Regulation of maturation and processing of ENaC subunits in the rat kidney - 《American Journal of Physiology》美国生理学杂志干细胞之家



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发表于 2009-4-22 08:36 |只看该作者 |倒序浏览 |打印

作者：Zuhal Ergonul, Gustavo Frindt, and Lawrence G. Palmer作者单位：Department of Physiology and Biophysics, Weill Medical College of Cornell University, New York, New York % Q+ B4 ~: u- g9 M! S: {

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      【摘要】* }/ _2 T) g. O: [) o
   Antibodies directed against subunits of the epithelial Na channel (ENaC) were used together with electrophysiological measurements in the cortical collecting duct to investigate the processing of the proteins in rat kidney with changes in Na or K intake. When animals were maintained on a low-Na diet for 7-9 days, the abundance of two forms of the -subunit, with apparent masses of 85 and 30 kDa, increased. Salt restriction also increased the abundance of the -subunit and produced an endoglycosidase H (Endo H)-resistant pool of this subunit. The abundance of the 90-kDa form of the -subunit decreased, whereas that of a 70-kDa form increased and this peptide also exhibited Endo H-resistant glycosylation. These changes in - and -subunits were correlated with increases in Na conductance elicited by a 4-h infusion with aldosterone. Changes in all three subunits were correlated with decreases in Na conductance when Na-deprived animals drank saline for 5 h. We conclude that ENaC subunits are mainly in an immature form in salt-replete rats. With Na depletion, the subunits mature in a process that involves proteolytic cleavage and further glycosylation. Similar changes occurred in - and - but not -subunits when animals were treated with exogenous aldosterone, and in - and - but not -subunits when animals were fed a high-K diet. Changes in the processing and maturation of the channels occur rapidly enough to be involved in the daily regulation of ENaC activity and Na reabsorption by the kidney.
      【关键词】 Na channels Na depletion Na repletion aldosterone potassium intake proteolytic cleavage
   REGULATION OF EPITHELIAL N A transport by salt depletion and aldosterone entails the stimulation of the activity of apical Na channels in epithelia such as the renal connecting tubule and collecting duct ( 8, 20 ). However, the mechanisms underlying this regulation are incompletely understood. The three subunits (,, and ) of the epithelial Na channel (ENaC) comprise the apical membrane the Na channel in absorptive epithelia ( 4 ). In the mammalian colon, aldosterone enhances the transcription and translation of - and -subunits ( 2 ). In A6 cells derived from Xenopus laevis kidney, the hormone has little effect on mRNA abundance but increases rates of translation of subunit protein ( 1, 17 ). In the mammalian kidney, all three subunits are constitutively expressed. Modest increases in the expression of -ENaC have been observed in response to elevated aldosterone levels in vivo at both the mRNA and protein levels ( 2, 15 ), but these changes do not appear to account for the stimulation of Na channel activity. Immunocytochemical evidence suggests that Na channel protein moves from intracellular sites to the plasma membrane of collecting duct and connecting tubule cells during Na depletion ( 13 - 15 ), suggesting that channel trafficking may play a dominant role in the response to the hormone in the kidney. These intracellular locations have not been precisely identified.
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Immunoblots indicate that one subunit of the channel, -ENaC, is cleaved during the process of stimulation ( 6, 15 ). Similar cleavage of both - and -ENaC was observed when the channel was expressed in heterologous systems ( 11 ). Two candidates for the enzyme(s) that could be responsible for cleavage in epithelial cells are the mCAPs (prostasin) ( 22 ), which are believed to be expressed in the plasma membrane, and furins ( 9 ), which are normally associated with the trans -Golgi network but can cycle to the cell surface. This proteolysis has been linked to increased channel activity. Expression of ENaC in oocytes ( 5, 22 ) and in fibroblasts ( 3 ) gives rise to channels that can be activated by exogenous extracellular proteases such as trypsin. Studies of ENaC expressed in heterologous systems suggest that processing of - and -ENaC by furin is essential for full activity of the channel ( 9 ).3 p( V# u$ \" U1 K5 ^3 {

The relationship between the putative proteolytic and trafficking events is unclear. In this study, we present evidence that the channels undergo a constellation of processing events during stimulation by aldosterone in vivo. These include proteolysis of both - and -subunits and the maturation of glycosylation of - and -subunits of ENaC. These events occur during both short-term and long-term activation of transport and may reflect hormone-dependent processing of the channels as they are moved from intracellular to surface membranes.9 ~3 F0 U, I- H4 g  X0 k6 L8 C
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METHODS5 k$ f& C2 D( Y0 e; `/ i3 R4 V8 G

Animals. Sprague-Dawley rats of either gender (150-170 g), raised free of viral infections (Charles River Laboratories, Kingston, NY), were fed a sodium-deficient diet (ICN Biomedicals) or a high-K (10% KCl) rat diet (Harlan-Teklad, Madison, WI) for 1, 3, or 7 days. Control animals were fed a modified diet that matches the low-Na diet but that contains 1% NaCl (ICN Biomedicals). All procedures involving animals were carried out under the guidelines of and were approved by the Institutional Animal Care and Use Committee of the Weill Medical College of Cornell University.
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In fourth group, animals were fed the 1% NaCl diet and implanted subcutaneously with osmotic minipumps (model 2002, Alza, Palo Alto, CA) for 4 h or 7 days to increase levels of circulating aldosterone. Aldosterone was dissolved in polyethylene glycol 300 at 2 mg/ml, to give a calculated infusion rate of 1 µg/h. Controls received a sham operation. The animals infused for 4 h were kept individually in metabolic cages, and urine was collected during the third and fourth hours after the implantation of the pumps. One kidney was homogenized for immunoblotting, whereas the other was dissected for electrophysiological measurements as described below.
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In a fifth group, rats were salt restricted for 7 days on the low-Na diet. Salt repletion was carried out with 0.9% NaCl in the drinking water for 5 h. Both fluid intake and urinary excretion rates were measured in metabolic cages. Control animals were given 3% sucrose in the drinking water to approximately match fluid intake. Kidneys were collected for immunoblots and electrophysiology as above.& l( O) d- f! M! T. C
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Urine Na and K concentrations were measured by flame photometry (IL Instruments, model 943).0 [- t: G# R( {* L" u
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Semiquantitative immunoblotting. To compare ENaC protein abundance between groups of rats, semiquantitative immunoblotting was carried out as described previously ( 12, 15 ). Animals were anesthetized with thiobutabarbital (Inactin, 150 mg/kg ip). Kidneys were perfused in situ through the abdominal aorta with 10 ml of PBS containing heparin. After the whole kidneys were minced, they were homogenized with a glass homogenizer (Wheaton, Millville, NJ) in ice-cold isolation solution containing 250 mM sucrose/10 mM triethanolamine buffer, pH 7.4, with 1 µg/ml leupeptin and 0.1 mg/ml phenylmethylsulfonyl fluoride (Sigma-Aldrich). The homogenate was centrifuged at 1,000 g for 10 min to sediment unbroken cells and nuclei. The supernatant was processed for immunodetection. Total protein was measured (Pierce BCA Kit). Equal amounts of protein (40-50 µg/sample) were solubilized at 70°C for 10 min in Laemmli sample buffer and resolved on 4-12% bis-Tris gels (Invitrogen, Carlsbad, CA) by SDS-PAGE. For immunoblotting, the proteins were transferred electrophoretically from unstained gels to PVDF membranes. After being blocked with BSA, membranes were incubated overnight at 4°C with primary antibodies against -, -, and -subunits at 1:500 or 1:1,000 dilutions. Anti-rabbit IgG conjugated with alkaline phosphatase was used as a secondary antibody. The sites of antibody-antigen reaction were visualized with a chemiluminescence substrate (Western Breeze, Invitrogen) before exposure to X-ray film (Biomax ML, Kodak). Band densities were quantitated using a Quantity One densitometer and acquisition system (Bio-Rad). For statistical analysis, we generally combined results from two or more blots. In these cases, each blot contained an equal number of lanes for two different conditions being compared. We normalized the densities from a given blot to the average value for all lanes in that blot to account for differences in signal strength. The results are reported in arbitrary units. In some experiments, gels were stained with Coomassie blue to check for equal loading of all lanes. Densities of arbitrary, strongly stained bands were quantitated using the same technique described above. These densities did not change with the physiological state of the animal.8 ?7 `; m0 C' E3 {& [5 P
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Antibodies. To produce polyclonal antibodies against the -, -, and -subunits of rat ENaC, short peptide sequences were used as follows, based on predicted amino acid sequences in the rat : -ENaC (amino acids 46-68), NH 2 -LGKGDKREEQGLGPEPSAPRQPTC-COOH; -ENaC (amino acids 617-638), NH 2 -CNYDSLRLQPLDTMESDSEVEAI-COOH; and -ENaC (amino acids 629-650), NH 2 -CNTLRLDRAFSSQLTDTQLTNEL-COOH. The antigens were purified by HPLC, conjugated to keyhole limpet hemocyanin, and used to immunize rabbits as described previously ( 15 ). The resulting antisera were purified using peptide-linked agarose gel affinity columns (Sulfolink Kit, Pierce Biotechnology).! c- u( }5 f2 V+ y& W/ l

Glycosidases. The state of glycosylation of the ENaC subunits was investigated by using glycosidases. Aliquots of supernatant from kidney homogenates were treated or untreated with peptide N -glycosidase F (PNGase F) or with endoglycosidase H (Endo H; New England Biolabs) to remove N-linked oligosaccharides or the mannose-sensitive component, respectively. Enzymes were used according to the manufacturer?s protocol. Twenty-five micrograms of protein were denatured in 0.5% SDS and 1% -mercaptoethanol containing glycoprotein-denaturing buffer at 100°C for 10 min and then incubated at 37°C for 1 h with either 0.05 M sodium phosphate buffer, pH 7.5, with or without 4 µl PNGase F reagent, or with 0.05 M sodium citrate buffer, pH 5.5, with or without 4 µl Endo H reagent.- R- i, P! d' K& g+ J9 [* ], m7 d
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Electrophysiology. Measurement of amiloride-sensitive currents in principal cells of the rat cortical collecting duct (CCD) followed procedures described previously ( 6, 7 ). Briefly, CCDs were dissected and opened manually to expose the luminal surface. The split tubules were attached to a small plastic rectangle coated with Cell-Tak (Collaborative Research, Bedford, MA) and placed in a perfusion chamber mounted on an inverted microscope.
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For whole cell clamp measurements, tubules were superfused with solutions prewarmed to 37°C containing (in mM) 135 Na methane sulfonate, 5 KCl, 2 CaCl 2, 1 MgCl 2, 2 glucose, 5 BaCl 2, and 10 HEPES, adjusted to pH 7.4 with NaOH. In some cases, a reduced Cl - solution was used in which CaCl 2 and BaCl 2 were replaced with methanesulfonate salts. The patch-clamp pipettes were filled with solutions containing (in mM) 7 KCl, 123 aspartic acid, 20 CsOH, 20 TEAOH, 5 EGTA, 10 HEPES, 3 MgATP, and 0.3 Na GDP S, with pH adjusted to 7.4 with KOH. Pipettes were pulled from hematocrit tubing, coated with Sylgard, and fire polished with a microforge. Pipette resistances ranged from 2 to 5 M. Amiloride-sensitive currents were measured as the difference in current with and without 10 µM amiloride in the bath solution.
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Statistics. Statistical significance was assessed using the unpaired two-tailed Student?s t -test.

RESULTS4 L' N! Z/ E$ }

Figure 1 shows the recognition of rat renal proteins by anti- -, -, and -ENaC antisera. The anti- -antibody recognizes protein bands of apparent masses of 85, 60, and 30 kDa. In previous studies, only the 85-kDa peptide was observed ( 15 ). Although our antibodies were raised against the same sequences as the ones used by Masilamini et al. ( 15 ), they were generated in different rabbits. The 60-kDa band is similar in size to a band from A6 cells recognized by an antibody raised against the COOH terminus of the X. laevis -ENaC ( 1 ). As discussed below, the 30-kDa band is similar in size to a proteolytic fragment of -ENaC observed in heterologous expression systems ( 11 ). We therefore show results below for all three bands that we observed. The anti- -antibody recognizes a predominant band at 85-90 kDa. In kidneys from control animals, the anti- -antibody marks a peptide at 85-90 kDa. Inclusion of peptides against which the antibodies were directed eliminates all of this staining ( Fig. 1 ) as does omission of the primary antibodies in the staining of the gels (not shown). Similar results for the anti- - and anti- -ENaC antibodies were presented by Masilamini et al. ( 15 ). To test whether changes in the intensity of the observed bands could be correlated with the amount of protein in them, we measured the density of scanned immunoblots in which different amounts of protein from the same sample were loaded into different lanes. A monotonic increase in density as a function of protein amount is shown in Fig. 1 B. The best-fit line to the data has a positive intercept on the y -axis, possibly due to uncertainties of background subtraction. This relationship suggests that the percent changes in band densities reported below reflect changes in protein abundance but will underestimate the change because the percent increase in density is less than that of the amount of protein loaded.
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Fig. 1. A : immunoblots of epithelial Na channel subunits -ENaC ( left ), -ENaC ( middle ), and -ENaC ( right ) in homogenates of kidneys from control rats. Forty micrograms of protein were loaded onto each lane. Primary antibodies were used at dilutions of 1:500 ( -ENaC) and 1:1,000 ( - and -ENaC). The right -hand blots were incubated with 10 µg/ml of the immunizing peptides added to the solution containing the primary antibody. B : quantitation of band densities. Different amounts of -ENaC were loaded onto each lane. Optical densities (OD; arbitrary units) of each band are plotted as a function of protein amount. The solid line represents least-squares fit of the data to a straight line./ b1 _) t7 F- S, Y# J

Figure 2 illustrates the essential qualitative and quantitative differences in the proteins during activation of Na channels by Na depletion for 7 days. These experiments were initially done by Masilamini et al. ( 6, 15 ), but since that time - and -subunits were shown to be cleaved as part of their trafficking and activation (see the beginning of this study). We therefore attempted to further study the ENaC subunits in an animal model. Consistent with previous reports ( 6, 15 ), the abundance of the 85-kDa -ENaC peptide increased. We also observed a concomitant increase in the 30-kDa peptide. The presence of this low-molecular-mass species is consistent with the idea that -ENaC is cleaved near the NH 2 terminus ( 11 ), as the antibody recognizes a sequence at the cytoplasmic NH 2 -terminal region of the protein. The intermediate band of 60 kDa did not change consistently with Na depletion or other maneuvers. We do not know whether it is related to -ENaC or whether it interacts nonspecifically with the antibody. Also in agreement with previous reports, Na depletion decreased the abundance of the 85- to 90-kDa form of -ENaC while increasing the amount of a lower mass 70-kDa species. This is also consistent with proteolysis near the NH 2 terminus, as this antibody was raised against a COOH-terminal sequence of this subunit. We also observed a consistent increase in the amount of -ENaC protein in response to Na depletion. This was not seen in previous studies ( 6, 15 ). Figure 2 also shows the effects of a chronic increase in dietary K on the ENaC subunits. These changes will be discussed in more detail below.6 v; l% Q5 q8 b9 Q

Fig. 2. Immunoblots of kidneys from rats on control, high-K, and low-Na diets. A : representative blots showing -ENaC ( top ), -ENaC ( middle ), and -ENaC ( bottom ). Forty micrograms of protein from whole kidney homogenates were loaded onto each lane. Primary antibodies were used at dilutions of 1:500 ( -ENaC) and 1:1,000 ( - and -ENaC). Each lane represents samples from a different rat. B : quantitation of band densities. Values are means ± SE for 6 measurements/condition. Bottom right : quantitation of Coomassie staining of an unidentified control band at 90 kDa to confirm equal protein loading of all lanes. * P
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Because the smaller fragment of -ENaC and the increased abundance of -ENaC have not been reported previously in kidney tissue, we checked whether these changes correlated in time with the other effects on ENaC subunits. Figure 3 shows a 7-day time course of salt depletion. There was little effect on any of the parameters probed in the animals after 1 day on a low-Na diet. After 3 days, all three subunits were affected. There were significant increases in the 85- and 30-kDa forms of -ENaC, increases in -ENaC, and shifts from the 85- to 70-kDa species of -ENaC. After 7 days of salt depletion, all of these effects were further enhanced. Thus at this level of resolution, all the effects are coordinated.
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Fig. 3. Time course of changes in ENaC protein. Rats were fed a low-Na diet for 1, 3, or 7 days. Values are means ± SE for 3 measurements/condition. Protein abundance plotted in arbitrary units.

We noted that part of the -ENaC band recognized by the antibody appeared to have an increased apparent mass, and in some blots appeared as a doublet. To test whether this might reflect a change in the glycosylation pattern of the protein, we examined the effects of both PNGase F and Endo H on -ENaC peptides. The former enzyme cleaves all N-linked sugars, whereas the latter attacks only those in an incompletely mature high-mannose state. As shown in Fig. 4 A, PNGase F decreased the apparent mass from 85 to 70 kDa in both control and Na-depleted animals. The effects of Endo H were different. All the -ENaC from control animals was reduced in size to a degree similar to that seen in response to PNGase F. However, in Na-depleted rats a second band with a smaller reduction in molecular mass appeared, consistent with the development of Endo H resistance in some of the protein. Endo H-resistant glycosylation of the -subunit was previously reported for ENaC in A6 cells from X. laevis kidney ( 1 ) and in Chinese hamster ovary (CHO) cells in which ENaC was exogenously expressed ( 11 ).) E+ _9 r5 w. B6 z1 w7 V

Fig. 4. Effects of glycosidases. Homogenates of kidneys from controls and from rats fed a low-Na diet for 7 days were treated with either peptide N -glycosidase F (PNGase F; A ) or endoglycosidase H (Endo H; B ). Top : -ENaC. Middle : -ENaC. Bottom : -ENaC. Each lane corresponds to a different animal.# L) s* C/ {: _0 c) f
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We also examined the effects of these glycosidases on - and -ENaC. There were no obvious differences in the effects of PNGase F and Endo H on -ENaC. In both cases, the major change in the staining pattern was a shift in the 85-kDa band to 70-75 kDa ( Fig. 3 A ). For -ENaC, both PNGase F and Endo H reduced the apparent size of the larger species from 85 to 60 kDa in both control and low-Na animals. However, the sizes of the smaller species were affected differently. PNGase F reduced the apparent mass from 70 to 50 kDa. Endo H decreased the mass by only 5 kDa ( Fig. 4 B ). Thus the lower-molecular-mass species of -ENaC also appears to possess Endo H-resistant glycosylation.

Most changes in ENaC protein in response to a low-Na diet occur over days ( Fig. 3 ), at least in part because it takes this long for circulating levels of aldosterone, the hormone presumed to mediate the effects of salt depletion, to increase maximally ( 18 ). However, Na transport begins to decrease in response to aldosterone within 1-2 h ( 23 ). Therefore, we wanted to see which whether any of these changes in protein occurred rapidly enough to be involved in this early response. We implanted rats with osmotic minipumps to deliver aldosterone at a constant rate and examined the kidneys after 4 h of infusion. To assess the physiological effects of the hormone, animals were kept in metabolic cages and excretion rates of Na and K were monitored. The aldosterone-infused rats excreted Na at rates that were 40% of controls, whereas K excretion was not changed ( Fig. 5 A ). In addition, in three of the animals we isolated CCDs and measured whole cell amiloride-sensitive current ( I Na ) at a voltage of -100 mV. I Na is not measurable in control rats on a normal-Na diet, as reported previously ( 6, 7 ). After 4 h of aldosterone infusion, I Na was 100 ± 33 pA/cell ( n = 28; Fig. 5 B ). For comparison, I Na measured in animals infused continuously for 6-8 days with the hormone was 740 ± 150 pA/cell ( n = 13). The short treatment with aldosterone had significant effects on ENaC protein ( Fig. 6 ). The amount of -ENaC increased, at both the 85- and 30-kDa bands. The amount of the higher-molecular-mass -ENaC form decreased, whereas the 70-kDa species increased in abundance. However, the amount of -ENaC did not increase.; P2 h2 ]: z5 I( p) J

Fig. 5. Physiological responses to acute and chronic aldosterone infusion. A : Na (U Na V) and K excretion rates (U K V) for rats infused with aldosterone (aldo) for 4 h. Urine was collected between 2 and 4 h after implantation of the osmotic minipumps. Values are means ± SE for 7 animals. * P . ^( v9 c2 G! }9 Z. s  V
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Fig. 6. Effects of acute aldosterone treatment. Left : representative blots showing -ENaC ( top ), -ENaC ( middle ), and -ENaC ( bottom ) for animals treated with aldosterone for 4 h and paired controls. Conditions are the same as in Fig. 2. Right : quantitation of band densities. Values are means ± SE for 4-6 measurements/condition from 2 similar blots. * P . x" @/ u( r! L
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We also examined changes in ENaC protein under conditions where Na transport in the distal nephron decreased rapidly. We salt deprived animals for 1 wk, then acutely repleted them with salt by giving them isotonic NaCl in the drinking water. On average, the animals consumed 3.5 mmol and excreted 2.1 mmol of Na over a 5-h period. This salt repletion had a profound effect on Na channel activity. I Na decreased from 530 ± 77 pA/cell in Na-depleted controls to 49 ± 18 pA/cell in Na-repleted animals ( Fig. 7 ). Several effects on ENaC protein were also evident ( Fig. 8 ). The amount of the 30-kDa form of -ENaC decreased, whereas a smaller fall in the 85-kDa form was not significant. The total amount of -ENaC also declined, accompanied by a fall in the amount of Endo H-resistant -ENaC protein ( Fig. 9 ). The 70-kDa forms of -ENaC decreased, without a significant change in the 85-kDa form.

Fig. 7. Physiological effects of acute salt repletion. Animals were fed a low-Na diet for 7 days and then were given saline drinking water for 5 h. Top : current-voltage relationships for typical principal cells without ( left ) and with ( right ) resalting. I Na is defined as in Fig. 5. Bottom : I Na from Na-depleted and acutely Na-repleted rats. Values are means ± SE for 16 and 18 measurements, respectively.
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Fig. 8. Acute salt repletion. Left : representative blots showing -ENaC ( top ), -ENaC ( middle ), and -ENaC ( bottom ) for animals given saline drinking water for 5 h and for controls given deionized drinking water. Conditions are the same as in Fig. 2. Right : quantitation of band densities in A. Values are means ± SE for 4 measurements/condition. * P ; X2 V, V) T6 t2 }- @
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Fig. 9. Endo H resistance of -ENaC after acute Na repletion. The blot shows results from acutely Na-depleted animals without enzyme treatment ( left lane) and with Endo H treatment ( middle 4 lanes) and salt-replete animals with Endo H treatment ( right 4 lanes).
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Because the effects of Na depletion on Na channels are thought to be mediated by aldosterone, we checked whether chronic infusion of the hormone for 6-8 days had the same effect as long-term dietary Na restriction. The results are shown in Fig. 10. Similar to Na depletion, the steroid increased the amount of both forms of -ENaC and produced reciprocal changes in the two forms of -ENaC. Unlike Na depletion, aldosterone did not change the amount of -ENaC. Furthermore, the amount of the Endo H-resistant form of this subunit did not increase ( Fig. 11 ). Thus the effects of Na depletion and aldosterone administration on the channels are not identical.
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Fig. 10. Effects of chronic aldosterone treatment. Left : representative blots showing -ENaC ( top ), -ENaC ( middle ), and -ENaC ( bottom ) for animals treated with aldosterone for 7 days and paired controls. Conditions as the same as in Fig. 2. Right : quantitation of band densities. Values are means ± SE for 3-7 measurements/condition. * P

Fig. 11. Endo H resistance of -ENaC after chronic aldosterone treatment. Top : kidney homogenates from control (con) and chronic aldosterone-treated rats were untreated ( left ) or treated ( right ) with Endo H. Bottom : blot run in parallel with kidney homogenates from control and Na-depleted rats were untreated ( left ) or treated ( right ) with Endo H., }1 ]: d2 a+ u0 L% i
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We also examined the effects of a high-K diet (see Fig. 2 ). K loading increases Na channel activity in the renal collecting duct by mechanisms that are, at least in part, independent of circulating aldosterone levels ( 19 ). This treatment did not change the abundance of -ENaC to a measurable extent, but it did increase -ENaC and the 70-kDa form of -ENaC. We also observed a small but significant increase in the amount of the Endo H-resistant form of -ENaC. Thus whereas all three maneuvers have the same effect on -ENaC, they have very different influences on the other subunits; Na depletion seems to combine the effects of aldosterone and K loading. These differences are summarized in Table 1.$ |, Z3 [7 d# u7 `
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Table 1. Summary of effects of low-Na diet, aldosterone, and high-K diet on ENaC subunits: R+ D0 O4 s2 a  y5 Q" K
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DISCUSSION4 ^! U! q: M# j; m: j
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Comparison with previous results in the rat kidney. Our findings confirm and extend previous results obtained in the rat kidney using antibodies directed against the same epitopes ( 15 ). One new finding that we report here is the existence of a 30-kDa peptide recognized by the anti- -ENaC antibody. This peptide is likely to be a proteolytic fragment of the -subunit. A similar product of proteolysis has been reported for -ENaC expressed in CHO cells ( 9, 11 ) and is thought to represent cleavage of the subunit by the endogenous protease furin. To our knowledge, this is the first time it has been observed in a cell expressing the channel endogenously. We propose that this fragment occurs naturally and is part of the processing of the channel protein. We cannot rule out the possibility that the cleavage occurs subsequent to the lysis of the kidney cells. However, the fact that it decreased specifically, without a change in the 85-kDa form of the protein, after acute Na repletion argues against this interpretation. A second new finding is the increase in the abundance of the -ENaC subunit in response to Na depletion. This is the first time changes in this subunit have been implicated in the regulation of Na channels in the kidney.

Correlation with channel regulation. The changes in ENaC protein described here and previously ( 15, 16 ) require a week or more of Na depletion to be fully realized. This is not surprising as both the increase in plasma aldosterone and the appearance of conducting Na channels in the apical membrane follow a similar time course ( 18 ). This leaves open the question of whether these changes can take place on a time scale consistent with the day-to-day regulation of Na excretion. In a previous study, we reported that changes in the -ENaC could be observed after 18 h of Na depletion, the earliest time point at that we have been able to record increases in channel activity in the CCD in response to decreases in salt intake. Here, we report that the changes in ENaC protein can occur on a much more rapid time scale. Elevation of -ENaC abundance (both 85- and 30-kDa forms) and of the 70-kDa form of -ENaC were observed within 4 h of administration of aldosterone via an osmotic minipump. We also documented the induction of channel activity in the CCD by this maneuver. We also developed a protocol in which channels could be acutely downregulated by salt repletion through saline drinking water. In this case, we observed selective decreases in the low-molecular-mass forms of - and -ENaC, and the Endo H-resistant form of -ENaC. Our interpretation of these findings is discussed below. This is the first time that qualitative or quantitative changes in ENaC protein have been demonstrated in the kidney over the time period of a few hours, corresponding to the "rapid" response to aldosterone ( 23 ). However, a redistribution of protein toward the apical membrane over a similar time course in response to aldosterone administration has been observed ( 14 ).) n" N% i6 K$ L# g# u1 E, G3 A  `
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Changes in glycosylation of ENaC subunits. Our results indicate that ENaC, particularly the - and -subunits, can exist in the cell glycosylated in either an immature, Endo H-sensitive or a mature, Endo H-resistant state. This is the first description of the glycosylation of ENaC in kidney tissue. The existence of the Endo H-resistant forms of the - and -subunits was documented previously in A6 cells, a cell-line from the X. laevis kidney ( 1 ). Hughey et al. ( 11 ) also found Endo H-sensitive and -resistant forms of ENaC expressed in CHO and Madin-Darby canine kidney cells ( 11 ). They correlated both the presence of these forms, as well as the proteolytic cleavage of the - and -subunits, with the maturation and processing of the channels. We have also correlated the amount of Endo H-resistant protein with the physiological state of the animal; increases were seen in both the both - and -subunits during Na depletion.

Maturation and trafficking of ENaC. Immunocytochemical data suggest that existing ENaC protein can be translocated from intracellular sites to the apical membrane in response to salt deprivation ( 13 - 15 ). Our results can be most easily explained if the intracellular ENaC protein resides in a compartment within the biosynthetic pathway and is in state of incomplete processing. This compartment would most likely be the late ER or cis -Golgi where membrane proteins are incompletely glycosylated. During salt depletion or aldosterone administration channels proceed through the remainder of the biosynthetic path and the glycosylation matures, achieving Endo H resistance.

The cleavage of the - and also perhaps the -subunit of ENaC may also occur during this process. This would be consistent with the findings of Hughey et al. ( 11 ), who found furin-dependent processing of ENaC expressed in CHO cells. Furin resides mainly in the trans -Golgi, although it can also be cycled to and from the plasma membrane ( 21 ). Analysis of furin-deficient cells indicated that the cleavage of the subunits is necessary for full channel activity ( 9 ).

We therefore interpret the presence of cleaved, maturely glycosylated subunits as representing ENaC that has reached the apical surface of the cells ( Fig. 12 ). According to this view, the action of the hormone is not necessarily a direct activation of a cleavage process but could be on the trafficking of the channels, leading to an accumulation of mature proteins at the apical surface. This could be accomplished by an increase in the rate of insertion of ENaC into the apical membrane or a slowing of the rate of retrieval. Our steady-state measurements do not distinguish these possibilities.9 E0 ]( l3 s5 H) _$ B6 E6 k

Fig. 12. Model of ENaC processing and maturation. The 3 subunits are represented by red ( ), yellow ( ), and orange ( ). Green and yellow balls represent immature and mature glycosylation patterns, respectively. Nicked ellipses represent subunits that have undergone proteolytic cleavage. The top right and left arrows indicate different pathways during Na depletion and aldosterone infusion, respectively.
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The results of the resalting experiment ( Figs. 8 and 9 ) suggest that the mature form of the subunits have short lifetimes, at least under conditions of Na loading. The amounts of the cleaved forms of - and -ENaC, as well as the Endo H-resistant form of -ENaC, decline rapidly and selectively under these conditions. We interpret this as a rapid clearing of mature, presumably conducting channel protein from the apical membrane. This could underlie the concomitant fall in channel conductance measured by whole cell clamp. This is consistent with a short lifetime ( t 15 min) of the subunits at the apical membrane measured in A6 cells ( 1 ), although other studies found much longer half-lives that varied among the different subunits ( 24 ).

Different effects of Na depletion and aldosterone administration. We were surprised by the differences in the effects of Na depletion and aldosterone infusion on ENaC subunits. To our knowledge, this is the first time such a difference has been reported in the effects on Na channels at either the electrophysiological or protein level. One important difference in these two conditions could be in the plasma K levels, which increase moderately with low Na and decrease with aldosterone administration (unpublished data). K loading, which increases plasma K at least transiently, produced a smaller but significant increase in -ENaC abundance. Thus the lack of increase in -ENaC with aldosterone infusion could be related to the fall in plasma K. It is possible that subtle differences in K intake could account for the discrepancy between the present results and the lack of effect of Na depletion on -ENaC abundance reported earlier ( 15 ).% k% _. z3 y" r

Aldosterone administration also failed to increase the amount of Endo H-resistant -ENaC. Thus this increase is not essential for the physiological response of increased channel activity, which is similar under Na depletion and aldosterone infusion conditions ( 18 ). If Endo H resistance reflects the mature form of the subunit, then one interpretation is that an increase in its abundance in the apical membrane is not required for increased channel activity. Alternatively, it is possible that the subunit can reach the membrane before it reaches its final glycosylation state. A precedent for the latter interpretation was reported by Hughey et al. ( 10 ), who found that immature ENaC subunits could reach the plasma membrane when expressed in Madin-Darby canine kidney cells.  g1 \  f* k0 B7 R
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GRANTS

This work was supported by National Institute of Diabetes and Digestive and Kidney Diseases Grant RO1-DK-59659.9 }" l! j9 k, o/ t3 [
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ACKNOWLEDGMENTS$ w) p% L3 S, e
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We thank Dr. Mark Knepper for donating the rabbits for anti- - and anti- -ENaC and for advice on immunoblotting.
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