Citrate and calcium effects on Tamm-Horsfall glycoprotein as a modifier of calcium oxalate crystal aggregation

Dietary management of hypocitraturia in children with urolithiasis: results from a systematic review

PURPOSE

Hypocitraturia is a low urinary excretion of citrate and a well-known risk factor for kidney stone development in children. This systematic review aimed to evaluate the dietary management of hypocitraturia in children with urolithiasis.

METHODS

Literature search was performed on 30th September 2022 using Embase, PubMed, and Cochrane Central Controlled Register of Trials. Studies were included if children with stones and hypocitraturia were managed with diet supplements.

RESULTS

Six papers were included. Four studies evaluated the role of oral potassium citrate associated with high fluid intake on stone resolution and recurrence. Two studies assessed the impact of oral potassium citrate on long-term stone recurrence after percutaneous nephrolithotomy and shock wave lithotripsy. All studies demonstrated that the association of potassium citrate and high fluid intake was well tolerated with no side effects and restored normal urine citrate excretion, allowed a reduction in stone size, and, following definitive treatments, was associated with a lower rate of stone regrowth and recurrence compared with controls. These effects were demonstrated across all pediatric ages.

CONCLUSIONS

Our review infers that oral potassium citrate and high fluid assumption are safe and effective in restoring urine citrate excretion, treating and preventing stone recurrence with no serious adverse events, and should probably be the first-line treatment of pediatric patients with asymptomatic stones and hypocitraturia.

Metabolic assessment in pure struvite stones formers: is it necessary?

BACKGROUND AND OBJECTIVE

Magnesium ammonium phosphate stones (MAP), also known as struvite stones, are associated with urinary infection and impairment of renal unit. The aim of this study is to evaluate the urinary metabolic risk factors for recurrence of renal calculi in patients submitted to nephrectomy due to MAP stones.

METHODS

We retrospectively reviewed the charts of patients > 18 years old submitted to total nephrectomy due to pure MAP stones and pure calcium oxalate (CaOx) stones from July 2006 to July 2016. Urinary metabolic parameters were assessed through 24-hour urine exams ≥ 3 months after nephrectomy. Urinary metabolic parameters and new event related to lithiasis were compared.

RESULTS

Twenty-eight and 39 patients were included in MAP and CaOx group, respectively. Abnormalities in 24-hour urine samples were similar between groups. Hypercalciuria occurred in 7.1 and 10.3% of patients in MAP and CaOx group, respectively (p = 0.66), whereas hypocitraturia was present in 65.2 and 59.0% of patients with MAP and CaOx group, respectively (p = 0.41). No significant difference in new events was found between MAP and CaOx groups (17.9 vs. 23.1%, respectively; p = 0.60).

CONCLUSION

A 24-hour urine evaluation should be offered to patients submitted to nephrectomy due to pure MAP stones in order to detect metabolic risk, improve treatment, and prevent stone recurrence.

Design of drug-like hepsin inhibitors against prostate cancer and kidney stones

Hepsin, a transmembrane serine protease abundant in renal endothelial cells, is a promising therapeutic target against several cancers, particularly prostate cancer. It is involved in the release and polymerization of uromodulin in the urine, which plays a role in kidney stone formation. In this work, we design new potential hepsin inhibitors for high activity, improved specificity towards hepsin, and promising ADMET properties. The ligands were developed in silico through a novel hierarchical pipeline. This pipeline explicitly accounts for off-target binding to the related serine proteases matriptase and HGFA (human hepatocyte growth factor activator). We completed the pipeline incorporating ADMET properties of the candidate inhibitors into custom multi-objective optimization functions. The ligands designed show excellent prospects for targeting hepsin via the blood stream and the urine and thus enable key experimental studies. The computational pipeline proposed is remarkably cost-efficient and can be easily adapted for designing inhibitors against new drug targets.

Protective potential of Angelica sinensis polysaccharide extract against ethylene glycol-induced calcium oxalate urolithiasis

Purpose: To evaluate a Angelica sinensis polysaccharide aqueous extract as a preventive agent in experimentally induced urolithiasis using in- vitro and vivo models.

Material and methods:Angelica sinensis polysaccharide was investigated in vitro to determine its antilithiatic effects on the formation and morphology of calcium oxalate (CaOx) crystals and was analyzed in vivo to determine its ability to prevent CaOx urolithiasis in rats subjected to ethylene glycol-induced urolithiasis. Potassium citrate administration was used in the positive control group. The urolithiasis-related biochemical parameters were evaluated in the rats urine, serum and kidney homogenates. Kidney sections were subjected to histopathological and immunohistochemical analyses, and urolithiasis-related phospho-c-Jun NH2-terminal protein kinase and kidney injury molecule-1proteins were evaluated by Western blot analyses.

Results:Angelica sinensis polysaccharide exhibited concentration-dependent inhibition of CaOx crystal formation. The in vitro assay revealed significant inhibition of crystal formation (6.99 ± 1.07) in the group treated with 4.0 mg/mL Angelica sinensis polysaccharide extract compared with the control group (58.38 ± 5.63; p < .05). In vivo, after treatment with ethylene glycol for 28 days, urinary oxidative stress, oxalate, creatinine, urea and urolithiasis-related protein were significantly increased (p < .05), except for serum oxidative stress (p > .05). The rats administered the extract of Angelica sinensis polysaccharide showed significantly decreased pathological change and CaOx deposition (p < .05) compared with the urolithiatic rats. Significantly reduced levels of urinary oxidative stress, oxalate, creatinine, urea and urolithiasis-related protein were observed in the Angelica sinensis polysaccharide treatment groups (p < .05) compared with the nephrolithic rats.

Conclusion: The results presented here suggest that Angelica sinensis polysaccharide has the potential to inhibit CaOx crystallization in vitro and may present anti-urolithiatic effects in vivo.

Kinetics of calcium oxalate crystal formation in urine

It is routinely observed that persons with increased urinary stone risk factors do not necessarily form uroliths. Furthermore, stone formers can present with urinalyses that do not reflect the clinical picture. We explain this discrepancy by differences in crystallization kinetics. In 1162 urines, crystallization of Ca-oxalate was induced according to the BONN-Risk-Index (BRI) method. The urine's relative light transmissivity (RLT) was recorded from 100 % at start of titration to 95 % due to nuclei formation and crystal growth. From the RLT changes, a measure of the thermodynamic inhibition threshold of crystal formation (BRI) and of crystal growth kinetics is derived ("turbidity slope" after crystallization onset). On average, subjects presenting with a low inhibition threshold, i.e., high BRI, also present significantly higher crystal growth rates compared with subjects in lower BRI classes. Only subjects in the highest BRI class show a lower growth rate than expected, probably due to a depletion of supersaturation by massive initial nucleation. With increasing thermodynamic risk of crystal formation (i.e., increasing BRI) due to an imbalance between inhibitors and promoters of crystal formation, an increase in the imbalance between inhibitors and promoters of crystal growth (i.e., increasing growth rate) is observed. Both lead to an increased urolith formation risk. Healthy subjects with increased BRI are an exception to this trend: their urine is thermodynamically prone to form stones, but they show a kinetic inhibition preventing nuclei from significant growth.

Potassium citrate decreases urine calcium excretion in patients with hypocitraturic calcium oxalate nephrolithiasis

Two previous studies (<10 patients each) have demonstrated that alkali therapy may reduce urine calcium excretion in patients with calcium oxalate nephrolithiasis. The hypothesized mechanisms are (1) a decrease in bone turnover due to systemic alkalinization by the medications; (2) binding of calcium by citrate in the gastrointestinal tract; (3) direct effects on TRPV5 activity in the distal tubule. We performed a retrospective review of patients on potassium citrate therapy to evaluate the effects of this medication on urinary calcium excretion. A retrospective review was performed of a metabolic stone database at a tertiary care academic hospital. Patients were identified with a history of calcium oxalate nephrolithiasis and hypocitraturia who were on potassium citrate therapy for a minimum of 3 months. 24-h urine composition was assessed prior to the initiation of potassium citrate therapy and after 3 months of therapy. Patients received 30-60 mEq potassium citrate by mouth daily. Inclusion criterion was a change in urine potassium of 20 mEq/day or greater, which suggests compliance with potassium citrate therapy. Paired t test was used to compare therapeutic effect. Twenty-two patients were evaluated. Mean age was 58.8 years (SD 14.0), mean BMI was 29.6 kg/m(2) (SD 5.9), and gender prevalence was 36.4% female:63.6% male. Mean pre-treatment 24-h urine values were as follows: citrate 280.0 mg/day, potassium 58.7 mEq/day, calcium 216.0 mg/day, pH 5.87. Potassium citrate therapy was associated with statistically significant changes in each of these parameters-citrate increased to 548.4 mg/day (p < 0.0001), potassium increased to 94.1 mEq/day (p < 0.0001), calcium decreased to 156.5 mg/day (p = 0.04), pH increased to 6.47 (p = 0.001). Urine sodium excretion was not different pre- and post-therapy (175 mEq/day pre-therapy versus 201 mEq/day post-therapy, p = NS). Urinary calcium excretion decreased by a mean of 60 mg/day on potassium citrate therapy-a nearly 30 % decrease in urine calcium excretion. These data lend support to the hypothesis that alkali therapy reduces urine calcium excretion.

An evaluation of Tamm-Horsfall protein glycans in kidney stone formers using novel techniques

Tamm-Horsfall protein (THP) is theorized to play a critical role in preventing kidney stone formation. There is conflicting literature on THP analysis in kidney stone patients; therefore, this study was conducted using sensitive and specific bio-analytical techniques to better understand differences in THP, which play a potential role in nephrolithiasis pathogenesis. THP was isolated from urine samples of 34 male and 19 female kidney stone patients and 30 male and 24 female control subjects using diatomaceous earth. Protein was quantified by Superdex-200 size-exclusion chromatography. Sialic acid was determined by 1,2-diamino-4,5-methylenedioxybenzene high-performance liquid chromatography. Neutral and amino sugars were determined by high pH anion-exchange chromatography (HPAEC) with pulsed amperometric detection. THP N-glycans were derivatized with 2-aminobenzamide (2-AB) and profiled by HPAEC with fluorescence detection. N-glycan structures were confirmed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). Results indicate that kidney stone patients had 32% lower protein content compared to controls, while sialic acid content was lower by 29 and 24% in male and female kidney stone patients, respectively, compared to controls. The neutral and amino sugars were also lower by 18 and 20% for male and female kidney stone patients, respectively, compared to controls. All results were statistically significant (p<0.001). These results are supported by 2-AB profiling of THP N-glycans and by MALDI-TOF MS of highly sialylated N-glycans in the range of m/z 3000-6000. This study demonstrates quantitative and qualitative differences in THP, which can be crucial contributing factors for nephrolithiasis.

Inhibition of urinary macromolecule heparin on aggregation of nano-COM and nano-COD crystals

Purpose: This research aims to study the influences of heparin (HP) on the aggregation of nano calcium oxalate monohydrate (COM) and nano calcium oxalate dihydrate (COD) with mean diameter of about 50 nm. Method: The influences of different concentrations of HP on the mean diameter and Zeta potential of nano COM and nano COD were investigated using a nanoparticle size Zeta potential analyzer. Results: HP could be adsorbed on the surface of nano COM and nano COD crystals, leading to an increase in the absolute value of Zeta potential on the crystals and an increase in the electrostatic repulsion force between crystals. Consequently, the aggregation of the crystals is reduced and the stability of the system is improved. The strong adsorption ability of HP was closely related to the -OSO₃⁻ and -COO⁻ groups contained in the HP molecules. X-ray photoelectron spectroscopy confirmed the coordination of HP with Ca²⁺ ions of COM and COD crystals. Conclusion: HP could inhibit the aggregation of nano COM and nano COD crystals and increase their stability in aqueous solution, which is conducive in inhibiting the formation of calcium oxalate stones.

Should we modify the principles of risk evaluation and recurrence preventive treatment of patients with calcium oxalate stone disease in view of the etiologic importance of calcium phosphate?

Prevention of recurrent calcium oxalate (CaOx) stone formation in the urinary tract is important to avoid negative effects on renal function, patient suffering and to reduce health care cost. Present shortcomings in this regard can be explained both by insufficient understanding of the mechanisms of stone formation and by poor patient compliance to those regimens that nevertheless have proven effective. During the past years, we have got increased insights in the nature of CaOx stone formation and it is suggested that the improved understanding of this process can be used for a more dynamic risk evaluation and treatment regimen directed to specific risk periods that can be identified in the individual patients. Some of the possibilities with regard to the important role of calcium phosphate are discussed in this article.

Immunohistochemical localization and mRNA quantification of osteopontin and Tamm-Horsfall protein in canine renal tissue after potassium oxalate injection

Background

Urinary macromolecules contribute to promoting or inhibiting crystal retention in renal tissue and stone formation. Osteopontin (OPN) and Tamm-Horsfall protein (THP) are the most important proteins involved in this process. Although these two proteins were discovered a long time ago, their role in setting kidney stone formation has not yet been fully investigated. We conducted a study to explore the role of OPN and THP in canine renal oxalosis. Ten dogs were carefully examined prior to the study. Six dogs were assigned to the treatment group and were injected intravenously with 0.5 M potassium oxalate (KOx). The other four dogs were assigned to a control group and were injected intravenously with 0.9% NaCl three times a day (tid) for 7 consecutive days. Then kidneys were harvested for pathological, immunohistochemical examination and OPN and THP mRNA expression levels were quantified by quantitative real-time PCR.

Results

Calcium oxalate crystals deposition was observed in both renal cortex and medulla. Immunohistochemistry examination revealed increased tissue expression of OPN in the renal tissue while THP was significantly decreased. OPN mRNA expression level significantly increased in treated dogs compared to that in the controls, while THP mRNA level significantly decreased.

Conclusion

Together, these results suggest that THP and OPN are both involved in the pathogenesis and response to oxalate exposure.

Urinary citrate excretion in healthy children depends on age and gender

BACKGROUND

Hypocitraturia is considered a major risk factor for calcium stone formation. However, there is no widely accepted reference database of urinary citrate excretion in children. The aim of our study was to determine the amount of citrate eliminated in the urine over a 24-h period in a pediatric cohort and to determine an optimal unit reflecting excretion.

METHODS

The study cohort comprised 2,334 healthy boys and girls aged 2-18 years. The levels of urinary citrate were assessed by an enzymatic method in 24-hour urine and expressed in absolute values, as urinary concentration, citrate/creatinine ratio, per kilogram of body weight, in relation to 1.73 m2, and as the calcium/citrate index.

RESULTS

Similar incremental age-related citraturia rates were observed in both male and female subjects until puberty during which time citrate excretion became significantly higher in girls. Urinary citrate adjusted for creatinine and for body weight showed a significantly decreasing trend with increasing age in both sexes. Urinary citrate corrected for body surface was weakly correlated with age. Thus, the assumption of 180 mg/1.73 m2/24 h for males and 250 mg/1.73 m2/24 h for females as lower cut-off values appeared to be reliable from a practical perspective.

CONCLUSIONS

We found distinct sex-dependent differences in citraturia at the start of puberty, with significantly higher values of urinary citrate in girls than in boys. Further prospective studies are warranted to elucidate whether this difference represents a differentiated risk of urolithiasis.

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.

SLC26A6 and NaDC-1 transporters interact to regulate oxalate and citrate homeostasis

The combination of hyperoxaluria and hypocitraturia can trigger Ca(2+)-oxalate stone formation, even in the absence of hypercalciuria, but the molecular mechanisms that control urinary oxalate and citrate levels are not understood completely. Here, we examined the relationship between the oxalate transporter SLC26A6 and the citrate transporter NaDC-1 in citrate and oxalate homeostasis. Compared with wild-type mice, Slc26a6-null mice exhibited increased renal and intestinal sodium-dependent succinate uptake, as well as urinary hyperoxaluria and hypocitraturia, but no change in urinary pH, indicating enhanced transport activity of NaDC-1. When co-expressed in Xenopus oocytes, NaDC-1 enhanced Slc26a6 transport activity. In contrast, Slc26a6 inhibited NaDC-1 transport activity in an activity dependent manner to restricted tubular citrate absorption. Biochemical and physiologic analysis revealed that the STAS domain of Slc26a6 and the first intracellular loop of NaDC-1 mediated both the physical and functional interactions of these transporters. These findings reveal a molecular pathway that senses and tightly regulates oxalate and citrate levels and may control Ca(2+)-oxalate stone formation.

Changes in urinary nanocrystallites in calcium oxalate stone formers before and after potassium citrate intake

The property changes of urinary nanocrystallites in 13 patients with calcium oxalate (CaOx) stones were studied before and after ingestion of potassium citrate (K3cit), a therapeutic drug for stones. The analytical techniques included nanoparticle size analysis, transmission electron microscopy, X-ray diffraction, and Fourier-transform infrared spectroscopy. The studied properties included the components, morphologies, zeta potentials, particle size distributions, light intensity autocorrelation curves, and polydispersity indices (PDIs) of the nanocrystallites. The main components of the urinary nanocrystallites before K3cit intake included uric acid, β-calcium phosphate, and calcium oxalate monohydrate. After K3cit intake, the quantities, species, and percentages of aggregated crystals decreased, whereas the percentages of monosodium urate and calcium oxalate dehydrate increased, and some crystallites became blunt. Moreover, the urinary pH increased from 5.96 ± 0.43 to 6.46 ± 0.50, the crystallite size decreased from 524 ± 320 nm to 354 ± 173 nm, and the zeta potential decreased from -4.85 ± 2.87 mV to -8.77 ± 3.03 mV. The autocorrelation curves became smooth, the decay time decreased from 11.4 ± 3.2 ms to 4.3 ± 1.7 ms, and the PDI decreased from 0.67 ± 0.14 to 0.53 ± 0.19. These changes helped inhibit CaOx calculus formation.

New pathophysiological aspects of growth and prevention of kidney stones

Kidney stones probably grow during crystalluria by crystal sedimentation and aggregation (AGN) on stone surfaces. This process has to occur within urinary transit time (UT) through the kidney before crystals are washed out by diuresis. To get more information, we studied by spectrophotometry the formation and AGN of Ca oxalate (Ca Ox) crystals which were directly produced in urine of 30 stone patients and 30 controls by an oxalate (Ox) titration. Some tests were also performed after removing urinary macromolecules (UMs) by ultrafiltration. To induce rapid crystallization, high Ox additions (0.5–0.8 mM) were necessary. The most important finding was retardation of crystal AGN by UM. In urine of 63% of controls but only 33% of patients, no AGN was observed during an observation of 60 minutes (P < 0.05). Also growth and sedimentation rate of crystals were significantly reduced by UM. For stone metaphylaxis, especially for posttreatment residuals, avoiding dietary Ox excesses to prevent crystal formation in the kidney and increasing diuresis to wash out crystals before they aggregate are recommended.

Major and trace elements in lithogenesis

The process of crystallization in the urinary tract occurs when the equilibrium between promoting and inhibiting factors is broken. Many theories have been published to explain the mechanism of urinary stones formation; however, none of these theories has paid attention to trace elements. Their role in lithogenesis is still unclear and under debate. The findings of some studies may support the thesis that some major and trace elements may take part in the initiation of stone crystallization for instance as a nucleus or nidus for the formation of the stone, or simply contaminate the stone structure. This review presents a comprehensive account of the basic principles of the basic data and the role of major and trace elements in lithogenesis.

Aggregation of freshly precipitated calcium oxalate crystals in urine of calcium stone patients and controls

Aggregation (AGN) of freshly precipitated calcium oxalate crystals was photometrically studied in urine of 30 calcium stone patients and 30 controls, in solutions containing urinary macromolecules (UMS) and in an inhibitor free control solution (CS). Crystals were produced by oxalate titration and crystallization was monitored measuring optical density (OD). Tests were repeated adding hydroxyapatite (HAP) to urine and UMS and adding citrate and pyrophosphate (PPi) to UMS of the controls. AGN was recognized as a rapid OD decrease being at least three times faster than sedimentation of single crystals (p < 0.001) and used to calculate an extent of AGN (EA%). The time between the end of titration and the beginning of AGN was determined as suspension stability (SS). The main effect of urinary inhibitors was retardation of AGN without changing EA, SS being higher in urine than UMS (p < 0.001) and in UMS than CS (p < 0.001). In urine of 63% of controls but only in 33% of patients, no AGN was recorded (p < 0.05). The high inhibitory activity of urine could not be reproduced in UMS even in combination with 3.5 mM citrate or 0.05 mM PPi. 0.05 mg/mL HAP reduced SS in all urine samples to low values and increased the rate of rapid OD decrease, being a measure for the size of aggregates. Retarding AGN of crystals during their passage through the kidney seems to be an important mechanism to prevent stone formation during crystalluria. The promotion of AGN by HAP reveals a new role of Randall's plaques in nephrolithiasis.

Aggregation of freshly precipitated calcium oxalate crystals in urine of calcium stone patients and controls

Aggregation (AGN) of freshly precipitated calcium oxalate crystals was photometrically studied in urine of 30 calcium stone patients and 30 controls, in solutions containing urinary macromolecules (UMS) and in an inhibitor free control solution (CS). Crystals were produced by oxalate titration and crystallization was monitored measuring optical density (OD). Tests were repeated adding hydroxyapatite (HAP) to urine and UMS and adding citrate and pyrophosphate (PPi) to UMS of the controls. AGN was recognized as a rapid OD decrease being at least three times faster than sedimentation of single crystals (p < 0.001) and used to calculate an extent of AGN (EA%). The time between the end of titration and the beginning of AGN was determined as suspension stability (SS). The main effect of urinary inhibitors was retardation of AGN without changing EA, SS being higher in urine than UMS (p < 0.001) and in UMS than CS (p < 0.001). In urine of 63% of controls but only in 33% of patients, no AGN was recorded (p < 0.05). The high inhibitory activity of urine could not be reproduced in UMS even in combination with 3.5 mM citrate or 0.05 mM PPi. 0.05 mg/mL HAP reduced SS in all urine samples to low values and increased the rate of rapid OD decrease, being a measure for the size of aggregates. Retarding AGN of crystals during their passage through the kidney seems to be an important mechanism to prevent stone formation during crystalluria. The promotion of AGN by HAP reveals a new role of Randall's plaques in nephrolithiasis.

Surface aggregation of urinary proteins and aspartic Acid-rich peptides on the faces of calcium oxalate monohydrate investigated by in situ force microscopy

The growth of calcium oxalate monohydrate in the presence of Tamm-Horsfall protein (THP), osteopontin, and the 27-residue synthetic peptides (DDDS)(6)DDD and (DDDG)(6)DDD (D = aspartic acid, S = serine, and G = glycine) was investigated via in situ atomic force microscopy. The results show that these four growth modulators create extensive deposits on the crystal faces. Depending on the modulator and crystal face, these deposits can occur as discrete aggregates, filamentary structures, or uniform coatings. These proteinaceous films can lead to either the inhibition of or an increase in the step speeds (with respect to the impurity-free system), depending on a range of factors that include peptide or protein concentration, supersaturation, and ionic strength. While THP and the linear peptides act, respectively, to exclusively increase and inhibit growth on the (101) face, both exhibit dual functionality on the (010) face, inhibiting growth at low supersaturation or high modulator concentration and accelerating growth at high supersaturation or low modulator concentration. Based on analyses of growth morphologies and dependencies of step speeds on supersaturation and protein or peptide concentration, we propose a picture of growth modulation that accounts for the observations in terms of the strength of binding to the surfaces and steps and the interplay of electrostatic and solvent-induced forces at the crystal surface.

Concentrated urine and diluted urine: the effects of citrate and magnesium on the crystallization of calcium oxalate induced in vitro by an oxalate load

Supplementation of certain calcium crystallization inhibitors, such as citrate and magnesium, and the dilution of urine with water are now considered consolidated practice for the prevention of calcium kidney stones. The aim of this study is to verify, using tried and true in vitro methods, whether the effect of these inhibitors can manifest itself in different ways depending on whether the urine is concentrated or diluted. Calcium oxalate crystallization was studied on 4-h urine of 20 male idiopathic calcium oxalate stone formers, first under low hydration conditions (non-diluted urine) and then under high hydration conditions (diluted urine). Both the diluted and the non-diluted urine samples were subjected to three types of load: (a) an oxalate concentration increment of 1.3 mmol/l only; (b) an oxalate concentration increment of 1.3 mmol/l with a citrate concentration increment of 1.56 mmol/l; (c) an oxalate concentration increment of 1.3 mmol/l with a magnesium concentration increment of 2.08 mmol/l. In non-diluted urine, the addition of the citrate and magnesium did not modify the crystallization parameters under study. In contrast, in the diluted urine the addition of the citrate and magnesium led to a reduction in the total quantity of crystals (equivalent to 35-45%) and their aggregates (equivalent to 30-40%); at the same time, there was an increase in the diameter of the monohydrate calcium oxalate crystals, which also underwent a morphological change. In conclusion, the inhibitory effects of citrate and magnesium on the crystallization of calcium oxalate do not manifest themselves in highly concentrated urine.

Tamm-Horsfall protein in recurrent calcium kidney stone formers with positive family history: abnormalities in urinary excretion, molecular structure and function

Tamm-Horsfall protein (THP) powerfully inhibits calcium oxalate crystal aggregation, but structurally abnormal THPs from recurrent calcium stone formers may promote crystal aggregation. Therefore, increased urinary excretion of abnormal THP might be of relevance in nephrolithiasis. We studied 44 recurrent idiopathic calcium stone formers with a positive family history of stone disease (RCSF(fam)) and 34 age- and sex-matched healthy controls (C). Twenty-four-hour urinary THP excretion was measured by enzyme linked immunosorbent assay. Structural properties of individually purified THPs were obtained from analysis of elution patterns from a Sepharose 4B column. Sialic acid (SA) contents of native whole 24-h urines, crude salt precipitates of native urines and individually purified THPs were measured. THP function was studied by measuring inhibition of CaOx crystal aggregation in vitro (pH 5.7, 200 mM sodium chloride). Twenty-four-hour urine excretion of THP was higher in RCSF(fam) (44.0 +/- 4.0 mg/day) than in C (30.9 +/- 2.2 mg/day, P = 0.015). Upon salt precipitation and lyophilization, elution from a Sepharose 4B column revealed one major peak (peak A, cross-reacting with polyclonal anti-THP antibody) and a second minor peak (peak B, not cross-reacting). THPs from RCSF(fam) eluted later than those from C (P = 0.021), and maximum width of THP peaks was higher in RCSF(fam )than in C (P = 0.024). SA content was higher in specimens from RCSF(fam) than from C, in native 24-h urines (207.5 +/- 20.4 mg vs. 135.2 +/- 16.1 mg, P = 0.013) as well as in crude salt precipitates of 24-h urines (10.4 +/- 0.5 mg vs. 7.4 +/- 0.9 mg, P = 0.002) and in purified THPs (75.3 +/- 9.3 microg/mg vs. 48.8 +/- 9.8 microg/mg THP, P = 0.043). Finally, inhibition of calcium oxalate monohydrate crystal aggregation by 40 mg/L of THP was lower in RCSF(fam) (6.1 +/- 5.5%, range -62.0 to +84.2%) than in C (24.9 +/- 6.0%, range -39.8 to +82.7%), P = 0.022, and only 25 out of 44 (57%) THPs from RCSF(fam )were inhibitory (positive inhibition value) vs. 25 out of 34 (74%) THPs from C, P < 0.05. In conclusion, severely recurrent calcium stone formers with a positive family history excrete more THP than healthy controls, and their THP molecules elute later from an analytical column and contain more SA. Such increasingly aggregated THP molecules predispose to exaggerated calcium oxalate crystal aggregation, an important prerequisite for urinary stone formation.

Molecular modulation of calcium oxalate crystallization

Calcium oxalate monohydrate (COM) is the primary constituent of the majority of renal stones. Osteopontin (OPN), an aspartic acid-rich urinary protein, and citrate, a much smaller molecule, are potent inhibitors of COM crystallization at levels present in normal urine. Current concepts of the role of site-specific interactions in crystallization derived from studies of biomineralization are reviewed to provide a context for understanding modulation of COM growth at a molecular level. Results from in situ atomic force microscopy (AFM) analyses of the effects of citrate and OPN on growth verified the critical role of site-specific interactions between these growth modulators and individual steps on COM crystal surfaces. Molecular modeling investigations of interactions of citrate with steps and faces on COM crystal surfaces provided links between the stereochemistry of interaction and the binding energy levels that underlie mechanisms of growth modification and changes in overall crystal morphology. The combination of in situ AFM and molecular modeling provides new knowledge that will aid rationale design of therapeutic agents for inhibition of stone formation.

Determinants of urinary excretion of Tamm-Horsfall protein in non-selected kidney stone formers and healthy subjects

BACKGROUND

The aim of the study was to measure urinary excretion of Tamm-Horsfall protein (THP), an important inhibitor of crystallization, and to identify possible determinants of urinary THP excretion in non-selected kidney stone formers (SF) and healthy subjects (C).

METHODS

By means of a commercially available ELISA (Pharmacia and Upjohn/Elias, Germany), we measured THP in 24-h urines of 104 SF (74 males/30 females, age 16-74 years) who had formed 8.7+/-2.4 stones (range 1-240), and of 71 C (41 males/30 females, age 22-62 years). Types of stones formed by SF were 88 calcium, eight uric acid, six infection, and two cystine. All values are means+/-SE.

RESULTS

The normal range (5th to 95th percentile) of U(THP)xV was 9.3-35.0 mg/day in males and 9.0-36.3 mg/day in females respectively. Mean U(THP)xV was 21.3+/-1.2 mg/day (range 3. 4-51.6) in male and 15.2+/-1.6 mg/day (range 1.8-32.3) in female SF (P=0.008 vs male SF). Since U(THP)xV was positively correlated with C(Crea) (r=0.312, P=0.001) in SF as well as with U(Crea)xV (r=0.346, P=0.0001) and with body surface (r=0.271, P=0.0003) in all study subjects, mean THP/Crea (mg/mmol) was used for all further calculations. Overall, THP/Crea was lower in SF (1.42+/-0.07 vs 1. 68+/-0.08, P:=0.015), mainly due to increased THP/Crea in female C (2.08+/-0.11, P=0.0036 vs female SF, P=0.0001 vs male C and vs male calcium SF), which also explains decreased THP/Crea values in calcium SF (1.46+/-0.08, P=0.041 vs C). In addition, THP/Crea was reduced in uric acid SF (1.11+/-0.21, P=0.049 vs C). Whereas THP/Crea was not related to age, urine volume, intake of dairy calcium, or urinary markers of protein intake, either in C or in SF, it correlated significantly with urinary Citrate/Crea, both in C (r=0.523, P=0.0001) and in SF (r=0.221, P=0.025). In C only, but not in SF, THP/Crea was correlated with urinary Calcium/Crea (r=0. 572, P=0.0001) and with Oxalate/Crea (r=0.274, P=0.022).

CONCLUSIONS

Both in C and SF, urinary THP excretion is related to body size, renal function and urinary citrate excretion, whereas dietary habits apparently do not affect THP excretion. Uric acid and calcium stone formation predict reduced THP excretion in comparison with C, whereas female gender goes along with increased urinary THP excretion in C. Possibly most relevant to kidney stone formation is the fact that THP excretion rises only in C in response to increasing urinary calcium and oxalate concentrations, whereas this self-protective mechanism appears to be missing in SF.

Citrate determines calcium oxalate crystallization kinetics and crystal morphology-studies in the presence of Tamm-Horsfall protein of a healthy subject and a severely recurrent calcium stone former

BACKGROUND

The aim of this study was to measure the effects of normal (nTHP) and abnormal stone former Tamm-Horsfall protein (SF-THP) on calcium oxalate (CaOx) nucleation and aggregation as well as on crystal morphology, in presence or absence of citrate.

METHODS

Nucleation and aggregation of CaOx crystals from a supersaturated, stirred solution (200 mM NaCl, 10 mM Na-acetate, pH 5.70, 5 mM Ca and 0.5 mM Ox) were studied by spectrophotometric time-course measurements of OD at 620 nm (OD(620)). Measured parameters were induction time t(I) (time to induce formation of detectable particles), S(N), (slope of increase of OD(620), mainly due to crystal nucleation), and S(A), (slope of decrease of OD(620) after equilibrium has been reached, due to crystal aggregation). Effects of citrate, nTHP and SF-THP on these parameters were measured, and scanning electron microscopy (SEM) was performed.

RESULTS

At 1.5, 2.5 and 3.5 mM, citrate increased t(I) and inhibited crystal nucleation (by 78-87%) as well as aggregation (by 63-70%), and smaller CaOx crystals (length/width ratio 1.7+/-0.1) than under standard conditions (length/width 3.9+/-0.5) were visible (P<0.001). Normal THP at 30 and 40 mg/l inhibited crystal nucleation and, more strongly, aggregation (inhibition 76-81%). SEM revealed a decrease in length/width ratio to 2.6+/-0.4 (P=0.051 vs standard conditions) and less aggregation than without nTHP. At all concentrations tested, SF-THP reduced t(I) (P=0.0001 vs standard conditions) and promoted aggregation (inhibition -48 to -33%); crystals were elongated with a length/width ratio of 4.3+/-0.6 (P<0. 05 vs nTHP). When simultaneously present with nTHP, citrate enhanced the inhibitory effects of nTHP, producing the smallest (length/width 1.5+/-0.1) and least aggregated crystals. Finally, 3.5 mM citrate turned promotory SF-THP into a crystallization inhibitor with abundant small and clustered, but not aggregated crystals.

CONCLUSION

Citrate appears to be the main determinant of CaOx crystallization rates and crystal morphology in the presence of nTHP as well as SF-THP. Its effects appear to predominate over those of THP, since even promotory SF-THP is turned into a crystallization inhibitor in the presence of citrate. This re-emphasizes at a morphological level what has been concluded from functional as well from clinical studies, namely that citrate is needed in urine at equimolar concentrations to calcium in order to prevent the formation of large crystal aggregates in presence of abnormal THP.

Renal citrate metabolism and urinary citrate excretion in the infant rat

BACKGROUND

Although hypercalciuria has the same prevalence in children as adults, children rarely develop renal stones. This may be explained by a greater urinary citrate excretion in infants compared with adults. The present study examines the renal excretion of citrate and renal cortical citrate metabolism in infant and adult rats.

METHODS

Adult male and newly weaned infant rats were acclimated to metabolic cages and fed synthetic diets. Urine was collected after two days, and renal cortical citrate metabolism was assayed.

RESULTS

Infant rats had a lower plasma [HCO3-] and higher plasma [K+] and had a fourfold higher urinary citrate:creatinine ratio and a twofold higher concentration of citrate in their urine compared with adult rats. This higher urinary citrate excretion was not due to a difference in renal proximal tubular Na/citrate cotransporter activity, nor renal cortical citrate synthase or ATP citrate lyase activities in infants as compared with adults. However, infant rat kidneys had significantly lower mitochondrial aconitase (m-aconitase) activity. Renal cortical citrate concentrations were comparable in infant and adult rats. Manipulation of plasma [K+] to adult levels did not affect the higher urinary citrate excretion in infant rats.

CONCLUSIONS

Urinary citrate excretion in infant rats is greater than in adults but does not parallel tissue [citrate]. Thus, this higher urinary citrate is likely due to maturational differences in the proximal tubule, other than Na/citrate cotransport, that directly affect citrate transport.

Agglomeration inhibition reflected stone-forming activity during long-term potassium citrate therapy in calcium stone formers

OBJECTIVES

The agglomeration of preformed crystals of calcium oxalate has been hypothesized to be the rate-limiting step in renal stone-forming activity (SFA). The effect of urine on the in vitro inhibition of agglomeration of seed crystals of calcium oxalate monohydrate, designated [tm], has been used to monitor SFA in calcium oxalate stone formers (CaOxSF). The objective of the present study was to determine whether [tm] could be used to help monitor the long-term effectiveness of oral potassium citrate therapy (K-Cit-Rx) in CaOxSF.

METHODS

Clinic and radiographic (or ultrasound) reports were evaluated for 80 patients, aged 20 to 72 years, 55 men and 25 women, who were treated with oral K-Cit for recurrent calcium oxalate urolithiasis at the Ochsner Stone Clinic between January 1992 and July 1996. Seventy-five of these patients had at least one 24-hour citrate excretion rate of less than 3.0 mm/day before or after K-Cit-Rx. SFA graded on a scale of -2 to +2 by radiographic criteria was combined with information on stone passage to evaluate clinical stone status, and 24-hour urine collections were evaluated for volume, pH, calcium, citrate, uric acid, oxalate, creatinine, and [tm] on free diet before and after 6 to 53 months of K-Cit-Rx. Historical information on procedures performed for urolithiasis before and on K-Cit-Rx was also reviewed.

RESULTS

K-Cit-Rx resulted in increased urine pH (P <0.0001) and decreased calcium (P=0.0475), [tm] (P=0.0045), number of stones passed per year (P=0.0016), and remedial procedures per year (P <0.0001). Patients taking allopurinol in addition to K-Cit required higher doses (P <0.0001) of K-Cit to control their disease, had lower pretreatment urine pH (P=0.0493), and showed greater increase in urine citrate (P=0.0092) than those on K-Cit alone. Those taking high-dose K-Cit were younger (P=0.0363) and showed greater decrease in SFA (P=0.0005) than those taking lower doses. A small group of 10 medication refractory patients, who retained (n=9) or increased (n=1) their stone burden during K-Cit-Rx, was identified. Compared with the medication-responsive group, the refractory patients were older (P=0.0124), and had greatly increased SFA (P <0.0001) and higher (P=0.0347) urine pH before and during (P=0.0173) treatment (data not shown).

CONCLUSIONS

The data confirm that [tm] can be used not only to verify previously documented stone formation rate but also to help evaluate the long-term effectiveness of therapy. In this report, changes in [tm] after K-Cit-Rx reflected decreased stone formation rate and decreased remedial procedures.

Biochemical and quantitative analysis of Tamm Horsfall protein in rats

The involvement of Tamm Horsfall protein (THP) in nephrolithiasis is currently under investigation in several laboratories. Although rat is a commonly used species as an in vivo model for such studies, there is only limited information available about the biochemical properties and excretion profile of THP in normal rats. In order to characterize rat THP, we purified and analyzed normal male rat THP, and compared it with normal human male urinary THP by gel electrophoresis. Both THPs migrated at approximately 90 KDa, and stained similarly for protein (Coomassie blue) as well as carbohydrates (periodic acid Schiff reagent). Compositional analysis revealed that rat THP was largely similar to human THP in amino acid and carbohydrate contents but showed differences in the individual sugar components from other mammals. There was considerable variation in the day-to-day urinary excretion of THP in normal rats, with values ranging from 552.96 micrograms to 2865.60 micrograms and a mean value of 1679.54 micrograms per 24 h. It was concluded from this study that rat THP did not contain any unusual biochemical components and was primarily similar to human THP in composition and mean urinary concentration.

Adenosine triphosphate citrate lyase mediates hypocitraturia in rats

Chronic metabolic acidosis increases proximal tubular citrate uptake and metabolism. The present study addressed the effect of chronic metabolic acidosis on a cytosolic enzyme of citrate metabolism, ATP citrate lyase. Chronic metabolic acidosis caused hypocitraturia in rats and increased renal cortical ATP citrate lyase activity by 67% after 7 d. Renal cortical ATP citrate lyase protein abundance increased by 29% after 3 d and by 141% after 7 d of acid diet. No significant change in mRNA abundance could be detected. Hypokalemia, which causes only intracellular acidosis, caused hypocitraturia and increased renal cortical ATP citrate lyase activity by 28%. Conversely, the hypercitraturia of chronic alkali feeding was associated with no change in ATP citrate lyase activity. Inhibition of ATP citrate lyase with the competitive inhibitor, 4S-hydroxycitrate, significantly abated hypocitraturia and increased urinary citrate excretion fourfold in chronic metabolic acidosis and threefold in K+-depletion. In summary, the hypocitraturia of chronic metabolic acidosis is associated with an increase in ATP citrate lyase activity and protein abundance, and is partly reversed by inhibition of this enzyme. These results suggest an important role for ATP citrate lyase in proximal tubular citrate metabolism.

Immunocytochemical localization of Tamm-Horsfall protein in the kidneys of normal and nephrolithic rats

Studies using in vitro systems have indicated that Tamm-Horsfall protein (THP) can interact with calcium oxalate (CaOx) crystals during kidney stone formation. However, information regarding the nature of its participation in this process remains controversial and unclear. In order to better understand the putative interaction of THP and crystals in vivo, we compared the localization of THP in normal rats and in chronic and semi-acute rat models of nephrolithiasis. In these rats, CaOx crystal deposits were induced in the kidneys by administering ethylene glycol (EG) in drinking water. The formation of CaOx mono- and dihydrate aggregates in the urine was confirmed by scanning electron microscopy. Immunohistochemical localization, as well as protein A-gold labeling at the ultrastructural level, demonstrated that in addition to its normal distribution, THP specifically associated with the renal crystal deposits. The THP-containing, organic matrix-like material consisted of a fine, fibrillar meshwork surrounding individual crystals and their aggregates. In addition, THP also appeared in the papilla, where it is normally absent, concurrent with the appearance of crystal deposits in the kidneys. These observations indicate that in nephrolithic rats the normal localization of THP is altered. Such an alteration may indicate an important physiological event related to crystal aggregation and kidney stone formation.

Isoelectric focusing of Tamm-Horsfall glycoproteins: a simple tool for recognizing recurrent calcium oxalate renal stone formers

Tamm-Horsfall glycoproteins (THPs) from healthy probands and a majority of recurrent calcium oxalate renal stone formers reveal different physicochemical properties when analyzed using isoelectric focusing (IEF). The pI values of THPs from healthy probands are approximately 3.5 while THPs from recurrent renal stone formers have pI values of between 4.5 and 6. The two groups of THPs exhibit completely different protein patterns. The differences in IEF analysis allow differentiation between THPs from healthy probands and recurrent calcium oxalate stone formers and may possibly be used as a simple diagnostic method for the recognition of recurrent calcium oxalate renal stone formers.

Is citrate an inhibitor of calcium oxalate crystal growth in high concentrations of urine?

The effect of citrate on calcium oxalate (CaOx) crystal growth was studied in a system in which series of samples containing [45Ca]calcium chloride were brought to different levels of supersaturation with various concentrations of oxalate. The crystallization was assessed by measuring the amount of isotope remaining in solution 30 min after the addition of CaOx seed crystals to samples containing citrate in concentrations corresponding to those in final urine. The experiments were carried out both in pure salt solutions and in solutions with dialysed urine. Increased concentrations of citrate resulted in a reduced crystallization of CaOx in both the presence and absence of dialysed urine, but with the lowest rate of crystallization in the samples containing urine. The increased concentration of 45Ca remaining in solution reflected a reduced crystallization, which could possibly be explained both by a reduced supersaturation and by an increased inhibition of CaOx crystal growth. The direct effects of citrate on CaOx crystal growth were assessed by calculating the ion-activity product of CaOx (APCaOx) at corresponding degrees of crystallization. The APCaOx recorded at a 30% reduction of the amount of isotope in solution increased with increasing concentrations of citrate between 1.0 and 1.5 mmol/l samples both with and without dialysed urine. These findings indicate that citrate has a weak direct inhibitory effect on CaOx crystal growth, which adds to the reduced growth rate brought about by urinary macromolecules and a decreased supersaturation.

A simple diagnostic method for the differentiation of Tamm-Horsfall glycoproteins from healthy probands and those from recurrent calcium oxalate renal stone formers

Tamm-Horsfall glycoproteins (THPs)* from healthy probands, and those from a majority of recurrent calcium oxalate renal stone formers, reveal different properties when analyzed using isoelectric focusing. The pl-values of THPs from healthy probands are approximately 3.5 while THPs from recurrent renal stone formers have pl-values between 4.5 and 6. The two groups of THPs exhibit completely different protein patterns in IEF. This proves the structural difference of these THPs. The differences in IEF analysis allow the differentiation between THPs from healthy probands and those from recurrent calcium oxalate stone formers. These differences could possibly be used as a simple diagnostic method for the recognition of recurrent calcium oxalate renal stone formers.