Calcium and oxalate concentrations in human renal tissue: the key to the pathogenesis of stone formation?

Calcium Oxalate Urolithiasis: A Case of Missing Microbes?

INTRODUCTION

Urinary stone disease (USD) has known associations with the gut microbiota. Approximately 80% of kidney stones contain oxalate as a primary constituent and diverse oxalate-degrading bacteria exist within the human gut, which may protect against USD. Although bacteriotherapy represents a promising strategy to eliminate oxalate and reduce the risk of USD, oxalate-degrading probiotics have had limited success. To identify limitations of oxalate-degrading probiotics and refine development of bacteriotherapies to prevent USD, we review the literature associated with the gut microbiota and USD.

MATERIALS AND METHODS

A literature search was performed to identify publications that examine the role of oxalate-degrading bacteria or the whole gut microbiota in oxalate metabolism and the pathophysiology of USD. We conducted a meta-analysis of studies that examined the association of the whole gut microbiota with USD. In addition, we evaluated the gut microbiota of healthy individuals and those with comorbidities related to USD using publically available data from the American Gut Project (AGP).

RESULTS

Studies on Oxalobacter formigenes reveal that colonization by this species is not a good predictor of USD risk or urinary oxalate excretion. The species of oxalate-degrading bacteria used in probiotics and duration of administration do not impact efficacy or persistence. Studies focused on the whole gut microbiota reveal broad shifts in the gut microbiota associated with USD and a diverse microbial network is associated with oxalate metabolism. AGP data analysis demonstrated a strong overlap in microbial genera depleted in diseased individuals among USD and comorbidities.

CONCLUSIONS

The associations between the gut microbiota and USD extend beyond individual functional microbial species. Common shifts in the gut microbiota may facilitate the onset of USD and/or comorbidities. The successful development of bacteriotherapies to inhibit USD will need to incorporate strategies that target a broad diversity of bacteria rather than focus on a few specialist species.

[Calcium oxalate stone formation. New pathogenetic aspects of an old disease]

Calcium oxalate (CaOx) urolithiasis is a very common disorder. Surprisingly, the pathogenetic mechanisms leading to CaOx stone formation have been largely unknown so far. The long-accepted simple explanation by an exceeding of the solubility product of lithogenic substances in the urine cannot sufficiently describe the complex processes. Deviating from the hypothesis that proclaims that the initial crystal deposition takes place in the lumens of renal tubules, new insights suggest a primary plaque formation in the interstitial space of the renal papilla. Initially, calcium phosphate (CaPh) crystals and organic matrix are deposited along the basement membranes of the thin loops of Henle and extend further in the interstitial space to the urothelium, constituting the so-called Randall's plaques that can be regularly found during endoscopy of CaOx-stone-forming patients. These CaPh crystals seem to be the origin for the development of future CaOx stones, which form by the attachment of further matrix molecules and CaOx from the urine to the plaque. The driving forces, the exact pathogenetic mechanisms, and the involved matrix molecules remain largely unknown. Possibly, completely different pathomechanisms lead to the common clinical diagnosis of"CaOx stone former."

Henle loop basement membrane as initial site for Randall plaque formation

Since the early description of Randall plaques in 1937, studies of the pathogenesis of stone formation mainly focused on the chemistry involving salt precipitation and crystallization, rather than tubular and interstitial medullar mechanisms of calcium concentration and supersaturation. In 2003, Bushinsky published a suggestive and inspiring sequence of events aimed to show that the basement membrane of the thin limb of the loop of Henle can be the first site of nucleation, as recently shown by the impressive work by Evan et al. The aim of this minireview is to verify the consistency of the Evan and Bushinsky theory with the current literature in the field.

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

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

Renal cell adaptation to oxalate

Renal manifestations of chronic hyperoxaluria include nephrolithiasis and, when extreme, interstitial scarring and progressive loss of function. Exposure of cultured renal cells to oxalate has been reported to cause cell death, as well as proliferation. The current study was performed to assess the time course and cell-type specificity of these responses. Proximal (LLC-PK(1)) and distal [cIMCD and primary human renal (HRC1)] renal epithelial cells, as well as interstitial KNRK cells, were exposed to oxalate (0.5-2.0 mM) for 24-72 h. The generation of reactive oxygen species (ROS) was measured using the fluorescent probe DCF, and cell number was determined with CyQuant reagent. HSP-70 expression was assessed via real time PCR and quantitative Western blot. In response to all oxalate concentrations (0.5-2.0 mM) and lengths of exposure (15 min-2 h), cultured proximal and distal renal epithelial cells and renal fibroblasts generated ROS. After 24 h, cells demonstrated initial cell death and decrease in cell numbers, but by 48-72 h adapted and grew, despite the continued presence of oxalate. This response was associated with increased expression of HSP-70 mRNA and protein. Renal cells in vivo may possess adaptive mechanisms to withstand chronic hyperoxaluria, including increased expression of chaperone molecules such as HSP-70.

Is there a role for pentosan polysulfate in the prevention of calcium oxalate stones?

The clinical role for pentosan polysulfate (PPS) in the prevention of calcium oxalate urolithiasis is not known. Crystallization and aggregation are important steps in calcium oxalate stone formation, and PPS has been shown to inhibit these steps, both in vitro and in vivo. In addition, PPS has a role in repairing injured urothelium and inhibiting adhesion to epithelial defects. A randomized double-blind placebo-controlled study appears warranted to assess the utility of PPS in the prevention of recurrent calcium oxalate stones.

Effect of oxalate on the growth of renal tubular epithelial cells

PURPOSE

Until recently, oxalate was considered merely as a major component of calcium oxalate stones, forming crystals in the lumen of the renal tubules. However, new evidence suggests that oxalate is not only a major constituent of calcium oxalate stones but also has effect on renal tubular epithelial cells, affecting the pathogenesis of nephrolithiasis. We tried to elucidate the effect of oxalate on the growth of renal tubular epithelial cells of different species and locations and also to interpret the possible role of the oxalate in the pathogenesis of urolithiasis.

MATERIALS AND METHODS

Porcine proximal renal tubular epithelial cells (LLC-PK1) and canine distal renal tubular epithelial cells (MDCK) were incubated with different concentrations of oxalate, and the effect of oxalate on the growth of the cells was assessed by methylthiazoletetrazolium assay.

RESULTS

Growth of the renal tubular epithelial cells was inhibited with increasing concentrations of oxalate in both proximal and distal lines.

CONCLUSION

Oxalate may cause stone formation by affecting the growth of renal tubular epithelial cells as well as by providing a constituent of calcium oxalate stones.

Functional effects of unilateral laser papillectomy in the pig

OBJECTIVES

To evaluate the use of endoscopic laser papillectomy in a multi-papillary animal model to unilaterally impair concentrating ability and increase the urinary flow rate.

METHODS

Domestic pigs underwent unilateral retrograde flexible nephroscopy. With a holmium:yttrium-aluminum-garnet laser, varying numbers of papillae were ablated. Four weeks after the procedure, renal function studies were performed during hydropenia and after hydration, the animals were killed, and the kidneys were examined histologically.

RESULTS

The urine flow rate per 100 mL creatinine clearance was significantly increased in the papillectomized kidney compared with the control kidney during hydropenia (1.50 versus 0.94, P <0.01). The papillectomized kidneys were unable to concentrate the urine as well as the control kidneys during both hydropenia (urine osmolarity 430 versus 534 mOsm/L, P <0.01) and after hydration (329 versus 362 mOsm/L, P = 0.02). The free water reabsorption per 100 mL creatinine clearance was impaired in the papillectomized kidneys compared with the control kidneys (0.48 versus 1.00, P = 0.02) after hydration. A significant correlation existed between the percentage of papillae ablated and the difference in osmolarity between the operated and control kidneys (r(2) = 0.50, P = 0.015). Histologic examination demonstrated transitional re-epithelialization with moderate collecting duct dilation and medullary fibrosis underlying the ablated papillae early in the series; however, the histologic features normalized and the creatinine clearance was less impaired with a more proficient technique later in the series.

CONCLUSIONS

Endoscopic laser papillectomy results in increased urine flow and impaired urinary concentrating ability. This surgical technique should be investigated further for its role in the prevention of nephrolithiasis.

The role of the papilla in idiopathic calcium oxalate nephrolithiasis

The complex and multifactorial phenomenon of urinary stone disease remains unclear. Anatomical and physiochemical theories do not adequately deal with certain aspects of idiopathic calcium oxalate nephrolithiasis in particular or of nephrolithiasis. One of the reasons for this could be that nephrolithiasis is not only a primary disorder but may also be a symptom of other disorders or various pathologic changes in the metabolism of lithogenic substances. Both affirmative and contradictory reports have been published since Randall's first description of papillary calcifications and their possible active role in the genesis of calcium oxalate nephrolithiasis. Our intention is to discuss focal calcified lesions as an etiologic factor of renal stone disease as well as the change from historical to modern concepts regarding the development of medullary calcifications and their relationship to idiopathic calcium oxalate nephrolithiasis.

Multielement analysis of kidney tissue with renal calculi

Tissue distribution and concentration gradients of macro- and micro-elements in the papilla, medulla, and cortex of human kidney with renal calculi were measured with spectrometer. An uninvolved portion of the hypernephroma kidney was used as control. Cadmium (Cd), cobalt (Co), and molybdenum (Mo) were least in amount in renal papilla as compared with the cortex and medulla in renal stone kidney. Overall there was less Mo in stone kidney tissue, and also significantly less when compared with that of control renal papilla. Considering the reported fact that Mo was found in high concentrations in urinary tract stones and our data that Mo was less in stone kidney, it is speculated that Mo may play some unexplained but significant role in certain stage(s) of the stone formation.

Calcium oxalate and other crystals associated with kidney diseases and arthritis

The recognition of tissue deposits of crystalline material in a variety of organs, including the kidney, predated the association of crystals and arthritic disease. Because of this, the pathophysiology of crystal formation and its resultant inflammation is based in part on studies of renal stones. A number of disease states involving renal and articular crystallization exist. The most common of these, uric acid precipitation, or gout, and calcium phosphate precipitation were not reviewed in this discussion. This review described a variety of less common disease states involving articular and renal crystal deposition. The renal diseases discussed included both parenchymal or ectopic crystal deposition, as seen in nephrocalcinosis or cystinosis, and ductal crystallization as seen in renal calculus disease. The crystals involved included not only calcium oxalate, but also aluminum, amino acids and proteins (cystine, hemoglobin, cryoglobulins, and immunoglobulins), purine metabolites (xanthine, hypoxanthine), and even lipids and their degradative enzymes (cholesterol, phospholipids, phospholipase, and fatty acids). The simultaneous occurrence of crystals in both kidneys and joints was found in some cases to result from the systemic deposition of an excess of a particular biological compound. However, of more interest, some renal deposits were shown to more selectively reflect the normal or abnormal function of the kidney in its secretory and excretory roles. This is particularly evident in the variety of arthritic states described in end-stage renal disease.

Plasma oxalate concentration in chronic renal disease

Plasma oxalate was measured with use of the enzyme oxalate oxidase (EC 1.2.3.4; normal values 3.3 +/- 1.5 mumol/L, n = 24) in 50 patients with different degrees of renal failure. The following mean concentrations +/- SD (in mumol/L) were found: for glomerular diseases, 12.7 +/- 7.8 (n = 21); tubular diseases, 20.4 +/- 14.0 (n = 16); chronic renal failure before dialysis, 32.5 +/- 13.5, and after dialysis, 17.8 +/- 3.8 (n = 10); and primary hyperoxalemia, 72.2 +/- 14.5 14.5 (n = 2). The course of plasma oxalate was followed in one of these two patients after renal transplantation and in a patient recovering from acute tubular necrosis. No significant differences were found between patients with glomerular and tubular disorders. Overall, plasma oxalate was correlated with plasma creatinine in patients with glomerular and tubular diseases and dialysis patients (r = .84, P less than .001). Patients with primary hyperoxalemia had values outside the 95% confidence area of the regression line. It is concluded that the values obtained with this method, although probably still tending to overestimate the true oxalate concentration to some extent, provide reliable information about relative differences in plasma oxalate levels. In patients with terminal renal failure, plasma oxalate sometimes rises to levels at which deposition of calcium oxalate in tissues can occur.

Calcium oxalate kidney stones in patients on continuous ambulatory peritoneal dialysis

Kidney stones were passed by ten out of 186 patients with endstage renal disease who were treated with continuous ambulatory peritoneal dialysis (CAPD). Stones from seven patients were examined by x-ray diffraction. In five of them the stones were composed of calcium oxalate monohydrate. The urine calcium oxalate activity product was determined in 44 CAPD patients, eight of whom were stone formers, and compared to that of 120 normal volunteers. In CAPD patients, mean urine ionic-calcium concentration was lower than in normal subjects whereas mean urine ionic-oxalate concentration was significantly higher than in normal subjects. In normal urine samples, the calcium oxalate activity product showed a significant correlation with both the urine ionic-calcium and the ionic-oxalate concentrations. In contrast, in CAPD patients the calcium oxalate activity product correlated with the ionic-calcium concentration but not with ionic-oxalate. Although the urine ionic-calcium concentration is lower in CAPD patients than in normal subjects, it is the relative increase in its concentration which appears to be associated with the increased risk of kidney stone formation in these patients. This relative hypercalciuria seems to follow 1,25(OH)2 vitamin D3 administration.

Concentration profiles of calcium and oxalate in urine, tubular fluid and renal tissue--some theoretical considerations

This paper analyzes some aspects of the pathophysiology of urolithiasis. It is emphasized that a better understanding of factors contributing to stone formation can only be gained when the primary nucleation site is identified. Three compartments are considered in which supersaturation as a precondition for stone formation could be present: urine in the urinary tract, tubular fluid from the glomerulus down to the duct of Bellini, and the interstitium of the medulla. From calculations based on micropuncture data it becomes apparent that the oxalate concentration in the tubular fluid at the bend of Henle's loop is 1 or 2 orders of magnitude lower than in the duct of Bellini and that the oxalate concentration maximum invariably must be located in the final urine. The calculation of a tubular concentration profile of oxalate shows, that the probability of intra luminal crystal formation is even less likely for plasma oxalate values of 2-3 microM as compared to 1.2 microM, which therefore should be the correct value. The time necessary for the growth of crystals up to a critical size which can obstruct tubules or ureter is not available in the urinary tract nor in the tubules. However, in the medullary interstitium, where solute concentration is highest, nearly unlimited time for crystal growth is available due to the fact, that in this compartment convective flow is very low. It is concluded that the interstitium of the inner medulla has the best chances to function as the primary nucleation site where particles can be formed of a size which subsequently can obstruct the urinary tract.

The fate of oxalic acid in the Wistar rat

1. An established procedure for determining oxalate in human urine has been modified for rat urine. The daily excretion of oxalate by the male Wistar rat is 570-650 microgram. 2. Oxalate excretion in rat urine following i.p. administration of [14C]oxalic acid (1-70 mg/kg) has been studied. The rate and degree of excretion are dose-dependent. 3. The excretion of urinary oxalate by the rat has been quantified after administration of two oxalate-producing xenobiotics, alpha-chlorohydrin and 1,2-dibromo-3-chloropropane. 4. Oxalate inhibits the metabolism of glucose and lactate by isolated rat kidney tubules in vitro.