Apical plasma membrane mispolarization of NaK-ATPase in polycystic kidney disease epithelia is associated with aberrant expression of the beta2 isoform

Sensational site: the sodium pump ouabain-binding site and its ligands

Cardiotonic steroids (CTS), used by certain insects, toads, and rats for protection from predators, became, thanks to Withering's trailblazing 1785 monograph, the mainstay of heart failure (HF) therapy. In the 1950s and 1960s, we learned that the CTS receptor was part of the sodium pump (NKA) and that the Na+/Ca2+ exchanger was critical for the acute cardiotonic effect of digoxin- and ouabain-related CTS. This "settled" view was upended by seven revolutionary observations. First, subnanomolar ouabain sometimes stimulates NKA while higher concentrations are invariably inhibitory. Second, endogenous ouabain (EO) was discovered in the human circulation. Third, in the DIG clinical trial, digoxin only marginally improved outcomes in patients with HF. Fourth, cloning of NKA in 1985 revealed multiple NKA α and β subunit isoforms that, in the rodent, differ in their sensitivities to CTS. Fifth, the NKA is a cation pump and a hormone receptor/signal transducer. EO binding to NKA activates, in a ligand- and cell-specific manner, several protein kinase and Ca2+-dependent signaling cascades that have widespread physiological effects and can contribute to hypertension and HF pathogenesis. Sixth, all CTS are not equivalent, e.g., ouabain induces hypertension in rodents while digoxin is antihypertensinogenic ("biased signaling"). Seventh, most common rodent hypertension models require a highly ouabain-sensitive α2 NKA and the elevated blood pressure is alleviated by EO immunoneutralization. These numerous phenomena are enabled by NKA's intricate structure. We have just begun to understand the endocrine role of the endogenous ligands and the broad impact of the ouabain-binding site on physiology and pathophysiology.

Existence and importance of Na+K+-ATPase in the plasma membrane of boar spermatozoa

Sodium-potassium-ATPase (Na+K+-ATPase), a target to treat congestive heart failure, is the only known receptor for cardiac glycosides implicated in intracellular signaling and additionally functions enzymatically in ion transport. Spermatozoa need transmembrane ion transport and signaling to fertilize, and Na+K+-ATPase is identified here for the first time in boar spermatozoa. Head plasma membrane (HPM) isolated from boar spermatozoa was confirmed pure by marker enzymes acid and alkaline phosphatase (218 ± 23% and 245 ± 38% enrichment, respectively, versus whole spermatozoa). Western immunoblotting detected α and β subunits (isoforms α1, α3, β1, β2, and β3) in different concentrations in whole spermatozoa and HPM. Immunofluorescence of intact sperm only detected α3 on the post-equatorial exterior membrane; methanol-permeabilized sperm also had α3 post-equatorially and other isoforms on the acrosomal ridge and cap. Mass spectrometry confirmed the presence of all isoforms in HPM. Incubating boar sperm in capacitating media to induce the physiological changes preceding fertilization significantly increased the percentage of capacitated sperm compared to 0 h control (33.0 ± 2.6% vs. 19.2 ± 2.6% capacitated sperm, respectively; p = 0.014) and altered the β2 immunofluorescence pattern. These results demonstrate the presence of Na+K+-ATPase in boar sperm HPM and that it changes during capacitation.

The Cyst Epithelium in Polycystic Kidney Disease Patients Displays Normal Apical-Basolateral Cell Polarity

The main characteristic of polycystic kidney disease is the development of multiple fluid-filled renal cysts. The discovery of mislocalized sodium-potassium pump (Na,K-ATPase) in the apical membrane of cyst-lining epithelia alluded to reversal of polarity as a possible explanation for the fluid secretion. The topic of apical Na,K-ATPase in cysts remains controversial. We investigated the localization of the Na,K-ATPase and assessed the apical-basolateral polarization of cyst-lining epithelia by means of immunohistochemistry in kidney tissue from six polycystic kidney disease patients undergoing nephrectomy. The Na,K-ATPase α1 subunit was conventionally situated in the basolateral membrane of all immunoreactive cysts. Proteins of the Crumbs and partitioning defective (Par) complexes were localized to the apical membrane domain in cyst epithelial cells. The apical targeting protein Syntaxin-3 also immunolocalized to the apical domain of cyst-lining epithelial cells. Proteins of the basolateral Scribble complex immunolocalized to the basolateral domain of cysts. Thus, no deviations from the typical epithelial distribution of basic cell polarity proteins were observed in the cysts from the six patients. Furthermore, we confirmed that cysts can originate from virtually any tubular segment with preserved polarity. In conclusion, we find no evidence of a reversal in apical-basolateral polarity in cyst-lining epithelia in polycystic kidney disease.

Ouabain enhances renal cyst growth in a slowly progressive mouse model of autosomal dominant polycystic kidney disease

Renal cyst progression in autosomal dominant polycystic kidney disease (ADPKD) is highly dependent on agents circulating in blood. We have previously shown, using different in vitro models, that one of these agents is the hormone ouabain. By binding to Na+-K+-ATPase (NKA), ouabain triggers a cascade of signal transduction events that enhance ADPKD cyst progression by stimulating cell proliferation, fluid secretion, and dedifferentiation of the renal tubular epithelial cells. Here, we determined the effects of ouabain in vivo. We show that daily administration of ouabain to Pkd1RC/RC ADPKD mice for 1-5 mo, at physiological levels, augmented kidney cyst area and number compared with saline-injected controls. Also, ouabain favored renal fibrosis; however, renal function was not significantly altered as determined by blood urea nitrogen levels. Ouabain did not have a sex preferential effect, with male and female mice being affected equally. By contrast, ouabain had no significant effect on wild-type mice. In addition, the actions of ouabain on Pkd1RC/RC mice were exacerbated when another mutation that increased the affinity of NKA for ouabain was introduced to the mice (Pkd1RC/RCNKAα1OS/OS mice). Altogether, this work highlights the role of ouabain as a procystogenic factor in the development of ADPKD in vivo, that the ouabain affinity site on NKA is critical for this effect, and that circulating ouabain is an epigenetic factor that worsens the ADPKD phenotype.NEW & NOTEWORTHY This work shows that the hormone ouabain enhances the progression of autosomal dominant polycystic kidney disease (ADPKD) in vivo. Ouabain augments the size and number of renal cysts, the kidney weight to body weight ratio, and kidney fibrosis in an ADPKD mouse model. The Na+-K+-ATPase affinity for ouabain plays a critical role in these effects. In addition, these outcomes are independent of the sex of the mice.

Polarized transport of membrane and secreted proteins during lumen morphogenesis

A ubiquitous feature of animal development is the formation of fluid-filled cavities or lumina, which transport gases and fluids across tissues and organs. Among different species, lumina vary drastically in size, scale, and complexity. However, all lumen formation processes share key morphogenetic principles that underly their development. Fundamentally, a lumen simply consists of epithelial cells that encapsulate a continuous internal space, and a common way of building a lumen is via opening and enlarging by filling it with fluid and/or macromolecules. Here, we discuss how polarized targeting of membrane and secreted proteins regulates lumen formation, mainly focusing on ion transporters in vertebrate model systems. We also discuss mechanistic differences observed among invertebrates and vertebrates and describe how the unique properties of the Na+/K+-ATPase and junctional proteins can promote polarization of immature epithelia to build lumina de novo in developing organs.

In vitro 3D phenotypic drug screen identifies celastrol as an effective in vivo inhibitor of polycystic kidney disease

Polycystic kidney disease (PKD) is a prevalent genetic disorder, characterized by the formation of kidney cysts that progressively lead to kidney failure. The currently available drug tolvaptan is not well tolerated by all patients and there remains a strong need for alternative treatments. The signaling rewiring in PKD that drives cyst formation is highly complex and not fully understood. As a consequence, the effects of drugs are sometimes difficult to predict. We previously established a high throughput microscopy phenotypic screening method for quantitative assessment of renal cyst growth. Here, we applied this 3D cyst growth phenotypic assay and screened 2320 small drug-like molecules, including approved drugs. We identified 81 active molecules that inhibit cyst growth. Multi-parametric phenotypic profiling of the effects on 3D cultured cysts discriminated molecules that showed preferred pharmacological effects above genuine toxicological properties. Celastrol, a triterpenoid from Tripterygium Wilfordii, was identified as a potent inhibitor of cyst growth in vitro. In an in vivo iKspCre-Pkd1^lox,lox mouse model for PKD, celastrol inhibited the growth of renal cysts and maintained kidney function.

Maturation of the Na,K-ATPase in the Endoplasmic Reticulum in Health and Disease

Abstract

The Na,K-ATPase establishes the electrochemical gradient of cells by driving an active exchange of Na⁺ and K⁺ ions while consuming ATP. The minimal functional transporter consists of a catalytic α-subunit and a β-subunit with chaperon activity. The Na,K-ATPase also functions as a cell adhesion molecule and participates in various intracellular signaling pathways. The maturation and trafficking of the Na,K-ATPase include co- and post-translational processing of the enzyme in the endoplasmic reticulum (ER) and the Golgi apparatus and subsequent delivery to the plasma membrane (PM). The ER folding of the enzyme is considered as the rate-limiting step in the membrane delivery of the protein. It has been demonstrated that only assembled Na,K-ATPase α:β-complexes may exit the organelle, whereas unassembled, misfolded or unfolded subunits are retained in the ER and are subsequently degraded. Loss of function of the Na,K-ATPase has been associated with lung, heart, kidney and neurological disorders. Recently, it has been shown that ER dysfunction, in particular, alterations in the homeostasis of the organelle, as well as impaired ER-resident chaperone activity may impede folding of Na,K-ATPase subunits, thus decreasing the abundance and function of the enzyme at the PM. Here, we summarize our current understanding on maturation and subsequent processing of the Na,K-ATPase in the ER under physiological and pathophysiological conditions.

Graphic Abstract

The Apical Localization of Na+, K+-ATPase in Cultured Human Retinal Pigment Epithelial Cells Depends on Expression of the β2 Subunit

Na⁺, K⁺-ATPase, or the Na⁺ pump, is a key component in the maintenance of the epithelial phenotype. In most epithelia, the pump is located in the basolateral domain. Studies from our laboratory have shown that the β₁ subunit of Na⁺, K⁺-ATPase plays an important role in this mechanism because homotypic β₁-β₁ interactions between neighboring cells stabilize the pump in the lateral membrane. However, in the retinal pigment epithelium (RPE), the Na⁺ pump is located in the apical domain. The mechanism of polarization in this epithelium is unclear. We hypothesized that the apical polarization of the pump in RPE cells depends on the expression of its β₂ subunit. ARPE-19 cells cultured for up to 8 weeks on inserts did not polarize, and Na⁺, K⁺-ATPase was expressed in the basolateral membrane. In the presence of insulin, transferrin and selenic acid (ITS), ARPE-19 cells cultured for 4 weeks acquired an RPE phenotype, and the Na⁺ pump was visible in the apical domain. Under these conditions, Western blot analysis was employed to detect the β₂ isoform and immunofluorescence analysis revealed an apparent apical distribution of the β₂ subunit. qPCR results showed a time-dependent increase in the level of β₂ isoform mRNA, suggesting regulation at the transcriptional level. Moreover, silencing the expression of the β₂ isoform in ARPE-19 cells resulted in a decrease in the apical localization of the pump, as assessed by the mislocalization of the α₂ subunit in that domain. Our results demonstrate that the apical polarization of Na⁺, K⁺-ATPase in RPE cells depends on the expression of the β₂ subunit.

Activation of the intrarenal renin-angiotensin-system in murine polycystic kidney disease

The mechanism for early hypertension in polycystic kidney disease (PKD) has not been elucidated. One potential pathway that may contribute to the elevation in blood pressure in PKD is the activation of the intrarenal renin-angiotensin-system (RAS). For example, it has been shown that kidney cyst and cystic fluid contains renin, angiotensin II (AngII), and angiotensinogen (Agt). Numerous studies suggest that ciliary dysfunction plays an important role in PKD pathogenesis. However, it is unknown whether the primary cilium affects the intrarenal RAS in PKD. The purpose of this study was to determine whether loss of cilia or polycystin 1 (PC1) increases intrarenal RAS in mouse model of PKD. Adult Ift88 and Pkd1 conditional floxed allele mice with or without cre were administered tamoxifen to induce global knockout of the gene. Three months after tamoxifen injection, kidney tissues were examined by histology, immunofluorescence, western blot, and mRNA to assess intrarenal RAS components. SV40 immortalized collecting duct cell lines from hypomorphic Ift88 mouse were used to assess intrarenal RAS components in collecting duct cells. Mice without cilia and PC1 demonstrated increased kidney cyst formation, systolic blood pressure, prorenin, and kidney and urinary angiotensinogen levels. Interestingly immunofluorescence study of the kidney revealed that the prorenin receptor was localized to the basolateral membrane of principal cells in cilia (−) but not in cilia (+) kidneys. Collecting duct cAMP responses to AngII administration was greater in cilia (−) vs. cilia (+) cells indicating enhanced intrarenal RAS activity in the absence of cilia. These data suggest that in the absence of cilia or PC1, there is an upregulation of intrarenal RAS components and activity, which may contribute to elevated blood pressure in PKD.

Cystic gene dosage influences kidney lesions after nephron reduction

Cystic kidney disease is characterized by the progressive development of multiple fluid-filled cysts. Cysts can be acquired, or they may appear during development or in postnatal life due to specific gene defects and lead to renal failure. The most frequent form of this disease is the inherited polycystic kidney disease (PKD). Experimental models of PKD showed that an increase of cellular proliferation and apoptosis as well as defects in apico-basal and planar cell polarity or cilia play a critical role in cyst development. However, little is known about the mechanisms and the mediators involved in acquired cystic kidney diseases (ACKD). In this study, we used the nephron reduction as a model to study the mechanisms underlying cyst development in ACKD. We found that tubular dilations after nephron reduction recapitulated most of the morphological features of ACKD. The development of tubular dilations was associated with a dramatic increase of cell proliferation. In contrast, the apico-basal polarity and cilia did not seem to be affected. Interestingly, polycystin 1 and fibrocystin were markedly increased and polycystin 2 was decreased in cells lining the dilated tubules, whereas the expression of several other cystic genes did not change. More importantly, Pkd1 haploinsufficiency accelerated the development of tubular dilations after nephron reduction, a phenotype that was associated to a further increase of cell proliferation. These data were relevant to humans ACKD, as cystic genes expression and the rate of cell proliferation were also increased. In conclusion, our study suggests that the nephron reduction can be considered a suitable model to study ACKD and that dosage of genes involved in PKD is also important in ACKD.

Epidermal growth factor-induced proliferation of collecting duct cells from Oak Ridge polycystic kidney mice involves activation of Na+/H+ exchanger

Epidermal growth factor (EGF) is linked to the pathogenesis of polycystic kidney disease (PKD). We explored signaling pathways activated by EGF in orpk cilia (-) collecting duct cell line derived from a mouse model of PKD (hypomorph of the Tg737/Ift88 gene) with severely stunted cilia, and in a control orpk cilia (+) cell line with normal cilia. RT-PCR demonstrated mRNAs for EGF receptor subunits ErbB1, ErbB2, ErbB3, ErbB4, and mRNAs for Na(+)/H(+) exchangers (NHE), NHE-1, NHE-2, NHE-3, NHE-4, and NHE-5 in both cell lines. EGF stimulated proton efflux in both cell lines. This effect was significantly attenuated by MIA, 5-(n-methyl-N-isobutyl) amiloride, a selective inhibitor of NHE-1 and NHE-2, and orpk cilia (-) cells were more sensitive to MIA than control cells (P < 0.01). EGF significantly induced extracellular signal-regulated kinase (ERK) phosphorylation in both cilia (+) and cilia (-) cells (63.3 and 123.6%, respectively), but the effect was more pronounced in orpk cilia (-) cells (P < 0.01). MIA significantly attenuated EGF-induced ERK phosphorylation only in orpk cilia (-) cells (P < 0.01). EGF increased proliferation of orpk cilia (+) cells and orpk cilia (-) cells, respectively, and MIA at 1-5 μM attenuated EGF-induced proliferation in orpk cilia (-) cells without affecting proliferation of orpk cilia (+) cells. EGF-induced proliferation of both cell lines was significantly decreased by the EGFR tyrosine kinase inhibitor AG1478 and MEK inhibitor PD98059. These results suggest that EGF exerts mitogenic effects in the orpk cilia (-) cells via activation of growth-associated amiloride-sensitive NHEs and ERK.

Trafficking to the apical and basolateral membranes in polarized epithelial cells

Renal epithelial cells must maintain distinct protein compositions in their apical and basolateral membranes in order to perform their transport functions. The creation of these polarized protein distributions depends on sorting signals that designate the trafficking route and site of ultimate functional residence for each protein. Segregation of newly synthesized apical and basolateral proteins into distinct carrier vesicles can occur at the trans-Golgi network, recycling endosomes, or a growing assortment of stations along the cellular trafficking pathway. The nature of the specific sorting signal and the mechanism through which it is interpreted can influence the route a protein takes through the cell. Cell type-specific variations in the targeting motifs of a protein, as are evident for Na,K-ATPase, demonstrate a remarkable capacity to adapt sorting pathways to different developmental states or physiologic requirements. This review summarizes our current understanding of apical and basolateral trafficking routes in polarized epithelial cells.

Urine microRNA as potential biomarkers of autosomal dominant polycystic kidney disease progression: description of miRNA profiles at baseline

BACKGROUND

Autosomal dominant polycystic kidney disease (ADPKD) is clinically heterogenic. Biomarkers are needed to predict prognosis and guide management. We aimed to profile microRNA (miRNA) in ADPKD to gain molecular insight and evaluate biomarker potential.

METHODS

Small-RNA libraries were generated from urine specimens of ADPKD patients (N = 20) and patients with chronic kidney disease of other etiologies (CKD, N = 20). In this report, we describe the miRNA profiles and baseline characteristics. For reference, we also examined the miRNA transcriptome in primary cultures of ADPKD cyst epithelia (N = 10), normal adult tubule (N = 8) and fetal tubule (N = 7) epithelia.

RESULTS

In primary cultures of ADPKD kidney cells, miRNA cistrons mir-143(2) (9.2-fold), let-7i(1) (2.3-fold) and mir-3619(1) (12.1-fold) were significantly elevated compared to normal tubule epithelia, whereas mir-1(4) members (19.7-fold), mir-133b(2) (21.1-fold) and mir-205(1) (3.0-fold) were downregulated (P<0.01). Expression of the dysregulated miRNA in fetal tubule epithelia resembled ADPKD better than normal adult cells, except let-7i, which was lower in fetal cells. In patient biofluid specimens, mir-143(2) members were 2.9-fold higher in urine cells from ADPKD compared to other CKD patients, while expression levels of mir-133b(2) (4.9-fold) and mir-1(4) (4.4-fold) were lower in ADPKD. We also noted increased abundance mir-223(1) (5.6-fold), mir-199a(3) (1.4-fold) and mir-199b(1) (1.8-fold) (P<0.01) in ADPKD urine cells. In ADPKD urine microvesicles, miR-1(2) (7.2-fold) and miR-133a(2) (11.8-fold) were less abundant compared to other CKD patients (P<0.01).

CONCLUSIONS

We found that in ADPKD urine specimens, miRNA previously implicated as kidney tumor suppressors (miR-1 and miR-133), as well as miRNA of presumed inflammatory and fibroblast cell origin (miR-223/miR-199), are dysregulated when compared to other CKD patients. Concordant with findings in the primary tubule epithelial cell model, this suggests roles for dysregulated miRNA in ADPKD pathogenesis and potential use as biomarkers. We intend to assess prognostic potential of miRNA in a followup analysis.

Novel role of ouabain as a cystogenic factor in autosomal dominant polycystic kidney disease

The classic role of the Na-K-ATPase is that of a primary active transporter that utilizes cell energy to establish and maintain transmembrane Na(+) and K(+) gradients to preserve cell osmotic stability, support cell excitability, and drive secondary active transport. Recent studies have revealed that Na-K-ATPase located within cholesterol-containing lipid rafts serves as a receptor for cardiotonic steroids, including ouabain. Traditionally, ouabain was viewed as a toxin produced only in plants, and it was used in relatively high concentrations to experimentally block the pumping action of the Na-K-ATPase. However, the new and unexpected role of the Na-K-ATPase as a signal transducer revealed a novel facet for ouabain in the regulation of a myriad of cell functions, including cell proliferation, hypertrophy, apoptosis, mobility, and metabolism. The seminal discovery that ouabain is endogenously produced in mammals and circulates in plasma has fueled the interest in this endogenous molecule as a potentially important hormone in normal physiology and disease. In this article, we review the role of the Na-K-ATPase as an ion transporter in the kidney, the experimental evidence for ouabain as a circulating hormone, the function of the Na-K-ATPase as a signal transducer that mediates ouabain's effects, and novel results for ouabain-induced Na-K-ATPase signaling in cystogenesis of autosomal dominant polycystic kidney disease.

Synchronization modulation increases transepithelial potentials in MDCK monolayers through Na/K pumps

Transepithelial potential (TEP) is the voltage across a polarized epithelium. In epithelia that have active transport functions, the force for transmembrane flux of an ion is dictated by the electrochemical gradient in which TEP plays an essential role. In epithelial injury, disruption of the epithelial barrier collapses the TEP at the wound edge, resulting in the establishment of an endogenous wound electric field (∼100 mV/mm) that is directed towards the center of the wound. This endogenous electric field is implicated to enhance wound healing by guiding cell migration. We thus seek techniques to enhance the TEP, which may increase the wound electric fields and enhance wound healing. We report a novel technique, termed synchronization modulation (SM) using a train of electric pulses to synchronize the Na/K pump activity, and then modulating the pumping cycles to increase the efficiency of the Na/K pumps. Kidney epithelial monolayers (MDCK cells) maintain a stable TEP and transepithelial resistance (TER). SM significantly increased TEP over four fold. Either ouabain or digoxin, which block Na/K pump, abolished SM-induced TEP increases. In addition to the pump activity, basolateral distribution of Na/K pumps is essential for an increase in TEP. Our study for the first time developed an electrical approach to significantly increase the TEP. This technique targeting the Na/K pump may be used to modulate TEP, and may have implication in wound healing and in diseases where TEP needs to be modulated.

Multiple roles for the Na,K-ATPase subunits, Atp1a1 and Fxyd1, during brain ventricle development

Formation of the vertebrate brain ventricles requires both production of cerebrospinal fluid (CSF), and its retention in the ventricles. The Na,K-ATPase is required for brain ventricle development, and we show here that this protein complex impacts three associated processes. The first requires both the alpha subunit (Atp1a1) and the regulatory subunit, Fxyd1, and leads to formation of a cohesive neuroepithelium, with continuous apical junctions. The second process leads to modulation of neuroepithelial permeability, and requires Atp1a1, which increases permeability with partial loss of function and decreases it with overexpression. In contrast, fxyd1 overexpression does not alter neuroepithelial permeability, suggesting that its activity is limited to neuroepithelium formation. RhoA regulates both neuroepithelium formation and permeability, downstream of the Na,K-ATPase. A third process, likely to be CSF production, is RhoA-independent, requiring Atp1a1, but not Fxyd1. Consistent with a role for Na,K-ATPase pump function, the inhibitor ouabain prevents neuroepithelium formation, while intracellular Na(+) increases after Atp1a1 and Fxyd1 loss of function. These data include the first reported role for Fxyd1 in the developing brain, and indicate that the Na,K-ATPase regulates three aspects of brain ventricle development essential for normal function: formation of a cohesive neuroepithelium, restriction of neuroepithelial permeability, and production of CSF.

Regulation of transport in the connecting tubule and cortical collecting duct

The central goal of this overview article is to summarize recent findings in renal epithelial transport,focusing chiefly on the connecting tubule (CNT) and the cortical collecting duct (CCD).Mammalian CCD and CNT are involved in fine-tuning of electrolyte and fluid balance through reabsorption and secretion. Specific transporters and channels mediate vectorial movements of water and solutes in these segments. Although only a small percent of the glomerular filtrate reaches the CNT and CCD, these segments are critical for water and electrolyte homeostasis since several hormones, for example, aldosterone and arginine vasopressin, exert their main effects in these nephron sites. Importantly, hormones regulate the function of the entire nephron and kidney by affecting channels and transporters in the CNT and CCD. Knowledge about the physiological and pathophysiological regulation of transport in the CNT and CCD and particular roles of specific channels/transporters has increased tremendously over the last two decades.Recent studies shed new light on several key questions concerning the regulation of renal transport.Precise distribution patterns of transport proteins in the CCD and CNT will be reviewed, and their physiological roles and mechanisms mediating ion transport in these segments will also be covered. Special emphasis will be given to pathophysiological conditions appearing as a result of abnormalities in renal transport in the CNT and CCD.

Apicobasal polarity in the kidney

The apicobasal polarization of epithelia is critical for many aspects of kidney function. Over the last decade there have been major advances in our understanding of the mechanisms that underlie this polarity. Critical to this understanding has been the identification of protein complexes on the apical and basolateral sides of epithelial cells that act in a mutually antagonistic manner to define these domains. Concomitant with the creation of apical and basolateral domains is the formation of highly specialized cell-cell junctions including adherens junctions and tight junctions. Recent research points to variability in the polarity and junctional complexes amongst different species and between different cell types of the kidney. Defects in apicobasal polarity are prominent in several disorders including acute renal failure and polycystic kidney disease.

Shear stress reverses dome formation in confluent renal tubular cells

BACKGROUND/AIMS

It has been shown that MDCK cells, a cell line derived from canine renal tubules, develop cell domes due to fluid pumped under cell monolayer and focal detachment from the adhesion surface. In vitro studies have shown that primary cilia of kidney tubular epithelial cells act as mechanosensors, increasing intracellular calcium within seconds upon changes in fluid shear stress (SS) on cell membrane. We then studied the effect of prolonged SS exposure on cell dome formation in confluent MDCK cell monolayers.

METHODS

A parallel plate flow chamber was used to apply laminar SS at 2 dynes/cm(2) to confluent cell monolayers for 6 hours. Control MDCK cell monolayers were maintained in static condition. The effects of Ca(2+) blockade and cell deciliation on SS exposure were also investigated.

RESULTS

Seven days after reaching confluence, static cultures developed liquid filled domes, elevating from culture plate. Exposure to SS induced almost complete disappearance of cell domes (0.4±0.8 vs. 11.4±2.8 domes/mm(2), p < 0.01, n=14). SS induced dome disappearance took place within minutes to hours, as shown by time-lapse videomicroscopy. Exposure to SS importantly affected cell cytoskeleton altering actin stress fibers expression and organization, and the distribution of tight junction protein ZO-1. Dome disappearance induced by flow was completely prevented in the presence of EGTA or after cell deciliation.

CONCLUSIONS

These data indicate that kidney tubular cells are sensitive to apical flow and that these effects are mediated by primary cilia by regulation of Ca(2+) entry in to the cell. SS induced Ca(2+) entry provokes contraction of cortical actin ring that tenses cell-cell borders and decreases basal stress fibers. These processes may increase paracellular permeability and decrease basal adhesion making dome disappear. Elucidation of the effects of apical fluid flow on tubular cell function may open new insights on the pathophysiology of kidney diseases associated with cilia dysfunction.

The Na+-K+-ATPase as self-adhesion molecule and hormone receptor

Thanks to the homeostasis of the internal milieu, metazoan cells can enormously simplify their housekeeping efforts and engage instead in differentiation and multiple forms of organization (tissues, organs, systems) that enable them to produce an astonishing diversity of mammals. The stability of the internal milieu despite drastic variations of the external environment (air, fresh or seawater, gastrointestinal fluids, glomerular filtrate, bile) is due to transporting epithelia that can adjust their specific permeability to H(2)O, H(+), Na(+), K(+), Ca(2+), and Cl(-) over several orders of magnitude and exchange substances with the outer milieu with exquisite precision. This exchange is due to the polarized expression of membrane proteins, among them Na(+)-K(+)-ATPase, an oligomeric enzyme that uses chemical energy from ATP molecules to translocate ions across the plasma membrane of epithelial cells. Na(+)-K(+)-ATPase presents two types of asymmetries: the arrangement of its subunits, and its expression in one pole of the epithelial cell ("polarity"). In most epithelia, polarity consists of the expression of Na(+)-K(+)-ATPase towards the intercellular space and arises in part from the interaction of the extracellular segment of the β-subunit with another β-subunit present in a Na(+)-K(+)-ATPase molecule expressed by a neighboring cell. In addition to enabling the Na(+)-K(+)-ATPase to transport ions and water vectorially, this position exposes its receptors to ouabain and analogous cardiotonic steroids, which are present in the internal milieu because these were secreted by endocrine cells.

Fluid transport and cystogenesis in autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is the most frequent inherited nephropathy. The development and enlargement of cysts in ADPKD requires tubular cell proliferation, abnormalities in the extracellular matrix and transepithelial fluid secretion. Multiple studies have suggested that fluid secretion across ADPKD cyst-lining cells is driven by the transepithelial secretion of chloride, mediated by the apical CFTR channel and specific basolateral transporters. The whole secretory process is stimulated by increased levels of cAMP in the cells, probably reflecting modifications in the intracellular calcium homeostasis and abnormal stimulation of the vasopressin V2 receptor. This review will focus on the pathophysiology of fluid secretion in ADPKD cysts, starting with classic, morphological and physiological studies that were followed by investigations of the molecular mechanisms involved and therapeutic trials targeting these pathways in cellular and animal models and ADPKD patients. This article is part of a Special Issue entitled: Polycystic Kidney Disease.

Apoptosis in polycystic kidney disease

Apoptosis is the process of programmed cell death. It is a ubiquitous, controlled process consuming cellular energy and designed to avoid cytokine release despite activation of local immune cells, which clear the cell fragments. The process occurs during organ development and in maintenance of homeostasis. Abnormalities in any step of the apoptotic process are associated with autoimmune diseases and malignancies. Polycystic kidney disease (PKD) is the most common inherited kidney disease leading to end-stage renal disease (ESRD). Cyst formation requires multiple mechanisms and apoptosis is considered one of them. Abnormalities in apoptotic processes have been described in various murine and rodent models of PKD as well as in human PKD kidneys. The purpose of this review is to outline the role of apoptosis in progression of PKD as well as to describe the mechanisms involved. This article is part of a Special Issue entitled: Polycystic Kidney Disease.

Na(+)/K (+)-ATPase β1 subunit interacts with M2 proteins of influenza A and B viruses and affects the virus replication

Interplay between the host and influenza virus has a pivotal role for the outcome of infection. The matrix proteins M2/BM2 from influenza (A and B) viruses are small type III integral membrane proteins with a single transmembrane domain, a short amino-terminal ectodomain and a long carboxy-terminal cytoplasmic domain. They function as proton channels, mainly forming a membrane-spanning pore through the transmembrane domain tetramer, and are essential for virus assembly and release of the viral genetic materials in the endosomal fusion process. However, little is known about the host factors which interact with M2/BM2 proteins and the functions of the long cytoplasmic domain are currently unknown. Starting with yeast two-hybrid screening and applying a series of experiments we identified that the β1 subunit of the host Na(+)/K(+)-ATPase β1 subunit (ATP1B1) interacts with the cytoplasmic domain of both the M2 and BM2 proteins. A stable ATP1B1 knockdown MDCK cell line was established and we showed that the ATP1B1 knockdown suppressed influenza virus A/WSN/33 replication, implying that the interaction is crucial for influenza virus replication in the host cell. We propose that influenza virus M2/BM2 cytoplasmic domain has an important role in the virus-host interplay and facilitates virus replication.

The polarized distribution of Na+,K+-ATPase: role of the interaction between {beta} subunits

Na⁺,K⁺-ATPase polarity depends on the interaction between the β subunits of Na⁺,K⁺-ATPases located on neighboring cells. In the present work, we use energy transfer methods (FRET), in vivo to demonstrate that these β subunits interact directly at the intercellular space of epithelial cells.

The very existence of higher metazoans depends on the vectorial transport of substances across epithelia. A crucial element of this transport is the membrane enzyme Na⁺,K⁺-ATPase. Not only is this enzyme distributed in a polarized manner in a restricted domain of the plasma membrane but also it creates the ionic gradients that drive the net movement of glucose, amino acids, and ions across the entire epithelium. In a previous work, we have shown that Na⁺,K⁺-ATPase polarity depends on interactions between the β subunits of Na⁺,K⁺-ATPases located on neighboring cells and that these interactions anchor the entire enzyme at the borders of the intercellular space. In the present study, we used fluorescence resonance energy transfer and coprecipitation methods to demonstrate that these β subunits have sufficient proximity and affinity to permit a direct interaction, without requiring any additional extracellular molecules to span the distance.

Cystic diseases of the kidney: molecular biology and genetics

CONTEXT

Cystic diseases of the kidney are a very heterogeneous group of renal inherited conditions, with more than 33 genes involved and encompassing X-linked, autosomal dominant, and autosomal recessive inheritance. Although mostly monogenic with mendelian inheritance, there are clearly examples of oligogenic inheritance, such as 3 mutations in 2 genes, while the existence of genetic modifiers is perhaps the norm, based on the extent of variable expressivity and the broad spectrum of symptoms.

OBJECTIVES

To present in the form of a mini review the major known cystic diseases of the kidney for which genes have been mapped or cloned and characterized, with some information on their cellular and molecular biology and genetics, and to pay special attention to commenting on the issues of molecular diagnostics, in view of the genetic and allelic heterogeneity. Data Sources.-We used major reviews that make excellent detailed presentation of the various diseases, as well as original publications.

CONCLUSIONS

There is already extensive genetic heterogeneity in the group of cystic diseases of the kidney; however, there are still many more genes awaiting to be discovered that are implicated or mutated in these diseases. In addition, the synergism and interaction among this repertoire of gene products is largely unknown, while a common unifying aspect is the expression of nearly all of them at the primary cilium or the basal body. A major interplay of functions is anticipated, while mutations in all converge in the unifying phenotype of cyst formation.

Unexpected roles of the Na-K-ATPase and other ion transporters in cell junctions and tubulogenesis

Recent work shows that transport-independent as well as transport-dependent functions of ion transporters, and in particular the Na-K-ATPase, are required for formation and maintenance of several intercellular junctions. Furthermore, these junctional and other nonjunctional functions of ion transporters contribute to development of epithelial tubes. Here, we consider what has been learned about the roles of ion pumps in formation of junctions and epithelial tubes in mammals, zebrafish, Drosophila, and C. elegans. We propose that asymmetric association of the Na-K-ATPase with cell junctions early in metazoan evolution enabled vectorial transcellular ion transport and control of intraorganismal environment. Ion transport-independent functions of the Na-K-ATPase arose as junctional complexes evolved.

Functional roles of Na,K-ATPase subunits

PURPOSE OF REVIEW

Na,K-ATPase is an oligomeric protein composed of alpha subunits, beta subunits and FXYD proteins. The catalytic alpha subunit hydrolyzes ATP and transports the cations. Increasing experimental evidence suggest that beta subunits and FXYD proteins essentially contribute to the variable physiological needs of Na,K-ATPase function in different tissues.

RECENT FINDINGS

Beta subunits have a crucial role in the structural and functional maturation of Na,K-ATPase and modulate its transport properties. The chaperone function of the beta subunit is essential, for example, in the formation of tight junctions and cell polarity. Recent studies suggest that beta subunits also have inherent functions, which are independent of Na,K-ATPase activity and which may be involved in cell-cell adhesiveness and in suppression of cell motility. As for FXYD proteins, they modulate Na,K-ATPase activity in a tissue-specific way, in some cases in close cooperation with posttranslational modifications such as phosphorylation.

SUMMARY

A better understanding of the multiple functional roles of the accessory subunits of Na,K-ATPase is crucial to appraise their influence on physiological processes and their implication in pathophysiological states.

Cystic disease of the kidney

This review focuses on the mechanisms that underlie the development of human renal cystic diseases. A pathological, clinical, and pathophysiological overview is given. Initial analysis of the cell biology of inappropriate hyperproliferation accompanied by fluid secretion of cyst-lining epithelia has been followed by the elucidation of fundamental defects in epithelial polarity, cell-matrix and cell-cell interactions, and apoptosis, all of which are discussed. Identification of the genes and proteins responsible for several renal cystic diseases has led to a more complete understanding of defects in renal developmental programming, differentiation, and morphogenesis, all of which underlie cystic diseases of the kidney.

Mechanoregulation of intracellular Ca2+ in human autosomal recessive polycystic kidney disease cyst-lining renal epithelial cells

Mutations of cilia-expressed proteins are associated with an attenuated shear-induced increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) in renal epithelial cell lines derived from murine models of autosomal recessive polycystic kidney disease (ARPKD). We hypothesized that human ARPKD cyst-lining renal epithelial cells also exhibited dysregulated mechanosensation. To test this, conditionally immortalized cell lines derived from human fetal ARPKD cyst-lining (pool and clone 5E) cell lines with low levels of fibrocystin/polyductin expression and age-matched normal collecting tubule [human fetal collecting tubule (HFCT) pool and clone 2C] cell lines were grown in culture, loaded with a Ca(2+) indicator dye, and subjected to laminar shear. Clonal cell lines were derived from single cells present in pools of cells from cyst-lining and collecting tubules, microdissected from human kidney. Resting and peak [Ca(2+)](i) were similar between ARPKD 5E and pool, and HFCT 2C and pool; however, the flow-induced peak [Ca(2+)](i) was greater in ARPKD 5E (700 +/- 87 nM, n = 21) than in HFCT 2C (315 +/- 58 nM, n = 12; P < 0.01) cells. ARPKD 5E cells treated with Gd(3+), an inhibitor of nonselective cation channels, inhibited but did not abolish the shear-induced [Ca(2+)](i) transient. Cilia were approximately 20% shorter in ARPKD than HFCT cells, but no difference in ciliary localization or total cellular expression of polycystin-2, a mechanosenory Gd(3+)-sensitive cation channel, was detected between ARPKD and HFCT cells. The intracellular Ca(2+) stores were similar between cells. In summary, human ARPKD cells exhibit an exaggerated Gd(3+)-sensitive mechano-induced Ca(2+) response compared with controls; whether this represents dysregulated polycystin-2 activity in ARPKD cells remains to be explored.

Mouse models of polycystic kidney disease

Polycystic kidney disease (PKD) is a diverse group of human monogenic lethal conditions inherited as autosomal dominant (AD) or recessive (AR) traits. Recent development of genetically engineered mouse models of ADPKD, ARPKD, and nephronophthisis/medullary cystic disease (NPHP) are providing additional insights into the molecular mechanisms governing of these disease processes as well as the developmental differentiation of the normal kidney. Genotypic and phenotypic mouse models are discussed and provide evidence for the fundamental involvement of cell-matrix, cell-cell, and primary cilia-lumen interactions, as well as epithelial proliferation, apoptosis, and polarization. Structure/function relationships between the PKD1, PKD2, PKHD1, and NPHP genes and proteins support the notion of a regulatory multiprotein cystic complex with a mechanosensory function that integrates signals from the extracellular environment. The plethora of intracellular signaling cascades that can impact renal cystic development suggest an exquisitely sensitive requirement for integrated downstream transduction and provide potential targets for therapeutic intervention. Appropriate genocopy models that faithfully recapitulate the phenotypic characteristics of the disease will be invaluable tools to analyze the effects of modifier genes and small molecule inhibitor therapies.

Polarized membrane distribution of potassium-dependent ion pumps in epithelial cells: different roles of the N-glycans of their beta subunits

The Na,K-ATPases and the H,K-ATPases are two potassium-dependent homologous heterodimeric P2-type pumps that catalyze active transport of Na+ in exchange for K+ (Na,K-ATPase) or H+ in exchange for K+ (H,K-ATPase). The ubiquitous Na,K-ATPase maintains intracellular ion balance and membrane potential. The gastric H,K-ATPase is responsible for acid secretion by the parietal cell of the stomach. Both pumps consist of a catalytic alpha-subunit and a glycosylated beta-subunit that is obligatory for normal pump maturation and trafficking. Individual N-glycans linked to the beta-subunits of the Na,K-ATPase and H,K-ATPase are important for stable membrane integration of their respective alpha subunits, folding, stability, subunit assembly, and enzymatic activity of the pumps. They are also essential for the quality control of unassembled beta-subunits that results in either the exit of the subunits from the ER or their ER retention and subsequent degradation. Overall, the importance of N-glycans for the maturation and quality control of the H,K-ATPase is greater than that of the Na,K-ATPase. The roles of individual N-glycans of the beta-subunits in the post-ER trafficking, membrane targeting and plasma membrane retention of the Na,K-ATPase and H,K-ATPase are different. The Na,K-ATPase beta1-subunit is the major beta-subunit isoform in cells with lateral location of the pump. All three N-glycans of the Na,K-ATPase beta1-subunit are important for the lateral membrane retention of the pump due to glycan-mediated interaction between the beta1-subunits of the two neighboring cells in the cell monolayer and cytosolic linkage of the alpha-subunit to the cytoskeleton. This intercellular beta1-beta1 interaction is also important for formation of cell-cell contacts. In contrast, the N-glycans unique to the Na,K-ATPase beta2-subunit,which has up to eight N-glycosylation sites, contain apical sorting information. This is consistent with the apical location of the Na,K-ATPase in normal and malignant epithelial cells with high abundance of the beta2-subunit. Similarly, all seven N-glycans of the gastric H,K-ATPase beta-subunit determine apical sorting of this subunit.

Zebrafish mutations affecting cilia motility share similar cystic phenotypes and suggest a mechanism of cyst formation that differs from pkd2 morphants

Zebrafish are an attractive model for studying the earliest cellular defects occurring during renal cyst formation because its kidney (the pronephros) is simple and genes that cause cystic kidney diseases (CKD) in humans, cause pronephric dilations in zebrafish. By comparing phenotypes in three different mutants, locke, swt and kurly, we find that dilations occur prior to 48 hpf in the medial tubules, a location similar to where cysts form in some mammalian diseases. We demonstrate that the first observable phenotypes associated with dilation include cilia motility and luminal remodeling defects. Importantly, we show that some phenotypes common to human CKD, such as an increased number of cells, are secondary consequences of dilation. Despite having differences in cilia motility, locke, swt and kurly share similar cystic phenotypes, suggesting that they function in a common pathway. To begin to understand the molecular mechanisms involved in cyst formation, we have cloned the swt mutation and find that it encodes a novel leucine rich repeat containing protein (LRRC50), which is thought to function in correct dynein assembly in cilia. Finally, we show that knock-down of polycystic kidney disease 2 (pkd2) specifically causes glomerular cysts and does not affect cilia motility, suggesting multiple mechanisms exist for cyst formation.

Characterization of a long-term rat mTAL cell line

A medullary thick ascending limb (mTAL) cell line, termed raTAL, has been established from freshly isolated rat mTAL tubules and cultured continuously for up to 75 passages; it retains characteristics of mTAL cells even after retrieval from storage in liquid nitrogen for several months. The cells express Tamm-Horsfall glycoprotein (THP), a TAL-specific marker, grow to confluence, exhibit a polygonal morphology characteristic of epithelial cells, and form “domes.” Detection of THP, Na+-K+-2Cl−cotransporter (NKCC2), Na+-K+-ATPase, and renal outer medullary K+channel (ROMK) was achieved using indirect immunofluorescence and confocal microscopy. Western blot analysis of NKCC2 expression using two different antibodies revealed a band of ∼160 kDa, and RT-PCR analysis demonstrated the presence of NKCC2 isoforms A and F, which was confirmed by DNA sequencing; transport of Cl−into raTAL cells was inhibited by furosemide. Ouabain- and bumetanide-sensitive oxygen consumption, an index of ion transport activity in the mTAL, was observed in raTAL cells, and the number of domes present was reduced significantly when cells were incubated in the presence of ouabain or bumetanide. The specific activity of Na+-K+-ATPase activity was determined in raTAL cells (0.67 ± 0.18 nmol Pi·μg protein−1·min−1), primary cultures of mTAL cells (0.39 ± 0.08 nmol Pi·μg protein−1·min−1), and freshly isolated mTAL tubules (1.10 ± 0.29 nmol Pi·μg protein−1·min−1), and ∼30–50% of total cellular ATPase activity was inhibited by ouabain, in accord with other mTAL preparations. This cell line will be used in studies that address biochemical, molecular, and physiological mechanisms in the mTAL.

Selective basolateral localization of overexpressed Na-K-ATPase β1- and β2- subunits is disrupted by butryate treatment of MDCK cells

The exclusive basolateral localization of the Na-K-ATPase in kidney epithelium is a critical aspect of nephron function. It has been suggested that mislocalized delivery of the Na-K-ATPase to the apical surface in autosomal dominant polycystic kidney disease (ADPKD) is due to the inappropriate expression of an alternative isoform of the β-subunit, the β2-isoform. It has been reported that heterologous expression of this β2-isoform in Madin-Darby canine kidney (MDCK) cells results in apical delivery of the Na-K-ATPase. We created a MDCK cell line containing a tetracycline-inducible promoter and expressed either myc-tagged β2- or flag-tagged β1-subunits to study the surface localization of these β-subunit isoforms in polarized monolayers. We find that the β2-isoform is targeted to the basolateral surface of the plasma membrane in a polarization pattern indistinguishable from the β1-isoform. However, inclusion of butyrate in the growth medium leads to upregulation of overexpressed β1- or β2-subunits and to their appearance at the apical surface. The β2-isoform expressed in MDCK cells does not assemble into α1β2heterodimers with the endogenous α1. Our findings demonstrate that expression of the β2-isoform does not lead to apical localization of the Na-K-ATPase in MDCK cells and provides evidence for an unexpected effect of butyrate on the trafficking of Na pump subunits.

Ouabain binds with high affinity to the Na,K-ATPase in human polycystic kidney cells and induces extracellular signal-regulated kinase activation and cell proliferation

In autosomal dominant polycystic kidney disease (ADPKD), cyst formation and enlargement require proliferation of mural renal epithelial cells and the transepithelial secretion of fluid into the cyst cavity. Na,K-ATPase is essential for solute and water transport in ADPKD cells, and ouabain blocks fluid secretion in these cells. By binding to the Na,K-ATPase, ouabain also induces proliferation in some cell types. Surprisingly, it was found that nanomolar concentrations of ouabain, similar to those circulating in blood, induced ADPKD cell proliferation but had no statistically significant effect on normal human kidney (NHK) cells. Ouabain, acting from the basolateral side of the cells, also caused an increase in the level of phosphorylated extracellular signal-regulated kinases (ERK). Mitogen-activated protein kinase kinase (MEK) inhibitor U0126 blocked ouabain-induced ERK activation and cell proliferation, suggesting that ouabain effect is mediated through the MEK-ERK pathway. In contrast to NHK cells, the dose-response curve for ouabain inhibition of Na,K-ATPase activity indicated that approximately 20% of the enzyme in ADPKD cells exhibits a higher affinity for ouabain. The increased ouabain affinity of ADPKD cells was not due to differences in Na,K-ATPase isoform expression because these cells, like NHK cells, possess only the alpha1 and beta1 subunits. The gamma variants of the Na,K-ATPase also are expressed in the cells but are elevated in ADPKD cells. Currently, the basis for the differences in ouabain sensitivity of NHK and ADPKD cells is unknown. It is concluded that ouabain stimulates proliferation in ADPKD cells by binding to the Na,K-ATPase with high affinity and via activation of the MEK-ERK pathway.

Subunit composition and role of Na+,K+-ATPases in ventricular myocytes

Na+,K+-ATPases are composed of one alpha and one beta subunit; four alpha and three beta isoforms have been found to date. We elucidated which alpha and beta subunits were present in the ventricular myocytes of rat and guinea-pig and what roles the Na+,K(+)-ATPase isozymes play in cardiac contraction. The presence of the alpha1, alpha2, and alpha3 subunits and the beta1 and beta2 subunits in rat and guinea-pig hearts were confirmed at the protein or mRNA level. Immunocytochemistry showed a patchy presence of alpha1 in the transverse tubules and surface sarcolemma, whereas alpha2 was distributed continuously in the transverse tubules alone. The alpha3 isoform was expressed prominently in the guinea-pig intercalated disc and slightly in the rat. On the other hand, the beta1 isoform was located in the transverse tubules and surface sarcolemma, whereas the beta2 was mainly located in the intercalated disc. The immunocytochemistry and immunoprecipitation findings indicated that the alpha1 and alpha2 form heterodimers with beta1 and the alpha3 with beta2 in ventricular myocytes. The application of low concentrations of ouabain enhanced the amplitudes of twitch without a change in resting tension in rat and guinea-pig ventricular stripts, whereas that of high concentrations resulted in a decrease in twitch with an increase in the resting tension. We thus conclude that the alpha2beta1 and alpha3beta2 isozymes are selectively located in the transverse tubules and intercalated disc of the ventricular myocytes, respectively, and the alpha2beta1 is involved in the regulation of the Ca2+ contents in the SR.

Inhibition of HER-2(neu/ErbB2) restores normal function and structure to polycystic kidney disease (PKD) epithelia

Autosomal Dominant Polycystic Kidney Disease (ADPKD) is a very common lethal monogenetic disease with significant morbidities and a high likelihood of progression to renal failure for which there is no proven disease-specific therapy currently available for clinical use. Human ADPKD cystic epithelia have proliferative abnormalities mediated by EGFR over-expression and mispolarization leading autocrine response to EGF family ligands. We now show that apical localization of EGFR complexes in normal fetal and ADPKD epithelia is associated with heterodimerization of EGFR(HER-1) with HER-2(neu/ErbB2), while basal membrane localization in normal adult renal epithelia is associated with EGFR(HER-1) homodimers. Since ADPKD epithelial cells have reduced migratory function, this was used as a bioassay to evaluate the ability of compounds to rescue the aberrant human ADPKD phenotype. General tyrosine kinase inhibition by herbimycin and specific inhibition of HER-2(neu/ErbB2) by AG825 or pretreatment with ErbB2 siRNA reversed the migration defect of ADPKD epithelia. Selective inhibition of EGFR(HER-1) showed partial rescue. Increased ADPKD cell migration after inhibition of p38MAP kinase but not of PI3-kinase implicated p38MAPK downstream of HER-2(neu/ErbB2) stimulation. Daily administration of AG825 to PKD1 null heterozygous mice significantly inhibited the development of renal cysts. These studies implicate HER2(neu/ErbB2) as an effector of apical EGFR complex mispolarization and that its inhibition should be considered a candidate for clinical therapy of ADPKD.

Defining a link with autosomal-dominant polycystic kidney disease in mice with congenitally low expression of Pkd1

Mouse models for autosomal-dominant polycystic kidney disease (ADPKD), derived from homozygous targeted disruption of Pkd1 gene, generally die in utero or perinatally because of systemic defects. We introduced a loxP site and a loxP-flanked mc1-neo cassette into introns 30 and 34, respectively, of the Pkd1 locus to generate a conditional, targeted mutation. Significantly, before excision of the floxed exons and mc1-neo from the targeted locus by Cre recombinase, mice homozygous for the targeted allele appeared normal at birth but developed polycystic kidney disease with a slower progression than that of Pkd-null mice. Further, the homozygotes continued to produce low levels of full-length Pkd1-encoded protein, suggesting that slight Pkd1 expression is sufficient for renal cyst formation in ADPKD. In this viable model, up-regulation of heparin-binding epidermal growth factor-like growth factor accompanied increased epidermal growth factor receptor signaling, which may be involved in abnormal proliferation of the cyst-lining epithelia. Increased apoptosis in cyst epithelia was only observed in the later period that correlated with the cyst regression. Abnormalities in Na(+)/K(+)-ATPase, aquaporin-2, and vasopressin V2 receptor expression were also identified. This mouse model may be suitable for further studies of progression and therapeutic interventions of ADPKD.

Tight junction biology and kidney dysfunction

The epithelial tight junction (TJ) has three major functions. As a "gate," it serves as a regulatory barrier separating and maintaining biological fluid compartments of different composition. As a "fence," it generates and maintains the apicobasal polarity of cells that form the confluent epithelium. Finally, the TJ proteins form a trafficking and signaling platform that regulates cell growth, proliferation, differentiation, and dedifferentiation. Six examples are selected that illustrate the emerging link between TJ dysfunction and kidney disease. First, the glomerular slit diaphragm (GSD) is evolved, in part, from the TJ and, on maturation, exhibits all three functions of the TJ. GSD dysfunction leads to proteinuria and, in some instances, podocyte dedifferentiation and proliferation. Second, accumulating evidence supports epithelial-mesenchymal transformation (EMT) as a major player in renal fibrosis, the final common pathway that leads to end-stage renal failure. EMT is characterized by a loss of cell-cell contact and apicobasal polarity, which are hallmarks of TJ dysfunction. Third, in autosomal dominant polycystic kidney disease, mutations of the polycystins may disrupt their known interactions with the apical junction complex, of which the TJ is a major component. This can lead to disturbances in epithelial polarity regulation with consequent abnormal tubulogenesis and cyst formation. Fourth, evidence for epithelial barrier and polarity dysregulation in the pathogenesis of ischemic acute renal failure will be summarized. Fifth, the association between mutations of paracellin-1, the first TJ channel identified, and clinical disorders of magnesium and calcium wasting and bovine renal fibrosis will be used to highlight an integral TJ protein that can serve multiple TJ functions. Finally, the role of WNK4 protein kinase in shunting chloride across the TJ of the distal nephron will be addressed.

Development of renal function

The mammalian metanephric kidney develops following a general principle of organogenesis of epithelial organs, i.e., along the tree-like structure of an arborizing ductal system (the ureteric bud and cortical collecting duct). In parallel, the proximal portions of the uriniferous tubule develop by mesenchymal-to-epithelial transition of the neighbouring mesenchyme. On one hand, vectorial transport systems in nephrogenesis should be functional at the onset of glomerular filtration in any of the newly formed nephron generations to prevent loss of salt, water and metabolites. On the other hand, developing nephron epithelia must serve the needs of organ-formation such as cell proliferation and fluid-secretion for morphogenic purposes. This review intends to summarize current data and concepts on the development of renal epithelial functions with an emphasis on ion channels. Current model systems are introduced, such as ureteric bud cell monolayer culture, in vitro nephron culture, HEK293 cell culture, and the dissection of tubular cells for direct analysis. The current data on the developmental expression and functions of ENaC Na(+) channels, the CFTR, ClC-2 Cl(ndash;) channels, L-type Ca(2+) channels, P2 purinoceptors, and the Kir6.1/SUR2, ROMK (Kir1.1), and Kv K(+) channels are presented.

Recombinant addition of N-glycosylation sites to the basolateral Na,K-ATPase beta1 subunit results in its clustering in caveolae and apical sorting in HGT-1 cells

In most polarized cells, the Na,K-ATPase is localized on the basolateral plasma membrane. However, an unusual location of the Na,K-ATPase was detected in polarized HGT-1 cells (a human gastric adenocarcinoma cell line). The Na,K-ATPase alpha1 subunit was detected along with the beta2 subunit predominantly on the apical membrane, whereas the Na,K-ATPase beta1 subunit was not found in HGT-1 cells. However, when expressed in the same cell line, a yellow fluorescent protein-linked Na,K-ATPase beta1 subunit was localized exclusively to the basolateral surface and resulted in partial redistribution of the endogenous alpha1 subunit to the basolateral membrane. The human beta2 subunit has eight N-glycosylation sites, whereas the beta1 isoform has only three. Accordingly, up to five additional N-glycosylation sites homologous to the ones present in the beta2 subunit were successively introduced in the beta1 subunit by site-directed mutagenesis. The mutated beta1 subunits were detected on both apical and basolateral membranes. The fraction of a mutant beta1 subunit present on the apical membrane increased in proportion to the number of glycosylation sites inserted and reached 80% of the total surface amount for the beta1 mutant with five additional sites. Clustered distribution and co-localization with caveolin-1 was detected by confocal microscopy for the endogenous beta2 subunit and the beta1 mutant with additional glycosylation sites but not for the wild type beta1 subunit. Hence, the N-glycans linked to the beta2 subunit of the Na,K-ATPase contain apical sorting information, and the high abundance of the beta2 subunit isoform, which is rich in N-glycans, along with the absence of the beta1 subunit, is responsible for the unusual apical location of the Na,K-ATPase in HGT-1 cells.

Disruption of polycystin-1 function interferes with branching morphogenesis of the ureteric bud in developing mouse kidneys

The polycystic kidney disease (PKD1) gene-encoded protein, polycystin-1, is developmentally regulated, with highest expression levels seen in normal developing kidneys, where it is distributed in a punctate pattern at the basal surface of ureteric bud epithelia. Overexpression in ureteric epithelial cell membranes of an inhibitory pMyr-GFP-PKD1 fusion protein via a retroviral (VVC) delivery system and microinjection into the ureteric bud lumen of embryonic day 11 mouse metanephric kidneys resulted in disrupted branching morphogenesis. Using confocal quantitative analysis, significant reductions were measured in the numbers of ureteric bud branch points and tips, as well as in the total ureteric bud length, volume and area, while significant increases were seen as dilations of the terminal branches, where significant increases in outer diameter and volumes were measured. Microinjection of an activating 5TM-GFP-PKD1 fusion protein had an opposite effect and showed significant increases in ureteric bud length and area. These are the first studies to experimentally manipulate polycystin-1 expression by transduction in the embryonic mouse kidney and suggest that polycystin-1 plays a critical role in the regulation of epithelial morphogenesis during renal development.

Phosphorylation of ezrin on threonine 567 produces a change in secretory phenotype and repolarizes the gastric parietal cell

Phosphorylation of the membrane-cytoskeleton linker protein ezrin has been functionally linked to acid secretion and vesicle recruitment to the apical secretory membrane in gastric parietal cells. Phosphorylation of the conserved T567 residue of ezrin has been shown to alter the N/C oligomerization of ezrin and promote the formation of actin-rich surface projections in other cells. To test the importance of T567 as a regulatory site for ezrin in parietal cell activation, we incorporated wild-type (WT) and mutant forms of ezrin, including the nonphosphorylatable T567A mutation and a mutant mimicking permanent phosphorylation, T567D. All ezrin constructs included C-terminal cyan-fluorescent protein (CFP) and were incorporated into adenoviral constructs for efficient introduction into cultured parietal cells from rabbit stomach. Fluorescence microscopy was used to localize CFP-ezrin and monitor morphological responses. Accumulation of a weak base (aminopyrine) was used to monitor receptor-mediated acid secretory response of the cultured cells. Similar to endogenous ezrin, WT and T567A CFP-ezrin localized heavily to apical membrane vacuoles with considerably lower levels associated with the surrounding basolateral membrane. Interestingly, H,K-ATPase within cytoplasmic tubulovesicles was incorporated into the apical vacuoles along with WT and T567A mutant ezrin. In these parietal cells secretagogue stimulation produced a striking vacuolar expansion associated with HCl secretion and the secretory phenotype. Expression of T567D CFP-ezrin was quite different, being rarely associated with apical vacuoles. T567D was more typically localized to the basolateral membrane, often associated with long spikes and fingerlike projections. Moreover, the cells did not display secretagogue-dependent morphological changes and, to our surprise, H,K-ATPase was recruited to the T567D CFP-ezrin-enriched basolateral projections. We conclude that T567 phosphorylation, which is probably regulated through Rho signaling pathway, may direct ezrin to membrane-cytoskeletal activity at the basolateral membrane and away from apical secretory activity. The large basolateral expansion is predicted to recruit membranes from sources not normally targeted to that surface.