Acid pH Increases the Stability of BSC1/NKCC2 mRNA in the Medullary Thick Ascending Limb

Abstract

Abstract. Chronic metabolic acidosis enhances the ability of the medullary thick ascending limb (MTAL) to absorb NH 4 ϩ at least in part by stimulating the mRNA and protein expression of BSC1/NKCC2, the MTAL apical Na ϩ -K ϩ (NH 4 ϩ )-2Cl Ϫ co-transporter. For assessing the mechanism by which an acid pH enhances the BSC1 mRNA abundance, MTAL were harvested from adrenalectomized rats and incubated in control (pH 7.35) and acid (pH 7.10) 1:1 mixtures of Ham's nutrient mixture F-12 and DME. rBSC1 mRNA abundance and gene transcription rate were quantified by quantitative reverse transcription-PCR and run-off assay, respectively. Acid incubation enhanced mRNA abundance within 4 h in whole cell (P Ͻ 0.02) but not in nucleus. BSC1 gene transcription rate was not affected by acid incubation. In contrast, under conditions in which gene transcription was blocked, rBSC1 mRNA decreased within 6 h by 38 Ϯ 11% in control but only by 15 Ϯ 15% in acid medium (P Ͻ 0.02), which represented an increase in the BSC1 mRNA half-life from approximately 7 to approximately 17 h. Furthermore, in a mouse TAL cell line, acid incubation for 16 h significantly increased (P Ͻ 0.02) the amount of BSC1 mRNA in cells transfected with the fulllength mBSC1 cDNA but not in cells transfected with a mBSC1 cDNA lacking the 3'-UTR. These results demonstrate that acid pH enhances the stability of BSC1 mRNA probably by activating pathways that act on the AU-rich 3'-UTR of BSC1 mRNA, which contributes to the renal response to metabolic acidosis. Increased urinary NH 4ϩ excretion, which augments acid excretion, has long been recognized to be quantitatively the major compensatory response of the kidney against chronic metabolic acidosis (CMA) (1). It is established that most of ammonia that leaves the proximal tubule is absorbed by the medullary thick ascending limb (MTAL), which causes ammonia accumulation in the medullary interstitium followed by its secretion in adjacent medullary collecting tubule (2). The role of MTAL ammonia absorption may be particularly important in states of CMA because micropuncture experiments have shown that the NH 4 ϩ amount absorbed by the loop of Henle is increased under the latter condition (3,4). In this regard, Good (5) has shown that rat MTAL isolated and perfused in vitro have an increased ability to absorb NH 4 ϩ in response to CMA. This adaptive response in MTAL would substantially augment NH 4 ϩ excretion during metabolic acidosis.It is established that the luminal step of MTAL NH 4 ϩ absorption largely involves the MTAL bumetanide-sensitive Na ϩ -K ϩ (NH 4 ϩ )-2Cl Ϫ apical co-transporter (6 -10) that was recently cloned in various species and named BSC1 or NKCC2 (11)(12)(13)(14)(15). We recently demonstrated that CMA upregulate BSC1 expression. Indeed, BSC1 mRNA abundance significantly increased 3 h after the creation of metabolic acidosis by peritoneal dialysis, which was followed within 24 h by an augmentation of the BSC1 protein abundance (16). The augmentation of both mRNA and protein abundance persisted after 6 d...

Acid-Base Status Determines the Renal Expression of Ca2+ and Mg2+ Transport Proteins

“…Acetazolamide treatment was associated with increased urine volume and Na ϩ excretion, which has been shown to result in extracellular volume contraction and enhanced NKCC2 activity (45,46). Furthermore, metabolic acidosis, which developed in acetazolamide-treated wild-type mice, was previously shown to increase NKCC2 mRNA and protein abundance (14,47). Thus, we postulate that these additive stimulatory effects enhance passive Mg 2ϩ reabsorption in TAL.…”

Hepcidin as a Major Component of Renal Antibacterial Defenses against Uropathogenic Escherichia coli

“…The mRNA levels of NHE3 and BSC1 that are sensitive to the body acid-base balance were not affected (Supplemental Figure 4). [32][33][34][35] Hepcidin as an Antimicrobial Peptide Active Against CFT073…”

Molecular Mechanisms of Acid-Base Sensing by the Kidney

“…50 A similar mechanism involving z-crystallin has been proposed to enhance mRNA and protein expression of the loop-diuretic-sensitive NKCC2 cotransporter in the TAL. [51][52][53] Whether any of the pH sensors described above are involved in these processes is unknown.…”

“…Indeed, an increase in basolateral CO 2 stimulates apical proton secretion by A-ICs. 52 A remaining puzzle is how A-type (acid-secreting) and B-type (bicarbonate-secreting) ICs differentially respond to variations in systemic acid/base levels to modulate V-ATPase trafficking into and out of their apical and basolateral membranes, respectively. 63 PC, principal cell.…”