Distinct expression patterns of different subunit isoforms of the V-ATPase in the rat epididymis

In the epididymis and vas deferens, the vacuolar H(+)ATPase (V-ATPase), located in the apical pole of narrow and clear cells, is required to establish an acidic luminal pH. Low pH is important for the maturation of sperm and their storage in a quiescent state. The V-ATPase also participates in the acidification of intracellular organelles. The V-ATPase contains many subunits, and several of these subunits have multiple isoforms. So far, only subunits ATP6V1B1, ATP6V1B2, and ATP6V1E2, previously identified as B1, B2, and E subunits, have been described in the rat epididymis. Here, we report the localization of V-ATPase subunit isoforms ATP6V1A, ATP6V1C1, ATP6V1C2, ATP6V1G1, ATP6V1G3, ATP6V0A1, ATP6V0A2, ATP6V0A4, ATP6V0D1, and ATP6V0D2, previously labeled A, C1, C2, G1, G3, a1, a2, a4, d1, and d2, in epithelial cells of the rat epididymis and vas deferens. Narrow and clear cells showed a strong apical staining for all subunits, except the ATP6V0A2 isoform. Subunits ATP6V0A2 and ATP6V1A were detected in intracellular structures closely associated but not identical to the TGN of principal cells and narrow/clear cells, and subunit ATP6V0D1 was strongly expressed in the apical membrane of principal cells in the apparent absence of other V-ATPase subunits. In conclusion, more than one isoform of subunits ATP6V1C, ATP6V1G, ATP6V0A, and ATP6V0D of the V-ATPase are present in the epididymal and vas deferens epithelium. Our results confirm that narrow and clear cells are well fit for active proton secretion. In addition, the diverse functions of the V-ATPase may be established through the utilization of specific subunit isoforms. In principal cells, the ATP6V0D1 isoform may have a physiological function that is distinct from its role in proton transport via the V-ATPase complex.

The B1-subunit of the H(+) ATPase is required for maximal urinary acidification

The multisubunit vacuolar-type H(+)ATPases mediate acidification of various intracellular organelles and in some tissues mediate H(+) secretion across the plasma membrane. Mutations in the B1-subunit of the apical H(+)ATPase that secretes protons in the distal nephron cause distal renal tubular acidosis in humans, a condition characterized by metabolic acidosis with an inappropriately alkaline urine. To examine the detailed cellular and organismal physiology resulting from this mutation, we have generated mice deficient in the B1-subunit (Atp6v1b1(-/-) mice). Urine pH is more alkaline and metabolic acidosis is more severe in Atp6v1b1(-/-) mice after oral acid challenge, demonstrating a failure of normal urinary acidification. In Atp6v1b1(-/-) mice, the normal urinary acidification induced by a lumen-negative potential in response to furosemide infusion is abolished. After an acute intracellular acidification, Na(+)-independent pH recovery rates of individual Atp6v1b1(-/-) intercalated cells of the cortical collecting duct are markedly reduced and show no further decrease after treatment with the selective H(+)ATPase inhibitor concanamycin. Apical expression of the alternative B-subunit isoform, B2, is increased in Atp6v1b1(-/-) medulla and colocalizes with the H(+)ATPase E-subunit; however, the greater severity of metabolic acidosis in Atp6v1b1(-/-) mice after oral acid challenge indicates that the B2-subunit cannot fully functionally compensate for the loss of B1. Our results indicate that the B1 isoform is the major B-subunit isoform that incorporates into functional, plasma membrane H(+)ATPases in intercalated cells of the cortical collecting duct and is required for maximal urinary acidification.

Stimulation of AQP2 membrane insertion in renal epithelial cells in vitro and in vivo by the cGMP phosphodiesterase inhibitor sildenafil citrate (Viagra)

Vasopressin-stimulated insertion of the aquaporin 2 (AQP2) water channel into the plasma membrane of kidney collecting duct principal cells is a key event in the urinary concentrating mechanism. The paradigm for vasopressin-receptor signaling involves cAMP-mediated protein kinase A activation, which results in the functionally critical phosphorylation of AQP2 on amino acid serine 256. We previously showed that a parallel cGMP-mediated signaling pathway also leads to AQP2 membrane insertion in AQP2-transfected LLC-PK1 (LLC-AQP2) cells and in outer medullary collecting duct principal cells in situ (Bouley R, Breton S, Sun T, McLaughlin M, Nsumu NN, Lin HY, Ausiello DA, and Brown D. J Clin Invest 106: 1115-1126, 2000). In the present report, we show by immunofluorescence microscopy, and Western blotting of plasma membrane fractions, that 45-min exposure of LLC-AQP2 cells to the cGMP phosphodiesterase type 5 (PDE5) inhibitors sildenafil citrate (Viagra) or 4-{[3',4'-methylene-dioxybenzyl]amino}-6-methoxyquinazoline elevates intracellular cGMP levels and results in the plasma membrane accumulation of AQP2; i.e., they mimic the vasopressin effect. Importantly, our data also show that acute exposure to PDE5 inhibitors for 60 min induces apical accumulation of AQP2 in kidney medullary collecting duct principal cells both in tissue slices incubated in vitro as well as in vivo after intravenous injection of Viagra into rats. These data suggest that AQP2 membrane insertion can be induced independently of vasopressin-receptor activation by activating a parallel cGMP-mediated signal transduction pathway with cGMP PDE inhibitors. These results provide proof-of-principle that pharmacological activation of vasopressin-independent, cGMP signaling pathways could aid in the treatment of those forms of nephrogenic diabetes insipidus that are due to vasopressin-2 receptor dysfunction.

V-ATPase B1-subunit promoter drives expression of EGFP in intercalated cells of kidney, clear cells of epididymis and airway cells of lung in transgenic mice

The kidney, epididymis, and lungs are complex organs with considerable epithelial cell heterogeneity. This has limited the characterization of pathophysiological transport processes that are specific for each cell type in these epithelia. The purpose of the present study was to develop new tools to study cell-specific gene and protein expression in such complex tissues and organs. We report the production of a transgenic mouse that expresses enhanced green fluorescent protein (EGFP) in a subset of epithelial cells that express the B1 subunit of vacuolar H(+)-ATPase (V-ATPase) and are actively involved in proton transport. A 6.5-kb portion of the V-ATPase B1 promoter was used to drive expression of EGFP. In two founders, quantitative real-time RT-PCR demonstrated expression of EGFP in kidney, epididymis, and lung. Immunofluorescence labeling using antibodies against the B1 and E subunits of V-ATPase and against carbonic anhydrase type II (CAII) revealed specific EGFP expression in all renal type A and type B intercalated cells, some renal connecting tubule cells, all epididymal narrow and clear cells, and some nonciliated airway epithelial cells. No EGFP expression was detected in collecting duct principal cells (identified using an anti-AQP2 antibody) or epididymal principal cells (negative for V-ATPase or CAII). This EGFP-expressing mouse model should prove useful in future studies of gene and protein expression and their physiological and/or developmental regulation in distinct cell types that can now be separated using fluorescence-assisted microdissection, fluorescence-activated cell sorting, and laser capture microdissection.

Modulation of the actin cytoskeleton via gelsolin regulates vacuolar H+-ATPase recycling

The role of the actin cytoskeleton in regulating membrane protein trafficking is complex and depends on the cell type and protein being examined. Using the epididymis as a model system in which luminal acidification is crucial for sperm maturation and storage, we now report that modulation of the actin cytoskeleton by the calcium-activated actin-capping and -severing protein gelsolin plays a key role in regulating vacuolar H(+)-ATPase (V-ATPase) recycling. Epididymal clear cells contain abundant V-ATPase in their apical pole, and an increase in their cell-surface V-ATPase expression correlates with an increase in luminal proton secretion. We have shown that apical membrane accumulation of V-ATPase is triggered by an elevation in cAMP following activation of bicarbonate-regulated soluble adenylyl cyclase in response to alkaline luminal pH (Pastor-Soler, N., Beaulieu, V., Litvin, T. N., Da Silva, N., Chen, Y., Brown, D., Buck, J., Levin, L. R., and Breton, S. (2003) J. Biol. Chem. 278, 49523-49529). Here, we show that clear cells express high levels of gelsolin, indicating a potential role in the functional activity of these cells. When jasplakinolide was used to overcome the severing action of gelsolin by polymerizing actin, complete inhibition of the alkaline pH- and cAMP-induced apical membrane accumulation of V-ATPase was observed. Conversely, when gelsolin-mediated actin filament elongation was inhibited using a 10-residue peptide (PBP10) derived from the phosphatidylinositol 4,5-bisphosphate-binding region (phosphoinositide-binding domain 2) of gelsolin, significant V-ATPase apical membrane mobilization was induced, even at acidic luminal pH. In contrast, the calcium chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis(acetoxymethyl ester) and the phospholipase C inhibitor U-73122 inhibited the alkaline pH-induced V-ATPase apical accumulation. Thus, maintenance of the actin cytoskeleton in a depolymerized state by gelsolin facilitates calcium-dependent apical accumulation of V-ATPase in response to luminal pH alkalinization. Gelsolin is present in other cell types that express the V-ATPase in their plasma membrane and recycling vesicles, including kidney intercalated cells and osteoclasts. Therefore, modulation of the actin cortex by this severing and capping protein may represent a common mechanism by which these cells regulate their rate of proton secretion.

Renal vacuolar H+-ATPase

Vacuolar H(+)-ATPases are ubiquitous multisubunit complexes mediating the ATP-dependent transport of protons. In addition to their role in acidifying the lumen of various intracellular organelles, vacuolar H(+)-ATPases fulfill special tasks in the kidney. Vacuolar H(+)-ATPases are expressed in the plasma membrane in the kidney almost along the entire length of the nephron with apical and/or basolateral localization patterns. In the proximal tubule, a high number of vacuolar H(+)-ATPases are also found in endosomes, which are acidified by the pump. In addition, vacuolar H(+)-ATPases contribute to proximal tubular bicarbonate reabsorption. The importance in final urinary acidification along the collecting system is highlighted by monogenic defects in two subunits (ATP6V0A4, ATP6V1B1) of the vacuolar H(+)-ATPase in patients with distal renal tubular acidosis. The activity of vacuolar H(+)-ATPases is tightly regulated by a variety of factors such as the acid-base or electrolyte status. This regulation is at least in part mediated by various hormones and protein-protein interactions between regulatory proteins and multiple subunits of the pump.

Expression of the 56-kDa B2 subunit isoform of the vacuolar H+-ATPase in proton-secreting cells of the kidney and epididymis

B1 and B2 are two highly homologous isoforms of the vacuolar H+-ATPase (V-ATPase) 56-kDa B subunit. We investigated whether the B2 subunit is expressed alongside B1 in proton-secreting cells of the rodent kidney collecting duct (intercalated cells, IC) and epididymis (clear cells) by using antibodies against distinct COOH-terminal peptides from the two B isoforms. B2 was detected not only in the kidney proximal tubule, thick ascending limb, distal convoluted tubule, and connecting segment but also in A- and B-type IC of collecting ducts (CD) in both rat and mouse. B2 had a predominant cytoplasmic localization in most IC but was clearly located in a tighter apical band together with the V-ATPase 31-kDa E subunit in some A-IC, especially in the medulla. Apical membrane staining was confirmed by immunogold electron microscopy. B2 was very weakly expressed on the basolateral membranes of B-IC in control kidney CD, but some connecting segment B-IC had more distinct basolateral staining. In response to chronic carbonic anhydrase inhibition by acetazolamide, many A-IC showed a strong apical membrane localization of B2, where it colocalized with E and B1. In rat and mouse epididymis, B2 isoform expression was detected in clear cells, where it was concentrated in subapical vesicles. Unlike B1, B2 did not colocalize with the E subunit in the apical microvilli. These findings indicate that in addition to its role in the acidification of intracellular organelles, the B2 isoform could also contribute to transepithelial proton secretion and the maintenance of acid-base homeostasis.

Differential expression and targeting of endogenous Arf1 and Arf6 small GTPases in kidney epithelial cells in situ

ADP-ribosylation factors (Arfs) are small GTPases that regulate vesicular trafficking in exo- and endocytotic pathways. As a first step in understanding the role of Arfs in renal physiology, immunocytochemistry and Western blotting were performed to characterize the expression and targeting of Arf1 and Arf6 in epithelial cells in situ. Arf1 and Arf6 were associated with apical membranes and subapical vesicles in proximal tubules, where they colocalized with megalin. Arf1 was also apically expressed in the distal tubule, connecting segment, and collecting duct (CD). Arf1 was abundant in intercalated cells (IC) and colocalized with V-ATPase in A-IC (apical) and B-IC (apical and/or basolateral). In contrast, Arf6 was associated exclusively with basolateral membranes and vesicles in the CD. In the medulla, basolateral Arf6 was detectable mainly in A-IC. Expression in principal cells became weaker throughout the outer medulla, and Arf6 was not detectable in principal cells in the inner medulla. In some kidney epithelial cells Arf1 but not Arf6 was also targeted to a perinuclear patch, where it colocalized with TGN38, a marker of the trans-Golgi network. Quantitative Western blotting showed that expression of endogenous Arf1 was 26–180 times higher than Arf6. These data indicate that Arf GTPases are expressed and targeted in a cell- and membrane-specific pattern in kidney epithelial cells in situ. The results provide a framework on which to base and interpret future studies on the role of Arf GTPases in the multitude of cellular trafficking events that occur in renal tubular epithelial cells.

Increased luminal pH in the epididymis of infertile c-ros knockout mice and the expression of sodium-hydrogen exchangers and vacuolar proton pump H+-ATPase

Transgenic mice targeted for the c-ros gene, which are fertile when heterozygous (HET), but infertile when homozygous (knockout, KO) and associated with failure in pubertal differentiation of the epididymal initial segment, provide a model for studying the role of the epididymal luminal environment in sperm development. Luminal fluid from the cauda epididymidis was measured by both ion-selective microelectrodes and pH strips to be 0.3 pH units higher in the KO than HET. Of the genes responsible for luminal acidification, expression of mRNA of vacuolar H(+)-ATPase was found in all epididymal regions, but with no difference between KO and HET. Immunohistochemistry showed its presence in epithelial apical cells and clear cells. The Na(+)-hydrogen exchanger NHE2 was expressed at mRNA and protein levels in the caput but only marginally detectable if at all in the distal epididymis. This was compensated for by NHE3 which was expressed strongest in the cauda region, in agreement with immunohistochemical staining. Quantification of Western blot data revealed slight, but significant, decreases of NHE2 in the caput and of NHE3 in the cauda in the KO mice. The increase in luminal fluid pH in the KO mice could also be contributed to by other epithelial regulating factors including the Na(+)-dependent glutamate transporter EAAC1 formerly reported to be down regulated in the KO.

Bicarbonate-regulated adenylyl cyclase (sAC) is a sensor that regulates pH-dependent V-ATPase recycling

Modulation of environmental pH is critical for the function of many biological systems. However, the molecular identity of the pH sensor and its interaction with downstream effector proteins remain poorly understood. Using the male reproductive tract as a model system in which luminal acidification is critical for sperm maturation and storage, we now report a novel pathway for pH regulation linking the bicarbonate activated soluble adenylyl cyclase (sAC) to the vacuolar H+ATPase (V-ATPase). Clear cells of the epididymis and vas deferens contain abundant V-ATPase in their apical pole and are responsible for acidifying the lumen. Proton secretion is regulated via active recycling of V-ATPase. Here we demonstrate that this recycling is regulated by luminal pH and bicarbonate. sAC is highly expressed in clear cells, and apical membrane accumulation of V-ATPase is triggered by a sAC-dependent rise in cAMP in response to alkaline luminal pH. As sAC is expressed in other acid/base transporting epithelia, including kidney and choroid plexus, this cAMP-dependent signal transduction pathway may be a widespread mechanism that allows cells to sense and modulate extracellular pH.

A rapid enzymatic method for the isolation of defined kidney tubule fragments from mouse

The increasing number of available genetically manipulated mice makes it necessary to develop tools and techniques for examining the phenotypes of these animals. We have developed a straightforward and rapid method for the isolation of large quantities of single tubule fragments from the mouse kidney. Immunohistochemistry, electron microscopy, and fluorescence microscopy were used to evaluate the viability, functional characteristics, and morphology of proximal tubules (PT), and collecting ducts from cortex (CCD) and inner stripe of the outer medulla (ISOMCD). Tubules were isolated using a modified collagenase digestion technique, and selected under light microscopy for experimentation. Electron microscopy and trypan blue exclusion showed that a large portion of unselected proximal tubules were damaged by the digestion procedure. The selected tubules, however, all excluded trypan blue, indicating that the plasma membrane had remained intact. Immunocytochemistry on isolated CCD showed normal distribution of H(+)-ATPase, pendrin, and anion exchanger-1 (AE-1) staining. The pH-sensitive dye 2',7'-bis(2-carboxylethyl)-5(6)-carboxyfluorescein (BCECF) was used to measure Na(+)-dependent and -independent intracellular pH (pH(i)) recovery rates in PT, and in single intercalated cells of CCD and ISOMCD fragments. Na(+)-dependent pH(i)-recovery was 0.144+/-0.008 (PT), 0.182+/-0.013 (CCD), and 0.112+/-0.010 pH units/min. (ISOMCD). Na(+)-independent pH(i) recovery was found in all three segments (PT: 0.021+/-0.002, CCD: 0.037+/-0.002, ISOMCD: 0.033+/-0.002 pH units/min) and was sensitive to concanamycin. In summary, we have developed a new technique for rapid and straightforward preparation of large quantities of defined tubule fragments from mouse kidney. Using this technique, the first measurements of plasma membrane vacuolar H(+)-ATPase activities in mouse PT and collecting duct were made. This technique will facilitate further characterization of kidney function in normal and genetically manipulated animals.

Modulation of the onset of postnatal development of H(+)-ATPase-rich cells by steroid hormones in rat epididymis

Vacuolar type H(+)-ATPase is involved in lumenal acidification of the epididymis. This protein is highly expressed in narrow and clear cells where it is located in the apical pole, and it contributes to proton secretion into the lumen. We have previously shown that in rats, epididymal cells rich in H(+)ATPase appear during postnatal development and reach maximal numbers at 3-4 wk of age. The factors that regulate the appearance of these cells have not been investigated, but androgens, estrogens, or both may be involved. This study examined whether neonatal administration of estrogens (diethylstilbestrol [DES] or ethinyl estradiol) or an antiandrogen (flutamide), or the suppression of androgen production via administration of a GnRH antagonist (GnRHa), was able to alter the appearance of cells rich in H(+)-ATPase in the rat epididymis when assessed at age 25 days. Surprisingly, all of these treatments were able to significantly reduce the number of H(+)-ATPase positive cells; this was determined by immunofluorescence and confirmed by Western blotting. In contrast, neonatal coadministration of DES and testosterone maintained the expression of H(+)-ATPase in the epididymis at Day 25 despite the high level of concomitant estrogen exposure. These findings indicate that androgens, acting via the androgen receptor, are essential for the normal development of epididymal cells rich in H(+)-ATPase, and that treatments that interfere directly or indirectly with androgen production (GnRHa, DES) or action (flutamide, DES) will result in reduced expression of H(+)-ATPase. Our findings do not exclude the possibility that estrogens can directly suppress the postnatal development of cells in the epididymis that are rich in H(+)-ATPase, but if this is the case, this suppression can be prevented by testosterone administration.

Hypertonicity is involved in redirecting the aquaporin-2 water channel into the basolateral, instead of the apical, plasma membrane of renal epithelial cells

In renal collecting ducts, vasopressin increases the expression of and redistributes aquaporin-2 (AQP2) water channels from intracellular vesicles to the apical membrane, leading to urine concentration. However, basolateral membrane expression of AQP2, in addition to AQP3 and AQP4, is often detected in inner medullary principal cells in vivo. Here, potential mechanisms that regulate apical versus basolateral targeting of AQP2 were examined. The lack of AQP2-4 association into heterotetramers and the complete apical expression of AQP2 when highly expressed in Madin-Darby canine kidney cells indicated that neither heterotetramerization of AQP2 with AQP3 and/or AQP4, nor high expression levels of AQP2 explained the basolateral AQP2 localization. However, long term hypertonicity, a feature of the inner medullary interstitium, resulted in an insertion of AQP2 into the basolateral membrane of Madin-Darby canine kidney cells after acute forskolin stimulation. Similarly, a marked insertion of AQP2 into the basolateral membrane of principal cells was observed in the distal inner medulla from normal rats and Brattleboro rats after acute vasopressin treatment of tissue slices that had been chronically treated with vasopressin to increase interstitial osmolality in the medulla, but not in tissues from vasopressin-deficient Brattleboro rats. These data reveal for the first time that chronic hypertonicity can program cells in vitro and in vivo to change the insertion of a protein into the basolateral membrane instead of the apical membrane.

Detection of ClC-3 and ClC-5 in epididymal epithelium: immunofluorescence and RT-PCR after LCM

Epithelial cells of the epididymis and vas deferens establish an optimum luminal environment in which spermatozoa mature and are stored. This is achieved by active transepithelial transport of various ions including Cl(-) and H(+). We investigated the localization of three closely related members of the ClC family, ClC-3, ClC-4, and ClC-5, in the epididymis and vas deferens. RT-PCR using mRNA isolated by laser capture microdissection (LCM)-detected ClC-3 and ClC-5 transcripts but did not detect any ClC-4-specific transcript. Western blot and immunofluorescence analysis demonstrated that ClC-3 and ClC-5 proteins are present in all regions of the epididymis and in the vas deferens. ClC-5 is expressed exclusively in H(+)-ATPase-rich cells (narrow and clear cells). Confocal microscopy showed that ClC-5 partially colocalizes with the H(+)-ATPase in the subapical pole of clear cells. ClC-3 is strongly expressed in the apical membrane of principal cells of the caput epididymidis and the vas deferens and is less abundant in principal cells of the body and cauda epididymidis. These findings are consistent with a potential role for ClC-3 in transepithelial chloride transport by principal cells and for ClC-5 in the acidification of H(+)-ATPase-containing vesicles in narrow and clear cells. ClC-5 might facilitate endosome trafficking in the epididymis, as has been proposed in the kidney.

Renal principal cell-specific expression of green fluorescent protein in transgenic mice

The purpose of this study is to develop transgenic mice with principal cell-specific expression of green fluorescent protein (GFP). After the cloning and sequencing of the mouse aquaporin-2 (AQP2) gene, 9.5 kb of the promoter were used to drive expression of GFP in transgenic mice. In transgenic mice, GFP was selectively expressed in principal cells of the renal collecting duct and not in intercalated cells. Expression was increased by dehydration of mice. AQP2 and GFP expression was maintained in primary cultures of renal medulla that were stimulated with cAMP or vasopressin analogs. GFP-expressing cells were then isolated by fluorescence-activated cell sorting. RT-PCR analysis showed expression of AQP2, AQP3, AQP4, vasopressin type 2 receptor, and cAMP response element binding protein but not H+-ATPase B1 subunit or anion exchanger 1. After expansion of these cells in culture, RT-PCR analysis showed continued expression of the same genes. This pattern of gene expression is that of principal cells rather than intercalated cells. This transgenic mouse model can be used in future studies of gene expression during the development, differentiation, and maturation of renal principal cells.

Effects of nicardipine and clonidine on cognitive functions and electroencephalography in hypertensive patients

The aim of this study was to investigate the cognitive and electroencephalography (EEG) short-term effects of a calcium antagonist, nicardipine, compared to placebo and clonidine (which, having known sedative effects, acted as a negative control) for 15 days in elderly hypertensive patients with memory complaints. Nicardipine and clonidine were compared with placebo in a double-blind, randomized, three-way cross-over controlled study after a 2-week placebo run-in period. This was a phase II clinical study carried out on out-patients in a single research centre. Fifteen elderly (63 +/- 10 years) hypertensive patients, without dementia but with memory complaints, were included. Psychomotor performance and cognition were assessed using both an extensive battery of validated psychometric tests (which evaluated attention and vigilance, body sway and memory) and an EEG profile. Cardiovascular parameters measured were blood pressure and heart rate. No detrimental effects of nicardipine were found on attention, vigilance, body sway or memory. Nicardipine produced a significant increase in alpha EEG energies, which may indicate possible alerting effects. In contrast, clonidine induced well-known deleterious sedative effects in psychometric tests and in EEG analysis (decrease in alpha and increase in delta and theta waves). The two drugs produced equivalent decreases in blood pressure at steady state. In conclusion, clonidine induced well-known sedative effects, while nicardipine did not impair central nervous system activity and may have had some short-term alerting effects in elderly hypertensive patients with memory complaints. This study supports the hypothesis of a dissociation between blood pressure and direct drug effects on the central nervous system.

Expression of aquaporin 9 in the adult rat epididymal epithelium is modulated by androgens

Reabsorption of fluid and solutes across the epithelium lining the male excurrent duct is important for adequate sperm maturation, concentration, and storage. Water channels contribute to water movement across epithelia in many tissues. Aquaporin 9 (AQP9) is abundantly expressed in the apical membrane of principal cells that line the epididymis, and in reabsorptive and secretory epithelial cells of the male reproductive tract. In this study we show that the nonsteroidal antiandrogen flutamide, given to adult rats at a dose of 50 mg x kg(-1) x day(-1) for 2 wk via osmotic minipumps significantly decreased the amount of AQP9 in the epididymis. This down-regulation was observed by immunofluorescence of cryostat tissue sections and by Western blotting of epididymal brush border membrane preparations. In addition, castrated adult rats showed lower levels of epididymal AQP9 compared with adult controls, whereas systemic testosterone treatment of castrated adult rats induced a recovery of the expression of AQP9 to control levels. These data indicate that the expression of AQP9, a likely candidate for apical transepithelial fluid and solute transport in several regions of the male reproductive tract, is modulated by androgens in the adult rat epididymis.

Antigen retrieval reveals widespread basolateral expression of syntaxin 3 in renal epithelia

Soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins play a key role in docking and fusion of intracellular transport vesicles and may regulate apical and basolateral membrane protein delivery in epithelial cells. In a previous study, syntaxin 3 (a target SNARE) protein was detectable in the kidney only in intercalated cells. We now report a more widespread distribution of syntaxin 3 in a variety of renal epithelial cells after antigen retrieval. Sections of rat kidney were treated with SDS and incubated with antisyntaxin 3 antibodies. Strong basolateral membrane staining was seen in descending and ascending thin limbs of Henle, thick ascending limbs of Henle, the macula densa, distal and connecting tubules, and all cells of the collecting duct including A- and B-intercalated cells. The papillary surface epithelium and the transitional epithelium of the ureter were also stained, but proximal tubules were negative. Western blotting revealed a strong signal at 37 kDa in all regions, and the antigen was restricted to membrane fractions. SDS treatment was not necessary to reveal syntaxin 3 in intercalated cells. These data show that syntaxin 3 might be involved in basolateral trafficking pathways in most renal epithelial cell types. The exclusive basolateral location of syntaxin 3 in situ, however, contrasts with the apical location of this SNARE protein in some kidney epithelial cells in culture.

Distribution of the vacuolar H+ atpase along the rat and human male reproductive tract

Luminal acidification in parts of the male reproductive tract generates an appropriate pH environment in which spermatozoa mature and are stored. The cellular mechanisms of proton (H+) secretion in the epididymis and the proximal vas deferens involve the activity of an apical vacuolar H+ ATPase in specialized cell types, as well as an apical Na+/H+ exchanger in some tubule segments. In this study we used Western blotting and immunocytochemistry to localize the H+ ATPase in various segments of the male reproductive tract in rat and man as a first step toward a more complete understanding of luminal acidification processes in this complex system of tissues. Immunoblotting of isolated total cell membranes indicated a variable amount of H+ ATPase in various segments of the rat reproductive tract. In addition to its known expression in distinct cell types in the epididymis and vas deferens, the H+ ATPase was also localized at the apical pole and in the cytoplasm of epithelial cells in the efferent duct (nonciliated cells), the ampulla of the vas deferens and the ventral prostate (scattered individual cells), the dorsal and lateral prostate, the ampullary gland, the coagulating gland, and all epithelial cells of the prostatic and penile urethra. Both apical and basolateral localization of the protein were found in epithelial cells of the prostatic ducts in the lateral prostate and in periurethral tissue. Only cytoplasmic, mostly perinuclear localization of the H+ ATPase was found in all epithelial cells of the seminal vesicles and in most cells of the ventral prostate and coagulating gland. No staining was detected in the seminiferous tubules, rete testis, and bulbourethral gland. In human tissue, H+ ATPase-rich cells were detected in the epididymis, prostate, and prostatic urethra. We conclude that the vacuolar H+ ATPase is highly expressed in epithelial cells of most segments of the male reproductive tract in rat and man, where it may be involved in H+ secretion and/or intracellular processing of the material endocytosed from the luminal fluid or destined to be secreted by exocytosis.

Aquaporin 9 expression along the male reproductive tract

Fluid movement across epithelia lining portions of the male reproductive tract is important for modulating the luminal environment in which sperm mature and reside, and for increasing sperm concentration. Some regions of the male reproductive tract express aquaporin (AQP) 1 and/or AQP2, but these transmembrane water channels are not detectable in the epididymis. Therefore, we used a specific antibody to map the cellular distribution of another AQP, AQP9 (which is permeable to water and to some solutes), in the male reproductive tract. AQP9 is enriched on the apical (but not basolateral) membrane of nonciliated cells in the efferent duct and principal cells of the epididymis (rat and human) and vas deferens, where it could play a role in fluid reabsorption. Western blotting revealed a strong 30-kDa band in brush-border membrane vesicles isolated from the epididymis. AQP9 is also expressed in epithelial cells of the prostate and coagulating gland where fluid transport across the epithelium is important for secretory activity. However, it was undetectable in the seminal vesicle, suggesting that an alternative fluid transport pathway may be present in this tissue. Intracellular vesicles in epithelial cells along the reproductive tract were generally poorly stained for AQP9. Furthermore, the apical membrane distribution of AQP9 was unaffected by microtubule disruption. These data suggest that AQP9 is a constitutively inserted apical membrane protein and that its cell-surface expression is not acutely regulated by vesicular trafficking. AQP9 was detectable in the epididymis and vas deferens of 1-wk postnatal rats, but its expression was comparable with adult rats only after 3--4 wk. AQP9 could provide a route via which apical fluid and solute transport occurs in several regions of the male reproductive tract. The heterogeneous and segment-specific expression of AQP9 and other aquaporins along the male reproductive tract shown in this and in our previous studies suggests that fluid reabsorption and secretion in these tissues could be locally modulated by physiological regulation of AQP expression and/or function.

The cellular physiology of carbonic anhydrases

Carbonic anhydrases are zinc metalloenzymes that catalyze the reversible hydration of CO(2) to form HCO(3)(-) and protons according to the following reaction: CO(2) + H(2)O <=> H(2)CO(3) <=> HCO(3)(-) + H(+). The first reaction is catalyzed by carbonic anhydrase and the second reaction occurs instantaneously. The carbonic anhydrase (CA) gene family includes ten enzymatically active members, which are major players in many physiological processes, including renal and male reproductive tract acidification, bone resorption, respiration, gluconeogenesis, signal transduction, and formation of gastric acid. The newly identified CA IX (previously called MN) and CA XII are related to cell proliferation and oncogenesis. Carbonic anhydrase isozymes have different kinetic properties and they are present in various tissues and in various cell compartments. CA I, II, III and VII are cytoplasmic, CA V is mitochondrial, and CA VI is present in salivary secretions. CA IV, IX, XII and XIV are membrane proteins: CA IV is a glycosyl-phosphatidylinositol-anchored protein, and CA IX, XII and XIV are transmembrane proteins. The present work will focus on the roles of CA II and CA IV in transepithelial proton secretion and bicarbonate reabsorption processes. The localization of these isoforms in selected epithelia that are involved in net acid/base transport, such as kidney proximal tubules and collecting ducts, and tubules from the male reproductive tract will be reviewed.

Remodeling the cellular profile of collecting ducts by chronic carbonic anhydrase inhibition

Factors regulating the differentiated phenotype of principal cells (PC) and A- and B-intercalated cells (IC) in kidney collecting ducts are poorly understood. However, we have shown previously that carbonic anhydrase II (CAII)-deficient mice have no IC in their medullary collecting ducts, suggesting a potential role for this enzyme in determining the cellular composition of this tubule segment. We now report that the cellular profile of the collecting ducts of adult rats can be remodeled by inhibiting CA activity in rats by using osmotic pumps containing acetazolamide. The 31-kDa subunit of the vacuolar H(+)-ATPase, the sodium/hydrogen exchanger regulatory factor NHE-RF, and the anion exchanger AE1 were used to identify IC subtypes by immunofluorescence staining, while aquaporin 2 and aquaporin 4 were used to identify PC. In the cortical collecting ducts of animals treated with acetazolamide for 2 wk, the percentage of B-IC decreased significantly (18 +/- 2 vs. 36 +/- 4%, P < 0.01) whereas the percentage of A-IC increased (82 +/- 2 vs. 64 +/- 4%, P < 0.01) with no change in the percentage of total IC in the epithelium. In some treated rats, B-IC were virtually undetectable. In the inner stripe of the outer medulla, the percentage of IC increased in treated animals (48 +/- 2 vs. 37 +/- 3%, P < 0.05) and the percentage of PC decreased (52 +/- 2 vs. 63 +/- 3%, P < 0.05). Moreover, IC appeared bulkier, protruded into the lumen, and showed a significant increase in the length of their apical (20.8 +/- 0.5 vs. 14.6 +/- 0.4 microm, P < 0.05) and basolateral membranes (25.8 +/- 0.4 vs. 23.8 +/- 0.5 microm, P < 0.05) compared with control rats. In the inner medullary collecting ducts of treated animals, the number of IC in the proximal third of the papilla was reduced compared with controls (11 +/- 4 vs. 40 +/- 11 IC/mm(2), P < 0.05). These data suggest that CA activity plays an important role in determining the differentiated phenotype of medullary collecting duct epithelial cells and that the cellular profile of collecting ducts can be remodeled even in adult rats. The relative depletion of cortical B-IC and the relative increase in number and hyperplasia of A-IC in the medulla may be adaptive processes that would tend to correct or stabilize the metabolic acidosis that would otherwise ensue following systemic carbonic anhydrase inhibition.

Na+/H+-exchange activity and immunolocalization of NHE3 in rat epididymis

An acidic luminal pH in the epididymis and vas deferens (VD) helps maintain mature sperm in an immotile state during storage. We have previously shown that the majority of proton secretion in the VD is due to the activity of the vacuolar H+-ATPase. Acidification is dependent on luminal sodium in more proximal regions of the epididymis, and we examined the distribution of the Na+/H+ exchanger, NHE3, by immunofluorescence and measured Na+/H+ exchange (NHE) activity in isolated epididymal tubules. NHE3 was detected in the apical pole of nonciliated cells of the efferent ducts and principal cells (PC) of the epididymis. No staining was seen in the distal cauda epididymidis and the VD. Isolated tubules from the distal initial segment (DIS) and proximal cauda epididymidis were perfused in vitro and loaded with the pH-sensitive dye 2',7'-bis(carboxyethyl)-5(6')-carboxyfluorescein. Ethylisopropyl amiloride (EIPA) (50 microM) reduced the initial rate of intracellular pH recovery (dpH(i)/dt), in response to an acute acid load, by 51% and 45% in the DIS and cauda epididymidis, respectively. In the DIS, removal of luminal sodium reduced dpH(i)/dt by 52%. HOE694 (50 microM) inhibited all EIPA-sensitive dpH(i)/dt in the DIS, despite the previously reported absence of NHE2 in this region (Cheng Chew SB, Leung GPH, Leung PY, Tse CM, and Wong PYD, Biol Reprod 62: 755-758, 2000). These data indicate that HOE694- and EIPA-sensitive Na+/H+ exchange may participate, together with the H+-ATPase, in luminal acidification in the male excurrent duct.

Nitric oxide and atrial natriuretic factor stimulate cGMP-dependent membrane insertion of aquaporin 2 in renal epithelial cells

In collecting duct principal cells, aquaporin 2 (AQP2) is shuttled from intracellular vesicles to the plasma membrane upon vasopressin (VP) stimulation. VP activates adenylyl cyclase, increases intracellular cAMP, activating protein kinase A (PKA) to phosphorylate AQP2 on the COOH-terminal residue, serine 256. Using rat kidney slices and LLC-PK1 cells stably expressing AQP2 (LLC-AQP2 cells), we now show that AQP2 trafficking can be stimulated by cAMP-independent pathways. In these systems, the nitric oxide (NO) donors sodium nitroprusside (SNP) and NONOate and the NO synthase substrate L-arginine mimicked the effect of VP, stimulating relocation of AQP2 from cytoplasmic vesicles to the plasma membrane. Unlike VP, these other agents did not increase intracellular cAMP. However, SNP increased intracellular cGMP, and exogenous cGMP stimulated AQP2-membrane insertion. Atrial natriuretic factor, which signals via cGMP, also stimulated AQP2 translocation. The VP and SNP effects were blocked by the kinase inhibitor H89. SNP did not stimulate membrane insertion of AQP2 in LLC-PK1 cells expressing the phosphorylation-deficient mutant 256SerAla-AQP2, indicating that phosphorylation of Ser256 is required for signaling. Both PKA and cGMP-dependent protein kinase G phosphorylated AQP2 on this COOH-terminal residue in vitro. These results demonstrate a novel, cAMP-independent and cGMP-dependent pathway for AQP2 membrane insertion in renal epithelial cells.

Defective interleukin-1 receptor antagonist production is associated with resistance of acute liver graft rejection to steroid therapy

Thirty percent of acute liver graft rejection episodes are resistant to steroids. As interleukin-1 (IL-1) is an important target of steroid therapy, we examined the possible involvement of reduced sensitivity of IL-1 production to steroids or defective production of its antagonist, IL-1Ra. Patients were assessed during steroid-sensitive or -resistant rejection and 2 years later. In situ IL-1beta and IL-1Ra expression was evaluated by immunohistochemistry; their production was assayed by enzyme-linked immunosorbent assay and the gene polymorphisms by reverse transcriptase-polymerase chain reaction on blood cells. Hepatic IL-1beta and IL-1Ra expression were enhanced during rejection. IL-1 production and its inhibition by steroids were similar in steroid-responsive and steroid-resistant rejection. However, IL-1Ra production was lower in steroid-resistant than in steroid-responsive rejection, and this difference was still observed 2 years after rejection. IL-1beta and IL-1Ra gene polymorphisms did not differ between patients with and without steroid resistance. Low IL-1Ra production is associated with steroid resistance of acute rejection and is due to a constitutional defect. The early identification of such patients might qualify them for stronger anti-rejection therapy, including IL-1Ra.