Marija Ljubojević

Revised immunolocalization of the Na+-D-glucose cotransporter SGLT1 in rat organs with an improved antibody

Previously, we characterized localization of Na(+)-glucose cotransporter SGLT1 (Slc5a1) in the rat kidney using a polyclonal antibody against the synthetic COOH-terminal peptide of the rat protein (Sabolić I, Skarica M, Gorboulev V, Ljubojević M, Balen D, Herak-Kramberger CM, Koepsell H. Am J Physiol Renal Physiol 290: 913-926, 2006). However, the antibody gave some false-positive reactions in immunochemical studies. Using a shortened peptide for immunization, we have presently generated an improved, more specific anti-rat SGLT1 antibody (rSGLT1-ab), which in immunochemical studies with isolated membranes and tissue cryosections from male (M) and female (F) rats exhibited 1) in kidneys and small intestine, labeling of a major protein band of approximately 75 kDa; 2) in kidneys of adult animals, localization of rSGLT1 to the proximal tubule (PT) brush-border membrane (S1 < S2 < S3) and intracellular organelles (S1 > S2 > S3), with zonal (cortex < outer stripe) and sex differences (M < F) in the protein expression, which correlated well with the tissue expression of its mRNA in RT-PCR studies; 3) in kidneys of castrated adult M rats, upregulation of the protein expression; 4) in kidneys of prepubertal rats, weak and sex-independent labeling of the 75-kDa protein band and immunostaining intensity; 5) in small intestine, sex-independent regional differences in protein abundance (jejunum > duodenum = ileum); and 6) thus far unrecognized localization of the transporter in cortical thick ascending limbs of Henle and macula densa in kidney, bile ducts in liver, enteroendocrine cells and myenteric plexus in the small intestine, and initial ducts in the submandibular gland. Our improved rSGLT1-ab may be used to identify novel sites of SGLT1 localization and thus unravel additional physiological functions of this transporter in rat organs.

cAMP stimulates apical V-ATPase accumulation, microvillar elongation, and proton extrusion in kidney collecting duct A-intercalated cells

Kidney proton-secreting A-intercalated cells (A-IC) respond to systemic acidosis by accumulating the vacuolar ATPase (V-ATPase) in their apical membrane and by increasing the length and number of apical microvilli. We show here that the cell-permeant cAMP analog CPT-cAMP, infused in vivo, results in an almost twofold increase in apical V-ATPase accumulation in AE1-positive A-IC within 15 min and that these cells develop an extensive array of apical microvilli compared with controls. In contrast, no significant change in V-ATPase distribution could be detected by immunocytochemistry in B-intercalated cells at the acute time point examined. To show a direct effect of cAMP on A-IC, we prepared cell suspensions from the medulla of transgenic mice expressing EGFP in IC (driven by the B1-subunit promoter of the V-ATPase) and exposed them to cAMP analogs in vitro. Three-dimensional reconstructions of confocal images revealed that cAMP induced a time-dependent growth of apical microvilli, starting within minutes after addition. This effect was blocked by the PKA inhibitor myristoylated PKI. These morphological changes were paralleled by increased cAMP-mediated proton extrusion (pHi recovery) by A-IC in outer medullary collecting ducts measured using the ratiometric probe BCECF. These results, and our prior data showing that the bicarbonate-stimulated soluble adenylyl cyclase (sAC) is highly expressed in kidney intercalated cells, support the idea that cAMP generated either by sAC, or by activation of other signaling pathways, is part of the signal transduction mechanism involved in acid-base sensing and V-ATPase membrane trafficking in kidney intercalated cells.

Expression of Na+-d-glucose cotransporter SGLT2 in rodents is kidney-specific and exhibits sex and species differences

With a novel antibody against the rat Na(+)-D-glucose cotransporter SGLT2 (rSGLT2-Ab), which does not cross-react with rSGLT1 or rSGLT3, the ∼75-kDa rSGLT2 protein was localized to the brush-border membrane (BBM) of the renal proximal tubule S1 and S2 segments (S1 > S2) with female-dominant expression in adult rats, whereas rSglt2 mRNA expression was similar in both sexes. Castration of adult males increased the abundance of rSGLT2 protein; this increase was further enhanced by estradiol and prevented by testosterone treatment. In the renal BBM vesicles, the rSGLT1-independent uptake of [(14)C]-α-methyl-D-glucopyranoside was similar in females and males, suggesting functional contribution of another Na(+)-D-glucose cotransporter to glucose reabsorption. Since immunoreactivity of rSGLT2-Ab could not be detected with certainty in rat extrarenal organs, the SGLT2 protein was immunocharacterized with the same antibody in wild-type (WT) mice, with SGLT2-deficient (Sglt2 knockout) mice as negative control. In WT mice, renal localization of mSGLT2 protein was similar to that in rats, whereas in extrarenal organs neither mSGLT2 protein nor mSglt2 mRNA expression was detected. At variance to the findings in rats, the abundance of mSGLT2 protein in the mouse kidneys was male dominant, whereas the expression of mSglt2 mRNA was female dominant. Our results indicate that in rodents the expression of SGLT2 is kidney-specific and point to distinct sex and species differences in SGLT2 protein expression that cannot be explained by differences in mRNA.

Low doses of ochratoxin A upregulate the protein expression of organic anion transporters Oat1, Oat2, Oat3 and Oat5 in rat kidney cortex.

Mycotoxin ochratoxin A (OTA) is nephrotoxic in various animal species. In rodents, OTA intoxication impairs various proximal tubule (PT) functions, including secretion of p-aminohippurate (PAH), possibly via affecting the renal organic anion (OA) transporters (Oat). However, an effect of OTA on the activity/expression of specific Oats in the mammalian kidney has not been reported. In this work, male rats were gavaged various doses of OTA every 2nd day for 10 days, and in their kidneys we studied: tubule integrity by microscopy, abundance of basolateral (rOat1, rOat3) and brush-border (rOat2, rOat5) rOat proteins by immunochemical methods, and expression of rOats mRNA by RT-PCR. The OTA treatment caused: a) dose-dependent damage of the cells in S3 segments of medullary rays, b) dual effect upon rOats in PT: low doses (50-250 microg OTA/kg b.m.) upregulated the abundance of all rOats, while a high dose (500 microg OTA/kg b.m.) downregulated the abundance of rOat1, and c) unchanged mRNA expression for all rOats at low OTA doses, and its downregulation at high OTA dose. Changes in the expression of renal Oats were associated with enhanced OTA accumulation in tissue and excretion in urine, whereas the indicators of oxidative stress either remained unchanged (malondialdehyde, glutathione, 8-hydroxydeoxyguanosine) or became deranged (microtubules). While OTA accumulation and downregulation of rOats in the kidney are consistent with the previously reported impaired renal PAH secretion in rodents intoxicated with high OTA doses, the post-transcriptional upregulation of Oats at low OTA doses may contribute to OTA accumulation and development of nephrotoxicity.

Functional and Immunochemical Characterization of a Novel Organic Anion Transporter Oat8 (Slc22a9) in Rat Renal Collecting Duct

In this study, we demonstrate that a putative membrane unknown solute transporter 1 of the rat kidney (UST1r; Slc22a9) is a multispecific transporter of organic anions (OAs). When expressed in Xenopus oocytes, UST1r mediated uptake of ochratoxin A (OTA; Km = 1.0 µM) and sulfate conjugates of steroids, such as estrone-3-sulfate (ES; Km = 3.1 µM) and dehydroepiandrosterone sulfate (DHEAS; Km = 2.1 µM) in a sodium-independent manner. We herein propose that UST1r be renamed OA transporter 8 (rOat8). rOat8 interacted with chemically heterogenous anionic compounds, such as nonsteroidal anti-inflammatory drugs, diuretics, probenecid, taurocholate, and methotrexate, but not with the organic cation tetraethylammonium. The rOat8-mediated ES transport was: a) cis-inhibited by 4-methylumbelliferyl sulfate and β-estradiol sulfate, but not by glucuronide conjugates of these compounds, b) cis-inhibited by four- and five- carbon (C4/C5) dicarboxylates (succinate and glutarate (GA)), and c) trans-stimulated by GA, whereas the efflux of GA was significantly trans-stimulated by ES. By RT-PCR, rOat8 mRNA was expressed in proximal convoluted tubules and cortical and outer medullary collecting ducts, whereas in immunochemical studies, Oat8 was identified as the ñ58 kDa protein that in the collecting duct colocalized with the V-ATPase in plasma membranes and intracellular vesicles in various subtypes of intercalated cells. Molecular identification of Oat8 in these cells indicates a possible novel role of OAT family in the renal secretion/reabsorption of OA and acids and bases via affecting the V-ATPase-dependent functions.

Distribution of organic anion transporters NaDC3 and OAT1-3 along the human nephron

The initial step in renal secretion of organic anions (OAs) is mediated by transporters in the basolateral membrane (BLM). Contributors to this process are primary active Na(+)-K(+)-ATPase (EC 3.6.3.9), secondary active Na(+)-dicarboxylate cotransporter 3 (NaDC3/SLC13A3), and tertiary active OA transporters (OATs) OAT1/SLC22A6, OAT2/SLC22A7, and OAT3/SLC22A8. In human kidneys, we analyzed the localization of these transporters by immunochemical methods in tissue cryosections and isolated membranes. The specificity of antibodies was validated with human embryonic kidney-293 cells stably transfected with functional OATs. Na(+)-K(+)-ATPase was immunolocalized to the BLM along the entire human nephron. NaDC3-related immunostaining was detected in the BLM of proximal tubules and in the BLM and/or luminal membrane of principal cells in connecting segments and collecting ducts. The thin and thick ascending limbs, macula densa, and distal tubules exhibited no reactivity with the anti-NaDC3 antibody. OAT1-OAT3-related immunostaining in human kidneys was detected only in the BLM of cortical proximal tubules; all three OATs were stained more intensely in S1/S2 segments compared with S3 segment in medullary rays, whereas the S3 segment in the outer stripe remained unstained. Expression of NaDC3, OAT1, OAT2, and OAT3 proteins exhibited considerable interindividual variability in both male and female kidneys, and sex differences in their expression could not be detected. Our experiments provide a side-by-side comparison of basolateral transporters cooperating in renal OA secretion in the human kidney.

Sex-dependent expression of water channel AQP1 along the rat nephron

In the mammalian kidney, nonglycosylated and glycosylated forms of aquaporin protein 1 (AQP1) coexist in the luminal and basolateral plasma membranes of proximal tubule and descending thin limb. Factors that influence AQP1 expression in (patho)physiological conditions are poorly known. Thus far, only angiotensin II and hypertonicity were found to upregulate AQP1 expression in rat proximal tubule in vivo and in vitro (Bouley R, Palomino Z, Tang SS, Nunes P, Kobori H, Lu HA, Shum WW, Sabolic I, Brown D, Ingelfinger JR, Jung FF. Am J Physiol Renal Physiol 297: F1575-F1586, 2009), a phenomenon that may be relevant for higher blood pressure observed in men and male experimental animals. Here we investigated the sex-dependent AQP1 protein and mRNA expression in the rat kidney by immunochemical methods and qRT-PCR in tissue samples from prepubertal and intact gonadectomized animals and sex hormone-treated gonadectomized adult male and female animals. In adult rats, the overall renal AQP1 protein and mRNA expression was ∼80% and ∼40% higher, respectively, in males than in females, downregulated by gonadectomy in both sexes and upregulated strongly by testosterone and moderately by progesterone treatment; estradiol treatment had no effect. In prepubertal rats, the AQP1 protein expression was low compared with adults and slightly higher in females, whereas the AQP1 mRNA expression was low and similar in both sexes. The observed differences in AQP1 protein expression in various experiments mainly reflect changes in the glycosylated form. The male-dominant expression of renal AQP1 in rats, which develops after puberty largely in the glycosylated form of the protein, may contribute to enhanced fluid reabsorption following the androgen- or progesterone-stimulated activities of sodium-reabsorptive mechanisms in proximal tubules.

In female rats, ethylene glycol treatment elevates protein expression of hepatic and renal oxalate transporter sat-1 (Slc26a1) without inducing hyperoxaluria

Aim To investigate whether the sex-dependent expression of hepatic and renal oxalate transporter sat-1 (Slc26a1) changes in a rat model of ethylene glycol (EG)-induced hyperoxaluria. Methods Rats were given tap water (12 males and 12 females; controls) or EG (12 males and 12 females; 0.75% v/v in tap water) for one month. Oxaluric state was confirmed by biochemical parameters in blood plasma, urine, and tissues. Expression of sat-1 and rate-limiting enzymes of oxalate synthesis, alcohol dehydrogenase 1 (Adh1) and hydroxy-acid oxidase 1 (Hao1), was determined by immunocytochemistry (protein) and/or real time reverse transcription polymerase chain reaction (mRNA). Results EG-treated males had significantly higher (in μmol/L; mean ± standard deviation) plasma (59.7 ± 27.2 vs 12.9 ± 4.1, P < 0.001) and urine (3716 ± 1726 vs 241 ± 204, P < 0.001) oxalate levels, and more abundant oxalate crystaluria than controls, while the liver and kidney sat-1 protein and mRNA expression did not differ significantly between these groups. EG-treated females, in comparison with controls had significantly higher (in μmol/L) serum oxalate levels (18.8 ± 2.9 vs 11.6 ± 4.9, P < 0.001), unchanged urine oxalate levels, low oxalate crystaluria, and significantly higher expression (in relative fluorescence units) of the liver (1.59 ± 0.61 vs 0.56 ± 0.39, P = 0.006) and kidney (1.77 ± 0.42 vs 0.69 ± 0.27, P < 0.001) sat-1 protein, but not mRNA. The mRNA expression of Adh1 was female-dominant and that of Hao1 male-dominant, but both were unaffected by EG treatment. Conclusions An increased expression of hepatic and renal oxalate transporting protein sat-1 in EG-treated female rats could protect from hyperoxaluria and oxalate urolithiasis.

Expression and immunolocalization of metallothioneins MT1, MT2 and MT3 in rat nephron

Rodent kidneys exhibit three isoforms of metallothioneins (MTs), MT1, MT2 and MT3, with poorly characterized localization along the nephron. Here we studied in adult male Wistar rats the renal expression of MTs mRNA by end-point RT-PCR and MT proteins by immunochemical methods The expression pattern of MT1 mRNA was cortex (CO)>outer stripe (OS)=inner stripe (IS)=inner medulla (IM), of MT2 mRNA was IM>CO>IS=OS, and of MT3 mRNA was IM>CO=OS=IM. MT1/2-antibody stained with heterogeneous intensity the cell cytoplasm and nuclei in proximal tubule (PT) and thin ascending limb, whereas MT3-antibody stained weakly the cell cytoplasm in various cortical tubules and strongly the nuclei in all nephron segments. However, the isolated nuclei exhibited an absence of MT1/2 and presence of MT3 protein. In MT1/2-positive PT cells, the intracellular staining appeared diffuse or bipolar, but the isolated brush-border, basolateral and endosomal membranes were devoid of MT1/2 proteins. In the lumen of some PT profiles, the heterogeneously sized MT1/2-rich vesicles were observed, with the limiting membrane positive for NHE3, but negative for V-ATPase, CAIV, and megalin, whereas their interior was positive for CAII and negative for cytoskeleton. They seem to be pinched off from the luminal membrane of MT1/2-rich cells, as also indicated by transmission electron microscopy. We conclude that in male rats, MTs are heterogeneously abundant in the cell cytoplasm and/or nuclei along the nephron. The MT1/2-rich vesicles in the tubule lumen may represent a source of urine MT and membranous material, whereas MT3 in nuclei may handle zink and locally-produced reactive oxygen species.

Presence of murine organic anion transporter 1(mOAT1) in kidneys and brain and interaction with tryptophan metabolites

Organic anion transporters (OATs) facilitate the renal secretion of weak organic acids including endogenous metabolites and drugs. Besides acidic metabolites of neurotransmitters, neuroactive tryptophan metabolites like kynurenate (KYNA), 3- hydroxykynurenine (3-HK) and quinolinate (QUIN) are postulated to be excreted via a probenecid-sensitive transport System. Immunohistochemical analyses in mouse kidneys revealed the localization of mouse OATl (mOAT1) in proximal convoluted tubules. Examinations of mouse brain sections showed positive stainings of cortical neurons. Additionally, we investigated the interaction of mOATl with several tryptophan metabolites. We transiently transfected COS 7 cells with mOAT1 and measured the influence of tryptophan metabolites on [~HIPAH u take 1mM of probenecid, KYNA, 3-HK, 5- hydroxyindoEte (i-HIAA) and anthranilate (ANTRA) decreased PAH uptake by 50-85%, demonstrating that all these compounds bind to OAT1. 1mM QUIN showed only a slight, but significant inhibition of PAH uptake. Preloading the cells with 1mM glutarate, KYNA, 3-HK, 5-HIAA, ANTRA or QUIN revealed a substantial trans-stimulation of PAH uptake by glutarate, and a significant effect for 5-HIAA, indicating that this compound is translocated by OAT1.