Contribution of thick ascending limb and distal convoluted tubule to glucose-induced hypercalciuria in healthy controls.

Carbohydrates increase kidney stone risk and increase urine calcium and magnesium. We hypothesize that the effects of glucose as an allosteric modulator of calcium-sensing receptors may mediate this effect. Six healthy subjects were on a low-sodium diet before consuming 100 g of glucose beverage. Timed fasting (3) and postglucose (6) urine and blood samples were collected every 30 min. Urine composition and serum markers were measured and microvesicular abundance of tubular transport proteins (NHE3, NKCC2, NCC, and TRPV5) were quantified. Postglucose, serum glucose, and insulin rose rapidly with a parallel increase in calcium and magnesium excretion and no change in fractional excretion of sodium. Both serum parathyroid hormone (PTH) and urine TRPV5 fell in the postglucose periods. The rise in the calcium and magnesium excretion likely occurred primarily in the thick ascending limb where they are both under control of the calcium-sensing receptor. The fall in PTH and TRPV5 support the role of glucose as an allosteric modulator of calcium-sensing receptor.NEW & NOTEWORTHY Sugar increases urine calcium and magnesium as well as kidney stone and bone disease risk. Our study provided new insights into the underlying mechanism as we gave healthy subjects an oral glucose load and used newer tools such as fractional excretion of lithium, serum parathyroid hormone, and microvesicular abundance of tubular transport proteins to characterize the mechanism and identify the thick ascending limb with possible calcium-sensing receptor mediation as a likely contributor to this mechanism.

Metabolomic profiles and pathogenesis of nephrolithiasis.

PURPOSE OF REVIEW: Kidney stone disease is caused by supersaturation of urine with certain metabolites and minerals. The urine composition of stone formers has been measured to prevent stone recurrence, specifically calcium, uric acid, oxalate, ammonia, citrate. However, these minerals and metabolites have proven to be unreliable in predicting stone recurrence. Metabolomics using high throughput technologies in well defined patient cohorts can identify metabolites that may provide insight into the pathogenesis of stones as well as offer possibilities in therapeutics. RECENT FINDINGS: Techniques including 1H-NMR, and liquid chromatography paired with tandem mass spectroscopy have identified multiple possible metabolites involved in stone formation. Compared to formers of calcium oxalate stones, healthy controls had higher levels of hippuric acid as well as metabolites involved in caffeine metabolism. Both the gut and urine microbiome may contribute to the altered metabolome of stone formers. SUMMARY: Although metabolomics has offered several potential metabolites that may be protective against or promote stone formation, the mechanisms behind these metabolomic profiles and their clinical significance requires further investigation.

A spatially anchored transcriptomic atlas of the human kidney papilla identifies significant immune injury in patients with stone disease.

Kidney stone disease causes significant morbidity and increases health care utilization. In this work, we decipher the cellular and molecular niche of the human renal papilla in patients with calcium oxalate (CaOx) stone disease and healthy subjects. In addition to identifying cell types important in papillary physiology, we characterize collecting duct cell subtypes and an undifferentiated epithelial cell type that was more prevalent in stone patients. Despite the focal nature of mineral deposition in nephrolithiasis, we uncover a global injury signature characterized by immune activation, oxidative stress and extracellular matrix remodeling. We also identify the association of MMP7 and MMP9 expression with stone disease and mineral deposition, respectively. MMP7 and MMP9 are significantly increased in the urine of patients with CaOx stone disease, and their levels correlate with disease activity. Our results define the spatial molecular landscape and specific pathways contributing to stone-mediated injury in the human papilla and identify associated urinary biomarkers.

End Point Considerations for Clinical Trials in Enteric Hyperoxaluria.

Enteric hyperoxaluria is a medical condition characterized by elevated urinary oxalate excretion due to increased gastrointestinal oxalate absorption. Causative features include fat malabsorption and/or increased intestinal permeability to oxalate. Enteric hyperoxaluria has long been known to cause nephrolithiasis and nephrocalcinosis, and, more recently, an association with CKD and kidney failure has been shown. Currently, there are no US Food and Drug Administration-approved therapies for enteric hyperoxaluria, and it is unclear what end points should be used to evaluate the efficacy of new drugs and biologics for this condition. This study represents work of a multidisciplinary group convened by the Kidney Health Initiative to review the evidence supporting potential end points for clinical trials in enteric hyperoxaluria. A potential clinical outcome is symptomatic kidney stone events. Potential surrogate end points include ( 1 ) an irreversible loss of kidney function as a surrogate for progression to kidney failure, ( 2 ) asymptomatic kidney stone growth/new stone formation observed on imaging as a surrogate for symptomatic kidney stone events, ( 3 ) urinary oxalate and urinary calcium oxalate supersaturation as surrogates for the development of symptomatic kidney stone events, and ( 4) plasma oxalate as a surrogate for the development of the clinical manifestations of systemic oxalosis. Unfortunately, because of gaps in the data, this Kidney Health Initiative workgroup was unable to provide definitive recommendations. Work is underway to obtain robust information that can be used to inform trial design and medical product development in this space.

Water Loading and Uromodulin Secretion in Healthy Individuals and Idiopathic Calcium Stone Formers.

BACKGROUND: Uromodulin is a protein made only by the kidney and released in urine, circulating in polymerizing and nonpolymerizing forms. This protein's multiple functions include inhibition of stone formation in the urine. The physiological determinants of uromodulin production are incompletely understood. METHODS: We investigated changes in uromodulin levels and key factors governing its production and release in urine and serum. We performed an experiment to determine whether water loading, a common intervention to prevent stone formation, will alter the rate of uromodulin production. During a 2-day period, 17 stone forming participants and 14 control participants were subjected to water loading (day 1) and normal fluid intake (day 2). Uromodulin levels were measured on timed hourly collections in urine and plasma during the period of the study. RESULTS: Water loading increased urinary uromodulin secretion (33±4 versus 10±4 µ g/min at baseline, P < 0.0001) in stone formers and control participants. Despite high urine volumes, most participants maintained relatively stable urinary uromodulin concentrations. Native Western blots for polymerizing and nonpolymerizing uromodulin suggest that polymerizing uromodulin was the predominant form at higher urinary flow volumes. Urine flow rates and sodium excretion were significant correlates of urinary uromodulin production. Water loading did not affect serum uromodulin levels, which were also not associated with urinary uromodulin. CONCLUSIONS: Water loading increases the secretion of polymerizing urinary uromodulin. This increased secretion reduces the variability of urinary uromodulin concentrations despite high urine volumes. Serum uromodulin levels were not affected by this treatment.

Clinical Effectiveness of Calcium Oxalate Stone Treatments.

INTRODUCTION: Lowering kidney stone risk and urine calcium oxalate supersaturation is a primary clinical focus for kidney stone prevention and can be achieved with multiple strategies. Common strategies include advice to increase fluid intake, restrict dietary sodium, or prescribing a thiazide-type diuretic. We investigated how physicians make these decisions in real-world practice and evaluate their efficacy based on 24-h urine collections. METHODS: We reviewed medical charts for 203 kidney stone formers with idiopathic calcium stones from University of Chicago Kidney Stone Clinic between 2005 and 2020. Patients had three 24-h urines before an initial pre-treatment clinic visit and one follow-up 24-h urine. We analyzed changes in urine composition based on treatment advice using t tests and ANOVA. RESULTS: Patients who received advice to increase fluid intake had lower urine volume at baseline (1.5 vs. 2.5 L/day, p < 0.001) and larger increase in urine volume at follow-up (0.6 vs. 0.1 L/day, p < 0.001) compared to those who did not receive the advice. Patients who were advised to restrict dietary sodium had a higher urine sodium at baseline (208 vs. 139 mEq/day, p < 0.001), a larger reduction in urine sodium (-28 vs. 13 mEq/day, p = 0.002), and larger reduction in urine calcium (-74 vs. -28 mg/day, p = 0.005) compared with those not advised to restrict dietary sodium. Patients started on a thiazide had a higher baseline urine calcium (281 vs. 213 mg/day) and larger reduction in urine calcium (-83 vs. -9 mg/day, p < 0.001) compared with patients not started on a thiazide. In combination, thiazide prescriptions with dietary sodium restriction reduced urine calcium by 99 mg/day and reduced calcium oxalate supersaturation from 8.0 to 5.5 and calcium phosphate supersaturation from 1.4 to 1.0. CONCLUSION: Providers use 24-h urine data to guide treatment strategy decisions. These strategies achieved the intended effects on urine composition and lowered kidney stone risk.

What treatments reduce kidney stone risk in patients with bowel disease?

We examined how physicians made therapeutic choices to decrease stone risk in patients with bowel disease without colon resection, many of whom have enteric hyperoxaluria (EH), at a single clinic. We analyzed clinic records and 24-h urine collections before and after the first clinic visit, among 100 stone formers with bowel disease. We used multivariate linear regression and t tests to compare effects of fluid intake, alkali supplementation, and oxalate-focused interventions on urine characteristics. Patients advised to increase fluid intake had lower initial urine volumes (L/day; 1.3 ± 0.5 vs. 1.7 ± 0.7) and increased volume more than those not so advised (0.7 ± 0.6 vs. 0.3 ± 0.6 p = 0.03; intervention vs. non-intervention). Calcium oxalate supersaturation (CaOx SS) fell (95% CI -4.3 to -0.8). Alkali supplementation increased urine pH (0.34 ± 0.53 vs. 0.22 ± 0.55, p = 0.26) and urine citrate (mg/d; 83 ± 256 vs. 98 ± 166, p = 0.74). Patients advised to reduce oxalate (mg/day) absorption had higher urine oxalate at baseline (88 ± 44 vs. 50 ± 26) which was unchanged on follow-up (88 (baseline) vs. 91 (follow-up), p = 0.90). Neither alkali (95% CI -1.4 to 2.1) nor oxalate-focused advice (95% CI -1.2 to 2.3) lowered CaOx SS. Physicians chose treatments based on baseline urine characteristics. Advice to increase fluid intake increased urine volume and decreased CaOx SS. Alkali and oxalate interventions were ineffective.

Primary hyperoxaluria: the adult nephrologist's point of view.

In adults, primary hyperoxaluria (PH) does not always present as obviously as in children, leading to delayed or even missed diagnosis. When diagnosed in adulthood, PH usually progresses at a slower rate and the focus is on the prevention of recurrent kidney stones as much as it is on the preservation of renal function. The most tragic presentation is when the diagnosis is made after primary non-function of a renal graft for treating previously unknown renal disease. Recurrent stones, nephrocalcinosis and features of systemic oxalosis can all be presenting features. For these reasons, consideration should be given to screening for this rare condition, using biochemical and/or genetic means, but being careful to exclude common differential diagnoses. Such efforts should be synchronized with diagnostic methods for other rare kidney diseases.

Collagen fibrils and cell nuclei are entrapped within Randall's plaques but not in CaOx matrix overgrowth: A microscopic inquiry into Randall's plaque stone pathogenesis.

Calcium oxalate (CaOx) stones can grow attached to the renal papillary calcification known as Randall's plaque. Although stone growth on Randall's plaque is a common phenomenon, this mechanism of stone formation is still poorly understood. The objective of this study was to investigate the microenvironment of mature Randall's plaque, explore its molecular composition and differentiate plaque from CaOx overgrowth using multimodal imaging on demineralized stone sections. Fluorescence imaging showed consistent differences in autofluorescence patterns between Randall's plaque and calcium oxalate overgrowth regions. Second harmonic generation imaging established the presence of collagen only in regions of decalcified Randall's plaque but not in regions of CaOx overgrowth matrix. Surprisingly, in these stone sections we observed cell nuclei with preserved morphology within regions of mature Randall's plaque. These conserved cells had variable expression of vimentin and CD45. The presence of nuclei in mature plaque indicates that mineralization is not necessarily associated with cell death. The markers identified suggest that some of the entrapped cells may be undergoing dedifferentiation or could emanate from a mesenchymal or immune origin. We propose that entrapped cells may play an important role in the growth and maintenance of Randall's plaque. Further characterization of these cells and thorough analyses of the mineralized stone forming renal papilla will be fundamental in understanding the pathogenesis of Randall's plaque and CaOx stone formation.

Stone Morphology Distinguishes Two Pathways of Idiopathic Calcium Oxalate Stone Pathogenesis.

Introduction: About 1 in 11 Americans will experience a kidney stone, but underlying causes remain obscure. The objective of the present study was to separate idiopathic calcium oxalate stone formers by whether or not they showed positive evidence of forming a stone on Randall's plaque (RP). Materials and Methods: In patients undergoing either percutaneous or ureteroscopic procedures for kidney stone removal, all stone material was extracted and analyzed using micro-CT imaging to identify those attached to RP. Twenty-four-hour urine samples were collected weeks after the stone removal procedure and patients were off of medications that would affect urine composition. The endoscopic video was analyzed for papillary pathology (RP, pitting, plugging, dilated ducts, and loss of papillary shape) by an observer blinded to the data on stone type. The percent papillary area occupied by RP and ductal plugging was quantified using image analysis software. Results: Patients having even one stone on RP (N = 36) did not differ from non-RP patients (N = 37) in age, sex, BMI, or other clinical characteristics. Compared with the non-RP group, RP stone formers had more numerous, but smaller, stones, more abundant papillary RP formation, and fewer ductal plugs, both by quantitative measurement of surface area (on average, three times more plaque area, but only 41% as much plug area as in non-RP patients) and by semiquantitative visual grading. Serum and blood values did not differ between RP and non-RP stone formers by any measure. Conclusions: Growth of many small stones on plaque seems the pathogenetic scheme for the RP stone-forming phenotype, whereas the non-RP phenotype stone pathogenesis pathway is less obvious. Higher papillary plugging in non-RP patients suggests that plugs play a role in stone formation and that these patients have a greater degree of papillary damage. Underlying mechanisms that create these distinctive phenotypes are presently unknown.

Evidence for abnormal linkage between urine oxalate and citrate excretion in human kidney stone formers.

BACKGROUND: Animal models have demonstrated an interactive relationship between the epithelial anion exchanger SLC26A6 and transporter NaDC-1 that regulates citrate and oxalate homeostasis. This relationship is a potential mechanism to protect against kidney stones as higher urine oxalate is accompanied by higher urine citrate but it has not been explored in humans. METHODS: We examined 24-h urine data on 13,155 kidney stone forming patients (SF) from separate datasets at the University of Chicago and Litholink, a national laboratory, and 143 non-kidney stone forming participants (NSF) to examine this relationship in humans. We used multivariate linear regression models to examine the association between oxalate and citrate in all study participants and separately in SF and NSF. RESULTS: Higher urinary oxalate was associated with higher urinary citrate in both SF and NSF. In NSF, the multivariate adjusted urine citrate excretion was 3.0 (1.5-4.6) (mmol)/creatinine (mmol) per oxalate (mmol)/creatinine (mmol). In SF, the multivariate adjusted urine citrate excretion was 0.3 (0.2-0.4) (mmol)/creatinine (mmol) per oxalate (mmol)/creatinine (mmol). CONCLUSIONS: Higher urinary oxalate excretion was associated with higher urinary citrate excretion and this effect was larger in non-kidney stone forming participants compared with those who form kidney stones.

Relative contributions of urine sulfate, titratable urine anion, and GI anion to net acid load and effects of age.

Models of acid-base balance include acid production from (1) oxidation of sulfur atoms on amino acids and (2) metabolically produced organic acid anions. Acid load is balanced by alkali from metabolism of GI anions; thus, net acid production is equivalent to the sum of urine sulfate and organic anion (measured by titration in urine), minus GI anion. However, the relative contributions of these three sources of acid production in people eating free choice diets, and presumably in acid-base balance, have not been well studied. We collected 26 urines from 18 normal subjects (10 male) and 43 urine samples from 34 stone formers (17 male) and measured sulfate, organic anion, and components of GI anion and acid excretion in each; values were expressed as mEq/mmol creatinine. Mean values of the urine components, except creatinine and pH, did not differ between the sexes or groups. Urine organic acid and acid production varied directly with age (p ≤ 0.03). In a general linear model of acid excretion, the coefficients for sulfate, organic anion, and GI anion were 0.34 ± 0.09, 0.49 ± 0.12, and -0.51 ± 0.06, respectively, p ≤ 0.005, and the model accounted for 54% of the variance. A model for urine ammonia gave similar results. Urine organic anion is a significant contributor to total acid production and may be responsible for an increase in acid production with age.

Sex Differences of Kidney Stone Urine Risk Factors after Roux-en-Y Gastric Bypass.

INTRODUCTION: Roux-en-Y gastric bypass (RYGB) is a bariatric surgical procedure that is associated with higher risk of kidney stones after surgery. We examined urine composition in 18 men and women before and after RYGB to examine differences in kidney stone risk. METHODS: Three 24-h urine collections were performed before and 1 year after RYGB. We analyzed mean urinary values for pre- and post-RYGB collections and compared men and women. RESULTS: Seven men and eleven women completed pre- and post-RYGB urine collections. Pre-RYGB, men had higher calcium oxalate supersaturation (CaOx SS) (7.0 vs. 5.0, p = 0.04) compared with women. Post-RYGB, women had higher urine CaOx SS (13.1 vs. 4.6, p = 0.002), calcium phosphate supersaturation (1.04 vs. 0.59, p = 0.05), and lower urine volumes (1.7 vs. 2.7L, p < 0.001) compared with men. DISCUSSION/CONCLUSION: There are important differences in urine composition by sex that may contribute to higher kidney stone risk in women after RYGB compared with men.

Demineralization and sectioning of human kidney stones: A molecular investigation revealing the spatial heterogeneity of the stone matrix.

The molecular mechanisms by which kidney stones grow are largely unknown. Organic molecules from the urine combine with mineral crystals to form stones, but analysis of the stone matrix has revealed over a thousand different proteins, with no clues as to which are important for stone growth. Molecules that are present in every layer of a stone would be candidates for having an essential function, and thus the analysis of the stone matrix at a microscopic level is necessary. For this purpose, kidney stones were demineralized, sectioned, stained, and imaged by microscopy, using micro CT for precise orientation. Histological staining demonstrated heterogeneity in the density of adjacent layers within stones. Additional results also showed brilliant and unique autofluorescence patterns in decalcified nephroliths, indicating heterogeneous organic composition in adjacent layers. Regions of calcium oxalate (CaOx) stones were dissected using laser microdissection (LMD) for protein analysis. LMD of broad regions of demineralized CaOx stone sections yielded the same proteins as those found in different specimens of pulverized CaOx stones. These innovative methodologies will allow spatial mapping of protein composition within the heterogeneous stone matrix. Proteins that consistently coincide spatially with mineral deposition would be candidates for molecules essential for stone growth. This kind of analysis will be required to assess which of the thousand proteins in the stone matrix may be fundamental for stone growth.

Pathophysiology and Treatment of Enteric Hyperoxaluria.

Enteric hyperoxaluria is a distinct entity that can occur as a result of a diverse set of gastrointestinal disorders that promote fat malabsorption. This, in turn, leads to excess absorption of dietary oxalate and increased urinary oxalate excretion. Hyperoxaluria increases the risk of kidney stones and, in more severe cases, CKD and even kidney failure. The prevalence of enteric hyperoxaluria has increased over recent decades, largely because of the increased use of malabsorptive bariatric surgical procedures for medically complicated obesity. This systematic review of enteric hyperoxaluria was completed as part of a Kidney Health Initiative-sponsored project to describe enteric hyperoxaluria pathophysiology, causes, outcomes, and therapies. Current therapeutic options are limited to correcting the underlying gastrointestinal disorder, intensive dietary modifications, and use of calcium salts to bind oxalate in the gut. Evidence for the effect of these treatments on clinically significant outcomes, including kidney stone events or CKD, is currently lacking. Thus, further research is needed to better define the precise factors that influence risk of adverse outcomes, the long-term efficacy of available treatment strategies, and to develop new therapeutic approaches.

Multimodal imaging reveals a unique autofluorescence signature of Randall's plaque.

Kidney stones frequently develop as an overgrowth on Randall's plaque (RP) which is formed in the papillary interstitium. The organic composition of RP is distinct from stone matrix in that RP contains fibrillar collagen; RP in tissue has also been shown to have two proteins that are also found in stones, but otherwise the molecular constituents of RP are unstudied. We hypothesized that RP contains unique organic molecules that can be differentiated from the stone overgrowth by fluorescence. To test this, we used micro-CT-guided polishing to expose the interior of kidney stones for multimodal imaging with multiphoton, confocal and infrared microscopy. We detected a blue autofluorescence signature unique to RP, the specificity of which was also confirmed in papillary tissue from patients with stone disease. High-resolution mineral mapping of the stone also showed a transition from the apatite within RP to the calcium oxalate in the overgrowth, demonstrating the molecular and spatial transition from the tissue to the urine. This work provides a systematic and practical approach to uncover specific fluorescence signatures which correlate with mineral type, verifies previous observations regarding mineral overgrowth onto RP and identifies a novel autofluorescence signature of RP demonstrating RP's unique molecular composition.