Kinetics and osmoregulation of Na(+)-and Cl(-)-dependent betaine transporter in rat renal medulla

Sickle cell disease up-regulates vasopressin, aquaporin 2, urea transporter A1, Na-K-Cl cotransporter 2, and epithelial Na channels in the mouse kidney medulla despite compromising urinary concentration ability

Sickle cell disease (SCD)-induced urinary concentration defect has been proposed as caused by impaired ability of the occluded vasa recta due to red blood cell sickling to serve as countercurrent exchangers and renal tubules to absorb water and solutes. However, the exact molecular mechanisms remain largely unknown. The present studies were undertaken to determine the effects of SCD on vasopressin, aquaporin2 (AQP2), urea transporter A1 (UTA1), Na-K-Cl co-transporter 2 (NKCC2), epithelial Na channels (ENaC), aquaporin1 (AQP1), nuclear factor of activated T cells 5 (NFAT5) and Src homology region-2 domain-containing phosphatase-1 (SHP-1), an important regulator of NFAT5, in the Berkeley SCD mouse kidney medulla. Under water repletion, SCD only induced a minor urinary concentration defect associated with increased urinary vasopressin level alone with the well-known effects of vasopressin: protein abundance of AQP2, UTA1 and ENaC-β and apical targeting of AQP2 as compared with non-SCD. SCD did not significantly affect AQP1 protein level. Water restriction had no further significant effect on SCD urinary vasopressin. NFAT5 is also critical to urinary concentration. Instead, water restriction-activated NFAT5 associated with inhibition of SHP-1 in the SCD mice. Yet, water restriction only elevated urinary osmolality by 28% in these mice as opposed to 104% in non-SCD mice despite similar degree increases of protein abundance of AQP2, NKCC2 and AQP2-S256-P. Water-restriction had no significant effect on protein abundance of ENaC or AQP1 in either strain. In conclusion, under water repletion SCD, only induces a minor defect in urinary concentration because of compensation from the up-regulated vasopressin system. However, under water restriction, SCD mice struggle to concentrate urine despite activating NFAT5. SCD-induced urinary concentration defect appears to be resulted from the poor blood flow in vasa recta rather than the renal tubules' ability to absorb water and solutes.

The betaine-GABA transporter (BGT1, slc6a12) is predominantly expressed in the liver and at lower levels in the kidneys and at the brain surface

The Na+- and Cl−-dependent GABA-betaine transporter (BGT1) has received attention mostly as a protector against osmolarity changes in the kidney and as a potential controller of the neurotransmitter GABA in the brain. Nevertheless, the cellular distribution of BGT1, and its physiological importance, is not fully understood. Here we have quantified mRNA levels using TaqMan real-time PCR, produced a number of BGT1 antibodies, and used these to study BGT1 distribution in mice. BGT1 (protein and mRNA) is predominantly expressed in the liver (sinusoidal hepatocyte plasma membranes) and not in the endothelium. BGT1 is also present in the renal medulla, where it localizes to the basolateral membranes of collecting ducts (particularly at the papilla tip) and the thick ascending limbs of Henle. There is some BGT1 in the leptomeninges, but brain parenchyma, brain blood vessels, ependymal cells, the renal cortex, and the intestine are virtually BGT1 deficient in 1- to 3-mo-old mice. Labeling specificity was assured by processing tissue from BGT1-deficient littermates in parallel as negative controls. Addition of 2.5% sodium chloride to the drinking water for 48 h induced a two- to threefold upregulation of BGT1, tonicity-responsive enhancer binding protein, and sodium- myo-inositol cotransporter 1 (slc5a3) in the renal medulla, but not in the brain and barely in the liver. BGT1-deficient and wild-type mice appeared to tolerate the salt treatment equally well, possibly because betaine is one of several osmolytes. In conclusion, this study suggests that BGT1 plays its main role in the liver, thereby complementing other betaine-transporting carrier proteins (e.g., slc6a20) that are predominantly expressed in the small intestine or kidney rather than the liver.

Nitric oxide production by mouse renal tubules can be increased by a sodium-dependent mechanism

Renal tubules process large amounts of NaCl that other investigators indicate increases tubular generation of nitric oxide. We questioned whether medullary or superficial cortical tubules would have the greater increase in nitric oxide concentration, [NO], when stressed by sodium and if the sodium/calcium exchanger was involved. Sodium stress in proximal tubules is due to the large amount of sodium absorbed and medullary tubules exist in a hypertonic sodium environment. To sodium stress the tissue, mouse kidney slices were exposed to monensin to allow passive entry of sodium ions from isotonic media and in separate studies, 400 and 600 mOsm NaCl was used. [NO] was measured with microelectrodes. Monensin (10 microM) caused a sustained increase in medullary and cortical [NO] to approximately 180% of control and 400 mOsm NaCl caused a similar initial increase in [NO] that then subsided. 600 mOsm NaCl caused a more sustained increase in [NO] of >250% of control. L-NAME strongly attenuated the increased [NO] during sodium stress. The increase in [NO] during NaCl elevation was due to sodium ions because mannitol hyperosmolarity caused approximately 20% of the increase in [NO]. Entry of sodium during NaCl hyperosmolarity was through bumetanide sensitive channels because the drug suppressed increased [NO]. Blockade of the sodium/calcium ion exchanger strongly suppressed the increased [NO] during monensin, to increase sodium entry into cells, and the elevated NaCl concentration. The data support a sodium-NO linkage that increased NO signaling in proportion to sodium stress by cortical tubules and was highly dependent upon sodium-calcium exchange.

Characterization of endogenous betaine gamma-amino-n-butyric acid cotransporter glycoform and its hyperosmotic regulation in MDCK cells

Increase in mRNA expression and transport activity of the betaine gamma-amino-n-butyric acid cotransporter (BGAT) in response to hyperosmolality has been previously shown in MDCK cells. However, the hyperosmolality-induced response of endogenous BGAT protein expression was not investigated in detail. We show two forms of endogenous BGAT immunoreactivity that are expressed in MDCK II cells. Both are sensitive to Peptide N-Glycosidase F (PNGase F), suggesting that they are N-glycosylated proteins. One band, about 75 kDa, is resistant to Endo H, while the other 55 kDa band is sensitive to it, suggesting that they are fully N-glycosylated mature form in the post-Golgi compartment and core-glycosylated immature form in the endoplasmic reticulum (ER), respectively. When treated with hyperosmolality, they are significantly increased. But the rate of BGAT processing, as assessed by the ratio of mature to immature form, is not increased, suggesting that hyperosmolality does not facilitate the export of BGAT from the ER to the secretory pathway. Surface biotinylation and confocal microscopy show that hyperosmolality significantly increases the amount of the mature form of BGAT on the basolateral membrane with a very small fraction on the apical membrane. We conclude that BGAT is an N-glycosylated protein with two glycoforms and endogenous BGAT synthesis rather than processing is involved in the adaptation to the hyperosmotic stress.

Impaired ability to increase water excretion in mice lacking the taurine transporter gene TAUT

Cellular taurine uptake or release counteracts alterations of cell volume. Na+-coupled taurine transporter TAUT mediates concentrative cellular uptake of taurine. Inhibition of vasopressin secretion by hypotonicity may involve taurine release from glial cells of supraoptic nucleus. We compared renal function of mice lacking TAUT (taut-/-) and wild-type littermates (taut+/+). We observed renal taurine loss and subsequent hypotaurinemia in taut-/- mice. With free access to water, plasma and urine osmolality, urinary flow rate as well as urinary excretion and plasma concentrations of Na+ and K+ were similar in taut-/- and taut+/+ mice, whereas plasma concentrations of urea were enhanced in taut-/- mice. An oral water load (1 ml/16 g body weight) induced a similar diuresis in both genotypes. Repeating the oral water load immediately after normalization of urine flow rate, however, resulted in delayed diuresis and higher urinary vasopressin/creatinine ratios in taut-/- mice. In comparison, the repeated diuretic response to vasopressin V2 receptor blockade was not different between genotypes. Water deprivation for 36 h led to similar antidiuresis and increases of urinary osmolality in both genotypes. Upon free access to water after deprivation, taut-/- mice continued to concentrate urine up to 6 days, while taut+/+ mice rapidly returned to normal urinary osmolality. Urinary vasopressin/creatinine ratios and plasma aldosterone concentrations were not different under basal conditions but were significantly higher in taut-/- mice than in taut+/+ mice at 6 days after water deprivation. In conclusion, taut-/- mice suffer from renal taurine loss and impaired ability to lower urine osmolality and to increase urinary water excretion. The latter defect could reside extrarenally and result from a role of taurine in the suppression of vasopressin release which may be attenuated in taut-/- mice.

Role of integrin alpha1beta1 in the regulation of renal medullary osmolyte concentration

The mechanism by which cells sense extracellular tonicity and trigger the accumulation of protective organic osmolytes is poorly understood. It has been proposed that changes in cell volume following alteration of extracellular toncity are important initiators of signaling events that lead to osmolyte accumulation. Because the extracellular matrix receptors integrins are linked to the cytoskeleton and can transduce signals that alter cell behavior, we investigated the role of these receptors in the modulation of osmolyte accumulation in the kidney medulla under different osmotic conditions. We show that integrin alpha1-null mice have impaired ability to accumulate organic osmolytes in the inner medulla due to altered signaling and decreased induction of osmolyte transporters or aldose reductase gene transcription. Utilizing inner medullary collecting duct cells, we demonstrate that the lack of integrin alpha1beta1 results in an impaired ability to induce the tonicity enhancer-binding protein TonEBP under hypertonic conditions. Furthermore, under the same conditions, integrin alpha1-null cells show prolonged ERK1/2 phosphorylation and decreased inositol uptake compared with control cells. The reduction of inositol uptake is significantly reversed by treatment with the MEK inhibitor PD-98059. Finally, integrin alpha1-null mice develop morphological changes of early tubular necrosis and increased apoptosis of renal medullary cells following dehydration. Together, these results show that integrin alpha1beta1 is an important mediator of the compatible osmolyte response in the medulla of the mammalian kidney.

Osmotic regulation of renal betaine transport: transcription and beyond

Cells in the kidney inner medulla are routinely exposed to high extracellular osmolarity during normal operation of the urinary concentrating mechanism. One adaptation critical for survival in this environment is the intracellular accumulation of organic osmolytes to balance the osmotic stress. Betaine is an important osmolyte that is accumulated via the betaine/gamma-aminobutyric acid transporter (BGT1) in the basolateral plasma membrane of medullary epithelial cells. In response to hypertonic stress, there is transcriptional activation of the BGT1 gene, followed by trafficking and membrane insertion of BGT1 protein. Transcriptional activation, triggered by changes in ionic strength and water content, is an early response that is a key regulatory step and has been studied in detail. Recent studies suggest there are additional post-transcriptional regulatory steps in the pathway leading to upregulation of BGT1 transport, and that additional proteins are required for membrane insertion. Reversal of this adaptive process, upon removal of hypertonic stress, involves a rapid efflux of betaine through specific release pathways, a reduction in betaine influx, and a slower downregulation of BGT1 protein abundance. There is much more to be learned about many of these steps in BGT1 regulation.

CELL SURVIVAL IN THE HOSTILE ENVIRONMENT OF THE RENAL MEDULLA

The countercurrent system in the medulla of the mammalian kidney provides the basis for the production of urine of widely varying osmolalities, but necessarily entails extreme conditions for medullary cells, i.e., high concentrations of solutes (mainly NaCl and urea) in antidiuresis, massive changes in extracellular solute concentrations during the transitions from antidiuresis to diuresis and vice versa, and low oxygen tension. The strategies used by medullary cells to survive in this hostile milieu include accumulation of organic osmolytes and heat shock proteins, the extensive use of the glycolysis for energy production, and a well-orchestrated network of signaling pathways coordinating medullary circulation and tubular work.

Expression of aldose reductase in developing rat kidney

Newborn rats are not capable of producing concentrated urine. With development of the concentrating system and a hypertonic medullary interstitium, intracellular osmolytes, such as sorbitol, accumulate in the renal medulla. Sorbitol is produced from glucose in a reaction catalyzed by aldose reductase (AR). The purpose of this study was to establish the time of expression and distribution of AR in the developing rat kidney. Kidneys from 16-, 18-, and 20-day-old fetuses and 1-, 3-, 4-, 5-, 7-, 14-, and 21-day-old pups were processed for immunohistochemistry and immunoblot analysis. In adult animals, AR was expressed only in the inner medulla, in which it was localized in ascending thin limbs (ATLs), inner medullary collecting ducts (IMCDs), and interstitial cells. AR immunoreactivity was not detected in fetal kidneys but was observed in the terminal part of the descending thin limb and IMCD in the renal papilla of 1-day-old pups. At birth, all of the loops of Henle are configured as short loops and there are no ATLs. After birth, papillary thick ascending limbs are gradually transformed into ATLs by a process that involves apoptotic deletion of cells from the thick ascending limb. During this time, AR immunoreactivity appeared in the cells undergoing transformation in the ascending limb, beginning at the papillary tip and ascending to the border between the outer medulla and the inner medulla. However, there was no labeling of apoptotic cells. The expression of AR in both the ATL and the IMCD gradually increased during kidney development. We conclude that AR expression in the inner medulla coincides with the increase in medullary tonicity that is known to occur during the first 3 wk after birth. On the basis of the observation that only AR-negative cells were deleted by apoptosis in the differentiating ATL, we propose that AR may protect ATL cells against apoptosis.

Osmotically relevant membrane signaling complex: association between HB-EGF, beta(1)-integrin, and CD9 in mTAL

The integral membrane proteins cluster of differentiation-9 (CD9), beta(1)-integrin, and heparin-binding epidermal growth factor-like (HB-EGF) exist in association in many cell lines and are linked to intracellular signaling mechanisms. Two of the proteins (CD9 and beta(1)-integrin) are induced by hypertonicity, suggesting that their related signaling processes may be relevant to osmotic stress. The validity of this hypothesis rests upon coexpression and physical association between these molecules in nephron segments that are normally exposed to high and variable ambient osmolality. In this work, we show that CD9 and beta(1)-integrin are induced in rat kidney medulla after dehydration. Immunohistochemistry and immunoprecipitation studies show that CD9, HB-EGF, and beta(1)-integrin are coexpressed and physically associated in medullary thick ascending limbs (mTAL), nephron segments that are normally exposed to high and variable extracellular osmolality. Our findings are consistent with the existence of a cluster of integral membrane proteins in mTAL that may initiate or modulate osmotically relevant signaling pathways.

Reabsorption of betaine in Henle's loops of rat kidney in vivo

This study was designed 1) to localize and 2) to characterize betaine reabsorption from the tubular lumen in rat kidney in vivo, and 3) to test whether reabsorption is modulated by the diuretic state. [(14)C]betaine (+ [(3)H]inulin) was microperfused through the proximal convoluted tubule (PCT) and microinfused into late proximal (LP) and early distal (ED) tubules, long loops of Henle (LLH), and vasa recta of the rat in vivo et situ, and the fractional recovery of the (14)C label was determined end proximally (PCT) and in the final urine, respectively. [(14)C]betaine was not reabsorbed during ED microinfusion, whereas fractional reabsorption during LP microinfusion was 82% at 0.06 mM betaine and decreased gradually to 4.8% at 60 mM. L-Proline had lower Michaelis-Menten constant (K(m)) and sarcosine a higher K(m) than betaine. Chronic, but not acute, diuresis inhibited betaine reabsorption in Henle's loops. Fractional [(14)C]betaine reabsorption in PCT was much smaller than that during LP microinfusion. [(14)C]betaine (7.28 mM) microinfused 1) into LLH was reabsorbed to 30% and 2) into vasa recta appeared in the ipsilateral urine to a much higher extent than contralaterally. In both cases, no saturation was detected at 70 mM. We conclude that betaine is reabsorbed by mediated transport from descending limbs of short Henle's loops by a proline-preferring carrier in a diuresis-modulated manner. In the deep medulla, bidirectional blood/urine betaine transport exists.