http://jasn.asnjournals.org Journal of the American Society of Nephrology RSS feed -- recent Cell and Transport Physiology articles 1533-3450 Journal of the American Society of Nephrology 1046-6673 http://jasn.asnjournals.org/icons/banner/title.gif http://jasn.asnjournals.org http://jasn.asnjournals.org/cgi/content/short/18/6/1652?rss=1 Activity of the epithelial Na⁺ channel (ENaC) is limiting for Na⁺ reabsorption in the aldosterone-sensitive distal nephron. Hormones, including aldosterone and insulin, increase ENaC activity, in part by stimulating phosphatidylinositide 3-OH kinase (PI3-K) signaling. Recent studies in heterologous expression systems reveal a close spatiotemporal coupling between PI3-K signaling and ENaC activity with the phospholipid product of this kinase, PI(3,4,5)P₃, in some cases, directly binding the channel and increasing open probability (P_o). This study tested whether this tight coupling plays a physiologic role in modulating ENaC activity in native tissue and polarized epithelial cells. IGF-I was found to increase Na⁺ reabsorption across mpkCCD_c14 principal cell monolayers in a PI3-K–sensitive manner. Inhibition of PI3-K signaling, moreover, rapidly decreased Na⁺ reabsorption and ENaC activity in mpkCCD_c14 cells that were treated with corticosteroids and IGF-I. These decreases paralleled changes in apical membrane PI(3,4,5)P₃ levels, demonstrating tight spatiotemporal coupling between ENaC activity and PI3-K/PI(3,4,5)P₃ signaling within this membrane. For further probing of the mechanism underpinning this coupling, cortical collecting ducts (CCD) were isolated from rat and split open to expose the apical membrane for patch-clamp analysis. Inhibition of PI3-K signaling with wortmannin and LY294002 but not its inactive analogue rapidly and markedly decreased the P_o of ENaC. Moreover, IGF-I acutely increased P_o of ENaC in CCD principal cells in a PI3-K–sensitive manner. Together, these observations stress the importance of tight spatiotemporal coupling between PI3-K signaling and ENaC within the apical membrane of principal cells to the physiologic control of this ion channel.

]]> 2007-05-25 info:doi/10.1681/ASN.2007010020 American Society of Nephrology 6 18 1661 2007-06-01 1652 Cell and Transport Physiology http://jasn.asnjournals.org/cgi/content/short/18/6/1662?rss=1 It was previously shown that cyclosporine A (CsA) increases transepithelial resistance in MDCK cells. Activation of the extracellular signal–regulated kinase 1/2 (ERK1/2) mitogen-activated protein kinase (MAPK) cascade seems to be pivotal to the CsA-induced increase in transepithelial electrical resistance (TER). This study examined the role played by TGF-ß in mediating the CsA-induced activation of ERK1/2 and the resulting increase in TER in MDCK cells. Paracellular permeability across MDCK monolayers after various treatments was assessed by measurement of TER. TGF-ß secretion was measured by Western blot and ELISA. Activation of the ERK1/2 pathway and tight junction protein expression were also assessed by Western blot analysis. CsA increased production and secretion of TGF-ß and expression of the TGF-ß receptor II. Exogenous addition of TGF-ß1 activated ERK1/2 and increased TER across MDCK monolayers, both of which were attenuated by the MEK inhibitor U0126. Neutralizing antibodies against TGF-ß1 and the TGF-ß receptor II significantly reduced the CsA-induced increase in TER. Both CsA and TGF-ß1 increased expression of tight junction proteins claudin-1 and zonula occludens 2. Inhibition of the p38 MAPK pathway also attenuated the TGF-ß1–induced increase in TER. The results presented here suggest that the CsA-induced modulation of paracellular permeability may be mediated, at least in part, by an increase in TGF-ß production.

]]> 2007-05-25 info:doi/10.1681/ASN.2006050527 American Society of Nephrology 6 18 1671 2007-06-01 1662 Cell and Transport Physiology http://jasn.asnjournals.org/cgi/content/short/18/6/1672?rss=1 The antidiuretic effect of vasopressin is mediated by V2 receptors (V2R) that are located in kidney connecting tubules and collecting ducts. This study provides evidence that V2R signaling is negatively regulated by regulator of G protein signaling 2 (RGS2), a member of the family of RGS proteins. This study demonstrates that (1) RGS2 expression in the kidney is restricted to the vasopressin-sensitive part of the nephron (thick ascending limb, connecting tubule, and collecting duct); (2) expression of RGS2 is rapidly upregulated by vasopressin; (3) the vasopressin-dependent accumulation of cAMP, the principal messenger of V2R signaling, is significantly higher in collecting ducts that are microdissected from the RGS2^–/– mice compared with their wild-type littermates; and (4) analysis of urine output of mice that were exposed to water restriction followed by acute water loading revealed that RGS2^–/– mice exhibit an increased renal responsiveness to vasopressin. It is proposed that RGS2 is involved in negative feedback regulation of V2R signaling.

]]> 2007-05-25 info:doi/10.1681/ASN.2007010032 American Society of Nephrology 6 18 1678 2007-06-01 1672 Cell and Transport Physiology http://jasn.asnjournals.org/cgi/content/short/18/6/1679?rss=1 Germline inactivation of the mineralocorticoid receptor (MR) gene in mice results in postnatal lethality as a result of massive loss of sodium and water. The knockout mice show impaired epithelial sodium channel (ENaC) activity in kidney and colon. For determination of the role of renal MR in aldosterone-driven ENaC-mediated sodium reabsorption, mice with principal cell MR deficiency were generated using the Cre-loxP system. For driving Cre recombinase expression in principal cells, the regulatory elements of the mouse aquaporin 2 (AQP2) gene were used. Mutant mice (MR^AQP2Cre) were obtained by crossing AQP2Cre mice with mice that carried a conditional MR allele. Under standard diet, MR^AQP2Cre mice develop normally and exhibit unaltered renal sodium excretion but show strongly elevated aldosterone levels. Increased renal sodium and water excretion, resulting in continuous loss of body weight, occur under low-sodium diet. Immunofluorescence revealed that the loss of MR and apical ENaC staining is restricted to principal cells of the collecting duct (CD) and late connecting tubule (CNT) and that MR is crucial for ENaC trafficking to the apical membrane. These results demonstrate that inactivation of MR in CD and late CNT can be compensated under standard diet but no longer when sodium supply is limited. Because the mutant mice show preserved renal ENaC activity, this study provides evidence that the late distal convoluted tubule and early CNT can compensate to a large extent deficient ENaC-mediated sodium reabsorption in late CNT and CD.

]]> 2007-05-25 info:doi/10.1681/ASN.2006090975 American Society of Nephrology 6 18 1687 2007-06-01 1679 Cell and Transport Physiology