Some Assembly Required: Putting the Epithelial Sodium Channel Together

The epithelial sodium channel (ENaC) 2The abbreviations used are:ENaCepithelial sodium channel(s)ERendoplasmic reticulum2The abbreviations used are:ENaCepithelial sodium channel(s)ERendoplasmic reticulum constitutes the rate-limiting step in Na+ re-absorption in the apical membrane of epithelia, including kidney, lung, colon, salivary glands, and sweat glands (1Garty H. Palmer L.G. Physiol. Rev. 1997; 77: 359-396Crossref PubMed Scopus (1035) Google Scholar). The channel is integral to homeostasis as knock-out of the α-subunit of ENaC is lethal in mice. Abnormal regulation of the channel that results in either gain or loss of function has been implicated in the pathogenesis of several disease states, including forms of salt-sensitive hypertension and pseudohypoaldosteronism type II, and may also contribute to the progression of pulmonary disease in cystic fibrosis (1Garty H. Palmer L.G. Physiol. Rev. 1997; 77: 359-396Crossref PubMed Scopus (1035) Google Scholar, 2Mall M. Grubb B.R. Harkema J.R. O'Neal W.K. Boucher R.C. Nat. Med. 2004; 10: 487-493Crossref PubMed Scopus (710) Google Scholar, 3Rossier B.C. J. Clin. Investig. 2003; 111: 947-950Crossref PubMed Scopus (33) Google Scholar). epithelial sodium channel(s) endoplasmic reticulum epithelial sodium channel(s) endoplasmic reticulum ENaC is a highly Na+-selective amiloride-sensitive channel of relatively low conductance that appears to be constitutively active when present in cell membranes (1Garty H. Palmer L.G. Physiol. Rev. 1997; 77: 359-396Crossref PubMed Scopus (1035) Google Scholar). The channel is a heterotrimeric protein, made up of three homologous subunits (α, β, and γ) with a likely stoichiometry of 1:1:1 (4Canessa C.M. Schild L. Buell G. Thorens B. Gautschi I. Horisberger J.-D. Rossier B.C. Nature. 1994; 367: 463-467Crossref PubMed Scopus (1766) Google Scholar, 5Jasti J. Furukawa H. Gonzales E.B. Gouaux E. Nature. 2007; 449: 316-323Crossref PubMed Scopus (849) Google Scholar). A δ-subunit that can co-assemble with α-, β-, and γ-ENaC has also been described (6Ji H.L. Su X.F. Kedar S. Li J. Barbry P. Smith P.R. Matalon S. Benos D.J. J. Biol. Chem. 2006; 281: 8233-8241Abstract Full Text Full Text PDF PubMed Scopus (91) Google Scholar). Given its essential roles in fluid clearance and regulation of blood volume, it is not surprising that ENaC activity is under strict control. The apical membrane abundance and open probability of ENaC are altered by a wide variety of hormonal and cellular effectors (1Garty H. Palmer L.G. Physiol. Rev. 1997; 77: 359-396Crossref PubMed Scopus (1035) Google Scholar, 7Butterworth M.B. Edinger R.S. Frizzell R.A. Johnson J.P. Am. J. Physiol. May 28 2008; 10.1152/ajprenal.90248.2008Google Scholar). Regulation of channel expression at the translational and post-translational levels is tissue-specific, and these differences are apparent for either whole channel levels or expression of individual subunits (8Weisz O.A. Johnson J.P. Am. J. Physiol. 2003; 285: F833-F842Crossref PubMed Scopus (41) Google Scholar). For example, message for the individual subunits is differentially expressed not only in distinct tissues such as lung and kidney, but also temporally, so the message for some subunits is expressed constitutively, whereas that for others is induced hormonally (8Weisz O.A. Johnson J.P. Am. J. Physiol. 2003; 285: F833-F842Crossref PubMed Scopus (41) Google Scholar). Differential translational efficiency of ENaC subunit message has also been noted. In endogenously expressing tissues and cell lines, it is clear that total cellular and apical membrane levels of some subunits remain unchanged, whereas others are either up- or down-regulated in response to factors that alter regulation of transport rate (9Weisz O.A. Wang J.-M. Edinger R.S. Johnson J.P. J. Biol. Chem. 2000; 275: 39886-39893Abstract Full Text Full Text PDF PubMed Scopus (96) Google Scholar). This phenomenon has been referred to as non-coordinate regulation of ENaC, a term first applied by Farman and co-workers (10Escoubet B. Coureau C. Bonvalet J.-P. Farman N. Am. J. Physiol. 1997; 272: C1482-C1491Crossref PubMed Google Scholar, 11Farman N. Talbot C.R. Boucher R. Fay M. Canessa C. Rossier B.C. Bonvalet J.-P. Am. J. Physiol. 1997; 272: C131-C141Crossref PubMed Google Scholar) to the differential steroid regulation of ENaC subunit mRNA in lung, kidney, and colon. We have previously proposed three possible mechanisms to explain this phenomenon: 1) post-translational assembly of channels, 2) the existence of channels with alternative stoichiometry that may be differentially regulated, and 3) disassembly and recombination of subunits at some post-translational site (8Weisz O.A. Johnson J.P. Am. J. Physiol. 2003; 285: F833-F842Crossref PubMed Scopus (41) Google Scholar). These mechanisms are not mutually exclusive, which highlights the likelihood that more than one pathway participates in the generation of the phenomenon broadly described as non-coordinate regulation. The purpose of this review is to consider the status of non-coordinate regulation of ENaC with respect to several recent developments in the field, in particular the appreciation of the probable trimeric stoichiometry of the channel and the accumulating evidence that sub-apical pools of ENaC subunits exist that can participate in recycling of the channel. Finally, we will discuss the impact that differing molecular mass forms of the ENaC subunits present at the apical membrane following proteolytic activation of the channel may have on our understanding of non-coordinate regulation. ENaC was originally identified and the primary structure of its three subunits elucidated through expression cloning (4Canessa C.M. Schild L. Buell G. Thorens B. Gautschi I. Horisberger J.-D. Rossier B.C. Nature. 1994; 367: 463-467Crossref PubMed Scopus (1766) Google Scholar). Initial studies of the channel in heterologous overexpression systems suggested that the channel oligomerizes co-translationally in a preferred stoichiometry that contains all three subunits (12Firsov D. Gautschi I. Merillat A.-M. Rossier B.C. Schild L. Embo J. 1998; 17: 344-352Crossref PubMed Scopus (368) Google Scholar). Although it is possible for channels containing only one or two subunits to reach the cell surface, expression of all three subunits results not only in greater membrane expression and activity, but also in stabilization of the channel (13Harris M. Garcia-Caballero A. Stutts M.J. Firsov D. Rossier B.C. J. Biol. Chem. 2008; 283: 7455-7463Abstract Full Text Full Text PDF PubMed Scopus (50) Google Scholar, 14Rotin D. Kanelis V. Schild L. Am. J. Physiol. 2001; 281: F391-F399Crossref PubMed Google Scholar). The majority of channel subunits are rapidly degraded prior to reaching the cell surface (14Rotin D. Kanelis V. Schild L. Am. J. Physiol. 2001; 281: F391-F399Crossref PubMed Google Scholar, 15Valentijn J.A. Fyfe G.K. Canessa C.M. J. Biol. Chem. 1998; 273: 30344-30351Abstract Full Text Full Text PDF PubMed Scopus (108) Google Scholar). Although several chaperones that participate in endoplasmic reticulum (ER)-associated degradation of ENaC have been identified, the details of this process remain unknown (16Kashlan O.B. Mueller G.M. Qamar M.Z. Poland P.A. Ahner A. Rubenstein R.C. Hughey R.P. Brodsky J.L. Kleyman T.R. J. Biol. Chem. 2007; 282: 28149-28156Abstract Full Text Full Text PDF PubMed Scopus (38) Google Scholar). The recent crystallographic demonstration that the chicken acid-sensing ion channel (a member of the ENaC/degenerin family of ion channels) is arranged as a heterotrimer has added a new dimension to our understanding of ENaC channel assembly (5Jasti J. Furukawa H. Gonzales E.B. Gouaux E. Nature. 2007; 449: 316-323Crossref PubMed Scopus (849) Google Scholar). The prevailing model posits that multisubunit channels form by assembly of newly synthesized proteins into heteromeric complexes of uniform stoichiometry in the ER. However, a considerable body of evidence suggests that this is not a requirement for the generation of functional Na+ channels. It is clear from studies in several model systems that channels comprising only one or two of the three subunits can assemble and traffic to the cell surface (12Firsov D. Gautschi I. Merillat A.-M. Rossier B.C. Schild L. Embo J. 1998; 17: 344-352Crossref PubMed Scopus (368) Google Scholar, 13Harris M. Garcia-Caballero A. Stutts M.J. Firsov D. Rossier B.C. J. Biol. Chem. 2008; 283: 7455-7463Abstract Full Text Full Text PDF PubMed Scopus (50) Google Scholar). There are several reports that ENaC subunits or channels composed of one or two subunits can accumulate at intracellular sites and assemble into fully functional channels upon subsequent expression of the remaining subunit (17Loffing J. Pietri L. Aregger F. Block-Faure M. Ziegler U. Meneton P. Rossier B.C. Kaissling B. Am. J. Physiol. 2000; 279: F252-F258Crossref PubMed Google Scholar, 18Hager H. Kwon T.H. Vinnikova A.K. Masilamani S. Brooks H.L. Frokiaer J. Knepper M.A. Nielsen S. Am. J. Physiol. 2000; 280: F1093-F1106Google Scholar, 19Masilamani S. Kim G.-H. Mitchell C. Wade J.B. Knepper M.A. J. Clin. Investig. 1999; 104: R19-R23Crossref PubMed Scopus (623) Google Scholar, 20Loffing J. Zecevic M. Feraille E. Kaissling B. Asher C. Rossier B.C. Firestone G.L. Pearce D. Verrey F. Am. J. Physiol. 2001; 280: F675-F682Crossref PubMed Google Scholar). Because the α-subunit seems to be required for channel activity (4Canessa C.M. Schild L. Buell G. Thorens B. Gautschi I. Horisberger J.-D. Rossier B.C. Nature. 1994; 367: 463-467Crossref PubMed Scopus (1766) Google Scholar, 12Firsov D. Gautschi I. Merillat A.-M. Rossier B.C. Schild L. Embo J. 1998; 17: 344-352Crossref PubMed Scopus (368) Google Scholar), it is possible for α-, αγ-, or αβ-channels to function at the cell membrane (12Firsov D. Gautschi I. Merillat A.-M. Rossier B.C. Schild L. Embo J. 1998; 17: 344-352Crossref PubMed Scopus (368) Google Scholar, 13Harris M. Garcia-Caballero A. Stutts M.J. Firsov D. Rossier B.C. J. Biol. Chem. 2008; 283: 7455-7463Abstract Full Text Full Text PDF PubMed Scopus (50) Google Scholar), whereas βγ-dimers would represent precursors for channels containing all three subunits (17Loffing J. Pietri L. Aregger F. Block-Faure M. Ziegler U. Meneton P. Rossier B.C. Kaissling B. Am. J. Physiol. 2000; 279: F252-F258Crossref PubMed Google Scholar, 18Hager H. Kwon T.H. Vinnikova A.K. Masilamani S. Brooks H.L. Frokiaer J. Knepper M.A. Nielsen S. Am. J. Physiol. 2000; 280: F1093-F1106Google Scholar, 19Masilamani S. Kim G.-H. Mitchell C. Wade J.B. Knepper M.A. J. Clin. Investig. 1999; 104: R19-R23Crossref PubMed Scopus (623) Google Scholar, 20Loffing J. Zecevic M. Feraille E. Kaissling B. Asher C. Rossier B.C. Firestone G.L. Pearce D. Verrey F. Am. J. Physiol. 2001; 280: F675-F682Crossref PubMed Google Scholar). Most of the early studies of channels formed from one or two subunits were performed using Xenopus oocytes or other heterologous expression models, but there are examples of alternative channel assemblies in vivo. Randrianarison et al. (21Randrianarison N. Clerici C. Ferreira C. Fontayne A. Pradervand S. Fowler-Jaeger N. Hummler E. Rossier B.C. Planès C. Am. J. Physiol. 2008; 294: L409-L416Google Scholar) examined fluid clearance in newborn mice with a disruption in the β-ENaC gene locus. The reduction in β-ENaC levels resulted in compensatory up-regulation of α- and γ-ENaC and, surprisingly, led only to a modest impairment in ENaC function. This study concluded that functional channels with alternative stoichiometries may traffic to the apical membrane of epithelial cells in vivo, consistent with the previous heterologous expression system studies. A selective increase in the synthesis of one or two ENaC subunits has also been previously noted (8Weisz O.A. Johnson J.P. Am. J. Physiol. 2003; 285: F833-F842Crossref PubMed Scopus (41) Google Scholar). This would suggest that a pool of “partial” ENaC channels exists within these cells and that production of a limiting third subunit can facilitate the formation of a heterotrimeric channel (Fig. 1, Model 1). The most dramatic example of this is in rat kidney, intracellular pools of and can be in when ENaC activity at the apical membrane is (17Loffing J. Pietri L. Aregger F. Block-Faure M. Ziegler U. Meneton P. Rossier B.C. Kaissling B. Am. J. Physiol. 2000; 279: F252-F258Crossref PubMed Google Scholar, 18Hager H. Kwon T.H. Vinnikova A.K. Masilamani S. Brooks H.L. Frokiaer J. Knepper M.A. Nielsen S. Am. J. Physiol. 2000; 280: F1093-F1106Google Scholar, 20Loffing J. Zecevic M. Feraille E. Kaissling B. Asher C. Rossier B.C. Firestone G.L. Pearce D. Verrey F. Am. J. Physiol. 2001; 280: F675-F682Crossref PubMed Google Scholar). for Na+ these subunits to the apical membrane with newly synthesized (17Loffing J. Pietri L. Aregger F. Block-Faure M. Ziegler U. Meneton P. Rossier B.C. Kaissling B. Am. J. Physiol. 2000; 279: F252-F258Crossref PubMed Google Scholar, 18Hager H. Kwon T.H. Vinnikova A.K. Masilamani S. Brooks H.L. Frokiaer J. Knepper M.A. Nielsen S. Am. J. Physiol. 2000; 280: F1093-F1106Google Scholar, 19Masilamani S. Kim G.-H. Mitchell C. Wade J.B. Knepper M.A. J. Clin. Investig. 1999; 104: R19-R23Crossref PubMed Scopus (623) Google Scholar, 20Loffing J. Zecevic M. Feraille E. Kaissling B. Asher C. Rossier B.C. Firestone G.L. Pearce D. Verrey F. Am. J. Physiol. 2001; 280: F675-F682Crossref PubMed Google Scholar). A may in the lung, Boucher and co-workers M. Grubb B.R. Harkema J.R. O'Neal W.K. Boucher R.C. Nat. Med. 2004; 10: 487-493Crossref PubMed Scopus (710) Google Scholar) that ENaC activity may be by expression of but not by γ-ENaC on these selective expression of a ENaC subunit in response to ENaC surface expression and have suggested that of mRNA translational efficiency can also contribute to differential subunit It appears that and γ-ENaC are than the In a study ENaC mRNA abundance and translational in lung, et al. G. B. H. Am. J. Biol. 2004; PubMed Scopus Google Scholar) that there was of mRNA and that this mRNA was not as as and γ-ENaC A study that mRNA in the lung be by in and G. B. M. H. Am. J. Biol. 2006; PubMed Scopus Google Scholar). factors such as can alter the of a pathway O.A. Am. J. Physiol. 2003; Scholar). This of mRNA expression translational efficiency to the formation of of subunits channels of differing on a phenomenon has been proposed for lung as a to to the regulation of Na+ re-absorption S. A. L. J. Physiol. PubMed Scopus Google Scholar, H. P. S. G. Am. J. Physiol. 2008; 294: PubMed Scopus Google Scholar, Am. J. Physiol. 2008; 294: PubMed Scopus Google Scholar, J. S. Matalon S. L. Am. 2004; PubMed Scopus Google Scholar). Given the differences in mRNA abundance and it is likely that the abundance of ENaC subunit would not be at all in the it is possible to one or two of the subunits in the of the third that the subunits that are synthesized are not degraded by the subunits in the cells with the to upon of by The site of this assembly is not The most likely would be in the transport to the and to the apical the abundance of the three subunits the of channels containing one or two subunits will be by heterotrimeric channels. in cell have not fully the regulation of ENaC assembly in vivo. Although increase production of expression in the kidney, overexpression of in cells not increase sodium the subunits were of the J.B. Am. J. Physiol. 2007; PubMed Scopus Google Scholar). of γ-ENaC a impact on but only when all three subunits were the increase in a to that with the of to ENaC These suggest that one or two of the subunits are of of a third subunit will increase ENaC It is possible that cells mechanisms described to ENaC subunit or A of the that channels may not be co-translationally is that subunits into channels of alternative stoichiometries of heteromeric has been (8Weisz O.A. Johnson J.P. Am. J. Physiol. 2003; 285: F833-F842Crossref PubMed Scopus (41) Google Scholar). of subunits of the cell and of the individual of the trimeric has been described S. H. Full Text PDF PubMed Scopus Google Scholar, A. A. M. E. R. A. J. Biol. PubMed Scopus Google Scholar). it was that the α- and of the which in the in the membrane and differential degradation S. J. 2008; PubMed Scopus Google Scholar). The of ENaC would likely on the of the channel to We previously that ENaC is from the apical surface and pathway M.B. Edinger R.S. Frizzell R.A. Johnson J.P. Am. J. Physiol. May 28 2008; 10.1152/ajprenal.90248.2008Google Scholar, H. M.J. N. Edinger R.S. L. M.B. Kleyman T.R. Frizzell R.A. Johnson J.P. J. Biol. Chem. 2006; 281: Full Text Full Text PDF PubMed Scopus Google Scholar). The of the channel is only to be ENaC to early H. M.J. N. Edinger R.S. L. M.B. Kleyman T.R. Frizzell R.A. Johnson J.P. J. Biol. Chem. 2006; 281: Full Text Full Text PDF PubMed Scopus Google Scholar, C. S. A. C. D. 2007; PubMed Scopus Google Scholar), it the proteins of the required for C. S. A. C. D. 2007; PubMed Scopus Google Scholar). B. A. and J. P. channels are for whereas others are to two that ENaC expression have been one of which is and in M.B. Edinger R.S. H. D. Johnson J.P. Frizzell R.A. J. Biol. Chem. 2007; 282: Full Text Full Text PDF PubMed Scopus Google Scholar, F. P. G. 2008; Full Text Full Text PDF PubMed Scopus Google Scholar). ENaC the exist for or recycling the pathway C. S. A. C. D. 2007; PubMed Scopus Google Scholar), or to the the R. Nat. Rev. Biol. 2006; PubMed Scopus Google Scholar). The would a for channel (Fig. proteins have been identified that transport and of is to ENaC and represent a pathway for selective generation of channels studies of ENaC and degradation have suggested the that individual subunits have reaching the cell surface (8Weisz O.A. Johnson J.P. Am. J. Physiol. 2003; 285: F833-F842Crossref PubMed Scopus (41) Google Scholar, O.A. Wang J.-M. Edinger R.S. Johnson J.P. J. Biol. Chem. 2000; 275: 39886-39893Abstract Full Text Full Text PDF PubMed Scopus (96) Google Scholar, S. J.R. Edinger R.S. J.B. Kleyman T.R. Johnson J.P. O.A. J. Biol. Chem. 2004; 279: Full Text Full Text PDF PubMed Scopus Google Scholar). Because ENaC is not in expressing tissues and only a of synthesized subunits reach the cell it has been to reach a the of functional channels or individual in endogenously expressing cell such as have from for channels that reach the apical membrane to for β-ENaC with for α- and M.B. Edinger R.S. Frizzell R.A. Johnson J.P. Am. J. Physiol. May 28 2008; 10.1152/ajprenal.90248.2008Google Scholar, 14Rotin D. Kanelis V. Schild L. Am. J. Physiol. 2001; 281: F391-F399Crossref PubMed Google Scholar). In the of functional channels appears to be apical may be the channels to in these cells M.B. Edinger R.S. Frizzell R.A. Johnson J.P. Am. J. Physiol. May 28 2008; 10.1152/ajprenal.90248.2008Google Scholar, M.B. Edinger R.S. Johnson J.P. Frizzell R.A. J. Physiol. PubMed Scopus Google Scholar). These differences in ENaC subunit may altered or selective of individual ENaC subunits or subunit in response to However, is by the recent appreciation that ENaC can exist in molecular mass forms as a of proteolytic of ENaC by was first described by et al. V. A. Horisberger J.-D. Rossier B.C. Nature. 1997; PubMed Scopus Google Scholar) in and activation of channels in cell membranes by proteolytic was by et al. R.A. Boucher R.C. Stutts M.J. Am. J. Physiol. 2004; PubMed Scopus Google Scholar, R.A. Boucher R.C. Stutts M.J. Am. J. Physiol. PubMed Scopus Google Scholar). A ENaC activation by in rat and the of a molecular mass form of γ-ENaC was by Masilamani et al. S. Kim G.-H. Mitchell C. Wade J.B. Knepper M.A. J. Clin. Investig. 1999; 104: R19-R23Crossref PubMed Scopus (623) Google Scholar). These suggested that ENaC may be by proteolytic a that was and in a of studies by Hughey et al. J.B. S. O.A. Hughey R.P. Kleyman T.R. J. Biol. Chem. 2007; 282: Full Text Full Text PDF PubMed Scopus Google Scholar, R.P. Mueller G.M. J.B. Poland P.A. Kleyman T.R. J. Biol. Chem. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar, R.P. J.B. Kleyman T.R. J. Biol. Chem. 2004; 279: Full Text Full Text PDF PubMed Scopus Google Scholar, R.P. J.B. Johnson J.P. Kleyman T.R. J. Biol. Chem. 2004; 279: Full Text Full Text PDF PubMed Scopus Google Scholar). These studies the of α- and γ-ENaC into molecular mass forms and that channels have a greater open probability than channels composed of on subunits are that the subunits have the to the apical A distinct of channels also to the apical the R.P. J.B. Kleyman T.R. J. Biol. Chem. 2004; 279: Full Text Full Text PDF PubMed Scopus Google Scholar). These subunits exist in alternative or this has not been It is also apparent that there is more than one site for in α- and γ-ENaC and that can on these so of activation can be noted J.B. S. O.A. Hughey R.P. Kleyman T.R. J. Biol. Chem. 2007; 282: Full Text Full Text PDF PubMed Scopus Google Scholar). composed of or subunits at the apical membrane be by by of apical or by traffic of ENaC through intracellular may with (Fig. 1, Model There are two mechanisms by which proteolytic activation of channel subunits contribute to the phenomenon of non-coordinate regulation. to ENaC subunits that only α- and γ-ENaC would not selective in more active forms of these For example, we that up-regulation of ENaC activity by either with or to is with selective in apical membrane β-ENaC in the apical membrane forms of γ-ENaC in cells (9Weisz O.A. Wang J.-M. Edinger R.S. Johnson J.P. J. Biol. Chem. 2000; 275: 39886-39893Abstract Full Text Full Text PDF PubMed Scopus (96) Google Scholar). In of ENaC by to of β-ENaC not from results are consistent with the that channels in the apical membrane from under these are composed of forms of α- and γ-ENaC with the of forms of ENaC was not in these studies. It has been that with results in the of ENaC these to be with in the total cellular levels of subunits G. Palmer L.G. Am. J. Physiol. 2006; PubMed Scopus Google Scholar). These results that in and of channels rapidly to ENaC activity in In a to this et al. G. Palmer L.G. J. Physiol. 2008; PubMed Scopus Google Scholar) a in whole rat that that or led to increase in surface expression of but the form of was by this but membrane studies suggested increase in and These to our in cells (9Weisz O.A. Wang J.-M. Edinger R.S. Johnson J.P. J. Biol. Chem. 2000; 275: 39886-39893Abstract Full Text Full Text PDF PubMed Scopus (96) Google Scholar), are consistent with the that to surface expression of fully channels in rat and not a of ENaC in rat kidney, of production by led to of α- and whereas led to the of but not G. Palmer L.G. Am. J. Physiol. 2006; PubMed Scopus Google Scholar). and may not all subunits of the channel. a phenomenon would have to be were to explain the by Planès et al. C. M. M.A. S. Clerici C. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar) that and increase the apical membrane of and γ-ENaC in epithelial cells the surface expression of or the whole cell of of the three The also suggest the by which may contribute to the phenomenon of non-coordinate the that differences exist in following hormonal channels composed of is at present channels made up of of subunits traffic We have a in the rate of of with channels containing all three subunits S. J.R. Edinger R.S. J.B. Kleyman T.R. Johnson J.P. O.A. J. Biol. Chem. 2004; 279: Full Text Full Text PDF PubMed Scopus Google Scholar), but the of proteolytic activation on is not evidence in overexpression system that the rate of ENaC is not altered by proteolytic R. R. J. Biol. Chem. 2008; 283: Full Text Full Text PDF PubMed Scopus Google Scholar). The and of subunits are to remain R.P. Mueller G.M. J.B. Poland P.A. Kleyman T.R. J. Biol. Chem. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar), and channels are in a R. R. J. Biol. Chem. 2008; 283: Full Text Full Text PDF PubMed Scopus Google Scholar). it has been that to increase in ENaC subunits R. U. S. A. 2006; PubMed Scopus Google Scholar) and fully subunits have been to be in H. M.J. N. Edinger R.S. L. M.B. Kleyman T.R. Frizzell R.A. Johnson J.P. J. Biol. Chem. 2006; 281: Full Text Full Text PDF PubMed Scopus Google Scholar), the exists that subunits may traffic from channels in vivo. differential of forms of α- γ-ENaC explain the of non-coordinate regulation suggested by distinct in expression levels of other including the differing of subunits (9Weisz O.A. Wang J.-M. Edinger R.S. Johnson J.P. J. Biol. Chem. 2000; 275: 39886-39893Abstract Full Text Full Text PDF PubMed Scopus (96) Google Scholar), the of cellular pools of individual subunits and γ) that traffic to the apical membrane following of (17Loffing J. Pietri L. Aregger F. Block-Faure M. Ziegler U. Meneton P. Rossier B.C. Kaissling B. Am. J. Physiol. 2000; 279: F252-F258Crossref PubMed Google Scholar, 18Hager H. Kwon T.H. Vinnikova A.K. Masilamani S. Brooks H.L. Frokiaer J. Knepper M.A. Nielsen S. Am. J. Physiol. 2000; 280: F1093-F1106Google Scholar), and the differential regulation of message for the remain to be our review of this examples of non-coordinate regulation of ENaC subunit or message have to in the G. B. H. Am. J. Biol. 2004; PubMed Scopus Google Scholar, G. B. M. H. Am. J. Biol. 2006; PubMed Scopus Google Scholar, S. A. C. F. U. M. M. J.-D. 2008; Full Text Full Text PDF PubMed Scopus Google Scholar, S. S. C.M. Am. J. Physiol. 2008; 294: PubMed Scopus Google Scholar, S. J.R. C.M. 2008; PubMed Scopus Google Scholar). It seems likely that more than one is required to fully explain this of this channel is more than we previously the other of non-coordinate regulation can be by these new