Purification and characterization of a calcium oxalate monohydrate crystal growth inhibitor from human kidney tissue culture medium

Current Status of Protein Biomarkers in Urolithiasis-A Review of the Recent Literature

Urolithiasis is an increasingly common clinical problem worldwide. The formation of stones is a combination of metabolic status, environmental factors, family history and many other aspects. It is important to find new ways to quickly detect and assess urolithiasis because it causes sudden, severe pain and often comes back. One way to do this is by exploring new biomarkers. Current advances in proteomic studies provide a great opportunity for breakthroughs in this field. This study focuses on protein biomarkers and their connection to kidney damage and inflammation during urolithiasis.

The Long Pentraxin PTX3 Is an Endogenous Inhibitor of Hyperoxaluria-Related Nephrocalcinosis and Chronic Kidney Disease

The long pentraxin 3 (PTX3) exerts a variety of regulatory functions in acute and chronic tissue inflammation. In particular, PTX3 acts as an opsonin for a variety of pathogens and endogenous particles. We hypothesized that PTX3 would exhibit opsonin-like functions toward calcium oxalate crystals, too, and inhibit crystal growth. This process is fundamental in kidney stone disease as well as in hyperoxaluria-related nephrocalcinosis, the paradigmatic cause of chronic kidney disease (CKD) in children with primary hyperoxaluria type I due to genetic defects in oxalate metabolism. Direct effects of PTX3 on calcium oxalate crystals were investigated in chemico by adding recombinant PTX3 to supersaturated calcium and oxalate solutions. PTX3, but not isomolar concentrations of albumin, dose-dependently inhibited crystal growth. In vivo, the PTX3 protein was undetectable in tubular epithelial cells and urine of wild-type mice under physiological conditions. However, its levels increased within 3 weeks of feeding an oxalate-rich diet, an exposure inducing hyperoxaluria-related nephrocalcinosis and CKD in selected mouse strains (male and female C57BL/6N and male Balb/c mice) but not in others (male and female 129SV and CD-1, male and female Balb/c mice). Genetic ablation of ptx3 in nephrocalcinosis un-susceptible B6;129 mice was sufficient to raise the oxalate nephropathy phenotype observed in susceptible strains. We conclude that PTX3 is an endogenous inhibitor of calcium oxalate crystal growth. This mechanism limits hyperoxaluria-related nephrocalcinosis, e.g., in primary or secondary hyperoxaluria, and potentially also in the more prevalent kidney stone disease.

Nephrolithiasis: molecular mechanism of renal stone formation and the critical role played by modulators

Urinary stone disease is an ailment that has afflicted human kind for many centuries. Nephrolithiasis is a significant clinical problem in everyday practice with a subsequent burden for the health system. Nephrolithiasis remains a chronic disease and our fundamental understanding of the pathogenesis of stones as well as their prevention and cure still remains rudimentary. Regardless of the fact that supersaturation of stone-forming salts in urine is essential, abundance of these salts by itself will not always result in stone formation. The pathogenesis of calcium oxalate stone formation is a multistep process and essentially includes nucleation, crystal growth, crystal aggregation, and crystal retention. Various substances in the body have an effect on one or more of the above stone-forming processes, thereby influencing a person's ability to promote or prevent stone formation. Promoters facilitate the stone formation while inhibitors prevent it. Besides low urine volume and low urine pH, high calcium, sodium, oxalate and urate are also known to promote calcium oxalate stone formation. Many inorganic (citrate, magnesium) and organic substances (nephrocalcin, urinary prothrombin fragment-1, osteopontin) are known to inhibit stone formation. This review presents a comprehensive account of the mechanism of renal stone formation and the role of inhibitors/promoters in calcium oxalate crystallisation.

Wu-Ling-San Formula Inhibits the Crystallization of Calcium OxalateIn Vitro

Urinary stone disease is a common disease and has a high rate of recurrence. There is no ideal long-term medical treatment to prevent the recurrence of urinary stones. Wu-Ling-San (WLS) formula has been used for centuries in China for long-term treatment of urological diseases. However, no pharmacological studies have been conducted to evaluate its effect on urinary stone disease. Therefore, using a photospectrometer, we studied the effects of WLS on nucleation, growth and aggregation of calcium oxalate in vitro. The results showed that WLS extract significantly slowed the speed of calcium oxalate ( CaOx ) crystal nucleation. WLS extracts at concentrations of 6.25, 12.5, 25, and 50 mg/ml inhibited nucleation of calcium oxalate crystallization by 344, 387, 543, and 943%, respectively. WLS extracts did not inhibit the growth of CaOx crystallization; however, WLS extracts at concentrations of 12.5 and 25 mg/ml significantly inhibited the aggregation of CaOx crystallization by 74.24% and 75.05%, respectively. WLS extract at a concentration of 50 mg/ml inhibited CaOx aggregation by 92.49%. In conclusion, our results indicate that WLS extract inhibited calcium oxalate nucleation and aggregation, and may have the potential to prevent stone recurrence.

Mass spectroscopic characteristics of low molecular weight proteins extracted from calcium oxalate stones: preliminary study

It is believed that boundary compositions of matrix proteins might play a role in stone formation; however, few proteomic studies concerning matrix proteins in urinary stones have been conducted. In this study, we extracted low molecular weight proteins from calcium oxalate stones and measured their characteristic patterns by mass spectroscopy. A total of 10 stones were surgically removed from patients with urolithiasis. Proteins were extracted from the stones and identified by one-dimensional electrophoresis (sodium dodecyl sulfate buffer [SDS]-polyacrylamide gel electrophoresis [SDS-PAGE]). After in-gel digest, samples were analyzed by the surface-enhanced laser desorption ionization-time of flight (SELDI-TOF) technique. The peptide sequences were analyzed from the data of mass spectroscopy. Proteins were identified from Database Search (SwissProt Protein Database; Swiss Institute of Bioinformatics; http://www.expasy.org/sprot) on a MASCOT server (Matrix Science Ltd.; http://www.matrixscience.com). A total of three bands of proteins (27, 18, and 14 kDa) were identified from SDS-PAGE in each stone sample. A database search (SwissProt) on a MASCOT server revealed that the most frequently seen proteins from band 1 (27 kDa) were leukocyte elastase precursor, cathepsin G precursor, azurocidin precursor, and myeloblastin precursor (EC 3.4.21.76) (leukocyte proteinase 3); band 2 (18 kDa) comprised calgranulin B, eosinophil cationic protein precursor, and lysozyme C precursor; band 3 (14 kDa) showed neutrophil defensin 3 precursor, calgranulin A, calgranulin C, and histone H4. The modifications and deamidations found from the mass pattern of these proteins may provide information for the study of matrix proteins. Various lower molecular weight proteins can be extracted from calcium oxalate stones. The characteristic patterns and their functions of those proteins should be further tested to investigate their roles in stone formation.

Urinary macromolecular inhibition of crystal adhesion to renal epithelial cells is impaired in male stone formers

BACKGROUND

Retention of microcrystals that form in tubular fluid could be a critical event in kidney stone formation. This study was performed to determine if urinary macromolecules from stone-forming (SF) individuals have reduced ability to inhibit crystal adhesion to renal cells.

METHODS

A first morning whole urine (WU) sample was obtained from 24 SF subjects (17 males and 7 females) and 24 age-, race-, and sex-matched controls (C). An aliquot of urine was centrifuged and an ultrafiltrate (UF) free of macromolecules >10 kD and 10x concentrate (U(conc)) were prepared.

RESULTS

Supplementing UF with increasing amounts of U(conc) to return the macromolecule concentration to 0.25x, 0.5x, or 1x of baseline progressively decreased crystal binding to cells. This effect was blunted in the male SF group compared to controls (P < 0.05, SF vs. C, for UF plus 0.25x macromolecules). No difference was apparent in the female groups. In order to identify responsible macromolecule(s), calcium oxalate monohydrate (COM) crystals were coated with U(conc) and adherent proteins then released and probed by Western blot. Coated COM crystals from male controls contained 3.5-fold more Tamm-Horsfall protein (THP) than SF subjects (P < 0.01). COM crystal coating with other proteins did not consistently differ between the groups. COM crystal coating by urinary prothrombin fragment 1 (UPTF1, P < 0.05) and crystal adhesion inhibitor (CAI) (P= 0.09) correlated with decreased crystal binding to cells, whereas coating with osteopontin (OPN) correlated with increased adhesion tendency (P < 0.05).

CONCLUSION

Urinary macromolecules >10 kD coat COM crystals and block their adhesion to renal cells. This capacity appears to be blunted in male but not female SF individuals. Multiple urinary proteins may play a role in renal cell-urinary crystal interactions, and THP appears to be one of the more important ones.

A simple procedure for isolating microgram quantities of biologically active bikunin from human urine

OBJECTIVE

To report a simple, relatively rapid protocol to isolate biologically active bikunin from human urine using ion-exchange-trypsin affinity chromatography. Bikunin is a protease inhibitor which has been shown to play a role in various processes, including inhibition of calcium oxalate crystallization, the regulation of proliferation and modulation of carcinogenesis. The unavailability of the purified protein has hampered studies on bikunin's expanding role in these processes.

MATERIALS AND METHODS

Female human urine was dialysed (15 kDa threshold) and crudely fractionated with a double-saturated ammonium sulphate precipitation. The first precipitation was with 35% saturated ammonium sulphate, and the supernatant was harvested, and the second with 90% saturated ammonium sulphate, and the precipitate collected. The protein mixture was then passed over Sepharose SP-fast-flow cation exchange and Sepharose Q-fast-flow anion exchange columns connected in series. The final purification was with a trypsin-affinity column which selectively bound bikunin.

RESULTS

This procedure could recover 1 microg of bikunin per 2 mL of urine, and the final product was essentially free of contaminating inter-alpha-trypsin inhibitor heavy chains or bikunin-heavy chain conjugates. Product purity was confirmed by two-dimensional polyacrylamide gel electrophoresis combined with silver staining or Western blot. All isolations contained the 17 kDa minimally glycoslyated/sulphated form of bikunin and the 28 kDa form of bikunin. Some preparations also contained 33-48 kDa forms of bikunin. The protein cores of all three proteins were confirmed to be bikunin by mass spectrometry and Western blot. Harvested bikunin retained its trypsin inhibitory activity (L-benzoylarginine-p-nitroanilide assay). Preparations containing the 33-45 kDa form had two to three times more trypsin inhibitory activity than preparations without this band.

CONCLUSIONS

This novel ion exchange-trypsin affinity chromatography protocol uses only two chromatographic steps. The product consists of three isomers of biologically active bikunin, free of contaminating heavy chains or bikunin-heavy chain conjugates. The ready availability of purified bikunin should facilitate future studies of bikunin's emerging role in urolithiasis, proliferation and carcinogenesis.

EXTRACT FROM HERNIARIA HIRSUTA COATS CALCIUM OXALATE MONOHYDRATE CRYSTALS AND BLOCKS THEIR ADHESION TO RENAL EPITHELIAL CELLS

PURPOSE

The interaction of calcium oxalate crystals with renal epithelial cells is a critical event in kidney stone formation. In this study we assessed the effect of aqueous extract from Herniaria hirsuta on the adhesion of calcium oxalate monohydrate (COM) crystals to cultured renal cells.

MATERIALS AND METHODS

Madin Darby canine kidney cells were used as a model for studying the adhesion of radioactive COM crystals in the presence and absence of plant extract.

RESULTS

COM crystal binding to cells was inhibited by extract in a concentration dependent manner. Prior exposure of crystals but not cells to extract blocked crystal binding, suggesting that plant molecules can coat and exert their effect at the crystal surface. Crystal attachment appeared related to membrane fluidity since crystal adhesion increased at higher vs lower temperatures (37C vs 0C) and Herniaria extract altered crystal adhesion only under conditions of increased fluidity (increased temperature). Extract also displaced a significant portion of prebound crystals without apparent effects on cell function or the morphology of preexisting calcium oxalate crystals. Herniaria extract exerted no adverse or toxic effect on cells, which proliferated normally in its presence even at relatively high concentrations.

CONCLUSIONS

Our current data suggest a mechanism whereby Herniaria hirsuta extract used in traditional medicine might prevent and possibly eliminate preexisting kidney stones. Further characterization of the active compound(s) could identify a new candidate drug for patients with nephrolithiasis.

Concentration of a potent calcium oxalate monohydrate crystal growth inhibitor in the urine of normal persons and kidney stone patients by ELISA-based assay system employing monoclonal antibodies

Standardized calcium oxalate monohydrate (COM) crystal growth assay system was employed to study the ability of various test samples to influence growth rates of COM crystals. The inhibitory activity (IA) of various samples was expressed in terms of inhibitory units. Urine samples obtained from normal persons and kidney stone patients were found to have IA of 3.18 +/- 0.62 and 1.02 +/- 0.08, respectively. A potent inhibitor having molecular weight between 14.2 and 16.2 kDa was found to be primarily responsible for the differences observed in the urinary IAs between normal persons and kidney stone patients. The potent inhibitor was found to be tightly associated with a chromophore resembling Urobilirubin. An ELISA based assay system, using monoclonal antibodies against the above most potent inhibitor confirmed the difference observed in the urinary IA between the normal persons and kidney stone patients. This assay system has the potential to be routinely used to screen human beings for potential stone formers.

The role of glycoproteins in calcium oxalate crystal development

OBJECTIVE

To assess the effects of a glycoprotein (mucine) on calcium oxalate crystal development in different conditions and situations, to clarify some of its possible effects.

MATERIALS AND METHODS

Crystallization was assessed using a batch system in presence of mucine suspensions, by kinetic-turbidimetric measurements, and using a flow system in the presence of retained agglomerates of mucine, evaluating the precipitated calcium oxalate.

RESULTS

In batch conditions low mucine concentrations (<15 mg/L) inhibited calcium oxalate nucleation and higher concentrations (<250 mg/L) inhibited calcium phosphate nucleation, whereas at high concentrations there was also promotion. The presence of an aggregate of mucine in the flow system provoked calcium oxalate monohydrate crystallization at 0.691 microg/h per mg of mucine. In flow conditions pyrophosphate at 11.5 micromol/L caused a decrease of 84% in the calcium oxalate crystallized on mucine, 1.32 mmol/L of citrate a decrease of a 83%, 20 mg/L of pentosan polysulphate a decrease by 80%, and 7.58 micromol/L phytate totally prevented the crystallization of calcium oxalate on mucine.

CONCLUSION

All substances inhibiting calcium oxalate crystallization with the capacity to interact with calcium ions also have crystallization promoting properties when they are at sufficiently high concentrations, because of their capacity to form agglomerates or the insolubility of their calcium salts.

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.

Effects of Tamm-Horsfall protein on the protection of MCDK cells from oxalate induced free radical injury

Renal cell injury and fixed particle formation is one of the theories of urinary stone formation. The exposure of renal epithelial cells to oxalate ions and calcium oxalate monohydrate crystals can cause free radical generation and increase lipid peroxidation. Tamm-Horsfall protein (THP) has a protective effect on the production of free radicals in vitro. We aimed to show that THP (and its deglycosylated products, D-THP) could protect culture cells from free radical injury in vivo as well as the possible mechanism by which this is done. Exposure of Madin-Darby canine kidney (MDCK) cells to Ox resulted in a significant increase in the release LDH, NBT and MDA, as well as an increase in caspase 3 activity, all of which were further elevated when COM crystals were added. With the addition of THP at 500 nM, there was a significant decrease in the release of LDH and the production of MDA and NBT. A decrease in capase 3 activity was observed when 500 nM THP was added to the culture medium that reached 32.7% and 40.4% of inhibition in CaOx+THP and CaOx+COM+THP, respectively. THP decreased the adhesion of COM crystals to the MDCK cells but lost its effect when THP was deglycosylated. The results indicate that both Ox and COM crystals cause the release of LDH, MDA, NBT and increase the activity of capase 3 in MDCK cells. As a free radical scavenger, THP reduces the amount of free radicals and provides significant protection at a critical concentration of 500 nM. The deglycosylated THP decreased the effect of the protection of the MDCK cells from oxalate-induced injury and an increase of adhesion of the COM crystals to the MDCK cells. Therefore, the effects of THP on the protection of oxalate induced radical injury may be partly due to its intact glycosylation and its adhesion to the cell membrane.

Effects of Tamm-Horsfall protein and albumin on calcium oxalate crystallization and importance of sialic acids

BACKGROUND

Tamm-Horsfall protein and human serum albumin are common urinary proteins that show uncertain inhibitory action on the crystallization of calcium oxalate monohydrate.

MATERIALS AND METHODS

Batch experiments on crystal nucleation, growth, and aggregation were performed using purified Tamm-Horsfall protein and albumin before and after enzymatic removal of sialic acids from the proteins.

RESULTS

At a concentration of 100 nM, both Tamm-Horsfall protein and albumin promoted the time of crystal nucleation by 18.4% and 8.9%, respectively, relative to the control. However, both of the proteins exerted an inhibitory effect on crystal growth, with the IC(50) being 7.27 nM for Tamm-Horsfall protein and 37.5 nM for albumin. The inhibition of crystal aggregation was 81.82% by Tamm-Horsfall protein 100 nM but only 54.55% at 50 nM after enzymatic removal of the sialic acid. Instead of increasing the inhibition, the effect was changed to promotion after an increase in the concentration of Tamm-Horsfall protein to more than 500 nM for native protein and to more than 100 nM for the enzymatic digest. Albumin showed little change after enzymatic treatment and maintained a maximal inhibitory effect of 72.73% on crystal aggregation when the concentration reached to 100 nM.

CONCLUSION

Because the promotion of nucleation could lessen the subsequent saturation of a calcium oxalate solution, it is concluded that Tamm-Horsfall protein and albumin show an overall effect of inhibition on crystallization in vitro. The inhibitory effect of Tamm-Horsfall protein is partly related to sialic acid.

Regulation of renal epithelial cell affinity for calcium oxalate monohydrate crystals

The binding and internalization of calcium oxalate monohydrate (COM) crystals by tubular epithelial cells may be a critical step leading to kidney stone formation. Exposure of MDCK cells to arachidonic acid (AA) for 3 days, but not oleic or linoleic acid, decreased COM crystal adhesion by 55%. Exogenous prostaglandin PGE(1) or PGE(2) decreased crystal binding 96% within 8 h, as did other agents that raise intracellular cAMP. Actinomycin D, cycloheximide, or tunicamycin each blocked the action of PGE(2), suggesting that gene transcription, protein synthesis, and N-glycosylation were required. Blockade of crystal binding by AA was not prevented by the cyclooxygenase inhibitor flurbiprofen, and was mimicked by the nonmetabolizable AA analog eicosatetryanoic acid (ETYA), suggesting that generation of PGE from AA is not the pathway by which AA exerts its effect. These studies provide new evidence that binding of COM crystals to renal cells is regulated by physiological signals that could modify exposure of cell surface molecules to which the crystals bind. Intrarenal AA, PGs, and/or other agents that raise the intracellular concentration of cAMP may serve a protective function by preventing crystal adhesion along the nephron, thereby defending the kidney against crystal retention and stone formation.

Identification of structural domains in inter-alpha-trypsin involved in calcium oxalate crystallization

The urinary glycoprotein that inhibits calcium oxalate (CaOx) crystallization in vitro shows a structural similarity to urinary trypsin inhibitor (UTI; recently termed bikunin), the light chain of inter-alpha-trypsin inhibitor (I alpha I). The functional domains of I alpha I involved in its inhibitory activity of CaOx crystallization have been investigated using isolated intact domains of I alpha I produced from controlled proteolytic digests of the protein. The fragments investigated include the heavy chains of I alpha I, UTI, chondroitinase AC-treated UTI, and the carboxyl-terminal domain of UTI (termed HI-8). The effects of I alpha I and its fragments on the inhibitory activity of CaOx crystallization were evaluated in vitro using CaOx crystal aggregation and growth assays, and seeded crystal generation assay as well as using crystal matrix protein generation assay. UTI, but not the heavy chains of I alpha I, had a discernible effect on CaOx crystallization inhibitory activity. Less requirement of the carbohydrate moiety of UTI is implicated by the observation that chondroitinase AC-treated UTI fragment was also found to inhibit CaOx crystallization with almost the same activity as UTI. HI-8 also efficiently inhibited CaOx crystallization, while I alpha I showed a weak inhibitory activity. The results are almost consistent with a seed crystal generation assay and a crystal adsorption inhibition assay, in which I alpha I or its derivatives inhibits prothrombin fragment 1 (F1) adsorption to CaOx crystals. In conclusion, these results suggest that the part of the I alpha I protein responsible for inhibition of CaOx crystallization is the carboxyl-terminal domain of UTI.

Control of calcium oxalate crystal structure and cell adherence by urinary macromolecules

Crystal polymorphism is exhibited by calcium oxalates in nephrolithiasis, and we have proposed that a shift in the preferred crystalline form of calcium oxalate (CaOx) from monohydrate (COM) to dihydrate (COD) induced by urinary macromolecules reduces crystal attachment to epithelial cell surfaces, thus potentially inhibiting a critical step in the genesis of kidney stones. We have tested the validity of this hypothesis by studying both the binding of monohydrate and dihydrate crystals to renal tubule cells and the effect of macromolecular urinary solutes on crystal structure. Renal tubule cells grown in culture bound 50% more CaOx monohydrate than dihydrate crystals of comparable size. The effects of macromolecules on the spontaneous nucleation of CaOx were examined in HEPES-buffered saline solutions containing Ca2+ and C2O4(2-) at physiologic concentrations and supersaturation. Many naturally occurring macromolecules known to be inhibitors of crystallization, specifically osteopontin, nephrocalcin and urinary prothrombin fragment 1, were found to favor the formation of calcium oxalate dihydrate in this in vitro system, while other polymers did not affect CaOx crystal structure. Thus, the natural defense against nephrolithiasis may include impeding crystal attachment by an effect of macromolecular inhibitors on the preferred CaOx crystal structure that forms in urine.

Adhesion, internalization and metabolism of calcium oxalate monohydrate crystals by renal epithelial cells

The interaction between crystals that nucleate in the nephron lumen and tubular cells could be an important determinant of renal calcification. Kidney epithelial cells in monolayer culture (BSC-1 line), used to model the tubule, rapidly bound and internalized crystals of calcium oxalate monohydrate (COM), the most common constituent of renal stones. Transmission and scanning electron microscopy, enzyme histochemistry, and kinetic analysis of [14C]-labeled crystals were used to study the interaction between renal cells and COM crystals. Electron microscopy revealed that adherent crystals on the apical cell surface can serve as sites for aggregation of additional crystals. Enhanced binding of exogenous crystals to plasma membrane domains overlying internalized crystals was observed for at least 24 hours after the initial cell-crystal interaction. Following internalization, crystals appeared to dissolve within lysosomal inclusion bodies during the ensuing five to seven weeks. Over this time, many cells still containing crystals clustered together in the monolayer. These observations suggest that adhesion and internalization can promote crystal retention in the nephron, whereas intracellular dissolution of crystals may serve as an important, hitherto unrecognized defense against pathologic renal calcification.

Renal cell osteopontin production is stimulated by calcium oxalate monohydrate crystals

Specific anions in tubular fluid, including uropontin (UP), the urinary form of human osteopontin (OPN), block adhesion to renal tubular cells of the most common crystal in kidney stones, calcium oxalate monohydrate (COM). In this study, monkey renal epithelial cells (BSC-1 line) in monolayer culture constitutively secreted UP into the culture medium. COM crystals added to the medium avidly bound previously secreted UP, reducing its concentration by 46% one hour later. However, the net UP content of cultures after a 24-hour exposure to COM crystals was increased by 18%. Northern blotting showed that the constitutively expressed gene encoding human OPN was maximally stimulated in BSC-1 cells after exposure to COM crystals for 12 hours. Two other calcium-containing crystals, hydroxyapatite and brushite, did not alter OPN gene expression or protein production. OPN mRNA expression was enhanced in canine renal epithelial cells (MDCK line) after exposure to COM crystals for six hours, whereas the constitutive expression of murine OPN mRNA by 3T3 fibroblasts was unchanged. In vivo this glycoprotein could defend the cell against adhesion of crystals in tubular fluid, and/or promote renal interstitial fibrosis in subjects with heavy crystalluria.

Glycosaminoglycans, proteins, and stone formation: adult themes and child's play

The relative infrequency of renal stones in children is probably the main reason for the paucity of literature devoted to the study of urolithiasis in pediatric patients. Nonetheless, when pediatricians do address the issue, the contents of their papers reflect those prevalent in the adult literature; with one notable exception. Papers dealing with the potential role of urinary macromolecules in pediatric stone disease are very scarce indeed; to my knowledge, only four have been published in the English literature in the last 15 years. One of these is to be found in this issue and, like the remaining three, it compares the urinary excretion of glycosaminoglycans in healthy children and those with stones. This article briefly reviews the history of the association of urinary macromolecules, particularly glycosaminoglycans and proteins, with calcium oxalate urolithiasis, and discusses in more detail the published experimental evidence for their fulfilling a determinant role in stone formation.

Is nephrocalcin related to the urinary derivative (bikunin) of inter-alpha-trypsin inhibitor?

OBJECTIVE

To isolate, purify, sequence and characterize nephrocalcin (NC), a urinary protein that may be an important determinant of calcium oxalate (CaOx) kidney-stone disease.

MATERIALS AND METHODS

Proteins were isolated from human urine using cellulose and resin columns and were sequenced using Edman degradation and SDS-polyacrylamide gel electrophoresis (SDS-PAGE). Inhibition of CaOx crystal growth by the isolated proteins was assessed by measuring the deposition of 14C-labelled CaOx.

RESULTS

A protein assumed to be NC on the basis of SDS-PAGE, inhibitory and gel filtration properties was isolated from healthy human urine. Its molecular weight and the amino acid sequences of two of its peptides suggested it was identical to fragment HI-14 of the light chain (bikunin) of inter-alpha-trypsin inhibitor (ITI).

CONCLUSIONS

NC represents a portion of the light chain of ITI, although this conclusion must remain tentative until confirmed using authentic NC.

Nephrocalcin in patients with renal cell carcinoma

Nephrocalcin, an acidic glycoprotein that inhibits calcium oxalate crystal growth, has been previously localized in proximal tubules of kidneys by an immunohistochemical staining method and purified from tissue culture media of 2 renal carcinoma cell lines. A polyclonal antibody specific to nephrocalcin was raised in rabbits and the level of nephrocalcin was quantitatively determined in urine of 19 renal cell carcinoma patients (0.241 +/- 0.341 microgram nephrocalcin per mg. creatinine) and compared to healthy controls (0.022 +/- 0.012 micrograms nephrocalcin per mg. creatinine). Nephrocalcin levels after tumor nephrectomy decreased dramatically in 5 patients and to a lesser degree in 7. A specific nephrocalcin fraction that was eluted from an anion exchange column with low ionic strength was detected in urine of the renal cell carcinoma patients, and this fraction decreased or disappeared after tumor nephrectomy in 6 of 9 patients studied. Amino acid composition, phosphate content and dissociation constants toward calcium oxalate monohydrate crystals were investigated in the nephrocalcin from tumor patients and compared to that from healthy controls. Our studies demonstrate that nephrocalcin in patients with renal cell carcinoma is atypical and usually in much higher quantity. Further studies are needed to determine the clinical significance of these observations.

Increased urinary saturation and kidney calcium content in genetic hypercalciuric rats

We have established a colony of genetic hypercalciuric (IH) rats as a model of idiopathic hypercalciuria in humans. To test the hypothesis that hypercalciuria can cause crystallization in kidneys through increased supersaturation, in the absence of confounding effects of diet and whatever complex inhibitor disorders underlay stone disease, we fed males and females of the 21st generation of IH rats 13 g per day of a low calcium (LCD, 0.02% Ca), followed by a normal calcium (NCD, 0.6% Ca) and then a high calcium (HCD, 1.2% Ca) diet, each for seven days. During the last 24 hours of each period complete urine collections were obtained and analyzed for all substances known to affect urinary calcium oxalate (CaOx) and brushite (CaHPO4) supersaturation. Relative supersaturation with respect to the solid phases of CaOx and CaHPO4 were then calculated. Compared to same gender controls (Ctl) urine calcium excretion was higher in the female IH rats on all diets and in the male IH rats on NCD and HCD. The female and male IH rats on NCD and HCD were supersaturated with respect to CaOx; however, the male and female Ctl were supersaturated with respect CaOx only on HCD. The female IH rats on NCD and HCD and the male IH rats on NCD were supersaturated with respect to CaHPO4; however, neither the male nor female Ctl rats were supersaturated with respect to CaHPO4 on any diet. On NCD and HCD urine supersaturation with respect to CaHPO4 by females exceeded that of males.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of warfarin on urine calcium oxalate crystal growth inhibition and urinary excretion of calcium and nephrocalcin

Urine contains inhibitors of calcium oxalate (CaOx) crystal growth. One such inhibitor is nephrocalcin (NC), a glycoprotein which is made in the kidney and contains several residues of gamma-carboxyglutamic acid (Gla) per molecule. The presence of Gla may be important to its ability to inhibit crystal growth. Several studies suggest that vitamin K-dependent proteins may also play a role in renal calcium (Ca) handling, and that vitamin D deficiency may lead to excess urinary Ca loss, but the effect of the vitamin K antagonist warfarin on urinary Ca excretion and CaOx growth inhibition in humans is not known. We studied 11 men while they were taking warfarin for a mean of 252 days, and again a mean of 64 days after its discontinuation. Urinary Ca excretion did not differ between those on or off warfarin, or between those on warfarin and normal controls. The ability of the subjects' urine to inhibit CaOx crystal growth did not differ on or off warfarin, or from that of control urine, and the excretion of immunoreactive NC also did not differ between these groups. NC was found to be responsible for approximately 16% of the CaOx growth inhibition seen. These results do not suggest that vitamin K-dependent proteins play a major role in renal Ca excretion in men, or that interference with vitamin K alters NC excretion or inhibitory activity of the urine.

Isolation and purification of a new glycoprotein from human urine inhibiting calcium oxalate crystallization

A calcium oxalate crystal growth inhibitor was isolated from human urine using DEAE-Sephacel gel followed by Sephacryl S-300 chromatography and FPLC column. The isolated inhibitor was a uronic-acid-rich protein (UAP). It was found to be a glycoprotein with a molecular weight of 35,000 Da as determined by SDS-polyacrylamide gel electrophoresis. Inhibitory activity was demonstrated using a calcium oxalate crystallization system. In addition UAP, nephrocalcin (NC) or nephrocalcin-like (NC-like) activity was an effective inhibitor in this system. However, the inhibitory activity of UAP appeared to be higher than that of NC or NC-like activity. This finding suggests that NC or NC-like activity is not only urinary protein with strong inhibitory activity. UAP and probably other proteins also play a role in the control of urinary crystal growth.

The calcium oxalate crystal growth inhibitor protein produced by mouse kidney cortical cells in culture is osteopontin

Urine contains proteins that inhibit the growth of calcium oxalate (CaOx) crystals and may prevent the formation of kidney stones. We have identified a potent crystal growth inhibitor in the conditioned media from primary cultures of mouse kidney cortical cells. Conditioned media, incubated with the kidney cells for 6-72 h, was assayed for crystal growth inhibition; inhibitory activity increased 15-fold by 24 h. Inhibitory activity was purified from serum-free media containing proteinase inhibitors using anion-exchange and gel-filtration chromatography. A single band of molecular weight 80,000 daltons was seen after SDS-polyacrylamide gel electrophoresis. The sequence of the N-terminal 21 amino acids of this protein matched that of osteopontin (OP), a phosphoprotein initially isolated from bone matrix. Antisera raised to fusion proteins produced by plasmids containing the N-terminal or C-terminal portions of OP cDNA also cross-reacted with the protein purified from cell culture media on western blots. The effect of the purified protein on the growth of CaOx crystals was measured using a constant composition assay. A 50% inhibition of growth occurred at a protein concentration of 0.85 micrograms/ml, and the dissociation constant of the protein with respect to CaOx crystal was 3.7 x 10(-8) M. The concentration of OP in mouse urine, measured using antibodies raised to the purified protein, was approximately 8 micrograms/ml. We conclude that OP is synthesized by kidney cortical tubule cells and functions as a crystal growth inhibitory protein in urine.

Inhibition of calcium phosphate precipitation by human salivary statherin: structure-activity relationships

Previous studies of human statherin showed the active region for inhibition of secondary calcium phosphate precipitation (crystal growth) to reside in the highly charged amino-terminal one-third of this molecule, and the neutral tyrosine-, glutamine- and proline-rich carboxy-terminal two-thirds of the molecule is required for maximal inhibition of primary (spontaneous) precipitation. The purpose of the present study was to define more clearly the activities of these different molecular segments of statherin with respect to the two kinds of inhibitory activities. Peptides from statherin were prepared by specific proteolysis using trypsin, endoproteinase Arg-C, and activated factor X to produce the amino-terminal hexa-, nona- and decapeptides, respectively, and carboxypeptidase-A was used to obtain a peptide extending from residue 1 to about residues 32-37. The peptides were purified by anion exchange and gel filtration chromatography, and characterized and quantified by amino-acid analysis. Serially diluted samples of statherin and derived peptides were assayed to determine the concentrations, giving a standard 50% inhibition of precipitation (C50%) in assay systems designed for this purpose using polyaspartate as a standard. Results are expressed as (C50% statherin)/(C50% peptide). For inhibition of primary precipitation, these values were peptide(1-6), 0.20; peptide(1-9), 0.15; peptide(1-31/35), 0.24. For inhibition of secondary precipitation, the values were peptide(1-6), 3.8; peptide(1-9), 2.8; peptide(1-10), 1.9; peptide(1-32/37), 1.5. These quantitative findings show that maximum inhibition of primary precipitation by statherin requires the entire molecule. Thus, removal of a relatively small segment of its carboxy-terminal region results in a substantial reduction in inhibitory activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibitors within the nephron

Kidney-derived inhibitors of crystal growth and aggregation prevent supersaturations created by water conservation from expressing themselves in pathological soft tissue calcifications and intranephronal crystallizations. These inhibitors include nephrocalcin (NC), an acidic glycoprotein produced in proximal tubules and thick ascending limbs of Henle's loop, and the Tamm-Horsfall glycoprotein (THP), produced only in the thick ascending limb. NC inhibits growth and aggregation of calcium oxalate monohydrate (COM), the major crystalline component of human renal stones, THP inhibits only COM aggregation. Patients who form COM stones produce abnormal NC molecules that lack gamma-carboxyglutamic acid and fail to inhibit COM crystallizations normally.

An unidentified macromolecular inhibitory constituent of calcium oxalate crystal growth in human urine

We have detected and isolated a macromolecular constituent in normal human urine possessing calcium crystal growth inhibitory activity. The purification procedure consisted of two anion exchange chromatographies and one affinity chromatography. The crystal growth inhibitor was found to be heterogeneous in net charge as well as in size. It has not been identified. It is not an uronic acid-containing glycosaminoglycan, hitherto presumed to be responsible for the inhibitory activity. Whether an urinary fragment of inter-alpha-trypsin inhibitor is responsible has yet to be resolved.

Calcium oxalate crystallisation kinetics and the effects of calcium and gamma-carboxyglutamic acid

gamma-carboxyglutamic acid (GLA) is an amino acid with a high affinity for calcium. It is found in urine both as the free amino acid and incorporated into proteins such as osteocalcin. Free and bound GLA have been reported to be found at higher concentrations in the urine of stone formers than controls. We have investigated the effect of GLA and calcium, at physiological levels, on the crystallisation of calcium oxalate using a mixed suspension mixed product removal continuous crystalliser. GLA caused very significant changes in the crystallisation kinetics, but the effect was dependent on the calcium concentration. At 4 mM calcium, GLA decreased the growth rate and increased the nucleation rate; at 12 mM the reverse occurred. At all concentrations of calcium tested, GLA caused a significantly increased crystal mass to be produced. Our evidence supports the hypothesis that GLA modifies calcium oxalate crystallisation and could be a promoter of stone formation in vivo, particularly at moderately elevated levels of calcium excretion.

Elucidation of multiple forms of nephrocalcin by 31P-NMR spectrometer

Four forms of nephrocalcin have been routinely isolated from mammalian kidney tissues and urine using DEAE-cellulose column chromatography with a linear NaCl gradient. We have demonstrated that these four forms of nephrocalcin, isolated from bovine kidneys, contain different amounts of phosphate residues, and that alkaline phosphatase digestion converts these to only one form of nephrocalcin. The changes in the nephrocalcin before and after removal of phosphate residues were measured by 31P-NMR spectrometer. Loss of phosphate residues decreased the dissociation constant of nephrocalcin 10-fold toward calcium oxalate monohydrate crystals, suggesting the phosphate residues appear to be important in the inhibitory effects of calcium oxalate monohydrate crystal growth.

Gestational hypercalciuria causes pathological urine calcium oxalate supersaturations

Although normal pregnant women are more hypercalciuric than women with calcium oxalate nephrolithiasis (243 +/- 23 mg/day vs. 194 +/- 5 mg/day), pregnancy is not an established stone-forming state and pregnant women do not exhibit pathological crystalluria. One hypothesis to explain their lack of overt stone formation and pathological crystalluria is that pregnancy does not raise urine supersaturation with respect to stone forming salts such as calcium oxalate or calcium monohydrogen phosphate (brushite) to levels as high as in stone forming women. To test this hypothesis, we studied eleven normal women during each trimester of pregnancy, and between six and eight weeks post-partum. During pregnancy, hypercalciuria occurs with unchanged urine volume, citrate and magnesium excretions do not increase proportionally with calcium excretion, and urine pH increases. Supersaturations with respect to calcium oxalate (CaOx) and brushite (Br) are as high as those of women with calcium nephrolithiasis. The lack of pathological crystalluria and stones during pregnancy is not due to a failure of supersaturations to increase; urinary potential for crystallization is as high as in patients with established stone disease.

Surface interaction between glycosaminoglycans and calcium oxalate

Molecules and macromolecules are known to alter the process of crystallization, either through inhibition or promotion of nucleation, growth, and/or aggregation. One particular group of macromolecules, glycosaminoglycans (GAGs), has been of interest in our laboratory. The GAGs chondroitin A, chondroitin C, heparan sulfate, dermatan sulfate, hyaluronic acid, and keratan sulfate have all been shown to be inhibitors of calcium oxalate crystallization. Heparin, the only GAG which is not naturally present in urine, is the most potent inhibitor of all GAGs. Using the method of Langmuir isotherm adsorption, we studied the adsorption of certain GAGs onto calcium oxalate crystals. Under standardized conditions, heparin, chondroitin C, hyaluronic acid, and pentosan polysulfate (a synthetic polyanionic molecule similar to, but a weaker inhibitor than, heparin) were adsorbed onto calcium oxalate. The total amount of GAG required to maximally cover the crystal surface, as well as the equilibrium concentration at which surface was half-covered with GAG (inversely related to the desorption energy) were measured. Chondroitin C was adsorbed in the greatest amount, followed by heparin, pentosan polysulfate, and finally hyaluronic acid. Using the method of fiducial limits, the only insignificant difference was between heparin and chondroitin C, and between hyaluronic acid and pentosan polysulfate. Pentosan polysulfate required significantly higher equilibrium concentration than heparin and hyaluronic acid to cover half of the surface of the calcium oxalate crystals. The principle of Langmuir isotherm adsorption can be useful in predicting the effects of macromolecules on crystallization. Weaker inhibitors bind with less affinity than do stronger inhibitors. Further work is underway to characterize other inhibitors and promoters.

Hydration properties of urinary compounds obtained from normal and stone former individuals. An NMR study

Proton magnetic resonance measurements were performed on lyophilized urine samples collected from 10 recurrent calcium oxalate stone former (SF) patients, 10 thiazide treated (TSF) patients and 14 normal (N) individuals. T1 and T2 relaxation times were measured with a Bruker PC Multispec at 20 MHz and 37C on the lyophilized sample and thereafter during gradual controlled rehydration. The prolongation of the relaxation times was found to be significantly different (p less than 0.001) for the SF and N groups, while it was similar for the SF and TSF groups. Water compartmentalization was then calculated according to the Fast Proton Infusion model. After the addition of similar amounts of water, significantly (p less than 0.01) different hydration fraction (HF) and fraction bound (FB) values were calculated for the N and SF groups, while similar values were obtained for the SF and TSF groups. The results reflect differences in the urinary contents and/or properties of N and SF groups. It seems that there are more hydrophilic compounds in the urine of N individuals than in that of SFs. It can be speculated that the urinary macromolecules of SFs differ from those of Ns in the amount of water binding sites and in the water multilayer thickness surrounding them. The present results point to possible pathogenic differences between SF and N individuals.

Mucopolysaccharidases from Pseudomonas sp. Isolation and partial characterization of constitutive enzymes involved in the degradation of keratan sulfate and chondroitin sulfate

Four constitutive enzymes, capable of degrading keratan sulfate, were isolated from Pseudomonas sp.: a particulate endoglycosidase, a soluble endoglycosidase, a soluble exo-beta-D-galactosidase and a soluble exo-beta-D-N-acetylglucosaminidase. The endoglycosidases were shown to act only upon keratan sulfate forming beta-D-2-acetamido-2-deoxy-6-O-sulfoglucosyl-(1----3)-D-galactose, as the main product. This results indicates that the enzyme catalyses the hydrolysis of beta-D-galactose-(1----4)-N-acetylglucosamine linkages. It was also shown that this monosulfated disaccharide inhibits the particulate keratan sulfate endoglycosidase. The bovine nucleus pulposus keratan sulfate is depolymerized at a lower rate and extent when compared to the corneal keratan sulfate. The soluble endoglycosidase is very labile, in contrast to the particulate enzyme, which has been stored at -20 degrees C or at 4 degrees C for at least 12 months with no loss in activity. The particulate endoglycosidase and the soluble exo-beta-D-galactosidase and exo-beta-D-N-acetylglucosaminidase are induced when the bacteria is grown in adaptative media containing either 0.1% keratan sulfate or 0.1% chondroitin sulfate. Furthermore, particulate forms of the exoenzymes were detected. The soluble endoglycosidase specific activity, in contrast, is approximately the same in extracts of cells grown in glucose, keratan sulfate or chondroitin sulfate. A chondroitin sulfate lyase was also identified in the soluble extracts of Pseudomonas sp. cells. This enzyme depolymerizes chondroitin 4-sulfate, chondroitin 6-sulfate and hyaluronic acid forming unsaturated disaccharides as main products. It is also active upon the glucuronic-acid-containing regions of the dermatan sulfate molecules. The properties of the soluble enzymes, further purified by ion-exchange chromatography, and of the particulate keratan sulfate endoglycosidase are presented.

Immunochemistry of urinary calcium oxalate crystal growth inhibitor (CGI)

Calcium oxalate crystal growth inhibitor (CGI) was isolated from human urine in monomeric form (14,000 daltons). Antibody was elicited and purified to monospecificity by affinity chromatography. Tamm-Horsfall protein was isolated from human urine and an antibody to Tamm-Horsfall protein compared to anti-CGI. The anti-CGI reacted with its antigen on immunodiffusion, by ELISA and by Western Blotting of polyacrylamide gel electrophoresis-separated antigen. Immunofluorescent localization of CGI was found in distal renal tubules. This was precisely the localization of Tamm-Horsfall protein. Isolated Tamm-Horsfall protein was found to bind CGI which could only be partially removed with EDTA. While anti-CGI is suitable to assay CGI in human urine by ELISA techniques, it will also detect CGI that is complexed to THP. While the CGI found in human urine possesses 90% of the urinary macromolecular crystal growth inhibitor activity, THP is without effect on crystal growth, in spite of bound CGI. The balance between free CGI and that bound to Tamm-Horsfall protein may be important in the overall balance of urinary macromolecules that affect calcium oxalate nephrolithiasis.

Bladder secretion of inhibitors of calcium oxalate crystal growth

Differences in calcium oxalate crystal growth inhibition were studied in normally voided urine (bladder urine) and in urine collected directly from the kidney (kidney urine) in nine dogs. Urine samples were collected before and 10 days after bilateral ureterostomies. Calcium oxalate crystal growth inhibition was measured in a standard seeded crystal growth system. The alcian blue-precipitable material of the urine samples was determined. Significantly lower values were observed in kidney urine than in bladder urine for calcium oxalate crystal inhibition (mean difference, 0.07 +/- 0.02 inhibitor units/mg creatinine; P less than 0.01) and for the alcian blue-precipitable material (mean difference, 0.07 +/- 0.02 mg/mg creatinine; P less than 0.01). We conclude that the bladder adds calcium oxalate crystal growth inhibition to urine. Glycosaminoglycans from the bladder mucosa may be responsible; however, other acidic polymers such as RNA fragments or glycopeptides have been shown to be a constituent of the alcian blue-precipitable material. These are potent inhibitors of calcium oxalate crystal growth, and their participation in the increase of inhibition observed in bladder urine cannot be excluded. Total calcium oxalate crystal growth inhibition present in normally voided urine may be an overestimation of the actual inhibition present at the level of the kidney, where calculi usually form.

Adsorption of molecules of biological interest onto hydroxyapatite

Equilibrium and kinetic experiments were conducted to investigate the factors determining the adsorption of salivary macromolecules onto hydroxyapatite. Using amino acids and other small adsorbates, it was determined that the carboxyl attached to the alpha carbon does not appear to adsorb onto HA and the affinities of side-chain carboxyls are much smaller than that of the phosphate group (phosphoserine). Hydroxyl (serine) displays an extremely high affinity, but its adsorption site on HA is different and the number of such sites is much smaller than found for the rest of the functional groups investigated. It is shown that the information obtained from small molecules cannot be readily applied to prediction of the adsorption behavior of salivary macromolecules and polypeptides. The kinetics of adsorption of the salivary phosphopeptide statherin, a polyaspartate, and the salivary prolinerich phosphoprotein PRP3 are consistent with the reversibility of the adsorption process; no conclusion was possible in the case of the protein PRP1. Apparent irreversibility cannot be explained on the basis of multipoint binding or the properties of the carboxyl versus phosphate group; it appears that secondary structure determines to a significant extent the adsorption properties of the macromolecules. Calculation of the thermodynamic molar quantities of adsorption of PRP1, PRP3, and L-ASP onto HA showed that the process is entropically driven. The functional relationship between partial molar entropy and adsorption coverage is similar for the two proteins, but quite different from that for aspartate. Explanations for these results are advanced on the bases of changes in structure configurations and displacement of water from the adsorbate and the adsorbent surface, the second factor being the dominant one in the adsorption of a small molecule such as L-ASP.