Characterization of ischemia-induced loss of epithelial polarity

Suppressing Syndecan-1 Shedding to Protect Against Renal Ischemia/Reperfusion Injury by Maintaining Polarity of Tubular Epithelial Cells

ABSTRACT

Syndecan-1 (SDC-1), a type of heparan sulfate proteoglycan on the surface of epithelial cells, is involved in maintaining cell morphology. Loss of cell polarity constitutes the early stage of ischemic acute kidney injury (AKI). This study investigated the role of SDC-1 shedding in I/R-induced AKI and the underlying mechanisms. Levels of the shed SDC-1 in the serum were measured with ELISA 12 and 24 h after reperfusion in renal I/R model mice. Na+/K+-ATPase-α1 expression was evaluated using western blotting in vivo and immunofluorescence in hypoxia/reoxygenation (H/R) cysts. Renal tubular epithelial cell apoptosis was measured using TUNEL in vivo and flow cytometry in vitro. Furthermore, plasma syndecan-1 (pSDC-1) levels were measured in patients at the time of anesthesia resuscitation after cardiac surgery. We found that shed SDC-1 levels increased and Na+/K+-ATPase-α1 expression decreased after H/R in the three-dimensional (3D) tubular model, and this state was exacerbated with extended period of hypoxia. After the inhibition of SDC-1 shedding by GM6001, SDC-1 and Na+/K+-ATPase-α1 expression was restored, while H/R-induced apoptosis was decreased. In vivo, SDC-1 shedding was induced by renal I/R and was accompanied with a loss of renal tubular epithelial cell polarity and increased apoptosis. GM6001 pretreatment protected against I/R injury by alleviating the disruption of cell polarity and apoptosis. pSDC-1 levels were significantly higher in AKI patients than in non-AKI patients. ROC curve showed that the accuracy of pSDC-1 for AKI prediction was 0.769. In conclusion, inhibition of I/R-induced SDC-1 shedding could contribute to renal protection by restoring the loss of cell polarity and alleviating apoptosis in tubular epithelial cells.

Post-mortem assessment of vimentin expression as a biomarker for renal tubular regeneration following acute kidney injury

BACKGROUND

Acute kidney injury (AKI) is a common cause of morbidity and mortality. It mainly targets the renal tubular epithelium with pathological changes, referred to as acute tubular injury. The latter is followed by a regenerative response that is difficult to visualize on routine hematoxylin and eosin (H&E) stains. In this study, we examined the regenerative capacity of renal tubules by correlating vimentin (VIM) immunohistochemical (IHC) expression and pathological findings of AKI and renal tubular regeneration (RTR) on H&E.

METHODS

We reviewed 23 autopsies performed in the clinical setting of AKI and RTR. VIM expression was scored in the renal cortical tubular epithelium using a statistical cutoff ≥ 3% for high expression and < 3% for low expression.

RESULTS

Of the 23 kidney tissues examined, seven (30.4%) had low VIM expression, and 16 (69.6%) had high VIM expression. Kidney tissues with evidence of AKI and RTR had significantly higher VIM expression. Renal peritubular microenvironment features showing regenerative changes on H&E were associated with high VIM expression. In the univariate model, kidney tissues with RTR were 18-fold more likely to have high VIM expression.

CONCLUSIONS

In conclusion, our findings suggest that VIM could serve as an IHC marker for RTR following AKI. However, correlation with H&E findings remains critical to excluding chronic tubular damage. Collectively, our preliminary results pave the way for future studies including a larger sample size to validate the use of VIM as a reliable biomarker for RTR.

Kidney Cells Regeneration: Dedifferentiation of Tubular Epithelium, Resident Stem Cells and Possible Niches for Renal Progenitors

A kidney is an organ with relatively low basal cellular regenerative potential. However, renal cells have a pronounced ability to proliferate after injury, which undermines that the kidney cells are able to regenerate under induced conditions. The majority of studies explain yielded regeneration either by the dedifferentiation of the mature tubular epithelium or by the presence of a resident pool of progenitor cells in the kidney tissue. Whether cells responsible for the regeneration of the kidney initially have progenitor properties or if they obtain a “progenitor phenotype” during dedifferentiation after an injury, still stays the open question. The major stumbling block in resolving the issue is the lack of specific methods for distinguishing between dedifferentiated cells and resident progenitor cells. Transgenic animals, single-cell transcriptomics, and other recent approaches could be powerful tools to solve this problem. This review examines the main mechanisms of kidney regeneration: dedifferentiation of epithelial cells and activation of progenitor cells with special attention to potential niches of kidney progenitor cells. We attempted to give a detailed description of the most controversial topics in this field and ways to resolve these issues.

Akt Substrate of 160 kD Regulates Na+,K+-ATPase Trafficking in Response to Energy Depletion and Renal Ischemia

Renal ischemia and reperfusion injury causes loss of renal epithelial cell polarity and perturbations in tubular solute and fluid transport. Na(+),K(+)-ATPase, which is normally found at the basolateral plasma membrane of renal epithelial cells, is internalized and accumulates in intracellular compartments after renal ischemic injury. We previously reported that the subcellular distribution of Na(+),K(+)-ATPase is modulated by direct binding to Akt substrate of 160 kD (AS160), a Rab GTPase-activating protein that regulates the trafficking of glucose transporter 4 in response to insulin and muscle contraction. Here, we investigated the effect of AS160 on Na(+),K(+)-ATPase trafficking in response to energy depletion. We found that AS160 is required for the intracellular accumulation of Na(+),K(+)-ATPase that occurs in response to energy depletion in cultured epithelial cells. Energy depletion led to dephosphorylation of AS160 at S588, which was required for the energy depletion-induced accumulation of Na,K-ATPase in intracellular compartments. In AS160-knockout mice, the effects of renal ischemia on the distribution of Na(+),K(+)-ATPase were substantially reduced in the epithelial cells of distal segments of the renal tubules. These data demonstrate that AS160 has a direct role in linking the trafficking of Na(+),K(+)-ATPase to the energy state of renal epithelial cells.

Regenerative medicine for the kidney: stem cell prospects & challenges

The kidney has key roles in maintaining human health. There is an escalating medical crisis in nephrology as growing numbers of patients suffer from kidney diseases that culminate in organ failure. While dialysis and transplantation provide life-saving treatments, these therapies are rife with limitations and place significant burdens on patients and healthcare systems. It has become imperative to find alternative ways to treat existing kidney conditions and preemptive means to stave off renal dysfunction. The creation of innovative medical approaches that utilize stem cells has received growing research attention. In this review, we discuss the regenerative and maladaptive cellular responses that occur during acute and chronic kidney disease, the emerging evidence about renal stem cells, and some of the issues that lie ahead in bridging the gap between basic stem cell biology and regenerative medicine for the kidney.

Overexpression of stanniocalcin-1 inhibits reactive oxygen species and renal ischemia/reperfusion injury in mice

Reactive oxygen species, endothelial dysfunction, inflammation, and mitogen-activated protein kinases have important roles in the pathogenesis of ischemia/reperfusion kidney injury. Stanniocalcin-1 (STC1) suppresses superoxide generation in many systems through induction of mitochondrial uncoupling proteins and blocks the cytokine-induced rise in endothelial permeability. Here we tested whether transgenic overexpression of STC1 protects from bilateral ischemia/reperfusion kidney injury. This injury in wild type mice caused a halving of the creatinine clearance; severe tubular vacuolization and cast formation; increased infiltration of macrophages and T cells; higher vascular permeability; greater production of superoxide and hydrogen peroxide; and higher ratio of activated ERK/activated JNK and p38, all compared to sham-treated controls. Mice transgenic for human STC1 expression, however, had resistance to equivalent ischemia/reperfusion injury indicated as no significant change from controls in any of these parameters. Tubular epithelial cells in transgenic mice expressed higher mitochondrial uncoupling protein 2 and lower superoxide generation. Pre-treatment of transgenic mice with paraquat, a generator of reactive oxygen species, before injury restored the susceptibility to ischemia/reperfusion kidney injury, suggesting that STC1 protects by an anti-oxidant mechanism. Thus, STC1 may be a therapeutic target for ischemia/reperfusion kidney injury.

The maximal cytoprotective function of the heat shock protein 27 is dependent on heat shock protein 70

Endogenous heat shock proteins (HSPs) 70 and 25/27 are induced in renal cells by injury from energy depletion. Transfected over-expression of HSPs 70 or 27 (human analogue of HSP25), provide protection against renal cell injury from ATP deprivation. This study examines whether over-expressed HSP27 depends on induction of endogenous HSPs, in particular HSP70, to afford protection against cell injury. LLC-PK1 cells transfected with HSP27 (27OE cells) were injured by ATP depletion for 2h and recovered for 4h in the presence of HSF decoy, HSP70 specific siRNA (siRNA-70) and their respective controls. Injury in the presence of HSF decoy, a synthetic oligonucleotide identical to the heat shock element, the nuclear binding site of HSF, decreased HSP70 induction by 80% without affecting the over-expression of transfected HSP27. The HSP70 stress response was completely ablated in the presence of siRNA-70. Protection against injury, provided by over-expression of HSP27, was reduced by treatment with HSF decoy and abolished by treatment with siRNA-70. Immunoprecipitation studies demonstrated association of HSP27 with actin that was not affected by either treatment with HSF decoy or siRNA. Therefore, HSP27 is dependent on HSP70 to provide its maximal cytoprotective effect, but not for its interaction with actin. This study suggests that, while it has specific action on the cytoskeleton, HSP 25/27 must have coordinated activity with other HSP classes, especially HSP70, to provide the full extent of resistance to injury from energy depletion.

Loss of galectin-3 impairs membrane polarisation of mouse enterocytes in vivo

Epithelial cells are characterised by distinct apical and basolateral membrane domains that are separated by tight junctions. Establishment and maintenance of this polarity depend on specific gene expression and protein targeting to their correct location. Our former studies, performed with renal epithelial MDCK cells, revealed a new function for galectin-3, a member of a conserved family of lectins. There, galectin-3 is required for intracellular sorting and correct targeting of non-raft-associated glycoproteins to the apical plasma membrane. In the present study, we found transport defects of the intestinal brush border hydrolases lactase-phlorizin hydrolase (LPH) and dipeptidylpeptidase IV (DPPIV) in galectin-3-null mutant mice. We could show that, in enterocytes of wild-type mice, both glycoproteins directly interact with galectin-3 and transit through non-raft-dependent apical transport platforms. Therefore, this genetic analysis provides definitive evidence for the involvement of galectin-3 in protein intracellular trafficking in vivo. Further investigations revealed that gal3-null enterocytes also exhibit striking cytoarchitecture defects, with the presence of numerous and regular protrusions located along basolateral membranes. Moreover, β-actin and villin, two characteristic markers of brush borders, become abnormally distributed along these atypical basolateral membranes in gal3–/– mice. Taken together, our results demonstrate that, in addition to a pivotal role in apical trafficking, galectin-3 also participates in epithelial morphogenesis in mouse enterocytes.

Dedifferentiation and proliferation of surviving epithelial cells in acute renal failure

In contrast to the heart or brain, the kidney can completely recover from an ischemic or toxic insult that results in cell death. During recovery from ischemia/reperfusion injury, surviving tubular epithelial cells dedifferentiate and proliferate, eventually replacing the irreversibly injured tubular epithelial cells and restoring tubular integrity. Repair of the kidney parallels kidney organogenesis in the high rate of DNA synthesis and apoptosis and in patterns of gene expression. As has been shown by proliferating cell nuclear antigen and 5-bromo 2'-deoxyuridine labeling studies and, in unpublished studies, by counting mitotic spindles identified by labeling with antitubulin antibody, the proliferative response is rapid and extensive, involving many of the remaining cells of the proximal tubule. This extensive proliferative capacity is interpreted to reflect the intrinsic ability of the surviving epithelial cell to adapt to the loss of adjacent cells by dedifferentiating and proliferating. Adhesion molecules likely play important roles in the regulation of renal epithelial cell migration, proliferation, and differentiation, as do cytokines and chemokines. Better understanding of all of the characteristics resulting in dedifferentiation and proliferation of the proximal tubule epithelial cell and cell-cell and cell-matrix interactions important for this repair function will lead to novel approaches to therapies designed to facilitate the processes of recovery in humans.

Induction of a laminin isoform and alpha(3)beta(1)-integrin in renal ischemic injury and repair in vivo

Ischemic injury to the kidney, a major cause of acute renal failure, leads to the detachment and loss of numerous tubular epithelial cells. Integrin-laminin interactions may promote regeneration of the damaged epithelium by influencing kidney epithelial cell adhesion and differentiation. Laminins are major structural components of basement membranes. Of the various laminin isoforms, laminin-5 is of particular interest because of its proposed role in the healing of skin wounds. In this study, we investigate the expression of laminin-5 in rat kidney after unilateral ischemia. Using a polyclonal antibody generated against laminin-5, we find that immunostaining is confined to the basement membranes of collecting ducts in the papilla and the major and minor calyces in normal kidney. With injury and regeneration, however, immunostaining becomes much more intense and widespread in basement membranes along the nephron. Immunoblotting of ischemic kidney extracts reveals significantly increased expression of a polypeptide of approximately 220 kDa, possibly corresponding to a precursor of one of the three laminin-5 chains. Immunoblotting and immunostaining also demonstrate significantly increased expression and altered localization of the alpha(3)-integrin subunit, a receptor for laminin-5. These results indicate that there is induction of a laminin isoform, possibly laminin-5, and alpha(3)beta(1)-integrin in the ischemic kidney and may implicate this receptor-ligand combination in the pathogenesis of acute renal failure and/or repair of the injured kidney epithelium.

Monitoring changes in gene expression in renal ischemia-reperfusion in the rat

BACKGROUND

Although acute renal failure (ARF) is a relatively common disorder with major morbidity and mortality, its molecular basis remains incompletely defined. The present study examined global gene expression in the well-characterized ischemia-reperfusion model of ARF using DNA microarray technology.

METHODS

Male Wistar rats underwent bilateral renal ischemia (30 min) or sham operation, followed by reperfusion for 1, 2, 3 or 4 days. Plasma creatinine increased approximately fivefold over baseline, peaking on day 1. Renal total RNA was used to probe cDNA microarrays.

RESULTS

Alterations in expression of 18 genes were identified by microarray analysis. Nine genes were up-regulated (ADAM2, HO-1, UCP-2, and thymosin beta4 in the early phase and clusterin, vanin1, fibronectin, heat-responsive protein 12 and FK506 binding protein in the established phase), whereas another nine were down-regulated (glutamine synthetase, cytochrome p450 IId6, and cyp 2d9 in the early phase and cyp 4a14, Xist gene, PPARgamma, alpha-albumin, uromodulin, and ADH B2 in the established phase). The identities of these 18 genes were sequence-verified. Changes in gene expression of ADAM2, cyp2d6, fibronectin, HO-1 and PPARgamma were confirmed by quantitative real-time polymerase chain reaction (PCR). ADAM2, cyp2d6, and PPARgamma have not previously been known to be involved in ARF.

CONCLUSION

Using DNA microarray technology, we identified changes in expression of 18 genes during renal ischemia-reperfusion injury in the rat. We confirmed changes in five genes (fibronectin, ADAM2, cyp 2d6, HO-1 and PPARgamma) by quantitative real-time PCR. Several genes, not previously been identified as playing a role in ischemic ARF, may have importance in this disease.

Altered cholesterol localization and caveolin expression during the evolution of acute renal failure

BACKGROUND

Renal cortical/proximal tubule cholesterol accumulation, with preferential localization within plasma membrane "detergent resistant microdomains" (DRMs: rafts/caveolae), is a hallmark of the maintenance phase of acute renal failure (ARF). This study addressed two related issues: (1) Are maintenance-phase cholesterol increases accompanied by an up-regulation of caveolin, a DRM/caveolar-associated cholesterol binding protein? (2) Is DRM cholesterol/caveolin homeostasis acutely altered during the induction phase of ARF?

METHODS

Mouse kidneys were subjected to ischemia +/- reperfusion (I/R) followed by assessment of cholesterol DRM partitioning. Acute cell injury effects on potential caveolin release from isolated proximal tubules or into urine also were assessed. Finally, renal cortical/isolated proximal tubule caveolin levels were determined 18 hours after I/R or myoglobinuric ARF.

RESULTS

Acute ischemia causes a rapid shift of cholesterol into cortical DRMs (>22%). Cholesterol migration into DRMs also was observed in ATP-depleted cultured proximal tubule (HK-2) cells. Acute hypoxic or toxic tubule injury induced plasma membrane caveolin release (Western blot). By the maintenance phase of ARF, marked renal cortical/proximal tubule caveolin increases resulted.

CONCLUSIONS

Acute proximal tubular injury damages caveolar/DRM structures, as determined by cholesterol maldistribution and caveolin release. Post-injury, there is a dramatic up-regulation of renal cortical/proximal tubule caveolin, suggesting an increased caveolar mass. These findings indicate, to our knowledge for the first time, that dysregulation of caveolae/raft microdomain expression is a correlate of, and potential participant in, the induction and maintenance phases of ischemic and toxic forms of experimental ARF.

Global analysis of gene expression in renal ischemia-reperfusion in the mouse

Ischemia-induced acute renal failure (ARF) is a relatively common disorder with major morbidity and mortality. To study global gene expression during ARF, 6-week-old C57BL/6 male mice underwent 30 min of bilateral renal ischemia followed by reperfusion [I/R] or sham operation. Oligonucleotide microarrays [Affymetrix] with approximately 10,000 genes, 6,643 of which were present in mouse kidney, were used to analyze mRNA expression for up to 4 days following I/R. Fifty-two genes at day 1 and 40 at day 4 were up-regulated more than 4-fold [400%]. Seventy genes at day 1 and 30 genes at day 4 were down-regulated to under 0.25-fold from baseline [25%]. Real-time quantitative RT-PCR confirmed changes in expression for 8 genes of interest. Most of the induced transcripts are involved in cell structure, extracellular matrix, intracellular calcium binding, and cell division/differentiation. Our data identified several novel genes that may be important in renal repair after ischemia.

Prevention of kidney ischemia/reperfusion-induced functional injury, MAPK and MAPK kinase activation, and inflammation by remote transient ureteral obstruction

Protection against ischemic kidney injury is afforded by 24 h of ureteral obstruction (UO) applied 6 or 8 days prior to the ischemia. Uremia or humoral factors are not responsible for the protection, since unilateral UO confers protection on that kidney but not the contralateral kidney. Prior UO results in reduced postischemic outer medullary congestion and leukocyte infiltration. Prior UO results in reduced postischemic phosphorylation of c-Jun N-terminal stress-activated protein kinase 1/2 (JNK1/2), p38, mitogen-activated protein kinase (MAPK) kinase 4 (MKK4), and MKK3/6. Very few cells stain positively for proliferating cell nuclear antigen after obstruction, indicating that subsequent protection against ischemia is not related to proliferation with increased numbers of newly formed daughter cells more resistant to injury. UO increases the expression of heat shock protein (HSP)-25 and HSP-72. The increased HSP-25 expression persists for 6 or 8 days, whereas HSP-72 does not. HSP-25 expression is increased in the proximal tubule cells in the outer stripe of the outer medulla postobstruction, prior to, and 24 h after ischemia. In LLC-PK(1) renal epithelial cells, adenovirus-expressed human HSP-27 confers resistance to chemical anoxia and oxidative stress. Increased HSP-27 expression in LLC-PK(1) cells results in reduced H(2)O(2)-induced phosphorylation of JNK1/2 and p38. In conclusion, prior transient UO renders the kidney resistant to ischemia. This resistance to functional consequences of ischemia is associated with reduced postischemic activation of JNK, p38 MAP kinases, and their upstream MAPK kinases. The persistent increase in HSP-25 that occurs as a result of UO may contribute to the reduction in phosphorylation of MAPKs that have been implicated in adhesion molecule up-regulation and cell death.

Hepatocyte growth factor alters renal epithelial cell susceptibility to uropathogenic Escherichia coli

The urinary tract is frequently the source of Escherichia coli bacteremia. Bacteria from the urinary tract must cross an epithelial layer to enter the bloodstream. Hepatocyte growth factor (HGF) alters the polarity of Madin-Darby canine kidney (MDCK) epithelial cells. The role of cell polarity in determining renal epithelial resistance to Escherichia coli invasion is not well known. A model of polarized and HGF-treated MDCK epithelial cells grown on filters was used to study the role of epithelial cell polarity during the interaction of nonvirulent (XL1-Blue) and uropathogenic (J96) strains of Escherichia coli with renal epithelium. Basolateral exposure of MDCK cells to J96, but not XL1-Blue, resulted in loss of transepithelial resistance (TER), which was due to epithelial cytotoxicity and not degradation of epithelial junctional proteins by bacterial proteases. Apical exposure to both J96 and XL1-Blue did not alter TER. Pretreatment of polarized MDCK cell monolayers with HGF renders the cells sensitive to loss of TER and cytotoxicity by apical exposure to J96. Analysis by confocal microscopy demonstrated that HGF treatment of MDCK cell monolayers also greatly enhances adherence of J96 to the apical surface of the cell monolayer. These data demonstrate that the basolateral surface of polarized epithelia is more susceptible to J96 cytotoxicity. The data also support the hypothesis that processes that alter epithelial cell polarity increase sensitivity of epithelia to bacterial injury and adherence from the apical compartment.

Cholesterol ester accumulation: an immediate consequence of acute in vivo ischemic renal injury

BACKGROUND

Cholesterol is a major constituent of plasma membranes, and recent evidence indicates that it is up-regulated during the maintenance phase of acute renal failure (ARF). However, cholesterol's fate and that of the cholesterol ester (CE) cycle [shuttling between free cholesterol (FC) and CEs] during the induction phase of ARF have not been well defined. The present studies sought to provide initial insights into these issues.

METHODS

FC and CE were measured in mouse renal cortex after in vivo ischemia (15 and 45 minutes)/reperfusion (0 to 120 minutes) and glycerol-induced myoglobinuria (1 to 2 hours). FC/CE were also measured in (1) cultured human proximal tubule (HK-2) cells three hours after ATP depletion and in (2) isolated mouse proximal tubule segments (PTSs) subjected to plasma membrane damage (with cholesterol oxidase, sphingomyelinase, phospholipase A2, or cytoskeletal disruption with cytochalasin B). The impact of cholesterol synthesis inhibition (with mevastatin) and FC traffic blockade (with progesterone) on injury-evoked FC/CE changes was also assessed.

RESULTS

In vivo ischemia caused approximately threefold to fourfold CE elevations, but not FC elevations, that persisted for at least two hours of reperfusion. Conversely, myoglobinuria had no effect. Isolated CE increments were observed in ATP-depleted HK-2 cells. Neither mevastatin nor progesterone blocked this CE accumulation. Plasma membrane injury induced with sphingomyelinase or cholesterol oxidase, but not with phospholipase A(2) or cytochalasin B, increased tubule CE content. High CE levels, induced with cholesterol oxidase, partially blocked hypoxic PTS attack.

CONCLUSIONS

In vivo ischemia/reperfusion acutely increases renal cortical CE, but not FC, content, indicating perturbed CE/FC cycling. The available data suggest that this could stem from specific types of plasma membrane damage, which then increase FC flux via aberrant pathways to the endoplasmic reticulum, where CE formation occurs. That CE levels are known to inversely correlate with both renal and nonrenal cell injury suggests the potential relevance of these observations to the induction phase of ischemic ARF.

Acute renal ischemia model in dogs: effects of metoprolol

INTRODUCTION

To study the functional and histological alterations in dog kidneys submitted to total ischemia for thirty minutes and the possible metoprolol protective action.

MATERIAL AND METHODS

Sixteen dogs anesthetized with sodium pentobarbital (SP) were studied and divided into two groups: G1-8 dogs submitted to left nephrectomy and right renal artery clamping for thirty minutes, and G2-8 dogs submitted to the same procedures of G1 and to the administration of 0.5 mg x kg(-1) metoprolol before ischemia. Attributes of renal function were studied.

RESULTS

There was acute tubular necrosis and a decrease of renal blood flow and glomerular filtration, and a increase of renal vascular resistance in both groups.

CONCLUSION

The thirty minute renal ischemia appears to have determined the alterations found in the renal function and histology in both groups. Metoprolol, used in G2, as to the time and dose applied didn't protect the kidney from the ischemic episode.

HCaRG, a novel calcium-regulated gene coding for a nuclear protein, is potentially involved in the regulation of cell proliferation

Since a negative calcium balance is present in spontaneously hypertensive rats, we searched for the gene(s) involved in this dysregulation. A cDNA library was constructed from the spontaneously hypertensive rat parathyroid gland, which is a key regulator of serum-ionized calcium. From seven overlapping DNA fragments, a 1100-base pair novel cDNA containing an open reading frame of 224 codons was reconstituted. This novel gene, named HCaRG (hypertension-related, calcium-regulated gene), was negatively regulated by extracellular calcium concentration, and its basal mRNA levels were higher in hypertensive animals. The deduced protein showed no transmembrane domain, 67% alpha-helix content, a mutated calcium-binding site (EF-hand motif), four putative "leucine zipper" motifs, and a nuclear receptor-binding domain. At the subcellular level, HCaRG had a nuclear localization. We cloned the human homolog of this gene. Sequence comparison revealed 80% homology between rats and humans at the nucleotide and amino acid sequences. Tissue distribution showed a preponderance in the heart, stomach, jejunum, kidney (tubular fraction), liver, and adrenal gland (mainly in the medulla). HCaRG mRNA was significantly more expressed in adult than in fetal organs, and its levels were decreased in tumors and cancerous cell lines. We observed that after 60-min ischemia followed by reperfusion, HCaRG mRNA declined rapidly in contrast with an increase in c-myc mRNA. Its levels then rose steadily to exceed base line at 48 h of reperfusion. HEK293 cells stably transfected with HCaRG exhibited much lower proliferation, as shown by cell count and [(3)H]thymidine incorporation. Taken together, our results suggest that HCaRG is a nuclear protein potentially involved in the control of cell proliferation.

Ischemia activates actin depolymerizing factor: role in proximal tubule microvillar actin alterations

Apical membrane of renal proximal tubule cells is extremely sensitive to ischemia, with structural alterations occurring within 5 min. These changes are felt secondary to actin cytoskeletal disruption, yet the mechanism responsible is unknown. Actin depolymerizing factor (ADF), a 19-kDa actin-binding protein, has recently been shown to play an important role in regulation of actin filament dynamics. Because ADF is known to mediate pH-dependent F-actin binding, depolymerization, and severing, and because ADF activation occurs by dephosphorylation, we questioned whether ADF played a role in microvilli microfilament disruption during ischemia. To test our hypothesis, we induced renal ischemia in the rat with the clamp model. Initial immunofluorescence and Western blot studies on cortical tissue documented the presence of ADF in proximal tubule cells. Under physiological conditions, ADF was distributed homogeneously throughout the cytoplasm, primarily in the Triton X-100-soluble fraction, and both phosphorylated (pADF) and nonphosphorylated forms were identified. During ischemia, marked alterations occurred. Intraluminal vesicle/bleb structures contained extremely high concentrations of ADF along with G-actin, but not F-actin. Western blot showed a rapidly occurring duration-dependent dephosphorylation of ADF. At 0-30 min of ischemia, total ADF levels were unchanged, whereas pADF decreased significantly to 72% and 19% of control levels, at 5 and 15 min, respectively. Urine collected under physiological conditions did not contain ADF or actin, whereas urine collected after 30 min of ischemia contained both ADF and actin. Reperfusion was associated with normalization of cellular pADF levels, pADF intracellular distribution, and repair of apical microvilli. These data suggest that activation of ADF during ischemia via dephosphorylation is, in part, responsible for apical actin disruption resulting in microvillar destruction and formation of intraluminal vesicles.

Sodium reabsorption and distribution of Na+/K+-ATPase during postischemic injury to the renal allograft

BACKGROUND

A loss of proximal tubule cell polarity is thought to activate tubuloglomerular feedback, thereby contributing to glomerular filtration rate depression in postischemic acute renal failure (ARF).

METHODS

We used immunomicroscopy to evaluate the segmental distribution of Na+/K+-ATPase in tubules of recipients of cadaveric renal allografts. Fractional excretion (FE) of sodium and lithium was determined simultaneously. Observations were made on two occasions: one to three hours after graft reperfusion (day 0) and again on post-transplant day 7. An inulin clearance below or above 25 ml/min on day 7 was used to divide subjects into groups with sustained (N = 15) or recovering (N = 16) ARF, respectively.

RESULTS

In sustained ARF, the fractional excretion of sodium (FENa) was 40 +/- 6% and 11 +/- 5%, and the fractional excretion of lithium (FELi) was 76 +/- 5% and 70 +/- 2% on days 0 and 7, respectively. Corresponding findings in recovering ARF were 28 +/- 2% and 6 +/- 2% for the FENa and 77 +/- 4% and 55 +/- 3% (P < 0.05 vs. sustained) for FELi. Na+/K+-ATPase distribution in both groups was mainly basolateral in distal straight and convoluted tubule segments and collecting ducts. However, Na+/K+-ATPase was poorly retained in the basolateral membrane of proximal convoluted and straight tubule segments in sustained and recovering ARF on both days 0 and 7.

CONCLUSIONS

We conclude that loss of proximal tubule cell polarity for Na+/K+-ATPase distribution is associated with enhanced delivery of filtered Na+ to the macula densa for seven days after allograft reperfusion. Whether an ensuing activation of tubuloglomerular feedback is an important cause of glomerular filtration rate depression in this form of ARF remains to be determined.

Polarity, integrin, and extracellular matrix dynamics in the postischemic rat kidney

Acute renal failure (ARF) as a consequence of ischemic injury is a common disease affecting 5% of the hospitalized population. Despite the fact that mortality from ARF is high, there has been little improvement in survival rates over the last 40 years. The pathogenesis of ARF may be related to substantial changes in cell-cell and cell-extracellular matrix interactions mediated by beta1-integrins. On the basis of in vitro and in vivo studies, reorganization of beta1-integrins from basal to apical surfaces of injured tubular epithelia has been suggested to facilitate epithelial detachment, contributing to tubular obstruction and backleak of glomerular filtrate. In this study, we examine integrin and extracellular matrix dynamics during epithelial injury and repair using an in vivo rat model of unilateral ischemia. We find that, soon after reperfusion, beta1-integrins newly appear on lateral borders in epithelial cells of the S3 segment but are not on the apical surface. At later times, as further injury and regeneration coordinately occur, epithelia adherent to the basement membrane localize beta1 predominantly to basal surfaces even while the polarity of other marker proteins is lost. At the same time, amorphous material consisting of depolarized exfoliated cells fills the luminal space. Notably, beta1-integrins are not detected on exfoliated cells. A novel finding is the presence of fibronectin, a glycoprotein of plasma and the renal interstitium, in tubular spaces of the distal nephron and to a lesser extent S3 segments. These results indicate that beta1-integrins dramatically change their distribution during ischemic injury and epithelial repair, possibly contributing to cell exfoliation initially and to epithelial regeneration at later stages. Together with the appearance of large amounts of fibronectin in tubular lumens, these alterations may play a significant role in the pathophysiology of ARF.

Recovery of cellular functions following oxidant injury

This study investigated the recovery of renal proximal tubule cellular (RPTC) functions following oxidant-induced sublethal injury. tert-Butylhydroperoxide (TBHP) treatment resulted in 24% cell death and loss 4 h following the exposure. The remaining sublethally injured RPTC proliferated, and monolayer DNA content returned to control values on day 4 following TBHP exposure. Basal oxygen consumption (Qo2) and ATP content in sublethally injured RPTC were decreased 64 and 63%, respectively, at 4 h and returned to control values on day 6. Net lactate consumption decreased 71% at 4 h and returned to control values on day 4. In contrast, net glutamine consumption increased 2.7-fold at 4 h and returned to control values on day 6. Ouabain-sensitive Qo2, Na(+)-K(+)-adenosinetriphosphatase (Na(+)-K(+)-ATPase) activity, and Na(+)-coupled glucose transport were inhibited 77, 88, and 83%, respectively, at 4 h and recovered to control values on day 6. These data show that 1) mitochondrial function, Na(+)-K(+)-ATPase activity, active Na+ transport, and Na(+)-coupled glucose transport are decreased in sublethally injured RPTC following oxidant exposure and are repaired over time; 2) monolayer regeneration precedes the recovery of mitochondrial and transport functions, and 3) sublethal injury and subsequent regeneration are associated with alterations in metabolic substrate utilization. These results suggest that oxidant-induced sublethal injury to RPTC may contribute to renal dysfunction and that RPTC can repair and regain cellular functions following oxidant injury.

Redistribution of villin to proximal tubule basolateral membranes after ischemia and reperfusion

After ischemia and reperfusion, severe alterations in the cytoskeletal organization of renal tubular epithelial cells have been reported. These effects, accompanied by a modification in the polarized distribution of some membrane transport proteins, are especially evident in the proximal tubule. In normal proximal tubule cells, actin is concentrated in apical brush border microvilli, along with the actin-binding protein villin. Because villin plays an important role in actin bundling and in microvillar assembly but can also act as an actin-fragmenting protein at higher calcium concentrations, we examined the effects of ischemic injury and reperfusion on the distribution of villin and actin in proximal tubule cells of rat kidney. Using specific antibodies against villin and actin, we show that these proteins redistribute in parallel from the apical to the basolateral plasma membrane within 1 h of reperfusion after ischemia. Ischemia alone had no effect on the staining pattern. Repolarization of villin to the apical membrane begins within hours after reperfusion with enhanced apical localization over time during the period of regeneration. This apical repolarization of villin is accompanied by the migration of actin back to the apical membrane. These results show not only that villin may be involved in the initial disruption of the actin cytoskeleton during reperfusion injury but also that its migration back to the apical domain of these cells accompanies the reestablishment of a normal actin distribution in the brush border.

Apical beta 1 integrin in polarized MDCK cells mediates tubulocyst formation in response to type I collagen overlay

A number of epithelia form tubulocysts in vitro when overlaid with type I collagen gel. Because collagen receptors are generally believed to be expressed on the basolateral domain, the mechanism by which collagen elicits this morphogenetic response from the apical surface is unclear. To investigate the role of beta 1 integrins, the major receptor family for collagen, in this process, we overlaid polarized monolayers of MDCK II cells grown on permeable supports with type I collagen gel and correlated integrin polarity with the polarity of other apical and basolateral membrane markers during tubulocyst formation. Polarized monolayers of one clone of MDCK II cells, referred to as Heidelberg MDCK, initially respond to collagen overlay by stratifying; within 48 hours, lumena develop between the cell layers giving rise to tubulocysts. Tight junctions remain intact during tubulocyst formation because transepithelial electrical resistance does not significantly change. Major alterations are observed, however, in the expression and localization of apical and basolateral membrane markers. beta 1 integrins are necessary for tubulocyst morphogenesis because a function-blocking antibody administered to the apical pole of the cells completely inhibits the formation of these structures. To determine how apical-cell collagen interactions elicit tubulocyst formation, we examined whether beta 1 integrins are mobilized to apical plasma membranes in response to collagen overlay. We found that in the absence of collagen, polarized monolayers of Heidelberg MDCK cells endogenously express on apical plasma membranes a small pool of the beta 1 family, including alpha 2 beta 1 and alpha 3 beta 1. Collagen overlay does not mobilize additional beta 1 integrins to apical domains. If beta 1 integrins are not already apically expressed, as in the C6 MDCK cell line (Schoenenberger et al. (1994) J. Cell Biol. 107, 527–541), beta 1 integrins are not directed apically and tubulocysts do not develop in response to collagen. Thus, interaction of beta 1 integrin pre-existing on apical plasma membranes of polarized epithelia with type I collagen gel is the mechanism by which apical application of collagen elicits the formation of tubulocysts. Depolarized integrins on apical plasma membranes of polarized epithelia may be relevant to the pathogenesis of disease and injury.

Surface membrane polarity of proximal tubular cells: alterations as a basis for malfunction

The surface membrane of proximal tubular cells is organized into distinct apical and basolateral membrane domains. The establishment and maintenance of these biochemically, structurally and physiologically distinct domains involves a multi-stage process involving cell-cell, cell-ECM interactions, and polarized targeting mechanisms. Ischemia, via cellular ATP depletion, results in a series of structural, biochemical and functional alterations that lead to loss of proximal tubular cell surface membrane polarity. Of central importance is the rapidly-occurring, duration-dependent disruption and dissociation of the actin cytoskeleton and associated surface membrane structures. This results in numerous cellular alterations including loss of cell-cell contact, cell-extracellular matrix adhesion and surface membrane polarity. Redistribution of surface membrane proteins and lipids into the alternate domain results in the cells inability to function properly. Repair of these disorders involves re-establishment of the actin cytoskeleton and apical and basolateral surface membrane domains. Recent information indicates growth factors may play a role in hastening this repair process.

Postischemic injury, delayed function and Na+/K(+)-ATPase distribution in the transplanted kidney

We evaluated the postischemic renal injury in 22 patients undergoing renal transplantation. Renal tissue obtained 45 to 60 minutes after reperfusion of the allograft was stained with specific antibodies against the delta subunit of Na+/K(+)-ATPase, fodrin and ankyrin. The distribution of each cytoskeletal protein was analyzed by laser confocal microscopy. Subsequent allograft function was assessed on two occasions, 1 to 3 and 36 hours post-reperfusion, respectively. Recipients were divided into two groups: those who achieved a normal GFR on post-transplant day 3 (group 1, N = 12) and those with persistent hypofiltration (group 2, N = 10). Patients of both groups exhibited impaired sodium reabsorption and isosthenuria one to three hours postoperatively, but these abnormalities persisted on day 3 only in group 2 subjects with persistent hypofiltration. Abnormalities of Na+/K(+)-ATPase, ankyrin and fodrin were confined to proximal tubule cells and were marked only in the subjects of group 2. They consisted of redistribution of each cytoskeletal protein from the basolateral membrane to the cytoplasm. We conclude that postischemic injury to a renal allograft results in a loss of polarity of proximal tubule cells. We propose that ensuing impairment of proximal sodium reabsorption could activate tubuloglomerular feedback, thereby contributing to the protracted hypofiltration that characterizes this form of postischemic, acute renal failure.

Plasmalogen phospholipid hydrolysis during hypoxic injury of rabbit proximal tubules

We have identified and quantified the major species of arachidonate-containing phospholipids in proximal tubules by high performance liquid chromatographic and gas chromatographic analyses. Arachidonate was found to comprise 53% of the total mass of fatty acids esterified at the sn-2 position of ethanolamine phospholipids, and 51% of that amount resides in three plasmenylethanolamine species containing the vinyl ethers of palmitaldehyde, oleylaldehyde or stearylaldehyde at the sn-1 position. Choline phospholipids contained 21% arachidonylated species and 33% of that amount resides in a single plasmenylcholine species containing the vinyl ether of palmitaldehyde at the sn-1 position. Ten minutes of hypoxia did not cause a significant change in the total phospholipid mass of ethanolamine or choline phospholipids; however, phosphate analysis of the individual phospholipid molecular species containing esterified arachidonic acid in isolated proximal tubules demonstrated a 24% reduction in the mass of the plasmenylethanolamine molecular species with the vinyl ether of oleylaldehyde at the sn-1 position and a 35% reduction in the mass of plasmenylcholine species with palmitaldehyde at the sn-1 position. These studies underscore the pathophysiological importance of plasmalogen phospholipid hydrolysis and suggest that activation of PLA2s, which utilize endogenous proximal tubule plasmalogen substrates, may play an important role in the early generation of arachidonic acid and accompanying phospholipid catabolism during hypoxic cell injury.

Characterization of the Han:SPRD rat model for hereditary polycystic kidney disease

The Han:SPRD rat model for inherited polycystic kidney disease (PKD) was characterized (clinical parameters, morphology, immunohistochemistry and in situ hybridization). Homozygous animals died of uremia after three to four weeks with severe cystic transformation of virtually all nephrons and collecting ducts (serum urea: 616 +/- 195 mg/dl; kidney-to-body weight ratio: > 20%). In heterozygotes, slow progression of the disease led to death between the 12th and 21st month (median: 17 months; serum urea levels above 200 mg/dl). Kidney enlargement was moderate, and cysts were restricted to the cortex and outer medulla. Immunohistochemical markers showed that approximately 75% of the cysts were derived from the proximal tubule. Cystic transformation started in the proximal tubule with a sharp onset of basement membrane alteration and a loss of epithelial differentiation restricted to small focal areas. In these areas, alpha 1(IV) collagen and laminin B1 mRNA were enhanced as revealed by isotopic and non-isotopic in situ hybridization. Fibroblasts underlying the affected tubular portions were involved in matrix overexpression resulting in subepithelial accumulation of immunoreactive collagen IV and laminin. In later stages of cystic transformation distal nephron segments were affected as well. A reversal in epithelial polarity as judged from Na,K-ATPase-immunoreactivity was not observed. Renal immunoreactive renin-status was significantly decreased. Hematocrit was lowered in heterozygotes (40.4 +/- 5.8 vol% compared to 46.7 +/- 1.99 vol% in controls; P < 0.05) and total renal EPO mRNA was reduced to 36 +/- 14% of the mean value of control animals, whereas serum EPO levels were not significantly altered. We conclude that the Han:SPRD rat is a useful model for the study of human ADPKD since both diseases are similar in several aspects. The model is particularly suitable for the study of epithelial-mesenchymal interactions at the beginning of tubular cystic transformation.

Role of cytosolic calcium-independent plasmalogen-selective phospholipase A2 in hypoxic injury to rabbit proximal tubules

Although the activation of calcium-independent phospholipase A2 (PLA2) enzymes has been described in the heart, the pathogenetic role of this enzyme(s) in hypoxic cell injury has not been previously examined in any tissue. Therefore, we characterized the time course of activation of calcium-independent PLA2 using both plasmalogen and diacylglycerophospholipid substrates during hypoxia in rabbit proximal tubules and examined whether inhibition of calcium-independent PLA2 activity is associated with a cytoprotective effect. Subjecting rabbit proximal tubules to hypoxia for 5 min resulted in at least a threefold increase in cytosolic calcium-independent PLA2, which was selective for plasmalogen substrates (control 444 +/- 69 vs hypoxia 1,675 +/- 194 pmol.mg protein-1.min-1, n = 5). In contrast, no changes in PLA2 activity were observed in the presence of 4 mM EGTA in the membrane fraction using plasmenylcholine substrates. 20 min of hypoxia resulted in an increase in arachidonate from 3 +/- 1 to 28 +/- 4 ng/mg protein and lactate dehydrogenase release from 7.5 +/- 2% to 38 +/- 5%, n = 4. Pretreatment of proximal tubules with 10 microM Compound I, a specific inhibitor of calcium-independent PLA2, resulted in reduction in the magnitude of both hypoxia-induced arachidonic acid release (11 +/- 3 ng/mg protein) and lactate dehydrogenase release (18 +/- 4%). Our data indicate that a significant fraction of PLA2 activity in the proximal tubule is calcium-independent and selective for plasmalogen substrates. Furthermore, the activation of this enzyme plays an important role in the pathogenesis of membrane injury during hypoxia in the proximal tubule.

Expression of parathyroid hormone-related protein in the rat glomerulus and tubule during recovery from renal ischemia

Parathyroid hormone-related protein (PTHrP) is widely expressed in normal adult and fetal tissues, where it acts in an autocrine/paracrine fashion, stimulates growth and differentiation, and shares early response gene characteristics. Since recovery from renal injury is associated with release of local growth factors, we examined the expression and localization of PTHrP in normal and ischemic adult rat kidney. Male Sprague-Dawley rats underwent complete bilateral renal artery occlusion for 45 min, followed by reperfusion for 15 min, and 2, 6, 24, 48, and 72 h. Renal PTHrP mRNA levels, when compared with sham-operated animals, increased twofold after ischemia, and peaked within 6 h after reperfusion. PTH receptor, beta-actin, and cyclophilin mRNA levels all decreased after ischemia. PTHrP immunohistochemical staining intensity increased in proximal tubular cells after ischemia, changing its location from diffusely cytoplasmic to subapical by 24 h after reperfusion. In addition, PTHrP localized to glomerular epithelial cells (visceral and parietal), but not to mesangial cells. PTHrP and PTH stimulated proliferation two- to threefold in cultured mesangial cells. We conclude that PTHrP mRNA and protein production are upregulated after acute renal ischemic injury, that PTHrP is present in glomerulus and in both proximal and distal tubular cells, and that PTHrP stimulates DNA synthesis in mesangial cells. The precise functions of PTHrP in normal and injured kidney remain to be defined.

Disease processes in epithelia: the role of the actin cytoskeleton and altered surface membrane polarity

The establishment and maintenance of cell polarity is essential for normal epithelial function. Disruption of the underlying processes, either as a primary inborn defect or as a secondary result of other pathologic processes, can lead to loss of epithelial polarity and further cellular and organ-level dysfunction. Continued elucidation of the processes involved may prove fruitful both in the understanding of basic cell biology and in the understanding and treatment of a variety of disease states.

Kid-1, a putative renal transcription factor: regulation during ontogeny and in response to ischemia and toxic injury

We have identified a new putative transcription factor from the rat kidney, termed Kid-1 (for kidney, ischemia and developmentally regulated gene 1). Kid-1 belongs to the C2H2 class of zinc finger genes. Its mRNA accumulates with age in postnatal renal development and is detected predominantly in the kidney. Kid-1 mRNA levels decline after renal injury secondary to ischemia or folic acid administration, two insults which result in epithelial cell dedifferentiation, followed by regenerative hyperplasia and differentiation. The low expression of Kid-1 early in postnatal development, and when renal tissue is recovering after injury, suggests that the gene product is involved in establishment of a differentiated phenotype and/or regulation of the proliferative response. The deduced protein contains 13 C2H2 zinc fingers at the COOH end in groups of 4 and 9 separated by a 32-amino-acid spacer. There are consensus sites for phosphorylation in the NH2 terminus non-zinc finger region as well as in the spacer region between zinc fingers 4 and 5. A region of the deduced protein shares extensive homology with a catalytic region of Raf kinases, a feature shared only with TFIIE among transcription factors. To determine whether Kid-1 can modulate transcription, a chimeric construct encoding the Kid-1 non-zinc finger region (sense or antisense) and the DNA-binding region of GAL4 was transfected into COS and LLC-PK1 cells together with a chloramphenicol acetyltransferase (CAT) reporter plasmid containing GAL4 binding sites, driven by either a minimal promoter or a simian virus 40 enhancer. CAT activity was markedly inhibited in cells transfected with the sense construct compared with the activity in cells transfected with the antisense construct. To our knowledge, this pattern of developmental regulation, kidney expression, and regulation of transcription is unique among the C2H2 class of zinc finger-containing DNA-binding proteins.

Kid-1, a putative renal transcription factor: regulation during ontogeny and in response to ischemia and toxic injury

We have identified a new putative transcription factor from the rat kidney, termed Kid-1 (for kidney, ischemia and developmentally regulated gene 1). Kid-1 belongs to the C2H2 class of zinc finger genes. Its mRNA accumulates with age in postnatal renal development and is detected predominantly in the kidney. Kid-1 mRNA levels decline after renal injury secondary to ischemia or folic acid administration, two insults which result in epithelial cell dedifferentiation, followed by regenerative hyperplasia and differentiation. The low expression of Kid-1 early in postnatal development, and when renal tissue is recovering after injury, suggests that the gene product is involved in establishment of a differentiated phenotype and/or regulation of the proliferative response. The deduced protein contains 13 C2H2 zinc fingers at the COOH end in groups of 4 and 9 separated by a 32-amino-acid spacer. There are consensus sites for phosphorylation in the NH2 terminus non-zinc finger region as well as in the spacer region between zinc fingers 4 and 5. A region of the deduced protein shares extensive homology with a catalytic region of Raf kinases, a feature shared only with TFIIE among transcription factors. To determine whether Kid-1 can modulate transcription, a chimeric construct encoding the Kid-1 non-zinc finger region (sense or antisense) and the DNA-binding region of GAL4 was transfected into COS and LLC-PK1 cells together with a chloramphenicol acetyltransferase (CAT) reporter plasmid containing GAL4 binding sites, driven by either a minimal promoter or a simian virus 40 enhancer. CAT activity was markedly inhibited in cells transfected with the sense construct compared with the activity in cells transfected with the antisense construct. To our knowledge, this pattern of developmental regulation, kidney expression, and regulation of transcription is unique among the C2H2 class of zinc finger-containing DNA-binding proteins.

Polycystic kidney disease: primary extracellular matrix abnormality or defective cellular differentiation?

Polycystic kidney disease (PKD) is inherited as a dominant or recessive trait or can be provoked by environmental factors. The disease is characterized by the growth of large epithelial-lined cysts derived from the nephrons and collecting ducts of affected kidneys. Cysts are thought to initiate as small dilations in renal tubules, which then expand into fluid-filled cavities of relatively large size. Cyst formation appears to involve increased cell proliferation, reversal of tubular epithelial polarity, and epithelial fluid secretion. In addition, a number of pronounced extracellular matrix changes have been found in the cystic kidneys of several animal models and in human autosomal dominant PKD. These abnormalities include thickened, laminated basement membrane, increased expression of alpha 1 type IV collagen and laminins B1 and B2, and changes in heparan sulfate proteoglycan and fibronectin. Some of these changes can also be seen in vitro, reflecting intrinsic abnormalities, and may be associated with abnormal tubular morphogenesis early in cyst formation as well as later in cyst expansion. We have been investigating gene expression in the C57BL/6J-cpk mouse, which has an autosomal recessive form of PKD, to determine the genetic basis of the abnormal tubule cell growth and morphology manifested during cyst formation.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of p35 (annexin I, lipocortin I) in normal adult rat kidney and during recovery from ischemia

The 35-kDa protein (p35, lipocortin I, annexin I), originally discovered as a Ca(++)-dependent substrate for the EGF receptor tyrosine kinase, binds Ca++ and phospholipids, is developmentally regulated in embryos and has restricted expression in adults. Immunohistochemistry of normal rat kidney shows that p35 is enriched in epithelia of Bowman's capsule, the macula densa, and medullary/papillary collecting ducts, suggesting that p35 is related to specialized renal functions. Light staining is observed in the thick ascending limb; elsewhere, immunoreactivity is nil. Since renal recovery from ischemia involves both hyperplasia and hypertrophy and reportedly is accelerated by EGF, we examined p35 distribution during this process. After 48 hours of recovery, both the distribution and amount of renal p35 are altered. Immunoblots show p35 levels increased at least threefold in whole-kidney homogenates. The expression of p35 is still highly restricted in recovering kidneys; however, the thick ascending limb now stains heavily. This is the first documentation of alterations in annexin levels during a pathophysiologic response. However, our attempts to discern effects of exogenous EGF on the recovery from ischemia were negative for both mitotic index and renal function assays.

C-fos expression is hypersensitive to serum-stimulation in cultured cystic kidney cells from the C57BL/6J-cpk mouse

Cystic kidneys of the C57BL/6J-cpk murine model of polycystic kidney disease show a marked overexpression of the proto-oncogenes c-fos, c-myc, and c-Ki-ras, consistent with an increased rate of cell proliferation and an altered state of differentiation. To determine if cystic cells have increased responsiveness to stimulation with mitogenic agents, quiescent primary cultures from normal and cystic cpk kidneys were treated with fetal bovine serum (FBS), 8-bromo-cAMP (cAMP), or epidermal growth factor (EGF). The level of c-fos induction following stimulation by FBS was found to be dramatically higher in cystic cells than in normal cells; whereas induction by cAMP or EGF was essentially the same in both cell types and much less than that seen in FBS-stimulated cells. To determine if this serum hypersensitivity reflects an increased proliferative state in vivo, c-fos induction was examined in cultures derived from normal kidneys stimulated to regenerate by folic acid-induced acute renal injury. As with cystic kidneys, the folic acid-injured kidneys showed increased c-fos responsiveness to FBS in cell culture. These experiments suggest that cystic and regenerating kidneys have an altered phenotypic state in vivo that is manifested in cell culture by serum hypersensitivity. However, whereas the folic acid-injured kidneys ultimately reestablish normal kidney function, cystic kidneys further progress to renal failure, suggesting that cystic epithelial cells are locked in this altered state of differentiation.

Ischemic acute renal failure

Underperfusion of the kidneys often results in the development of ischemic acute renal failure. This review summarizes the recent developments in the understanding of the pathophysiology, diagnosis, and treatment of this serious and costly disorder that affects almost 5% of hospitalized patients.

Incorporation into a planar lipid bilayer of K channels from the luminal membrane of rabbit proximal tubule

The presence of ionic channels at the apical membrane of rabbit proximal tubule cells was investigated by fusion of brush-border membrane vesicles (BBMV) with a planar lipid bilayer (PE/PC, 1:1). The BBMV obtained from native membranes showed poor fusogenic properties. The probability of vesicles fusion with a planar bilayer was, however, enhanced by preincubating the BBMV with liposomes made of azolectin. We report here the presence in BBMV preparations of two K(+)-selective channels of 65 pS and 40 pS, respectively, in asymmetrical 200 parallel 50 mM KCl solutions. The channel of 65 pS appeared highly selective to K+ over Na+ and Cl- ions, while the 40 pS channel discriminated poorly between K+ and Na+ with a permeability ratio PK/PNa = 4. The open probability Po of both channels was found to be voltage-independent within the potential range -60 mV to +60 mV. These K+ channels may be related to channels identified using other methods.

Mechanism for regulating cell surface distribution of Na+,K(+)-ATPase in polarized epithelial cells

Restriction of sodium, potassium adenosine triphosphatase (Na+,K(+)-ATPase) to either the apical or basal-lateral membrane domain of polarized epithelial cells is fundamental to vectorial ion and solute transport in many tissues and organs. A restricted membrane distribution of Na+,K(+)-ATPase in Madin-Darby canine kidney (MDCK) epithelial cells was found experimentally to be generated by preferential retention of active enzyme in the basal-lateral membrane domain and selective inactivation and loss from the apical membrane domain, rather than by vectorial targeting of newly synthesized protein from the Golgi complex to the basal-lateral membrane domain. These results show how different distributions of the same subunits of Na+,K(+)-ATPase may be generated in normal polarized epithelial and in disease states.

The SGP-2 gene is developmentally regulated in the mouse kidney and abnormally expressed in collecting duct cysts in polycystic kidney disease

Sulfated glycoprotein-2 (SGP-2) is a secreted, dimeric, glycosylated protein synthesized by a number of different epithelial cell types. Although its function is not yet understood, SGP-2 has been hypothesized to be involved in such diverse processes as the promotion of cell-cell interactions, spermatogenesis, modulation of the complement system, and programmed cell death. We have now found that the SGP-2 gene is developmentally regulated in the mouse kidney. SGP-2 gene expression is first detected in the condensing nephrogenic mesenchyme and is subsequently down-regulated during the maturation of the glomerular epithelia, proximal tubules, and collecting ducts. SGP-2 continues to be expressed in the mature kidney in distal tubules and in the urothelial lining of the calyx and papilla. We have also examined the expression of the SGP-2 gene in polycystic kidneys of the C57BL/6J-cpk mouse, a model of autosomal recessive polycystic kidney disease in which there is development of epithelial-lined cysts arising primarily from the collecting duct system. Abnormally high levels of SGP-2 mRNA were found in the cyst wall epithelium of polycystic kidneys. The expression of the SGP-2 gene in normal development suggests that it plays a role in differentiating epithelial structures; and the abnormally high levels of SGP-2 gene expression in polycystic kidneys suggests that the cells lining cysts are not fully differentiated. It is possible, therefore, that polycystic kidney disease is caused by a defective developmental process in which there is a delay in terminal differentiation.

Dissociation and redistribution of Na+,K(+)-ATPase from its surface membrane actin cytoskeletal complex during cellular ATP depletion

Establishment and maintenance of a polar distribution of Na+,K(+)-ATPase is essential for efficient Na+ reabsorption by proximal tubule cells and is dependent upon the formation of a metabolically stable, detergent-insoluble complex of Na+,K(+)-ATPase with the actin membrane cytoskeleton. The present studies show that cellular ATP depletion results in a rapid duration-dependent dissociation of Na+,K(+)-ATPase from the actin cytoskeleton and redistribution of Na+,K(+)-ATPase to the apical membrane. During ATP depletion, total cellular Na+,K(+)-ATPase activity was unaltered, but the Triton-X-100-insoluble fraction (cytoskeleton associated) of Na+,K(+)-ATPase activity decreased (P less than 0.01), with a corresponding increase in the detergent-soluble fraction of Na+,K(+)-ATPase (P less than 0.01). Indirect immunofluorescent studies of cells with depleted ATP revealed a redistribution of Na+,K(+)-ATPase from the basolateral membrane into the apical membrane and throughout the cytoplasm. ATP depletion also resulted in the redistribution of F-actin from a primarily cortical concentration to a perinuclear location. There was also a rapid, duration-dependent conversion of monomeric G-actin to F-actin starting during the first 5 min of ATP depletion. Taken together, these data suggest that ATP depletion causes profound alterations in cell polarity by inducing major changes in the actin cytoskeletal architecture.