-citrate concentration (preferred dicarboxylate) and increasing citrate concentration(a tricarboxylate) as pH increases beyond 6.4 (pKa3 6.4; H-citrate ¡ H citrate ). This pH shift, thus results in less of the transported substrate (H-citrate ). That is, the proximal tubule responds by limiting citrate absorption, resulting in higher urinary citrate (3, 7). Low urinary citrate (hypocitraturia) is associated with kidney stones (nephrolithiasis) (12) as is acidosis (3, 17). In addition, as above, acidosis increases apical NaDC1 protein (2) as well as activity (citrate transport) (1). In fact, one of the most common treatments for physiologically dissolving uric acid kidney stones is to alkalinize the urine, usually with K-citrate or possibly lemon juice (high citrate). Medically, calcium stones are also often treated by giving K-citrate. While from an acid-base prospective this treatment seems a bit counterintuitive, there is an additional role of citrate per se. Citrate can complex ionized Ca . This means that there is a dual role for citrate: H buffer and Ca chelator. For Ca-oxalate stones, the chelation role is critical. If free-ionized Ca is complexed, the Ca cannot complex with oxalate (only slightly soluble and therefore stone initiating). Accordingly, high urine citrate should decrease the ability of Caoxalate stones to initiate (protective against stones). Similarly, low urine citrate (hypocitraturia) would result in more ionized Ca and increase the likelihood that a stone could form. With the cloning of NaDC1 (14), it was hypothesized that this cotransporter might be associated with controlling urinary citrate, and thus urinary pH, and subsequently the ability to form urinary stones. In fact, one allele (I550V) has been implicated as causative in recurrent stone formers (13). Pajor and Sun (16) have examined the protein and physiological impact of allelic variation (single nucleotide polymorphisms; SNPs) in NaDC1 (SLC13A2). After examining the published and database (http://www.ncbi.nlm.nih.gov/projects/ SNP/) SNPs, they have determined the biophysical impact of the coding SNPs (cSNPs) on NaDC1 function (see Table 1). While these data are of themselves interesting for the function of NaDC1, there is a larger implication. Due to decreased protein and function, all of these NaDC1-SNPs are expected to decrease intestinal and renal citrate absorption, thereby increasing urinary citrate. Importantly, since nephrolithiasis is associated with decreased urinary citrate, these allelic variations would not be causative of nephrolithiasis and perhaps even preventative. Is the case closed for NaDC1 being causative of nephrolithiasis? Probably not. As we have learned with many rare disorders, the rare allelic variations (cSNPs) are typically not represented in these population databases. What Pajor and Sun’s work (16) does indicate is that NaDC1 mutations are unlikely to be a common cause of hypocitraturia and nephrolithiasis. These results do illustrate that for NaDC1 to result in hypocitraturia would require either increases in NaDC1 protein expression or hyperactivity of the transporter. Thus far, such increases in NaDC1 protein or activity have not been reported. However, as with oxalate transport (5, 10), decreased NaDC1 activity under certain circumstances could result in low urinary citrate. For example, decreased intestinal absorption of citrate (through decreased gut NaDC1 activity) would tend to Address for reprint requests and other correspondence: M. F. Romero, Physiology and Biomedical Engineering, Nephrology and Hypertension, and O’Brien Urology Research Center, Mayo Clinic College of Medicine, Rochester, MN 55905 (e-mail: [email protected]). Am J Physiol Renal Physiol 299: F702–F703, 2010; doi:10.1152/ajprenal.00432.2010. Editorial Focus