Cloning and preliminary characterization of a calcium-binding protein closely related to nucleolin on the apical surface of inner medullary collecting duct cells

Regulatory Effects of Damaged Renal Epithelial Cells After Repair by Porphyra yezoensis Polysaccharides with Different Sulfation Degree on the Calcium Oxalate Crystal-Cell Interaction

BACKGROUND

The interaction between urinary microcrystals and renal epithelial cells is closely related to kidney stone formation. However, the mechanism of cell state changes that affect crystal-cell interaction remains unclear.

METHODS

This study investigated the relationship between the sulfate group (-OSO3 -) content in Porphyra yezoensis polysaccharide (PYP) and the ability to repair damaged cells, as well as the changes in cell adhesion and endocytosis of nano-calcium oxalate monohydrate (COM) crystals before and after PYP repair of damaged renal tubular epithelial cells. The sulfur trioxide-pyridine method was used to sulfate PYP (-OSO3 - content of 14.14%), and two kinds of sulfated PYPs with -OSO3 - content of 20.28% (SPYP1) and 27.14% (SPYP2) were obtained. The above three PYPs were used to repair oxalate-damaged human proximal tubular epithelial cells (HK-2), and the changes in the biochemical indicators of the cells before and after the repair and the changes in cell adhesion and endocytosis of nano-COM crystals were detected.

RESULTS

After repair by PYPs, the cell viability increased, the number of reactive oxygen species decreased, and the reduction of mitochondrial membrane potential and the release of intracellular Ca2+ were suppressed. The cells repaired by PYPs inhibited the adhesion of nano-COM crystals while promoting the endocytosis of the adhered crystals. The endocytosed crystals mainly accumulated in the lysosome. The ability of PYPs to repair cell damage, inhibit crystal adhesion, and promote crystal endocytosis was enhanced when the -OSO3 - content increased. Among them, SPYP2 with the highest -OSO3 - content showed the best biological activity.

CONCLUSION

SPYP2 showed the best ability to repair damaged cells, followed by SPYP1 and PYP. SPYP2 may become a potential green drug that inhibits the formation and recurrence of calcium oxalate stones.

Alpha-enolase on apical surface of renal tubular epithelial cells serves as a calcium oxalate crystal receptor

To search for a strategy to prevent kidney stone formation/recurrence, this study addressed the role of α-enolase on apical membrane of renal tubular cells in mediating calcium oxalate monohydrate (COM) crystal adhesion. Its presence on apical membrane and in COM crystal-bound fraction was confirmed by Western blotting and immunofluorescence staining. Pretreating MDCK cells with anti-α-enolase antibody, not isotype-controlled IgG, dramatically reduced cell-crystal adhesion. Immunofluorescence staining also confirmed the direct binding of purified α-enolase to COM crystals at {121} > {100} > {010} crystal faces. Coating COM crystals with urinary proteins diminished the crystal binding capacity to cells and purified α-enolase. Moreover, α-enolase selectively bound to COM, not other crystals. Chemico-protein interactions analysis revealed that α-enolase interacted directly with Ca²⁺ and Mg²⁺. Incubating the cells with Mg²⁺ prior to cell-crystal adhesion assay significantly reduced crystal binding on the cell surface, whereas preincubation with EDTA, a divalent cation chelator, completely abolished Mg²⁺ effect, indicating that COM and Mg²⁺ competitively bind to α-enolase. Taken together, we successfully confirmed the role of α-enolase as a COM crystal receptor to mediate COM crystal adhesion at apical membrane of renal tubular cells. It may also serve as a target for stone prevention by blocking cell-crystal adhesion and stone nidus formation.

Surface expressed nucleolin is constantly induced in tumor cells to mediate calcium-dependent ligand internalization

BACKGROUND

Nucleolin is one of the major proteins of the nucleolus, but it is also expressed on the cell surface where is serves as a binding protein for variety of ligands implicated in tumorigenesis and angiogenesis. Emerging evidence suggests that the cell-surface expressed nucleolin is a strategic target for an effective and nontoxic cancer therapy.

METHODOLOGY/PRINCIPAL FINDINGS

By monitoring the expression of nucleolin mRNA, and by measuring the level of nucleolin protein recovered from the surface and nucleus of cells, here we show that the presence of nucleolin at the cell surface is dependent on the constant induction of nucleolin mRNA. Indeed, inhibitors of RNA transcription or translation block expression of surface nucleolin while no apparent effect is observed on the level of nucleolin in the nucleus. The estimated half-life of surface nucleolin is less than one hour, whereas that of nuclear nucleolin is more than 8 hours. Nucleolin mRNA induction is reduced markedly in normal fibroblasts that reach confluence, while it occurs continuously even in post-confluent epithelial tumor cells consistent with their capacity to proliferate without contact inhibition. Interestingly, cold and heat shock induce nucleolin mRNA concomitantly to enhanced mRNA expression of the heat shock protein 70, thus suggesting that surface nucleolin induction also occurs in response to an environmental insult. At the cell surface, one of the main functions of nucleolin is to shuttle specific extracellular ligands by an active transport mechanism, which we show here to be calcium dependent.

CONCLUSION/SIGNIFICANCE

Our results demonstrate that the expression of surface nucleolin is an early metabolic event coupled with tumor cell proliferation and stress response. The fact that surface nucleolin is constantly and abundantly expressed on the surface of tumor cells, makes them a preferential target for the inhibitory action of anticancer agents that target surface nucleolin.

The tubular epithelium in the initiation and course of intratubular nephrocalcinosis

Intratubular nephrocalcinosis is defined as the histological observation of calcium oxalate and/or calcium phosphate deposits retained within the lumen of the renal tubules. As the tubular epithelium is the primary interaction partner of crystals formed in the tubular fluid, the role of the epithelial cells in nephrocalcinosis has been investigated intensively. This review summarizes our current understanding on how the tubular epithelium mechanistically appears to be involved both in the initiation and in the course of nephrocalcinosis, with emphasis on in vivo observations.

Induction of apoptosis with cisplatin enhances calcium oxalate crystal adherence to inner medullary collecting duct cells

Attachment of stone crystals to tubular epithelium may initiate kidney stone formation. We previously reported that apical nucleolin related protein (NRP) expression during mitosis enhance attachment of Ca oxalate monohydrate crystals (COM). Some forms of injury may also increase affinity for crystals. We examined changes in subcellular localization of NRP during the course of cisplatin-induced apoptosis in cultured inner medullary collecting duct cells. Caspase-3 activation and chromatin condensation followed by nuclear fragmentation occurred after 20 h exposure to cisplatin, indicating the development of apoptosis. Cells were fixed without permeabilization and stained for surface NRP. Cells with condensed chromatin showed little or no cytoplasmic or apical NRP. Those at an early stage of nuclear fragmentation had cytoplasmic but not apical NRP and cells with advanced nuclear fragmentation were positively stained for apical NRP. Membrane proteins isolated by apical biotinylation and precipitated with avidin were analyzed by Western blot. Apical NRP was markedly increased after cisplatin compared to control, while expression of the apical marker, GP-135, and other putative attachment protein were unchanged. Hyaluronic acid was decreased. Cultures with apoptotic cells demonstrated increased adherence of COM that was inhibited by the polyanion (poly)aspartic acid. We conclude that pre-existing apoptotic injury may promote calcium oxalate crystals attachment to renal tubular epithelium via apical NRP expression.

The cell surface expressed nucleolin is a glycoprotein that triggers calcium entry into mammalian cells

Nucleolin is an ubiquitous nucleolar phosphoprotein involved in fundamental aspects of transcription regulation, cell proliferation and growth. It has also been described as a shuttling molecule between nucleus, cytosol and the cell surface. Several studies have demonstrated that surface nucleolin serves as a receptor for various extracellular ligands implicated in cell proliferation, differentiation, adhesion, mitogenesis and angiogenesis. Previously, we reported that nucleolin in the extranuclear cell compartment is a glycoprotein containing N- and O-glycans. In the present study, we show that glycosylation is an essential requirement for surface nucleolin expression, since it is prevented when cells are cultured in the presence of tunicamycin, an inhibitor of N-glycosylation. Accordingly, surface but not nuclear nucleolin is radioactively labeled upon metabolic labeling of cells with [(3)H]glucosamine. Besides its well-demonstrated role in the internalization of specific ligands, here we show that ligand binding to surface nucleolin could also induce Ca(2+) entry into cells. Indeed, by flow cytometry, microscopy and patch-clamp experiments, we show that the HB-19 pseudopeptide, which binds specifically surface nucleolin, triggers rapid and intense membrane Ca(2+) fluxes in various types of cells. The use of several drugs then indicated that Store-Operated Ca(2+) Entry (SOCE)-like channels are involved in the generation of these fluxes. Taken together, our findings suggest that binding of an extracellular ligand to surface nucleolin could be involved in the activation of signaling pathways by promoting Ca(2+) entry into cells.

Recent advances in the pathophysiology of nephrolithiasis

Over the past 10 years, major progress has been made in the pathogenesis of uric acid and calcium stones. These advances have led to our further understanding of a pathogenetic link between uric acid nephrolithiasis and the metabolic syndrome, the role of Oxalobacter formigenes in calcium oxalate stone formation, oxalate transport in Slc26a6-null mice, the potential pathogenetic role of Randall's plaque as a precursor for calcium oxalate nephrolithiasis, and the role of renal tubular crystal retention. With these advances, we may target the development of novel drugs including (1) insulin sensitizers; (2) probiotic therapy with O. formigenes, recombinant enzymes, or engineered bacteria; (3) treatments that involve the upregulation of intestinal luminal oxalate secretion by increasing anion transporter activity (Slc26a6), luminally active nonabsorbed agents, or oxalate binders; and (4) drugs that prevent the formation of Randall's plaque and/or renal tubular crystal adhesions.

Proposed mechanisms in renal tubular crystal retention

The production of concentrated urine inevitably leads to the precipitation of poorly soluble waste salts in the renal tubular fluid. These crystallization processes are physiologic and without consequences as long as all crystals are excreted with the urine. The retention of crystals in the renal tubules, however, may lead to tubular nephrocalcinosis. Here, we present a brief survey of the possible mechanisms involved in this process, which seems to depend predominantly on the presence of regenerating/(re)differentiating cells in the renal tubules. Crystal binding to the surface of these cells can be mediated by a number of luminal membrane molecules, including acidic fragment of nucleolin-related protein, annexin-II, osteopontin, and hyaluronan.

Overexpression of nucleolin in chronic lymphocytic leukemia cells induces stabilization of bcl2 mRNA

B-cell chronic lymphocytic leukemia (CLL) is characterized by the accumulation of clonal B cells that are resistant to apoptosis as a result of bcl2 oncogene overexpression. Studies were done to determine the mechanism for the up-regulation of bcl-2 protein observed in CD19+ CLL cells compared with CD19+ B cells from healthy volunteers. The 11-fold higher level of bcl-2 protein in CLL cells was positively correlated with a 26-fold elevation in the cytosolic level of nucleolin, a bcl2 mRNA-stabilizing protein. Measurements of the bcl2 heterogeneous nuclear/bcl2 mRNA (hnRNA)/mRNA ratios and the rates of bcl2 mRNA decay in cell extracts indicated that the 3-fold higher steady-state level of bcl2 mRNA in CLL cells was the result of increased bcl2 mRNA stability. Nucleolin was present throughout the nucleus and cytoplasm of CLL cells, whereas in normal B cells nucleolin was only detected in the nucleus. The addition of recombinant human nucleolin to extracts of normal B cells markedly slowed the rate of bcl2 mRNA decay. SiRNA knockdown of nucleolin in MCF-7 cells resulted in decreased levels of bcl2mRNA and protein but no change in beta-actin. These results indicate that bcl-2 overexpression in CLL cells is related to stabilization of bcl2 mRNA by nucleolin.

Cell biology of pathologic renal calcification: contribution of crystal transcytosis, cell-mediated calcification, and nanoparticles

INTRODUCTION

The earliest lesion in the kidneys of idiopathic calcium oxalate stone formers is deposition of calcium phosphate in the interstitium, termed a Randall's plaque. Yet the cellular and molecular factors leading to their formation are unknown.

METHODS

The influence of urinary proteins on adhesion of preformed calcium oxalate crystals to rat continuous inner medullary collecting duct (cIMCD) cells was studied in vitro, and cIMCD cells were also exposed to calcifying media containing beta-glycerophosphate for up to 28 days. Renal tissue was obtained from a stone-forming and non-stone-forming individual at the time of nephrectomy. These nanoparticles, isolated from renal stones obtained at the time of surgical resection, were analyzed and propagated in standard cell culture medium.

RESULTS

Urinary proteins influence crystal adhesion to renal epithelial cells, and this activity is abnormal in the urine of stone-forming patients. cIMCD cells assumed an osteoblastic phenotype when exposed to the calcifying medium, expressing two bone matrix proteins (osteopontin and bone sialoprotein) that were also identified in the kidney of the stone-forming patient and associated with crystal deposition. Nanoparticles were propagated from the majority of renal stones. Isolates were susceptible to selected metabolic inhibitors and antibiotics and contained conserved bacterial proteins and deoxyribonucleic acid (DNA).

CONCLUSIONS

These results suggest new paradigms for Randall's plaque formation and idiopathic calcium oxalate stone disease. It seems unlikely that these events are driven solely by physical chemistry; rather, they are critically influenced by specific proteins and cellular responses, and understanding these events will provide clues toward novel therapeutic targets.

Role of crystal surface adhesion in kidney stone disease

PURPOSE OF REVIEW

Atomic force microscopy has been used recently to characterize the adhesion force between selected calcium oxalate crystal surfaces and biologically relevant chemical groups attached to the atomic force microscopy probe tip. These measurements have permitted comparisons of the adhesion properties of different, well defined crystal faces, as well as determination of the influence of solution-phase macromolecules on adhesion. These studies have produced new insight into the specific chemical interactions that regulate kidney stone formation.

RECENT FINDINGS

The adhesion force measurements have demonstrated that the large hexagonal (100) face of calcium oxalate monohydrate is the most adhesive. In contrast, the large (101) face of calcium oxalate dihydrate is the least adhesive. Carboxylate and amidinium groups on the atomic force microscopy tip exhibit equivalently large adhesion at a given crystal face, implicating specific binding to crystal surface lattice ions. Solution-phase macromolecules modulate adhesion in a face-selective manner, dependent on their chemical structures.

SUMMARY

The low adhesion force for calcium oxalate dihydrate predicts a decreased ability of these crystals to aggregate or attach to cells, and correlates with the relative absence of calcium oxalate dihydrate in kidney stones. These measurements provide new understanding of the macromolecular regulation of crystal aggregation and attachment to cells in stone formation.

Protein regulation of intrarenal crystallization

PURPOSE OF REVIEW

In this review we discuss the key role renal proteins appear to play during initiation and growth of renal stones.

RECENT FINDINGS

Specific macromolecules have been identified in urine that can regulate crystal nucleation, growth, aggregation, and adhesion to renal cells. Many are incorporated into the matrix of crystals while they grow, including urinary prothrombin fragment 1, thereby increasing crystal susceptibility to degradation by cellular proteases. None of these macromolecular inhibitors appears to be quantitatively decreased in the urine of stone formers, although functional deficiencies thought due to abnormal glycosylation have been implicated, especially in the case of Tamm Horsfall protein. Increasing information is available on the nature and expression of crystal binding molecules on the renal cell surface, and they appear to be maximally expressed in response to stressful stimuli. Studies that employ atomic force microscopy and knockout mice are now being used to further clarify macromolecule-crystal interactions.

SUMMARY

The exact series of events that transform supersaturation to crystal formation and renal stones are poorly defined. Multiple anchored and soluble renal proteins potentially modulate or even regulate these events. A combination of proteomics and molecular biology seems likely to unravel these critical mediators in the coming years.

Crystal Retention in Renal Stone Disease: A Crucial Role for the Glycosaminoglycan Hyaluronan?

The mechanisms that are involved in renal stone disease are not entirely clear. In this article, the various concepts that have been proposed during the past century are reviewed briefly and integrated into current insights. Much attention is dedicated to hyaluronan (HA), an extremely large glycosaminoglycan that may play a central role in renal stone disease. The precipitation of poorly soluble calcium salts (crystal formation) in the kidney is the inevitable consequence of producing concentrated urine. HA is a major constituent of the extracellular matrix in the renal medullary interstitium and the pericellular matrix of mitogen/stress-activated renal tubular cells. HA is an excellent crystal-binding molecule because of its size, negative ionic charge, and ability to form hydrated gel-like matrices. Crystal binding to HA leads to crystal retention in the renal tubules (nephrocalcinosis) and to the formation of calcified plaques in the renal interstitium (Randall's plaques). It remains to be determined whether one or both forms of renal crystal retention are involved in the development of kidney stones (nephrolithiasis).

Role of nucleolin in human parainfluenza virus type 3 infection of human lung epithelial cells

Human parainfluenza virus type 3 (HPIV-3) is an airborne pathogen that infects human lung epithelial cells from the apical (luminal) plasma membrane domain. In the present study, we have identified cell surface-expressed nucleolin as a cellular cofactor required for the efficient cellular entry of HPIV-3 into human lung epithelial A549 cells. Nucleolin was enriched on the apical cell surface domain of A549 cells, and HPIV-3 interacted with nucleolin during entry. The importance of nucleolin during HPIV-3 replication was borne out by the observation that HPIV-3 replication was significantly inhibited following (i). pretreatment of cells with antinucleolin antibodies and (ii). preincubation of HPIV-3 with purified nucleolin prior to its addition to the cells. Moreover, HPIV-3 cellular internalization and attachment assays performed in the presence of antinucleolin antibodies and purified nucleolin revealed the requirement of nucleolin during HPIV-3 internalization but not during attachment. Thus, these results suggest that nucleolin expressed on the surfaces of human lung epithelial A549 cells plays an important role during HPIV-3 cellular entry.

Phagocytosis of M. paratuberculosis fails to activate expression of NADH dehydrogenase and nucleolin-related protein in bovine macrophages

Mycobacterium avium subspecies paratuberculosis (M. paratuberculosis) is a facultative intracellular bacterium and causal agent of Johne's disease in cattle. Following phagocytosis, M. paratuberculosis resides and replicates in macrophage phagosomes that fail to mature. Differential display reverse transcription polymerase chain reaction (DDRT-PCR) was used as a high throughput initial screen to begin to test the hypothesis that macrophage gene expression patterns would be differentially affected by M. paratuberculosis when compared to readily degraded bacteria or non-degradable latex beads. Gene expression profiles from immortalized bovine macrophage cells (BOMAC) exposed to M. paratuberculosis were compared to gene expression profiles for BOMAC cells exposed to Escherichia coli, latex beads or PBS. Amplicons representing genes specifically activated or repressed during M. paratuberculosis phagocytosis were cloned for further investigation. Northern blot hybridizations preformed using DDRT-PCR-derived amplicons 3-1-4, 5-2-10, 5-4-2 and 4-1-6 confirmed stimuli dependent differential gene expression. Expression pattern observed for amplicon 3-1-4 represents genes that are up-regulated following phagocytosis of E. coli or latex beads, but not M. paratuberculosis. Amplicon 5-2-10 exhibited a pattern of expression representative of genes that are up-regulated strongly following phagocytosis of E. coli or latex beads but only moderately following M. paratuberculosis phagocytosis. Expression pattern of the gene for amplicon 5-4-2 was representative of genes that are specifically suppressed following M. paratuberculosis phagocytosis, while the amplicon 4-1-6 gene expression pattern represented genes that are generally suppressed following phagocytosis of any of the three stimuli. DNA sequencing and Genbank database analysis of these amplicons revealed that amplicon 3-1-4, whose expression failed to activate following M. paratuberculosis phagocytosis, had high levels of similarity to a Rattus norvegicus nucleolin-related protein (NRP). Amplicon 5-2-10, which increased expression moderately following M. paratuberculosis phagocytosis, was a near perfect match to bovine nicotinamide adenine dinucleotide dehydrogenase (FNADH dehydrogenase) subunit 1 (ND1). Failure to activate these two genes at levels observed following phagocytosis of either E. coli or latex beads may uncover new mechanisms for the survival of M. paratuberculosis within bovine macrophage cells.

Calcium Oxalate Crystal Adherence to Hyaluronan-, Osteopontin-, and CD44-Expressing Injured/Regenerating Tubular Epithelial Cells in Rat Kidneys

ABSTRACT. Retention of crystals in the kidney is an essential early step in renal stone formation. Studies with renal tubular cells in culture indicate that hyaluronan (HA) and osteopontin (OPN) and their mutual cell surface receptor CD44 play an important role in calcium oxalate (CaOx) crystal binding during wound healing. This concept was investigated in vivo by treating rats for 1, 4, and 8 d with ethylene glycol (0.5 and 0.75%) in their drinking water to induce renal tubular cell damage and CaOx crystalluria. Tubular injury was morphologically scored on periodic acid-Schiff–stained renal tissue sections and tissue repair assessed by immunohistochemical staining for proliferating cell nuclear antigen. CaOx crystals were visualized in periodic acid-Schiff–stained sections by polarized light microscopy, and renal calcium deposits were quantified with von Kossa staining. HA was visualized with HA-binding protein and OPN and CD44 immunohistochemically with specific antibodies and quantified with an image analyzer system. Already after 1 d of treatment, both concentrations of ethylene glycol induced hyperoxaluria and CaOx crystalluria. At this point, there was neither tubular injury nor crystal retention in the kidney, and expression of HA, OPN, and CD44 was comparable to untreated controls. After 4 and 8 d of ethylene glycol, however, intratubular crystals were found adhered to injured/regenerating (proliferating cell nuclear antigen positive) tubular epithelial cells, expressing HA, OPN, and CD44 at their luminal membrane. In conclusion, the expression of HA, OPN, and CD44 by injured/regenerating tubular cells seems to play a role in retention of crystals in the rat kidney.

Pericellular matrix formation by renal tubule epithelial cells in relation to crystal binding

BACKGROUND/AIM

Retention of crystals in the kidney ultimately leads to renal stone formation. Hyaluronan (HA) has been identified as binding molecule for calcium oxalate monohydrate crystals. The association of high molecular mass (M(r)) HA with cell surface receptors such as CD44 gives rise to pericellular matrix (PCM) formation by many eukaryotic cells in culture. Here, we study the ability of several renal tubular cell lines to assemble PCMs and to synthesize high-M(r) HA during proliferation in relation to crystal retention.

METHODS

PCM assembly by MDCK-I, MDCK-II, and LLC-PK1 cells was visualized by particle exclusion assay. Metabolic labeling studies were performed to estimate the cellular production of HA. The expression of CD44 and HA was studied using fluorescent probes, and crystal binding was quantified with radiolabeled calcium oxalate monohydrate.

RESULTS

PCMs were formed, and HA was expressed by most MDCK-I and some MDCK-II, but not by LLC-PK1 cells. All cell types expressed CD44 at their apical surface. MDCK-I and MDCK-II cells secreted, respectively, 14.7 +/- 1.6 and 0.5 +/- 0.2 pmol [3H]glucosamine incorporated in high-M(r) HA, whereas LLC-PK1 cells did not secrete HA. Streptomyces hyaluronidase treatment significantly decreased crystal binding (microg/cm2) to MDCK-I cells (from 8.6 +/- 0.4 to 3.9 +/- 0.9), but hardly to MDCK-II cells (from 10.2 +/- 0.2 to 9.6 +/- 0.1) or LLC-PK1 cells (from 10.2 +/- 0.8 to 9.9 +/- 0.3).

CONCLUSIONS

There are various forms of crystal binding to renal tubular cells in culture. Crystal attachment to MDCK-I and some MDCK-II cells involves PCM assembly that requires high-M(r) HA synthesis. HA production and PCM formation do not play a role in crystal binding to LLC-PK1 and the majority of MDCK-II cells. It remains to be determined which form of binding is involved in renal stone disease.

Whole urinary proteins coat calcium oxalate monohydrate crystals to greatly decrease their adhesion to renal cells

PURPOSE

Adhesion of urinary crystals to renal tubular cells could be a critical event that triggers a cascade of responses ending in kidney stone formation. We clarified the role of urinary macromolecules during calcium oxalate monohydrate (COM) crystal adhesion to cells.

MATERIALS AND METHODS

To assess COM crystal binding to cells in the presence of whole urine and fractions thereof we used monolayer cultures of distal nephron derived Madin-Darby canine kidney, type I cells as a model system.

RESULTS

COM crystal adhesion to cells was decreased in the presence of whole urine compared with an ultrafiltrate prepared by passing urine through a 10 kDa cutoff membrane. Supplementing the ultrafiltrate with urinary concentrate containing proteins greater than 10 kDa returned crystal adhesion to low levels, similar to whole urine. Macromolecules in whole urine acted to decrease binding to cells by coating crystals and 4 proteins previously implicated in the pathogenesis of nephrolithiasis were detected on coated crystals (bikunin, osteopontin, prothrombin fragment 1 + 2 and Tamm-Horsfall glycoprotein). Crystals precipitated and grown in whole urine also bound less avidly to cells than crystals precipitated in artificial urine.

CONCLUSIONS

This study confirms that macromolecules present in whole urine can coat crystals and, thereby, block their adhesion to renal tubular cells. Preventing crystal retention in the kidney could be an important mechanism whereby these macromolecules protect against kidney stones.

Annexin II is present on renal epithelial cells and binds calcium oxalate monohydrate crystals

Attachment of newly formed crystals to renal epithelial cells appears to be a critical step in the development of kidney stones. The current study was undertaken to identify potential calcium oxalate monohydrate (COM) crystal-binding proteins on the surface of renal tubular cells. Apical membranes were prepared from confluent monolayers of renal epithelial cells (MDCKI line), and COM crystal affinity was used to isolate crystal-binding proteins that were then subjected to electrophoresis and electroblotting. Microsequencing of the most prominent COM crystal-binding protein (M(r) of 37 kD) identified it as annexin II (Ax-II). When exposed proteins on the surface of intact monolayers were biotinylated and then isolated using streptavidin agarose beads, Ax-II was detected, suggesting that at least a portion is exposed on the apical cell surface. Ax-II was not completely extracted by 0.1 M Na(2)CO(3), suggesting that at least a portion of cellular Ax-II is an intrinsic membrane-bound protein. Using confocal immunofluorescence microscopy, Ax-II was visualized together with Caveolin-1 (Cav-1) on the apical membrane of intact MDCKI cells. Cells pretreated with a monoclonal anti-Ax-II antibody bound significantly fewer COM crystals, whereas anti-LDL receptor antibody did not decrease COM binding, further suggesting a functional role for Ax-II during adhesion of crystals to intact cells. These results suggest that Ax-II avidly binds COM crystals and is present on the apical surface of MDCKI cells. Therefore, in the intact nephron, Ax-II could mediate adhesion of COM crystals to cells, and altered exposure of Ax-II on the surface of renal tubular cells could promote crystal retention and possibly kidney stone formation.

Crystal retention capacity of cells in the human nephron: involvement of CD44 and its ligands hyaluronic acid and osteopontin in the transition of a crystal binding- into a nonadherent epithelium

Nephrolithiasis requires formation of crystals followed by their retention and accumulation in the kidney. Crystal retention can be caused by the association of crystals with the epithelial cells lining the renal tubules. The present study investigated the interaction between calcium oxalate monohydrate (COM) crystals and primary cultures of human proximal (PTC) and distal tubular/collecting duct cells (DTC). Both PTC and DTC were susceptible to crystal binding during the first days post-seeding (4.9 +/- 0.8 micro g COM/cm2), but DTC lost this affinity when the cultures developed into confluent monolayers with functional tight junctions (0.05 +/- 0.02 micro g COM/cm2). Confocal microscopy demonstrated the expression of the transmembrane receptor protein CD44 and its ligands osteopontin (OPN) and hyaluronic acid (HA) at the apical membrane of proliferating tubular cells; at confluence, CD44 was expressed at the basolateral membrane and OPN and HA were no longer detectable. In addition, a particle exclusion technique revealed that proliferating cells were surrounded by HA-rich pericellular matrices or "cell coats" extending several microns from the cell surface. Disintegration of these coats with hyaluronidase significantly decreased the cell surface affinity for crystals. Furthermore, CD44, OPN, and HA were also expressed in vivo at the luminal side of tubular cells in damaged kidneys. These results suggest (1) that the intact distal tubular epithelium of the human kidney does not bind crystals, and (2) that crystal retention in the human kidney may depend on the expression of CD44-, OPN-, and-HA rich cell coats by damaged distal tubular epithelium.

A novel protein localized to the fibrillar compartment of the nucleolus and to the brush border of a secretory cell

We report the identification and molecular characterization of a novel abundant nucleolar protein of the dipteran Chironomus tentans. As shown by Western blot analysis, this protein is present in nuclear extracts in a phosphorylated form with a mobility corresponding to 100 kDa. Therefore, the protein has been termed Chironomus tentans p100, or p100 for short. Analysis of the cDNA-derived primary structure of p100 indicates a protein that contains a combination of structural domains which could be involved in interactions with proteins and nucleic acids: twelve alternating acidic and basic repeats, a glycine-arginine-rich domain and a region with two zinc fingers of the C4-type. Acidic and basic repeats are typical for a group of nonribosomal nucleolar proteins. The best-studied representatives of this group are Nopp140 and nucleolin, proteins with structural and regulatory functions in rDNA transcription. Immunocytology and immunoelectron microscopy of Chironomus tentans salivary gland cells have shown that the p100 protein is located in the fibrillar compartment of the nucleolus, while it is almost absent from the granular compartment and from the nucleoplasm. The p100 protein remains in the nucleolus after removal of RNA and DNA by digestion with nucleases. This indicates that p100 might be a constituent of the nucleolar proteinaceous framework. Remarkably, p100 is also localized in the brush border in the apical part of the salivary gland cell. The presence of p100 both in the nucleolus and at the apical plasma membrane suggests that it could be involved in coordination of the level of protein production and export from the cell through regulation of the level of rRNA production in the nucleolus.

Cell surface-localized nucleolin is a eukaryotic receptor for the adhesin intimin-gamma of enterohemorrhagic Escherichia coli O157:H7

Intimin-gamma is an outer membrane protein of enterohemorrhagic Escherichia coli (EHEC) O157:H7 that is required for the organism to adhere tightly to HEp-2 cells and to colonize experimental animals. Another EHEC O157:H7 protein, the Transferred intimin receptor (Tir), is considered the primary receptor for intimin-gamma. Nevertheless, Tir-independent binding of intimin-gamma to HEp-2 cells has been reported. This observation suggests the existence of a eukaryotic receptor(s) for intimin-gamma. In this study, we sought to identify that receptor(s). First, we determined by equilibrium binding titration that the association of purified intimin-gamma with HEp-2 cells was specific and consistent with a single host cell receptor. Second, we isolated a protein from lysates of HEp-2 cells that bound intimin-gamma and subsequently identified this molecule as nucleolin, a protein involved in cell growth regulation that can be cell surface-expressed. Third, we established that purified intimin-gamma and nucleolin were co-localized on the surface of HEp-2 cells and that the site of EHEC O157:H7 attachment was associated with regions of nucleolin expression. Finally, we demonstrated that mouse anti-nucleolin sera significantly decreased the adherence of EHEC O157:H7 to HEp-2 cells. From this, we conclude that nucleolin is the HEp-2 cell receptor for intimin-gamma expressed by EHEC O157:H7.

Nucleolin is a calcium-binding protein

We have purified a prominent 110-kDa protein (p110) from 1.6 M NaCl extracts of rat liver nuclei that appears to bind Ca2+. p110 was originally identified by prominent blue staining with 'Stains-All' in sodium dodecyl sulfate-polyacrylamide gels and was observed to specifically bind ruthenium red and 45Ca2+ in nitrocellulose blot overlays. In spin-dialysis studies, purified p110 saturably bound approximately 75 nmol Ca2+/mg protein at a concentration of 1 mM total Ca2+ with half-maximal binding observed at 105 microM Ca2+. With purification, p110 became increasingly susceptible to proteolytic (likely autolytic) fragmentation, although most intermediary peptides between 40 and 90 kDa retained "Stains-All", ruthenium red, and 45Ca2+ binding. N-terminal sequencing of intact p110 and a 70-kDa autolytic peptide fragment revealed a strong homology to nucleolin. Two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE)/IEF revealed autolysis produced increasingly acidic peptide fragments ranging in apparent pI's from 5.5 for intact p110 to 3.5 for a 40 kDa peptide fragment. Intact p110 and several peptide fragments were immunostained with a highly specific anti-nucleolin antibody, R2D2, thus confirming the identity of this protein with nucleolin. These annexin-like Ca2+-binding characteristics of nucleolin are likely contributed by its highly acidic argyrophilic N-terminus with autolysis apparently resulting in largely selective removal of its basic C-terminal domain. Although the Ca2+-dependent functions of nucleolin are unknown, we discuss the possibility that like the structurally analogous HMG-1, its Ca2+-dependent actions may regulate chromatin structure, possibly during apoptosis.

Inhibition of DNA replication and induction of S phase cell cycle arrest by G-rich oligonucleotides

The discovery of G-rich oligonucleotides (GROs) that have non-antisense antiproliferative activity against a number of cancer cell lines has been recently described. This biological activity of GROs was found to be associated with their ability to form stable G-quartet-containing structures and their binding to a specific cellular protein, most likely nucleolin (Bates, P. J., Kahlon, J. B., Thomas, S. D., Trent, J. O., and Miller, D. M. (1999) J. Biol. Chem. 274, 26369-26377). In this report, we further investigate the novel mechanism of GRO activity by examining their effects on cell cycle progression and on nucleic acid and protein biosynthesis. Cell cycle analysis of several tumor cell lines showed that cells accumulate in S phase in response to treatment with an active GRO. Analysis of 5-bromodeoxyuridine incorporation by these cells indicated the absence of de novo DNA synthesis, suggesting an arrest of the cell cycle predominantly in S phase. At the same time point, RNA and protein synthesis were found to be ongoing, indicating that arrest of DNA replication is a primary event in GRO-mediated inhibition of proliferation. This specific blockade of DNA replication eventually resulted in altered cell morphology and induction of apoptosis. To characterize further GRO-mediated inhibition of DNA replication, we used an in vitro assay based on replication of SV40 DNA. GROs were found to be capable of inhibiting DNA replication in the in vitro assay, and this activity was correlated to their antiproliferative effects. Furthermore, the effect of GROs on DNA replication in this assay was related to their inhibition of SV40 large T antigen helicase activity. The data presented suggest that the antiproliferative activity of GROs is a direct result of their inhibition of DNA replication, which may result from modulation of a replicative helicase activity.

The voltage-dependent Cl(-) channel ClC-5 and plasma membrane Cl(-) conductances of mouse renal collecting duct cells (mIMCD-3)

1. We have tested the hypothesis that the voltage-dependent Cl(-) channel, ClC-5 functions as a plasma membrane Cl(-) conductance in renal inner medullary collecting duct cells. 2. Full-length mouse kidney ClC-5 (mClC-5) was cloned and transiently expressed in CHO-K1 cells. Fast whole-cell patch-clamp recordings confirmed that mClC-5 expression produces a voltage-dependent, strongly outwardly rectifying Cl(-) conductance that was unaffected by external DIDS. 3. Slow whole-cell recordings, using nystatin-perforated patches from transfected CHO-K1 cells, also produced voltage-dependent Cl(-) currents consistent with ClC-5 expression. However, under this recording configuration an endogenous DIDS-sensitive Ca(2+)-activated Cl(-) conductance was also evident, which appeared to be activated by green fluorescent protein (GFP) transfection. 4. A mClC-5-GFP fusion protein was transiently expressed in CHO-K1 cells; confocal laser scanning microscopy (CLSM) showed localization at the plasma membrane, consistent with patch-clamp experiments. 5. Endogenous expression of mClC-5 was demonstrated in mouse renal collecting duct cells (mIMCD-3) by RT-PCR and by immunocytochemistry. 6. Using slow whole-cell current recordings, mIMCD-3 cells displayed three biophysically distinct Cl(-)-selective currents, which were all inhibited by DIDS. However, no cells exhibited whole-cell currents that had mClC-5 characteristics. 7. Transient transfection of mIMCD-3 cells with antisense mClC-5 had no effect on the endogenous Cl(-) conductances. Transient transfection with sense mClC-5 failed to induce the Cl(-) conductance seen in CHO-K1 cells but stimulated levels of the endogenous Ca(2+)-activated Cl(-) conductance 24 h post-transfection. 8. Confocal laser scanning microscopy of mIMCD-3 cells transfected with mClC-5-GFP showed that the protein was absent from the plasma membrane and was instead localized to acidic endosomal compartments. 9. These data discount a major role for ClC-5 as a plasma membrane Cl(-) conductance in mIMCD-3 cells but suggest a role in endosomal function.

Nanobacteria: controversial pathogens in nephrolithiasis and polycystic kidney disease

Nanobacteria are unconventional agents 100-fold smaller than common bacteria that can replicate apatite-forming units. Nanobacteria are powerful mediators of biogenic apatite nucleation (crystal form of calcium phosphate) and crystal growth under conditions simulating blood and urine. Apatite is found in the central nidus of most kidney stones and in mineral plaques (Randall's plaques) in renal papilla. The direct injection of nanobacteria into rat kidneys resulted in stone formation in the nanobacteria-injected kidney during one month follow-up, but not in the control kidney injected with vehicle. After intravenous administration in rats and rabbits, nanobacteria are rapidly excreted from the blood into the urine, as a major elimination route, and damage renal collecting tubuli. Nanobacteria are cytotoxic to fibroblasts in vitro. Human kidney cyst fluids contain nanobacteria. Nanobacteria thus appear to be potential provocateurs and initiators of kidney stones, tubular damage, and kidney cyst formation. It is hypothesized that nanobacteria are the initial nidi on which kidney stone is built up, at a rate dependent on the supersaturation status of the urine. Those individuals having both nanobacteria and diminished defences against stone formation (i.e. genetic factors, diet and drinking habits) could be at high risk. Kidney cyst formation is hypothesized to involve nanobacteria-induced tubular damage and defective tissue regeneration yielding cyst formation, the extent of which is dependent on genetic vulnerability.

The cell-surface-expressed nucleolin is associated with the actin cytoskeleton

Nucleolin is a RNA- and protein-binding multifunctional protein. Mainly characterized as a nucleolar protein, nucleolin is continuously expressed on the surface of different types of cells along with its intracellular pool within the nucleus and cytoplasm. By confocal and electron microscopy using specific antibodies against nucleolin, we show that cytoplasmic nucleolin is found in small vesicles that appear to translocate nucleolin to the cell surface. Translocation of nucleolin is markedly reduced at low temperature or in serum-free medium, whereas conventional inhibitors of intracellular glycoprotein transport have no effect. Thus, translocation of nucleolin is the consequence of an active transport by a pathway which is independent of the endoplasmic reticulum-Golgi complex. The cell-surface-expressed nucleolin becomes clustered at the external side of the plasma membrane when cross-linked by the nucleolin-specific monoclonal antibody mAb D3. This clustering, occurring at 20 degrees C and in a well-organized pattern, is dependent on the existence of an intact actin cytoskeleton. At 37 degrees C, mAb D3 becomes internalized, thus illustrating that surface nucleolin can mediate intracellular import of specific ligands. Our results point out that nucleolin should also be considered a component of the cell surface where it could be functional as a cell surface receptor for various ligands reported before.