Regulation of cell surface polarity in renal epithelia

KCNQ1 and KCNE1 K+ channel components are involved in early left-right patterning in Xenopus laevis embryos

Several ion transporters have been implicated in left-right (LR) patterning. Here, we characterize a new component of the early bioelectrical circuit: the potassium channel KCNQ1 and its accessory subunit KCNE1. Having cloned the native Xenopus versions of both genes, we show that both are asymmetrically localized as maternal proteins during the first few cleavages of frog embryo development in a process dependent on microtubule and actin organization. Molecular loss-of-function using dominant negative constructs demonstrates that both gene products are required for normal LR asymmetry. We propose a model whereby these channels provide an exit path for K(+) ions brought in by the H(+),K(+)-ATPase. This physiological module thus allows the obligate but electroneutral H(+),K(+)-ATPase to generate an asymmetric voltage gradient on the left and right sides. Our data reveal a new, bioelectrical component of the mechanisms patterning a large-scale axis in vertebrate embryogenesis.

Cell responses regulated by early reorganization of actin cytoskeleton

Microfilaments exist in a dynamic equilibrium between monomeric and polymerized actin and the ratio of monomers to polymeric forms is influenced by a variety of extracellular stimuli. The polymerization, depolymerization and redistribution of actin filaments are modulated by several actin-binding proteins, which are regulated by upstream signalling molecules. Actin cytoskeleton is involved in diverse cellular functions including migration, ion channels activity, secretion, apoptosis and cell survival. In this review we have outlined the role of actin dynamics in representative cell functions induced by the early response to extracellular stimuli.

Retention of membrane-localized beta-catenin in cells lacking functional polycystin-1 and tuberin

The tuberous sclerosis (TSC) 2 tumor suppressor gene encodes the protein tuberin, which has recently been shown to play a crucial role in the intracellular trafficking of polycystin-1, the product of the polycystic kidney disease (PDK) 1 gene. PKD1 is responsible for most cases of autosomal dominant polycystic kidney disease, which has been described as "neoplasia in disguise." Polycystin-1 is a membrane protein localized to adherens junctions in a complex containing E-cadherin and alpha-, beta-, and gamma-catenins. To determine whether loss of membrane localization of polycystin-1 and E-cadherin affects the function of beta-catenin, beta-catenin localization and signaling were characterized in tuberin-null EKT2 and ERC15 cells and in tuberin-positive TRKE2 cells derived from polycystic, neoplastic, and normal rat kidney epithelial cells, respectively. EKT2 cells lacking tuberin because of inactivation of the Tsc2 gene fail to localize polycystin-1 and E-cadherin appropriately to these junctions. However, beta-catenin was retained at lateral cell membranes in both tuberin-null and tuberin-positive cells. Moreover, gene transcription mediated by beta-catenin T-cell--specific transcription factor complexes showed no differences among EKT2, ERC15, and TRKE2 cells. Thus, beta-catenin was stably retained at the lateral cell membrane in tuberin-null renal cells lacking membrane-localized polycystin-1 and E-cadherin. These data suggest that, although loss of Tsc2 tumor suppressor gene function disrupts normal polycystin-1 function and membrane localization of E-cadherin, normal beta-catenin signaling is retained in tuberin-null cells.

Epithelial sodium channels regulate cystic fibrosis transmembrane conductance regulator chloride channels in Xenopus oocytes

The cystic fibrosis transmembrane conductance regulator (CFTR), in addition to its well defined Cl(-) channel properties, regulates other ion channels. CFTR inhibits epithelial Na(+) channel (ENaC) currents in many epithelial and nonepithelial cells. Because modulation of net NaCl reabsorption has important implications in extracellular fluid volume homeostasis and airway fluid volume and composition, we investigated whether this regulation was reciprocal by examining whether ENaC regulates CFTR. Co-expression of human (h) CFTR and mouse (m) alphabetagammaENaC in Xenopus oocytes resulted in a significant, 3.7-fold increase in whole-cell hCFTR Cl(-) conductance compared with oocytes expressing hCFTR alone. The forskolin/3-isobutyl-1-methylxanthine-stimulated whole-cell conductance in hCFTR-mENaC co-injected oocytes was amiloride-insensitive, indicating an inhibition of mENaC following hCFTR activation, and it was blocked by DPC (diphenylamine-2-carboxylic acid) and was DIDS (4, 4'-diisothiocyanatostilbene-2,2'-disulfonic acid)-insensitive. Enhanced hCFTR Cl(-) conductance was also observed when either the alpha- or beta-subunit of mENaC was co-expressed with hCFTR, but this was not seen when CFTR was co-expressed with the gamma-subunit of mENaC. Single Cl(-) channel analyses showed that both CFTR Cl(-) channel open probability and the number of CFTR Cl(-) channels detected per patch increased when hCFTR was co-expressed with alphabetagammamENaC. We conclude that in addition to acting as a regulator of ENaC, CFTR activity is regulated by ENaC.

Polycystin-1, the PKD1 gene product, is in a complex containing E-cadherin and the catenins

Autosomal dominant polycystic kidney disease (ADPKD) is a common human genetic disease characterized by cyst formation in kidney tubules and other ductular epithelia. Cells lining the cysts have abnormalities in cell proliferation and cell polarity. The majority of ADPKD cases are caused by mutations in the PKD1 gene, which codes for polycystin-1, a large integral membrane protein of unknown function that is expressed on the plasma membrane of renal tubular epithelial cells in fetal kidneys. Because signaling from cell-cell and cell-matrix adhesion complexes regulates cell proliferation and polarity, we speculated that polycystin-1 might interact with these complexes. We show here that polycystin-1 colocalized with the cell adhesion molecules E-cadherin and alpha-, beta-, and gamma-catenin. Polycystin-1 coprecipitated with these proteins and comigrated with them on sucrose density gradients, but it did not colocalize, coprecipitate, or comigrate with focal adhesion kinase, a component of the focal adhesion. We conclude that polycystin-1 is in a complex containing E-cadherin and alpha-, beta-, and gamma-catenin. These observations raise the question of whether the defects in cell proliferation and cell polarity observed in ADPKD are mediated by E-cadherin or the catenins.

Expression of the beta2-subunit and apical localization of Na+-K+-ATPase in metanephric kidney

During kidney organogenesis, the Na+-K+-ATPase pump is not restricted to the basolateral plasma membrane of the renal epithelial cell but is instead either localized to the apical and lateral membrane sites of the early nephron or expressed in a nonpolarized distribution in the newly formed collecting ducts. The importance of Na+-K+-ATPase beta-subunit expression in the translocation of the Na+-K+-ATPase to the plasma membrane raises the question as to which beta-subunit isoform is expressed during kidney organogenesis. Immunocytochemical, Western analysis and RNase protection studies showed that both beta2-subunit protein and beta2 mRNA are expressed in the early gestation to midgestation human metanephric kidney. In contrast, although beta1 mRNA abundance is equivalent to that of the beta2-subunit in the metanephric kidney, the beta1-subunit protein was not detected in early to midgestation metanephric kidney samples. Immunocytochemical analysis revealed that both alpha1- and beta2-subunits were present in the apical epithelial plasma membranes of distal nephron segments of early stage nephrons, maturing loops of Henle, and collecting ducts during kidney development. We also detected a significant increase in alpha1 and beta1 mRNA after birth with a marked reduction in beta2 mRNA abundance associated with an increase in alpha1- and beta1-subunit proteins and loss of beta2 protein expression. These studies support the conclusion that the expression of the beta2-subunit in the fetal kidney may be an important mechanism controlling polarization of the Na+-K+-ATPase pump in the epithelia of the developing nephron during kidney organogenesis.

Transformations between epithelium and mesenchyme: normal, pathological, and experimentally induced

In this review, we define the two major tissue types, epithelium and mesenchyme, and we describe the transformations (transdifferentiations) of epithelium to mesenchyme (EMT) and mesenchyme to epithelium (MET) that occur during embryonic development. The differentiation of the metanephric blastema provides a striking example of MET. Differentiation of metanephric epithelium is promoted by matrix molecules and receptors (nidogen, laminins, alpha 6 integrins), hepatic growth factor/scatter factor, and products of the genes wnt-1, wnt-4, and Pax-2. Transformation of MDCK epithelium to mesenchyme-like cells is promoted in vitro by antibodies to E-cadherin, products of v-src, v-ras, and v-mos, and by manipulation of the epithelium on collagen gels. Suspension in collagen gel, transforming growth factors, and c-fos have also been shown to promote EMT in epithelia. We present studies from our laboratory showing that alpha 5 beta 1 integrin has a role in the EMT of lens epithelium that is brought about by suspension in collagen gel. Our laboratory has also shown that transfection with the E-cadherin gene induces embryonic corneal fibroblasts to undergo MET and that this MET is enhanced by interaction of the differentiating epithelium with living fibroblasts. This review calls attention to the roles that EMT and MET might have in kidney pathologies and urges further study of the involvement of these phenomena in renal development, renal injury, and renal malignancy.

Epidermal growth factor receptor expression is abnormal in murine polycystic kidney

Renal tubular cyst formation and progressive enlargement in autosomal recessive polycystic kidney disease (ARPKD) are mediated by increased epithelial cell proliferation and altered transtubular fluid transport. Epidermal growth factor (EGF)-like peptides have been proposed to play roles in normal nephrogenesis and cystic tubular mitogenesis. Therefore, renal expression of EGF receptor (EGFR) protein and mRNA was examined in an animal model for ARPKD, the C57BL/6Jcpk/cpk (CPK) mouse. Both quantitative and qualitative abnormalities of EGFR expression were demonstrated. While both control and cystic proximal tubules, as well as control collecting tubules, demonstrated exclusive basalateral EGFR protein expression, cystic collecting tubules exhibited significant apical-lateral receptor localization. During nephrogenesis, EGFR protein expression was elevated in CPK renal tissue when compared to developmentally staged controls. Control and CPK kidneys expressed the same species of EGFR mRNA. Levels increased with developmental age, but were significantly higher at each stage of development in CPK kidneys. Overexpression of both EGFR protein and mRNA in CPK mice suggests altered control of EGFR protein and/or gene expression. EGFR mislocalization and overexpression may be mechanisms whereby the EGF-like factors in cyst fluid stimulate cystogenesis through an autocrine-paracrine cycle in ARPKD.