Intestinal transport of an obdurate anion: oxalate

From Smoothies to Dialysis: The Impact of Oxalate Nephropathy

Oxalate nephropathy is a rare cause of acute kidney injury that can lead to end-stage renal disease. This case report describes a 54-year-old male with type 2 diabetes mellitus and chronic kidney disease who presented for a routine clinic follow-up. Laboratory tests revealed significant deterioration in renal function with an unrevealing history and symptoms suggestive of the process. Initial investigations for worsening renal function were inconclusive, prompting a renal biopsy that confirmed acute tubular injury with abundant calcium oxalate deposits. Further investigation into dietary history revealed that the patient regularly consumed high-oxalate foods, such as spinach and kale smoothies, under the impression they were beneficial for his diabetes. Despite the initiation of hemodialysis, the patient did not recover renal function and remains dialysis-dependent. This case underscores the need for a high index of suspicion for oxalate nephropathy in chronic kidney disease patients presenting with unexplained acute kidney injury. Diagnosis is confirmed through renal biopsy and should be considered in patients with relevant dietary histories.

Engineered microorganisms: A new direction in kidney stone prevention and treatment

Numerous studies have shown that intestinal and urinary tract flora are closely related to the formation of kidney stones. The removal of probiotics represented by lactic acid bacteria and the colonization of pathogenic bacteria can directly or indirectly promote the occurrence of kidney stones. However, currently existing natural probiotics have limitations. Synthetic biology is an emerging discipline in which cells or living organisms are genetically designed and modified to have biological functions that meet human needs, or even create new biological systems, and has now become a research hotspot in various fields. Using synthetic biology approaches of microbial engineering and biological redesign to enable probiotic bacteria to acquire new phenotypes or heterologous protein expression capabilities is an important part of synthetic biology research. Synthetic biology modification of microorganisms in the gut and urinary tract can effectively inhibit the development of kidney stones by a range of means, including direct degradation of metabolites that promote stone production or indirect regulation of flora homeostasis. This article reviews the research status of engineered microorganisms in the prevention and treatment of kidney stones, to provide a new and effective idea for the prevention and treatment of kidney stones.

Oxalate (dys)Metabolism: Person-to-Person Variability, Kidney and Cardiometabolic Toxicity

Oxalate is a metabolic end-product whose systemic concentrations are highly variable among individuals. Genetic (primary hyperoxaluria) and non-genetic (e.g., diet, microbiota, renal and metabolic disease) reasons underlie elevated plasma concentrations and tissue accumulation of oxalate, which is toxic to the body. A classic example is the triad of primary hyperoxaluria, nephrolithiasis, and kidney injury. Lessons learned from this example suggest further investigation of other putative factors associated with oxalate dysmetabolism, namely the identification of precursors (glyoxylate, aromatic amino acids, glyoxal and vitamin C), the regulation of the endogenous pathways that produce oxalate, or the microbiota's contribution to oxalate systemic availability. The association between secondary nephrolithiasis and cardiovascular and metabolic diseases (hypertension, type 2 diabetes, and obesity) inspired the authors to perform this comprehensive review about oxalate dysmetabolism and its relation to cardiometabolic toxicity. This perspective may offer something substantial that helps advance understanding of effective management and draws attention to the novel class of treatments available in clinical practice.

Sel1-like proteins and peptides are the major Oxalobacter formigenes-derived factors stimulating oxalate transport by human intestinal epithelial cells

Kidney stones (KSs) are very common, excruciating, and associated with tremendous healthcare cost, chronic kidney disease (CKD), and kidney failure (KF). Most KSs are composed of calcium oxalate and small increases in urinary oxalate concentration significantly enhance the stone risk. Oxalate also potentially contributes to CKD progression, kidney disease-associated cardiovascular diseases, and poor renal allograft survival. This emphasizes the urgent need for plasma and urinary oxalate lowering therapies, which can be achieved by enhancing enteric oxalate secretion. We previously identified Oxalobacter formigenes (O. formigenes)-derived factors secreted in its culture-conditioned medium (CM), which stimulate oxalate transport by human intestinal Caco2-BBE (C2) cells and reduce urinary oxalate excretion in hyperoxaluric mice by enhancing colonic oxalate secretion. Given their remarkable therapeutic potential, we now identified Sel1-like proteins as the major O. formigenes-derived secreted factors using mass spectrometry and functional assays. Crystal structures for six proteins were determined to confirm structures and better understand functions. OxBSel1-14-derived small peptides P8 and P9 were identified as the major factors, with P8 + 9 closely recapitulating the CM's effects, acting through the oxalate transporters SLC26A2 and SLC26A6 and PKA activation. Besides C2 cells, P8 + 9 also stimulate oxalate transport by human ileal and colonic organoids, confirming that they work in human tissues. In conclusion, P8 and P9 peptides are identified as the major O. formigenes-derived secreted factors and they have significant therapeutic potential for hyperoxalemia, hyperoxaluria, and related disorders, impacting the outcomes of patients suffering from KSs, enteric hyperoxaluria, primary hyperoxaluria, CKD, KF, and renal transplant recipients.NEW & NOTEWORTHY We previously identified Oxalobacter formigenes-derived secreted factors stimulating oxalate transport by human intestinal epithelial cells in vitro and reducing urinary oxalate excretion in hyperoxaluric mice by enhancing colonic oxalate secretion. We now identified Sel1-like proteins and small peptides as the major secreted factors and they have significant therapeutic potential for hyperoxalemia and hyperoxaluria, impacting the outcomes of patients suffering from kidney stones, primary and secondary hyperoxaluria, chronic kidney disease, kidney failure, and renal transplant recipients.

Increased abundance of bacteria of the family Muribaculaceae achieved by fecal microbiome transplantation correlates with the inhibition of kidney calcium oxalate stone deposition in experimental rats

Background

The incidence of nephrolithiasis is increasing rapidly worldwide. Calcium oxalate is the most common constituent, contributing to approximately 80% of all kidney stones. The gut microbiome, through its oxalate-degrading ability, may play a role in decreasing morbidity due to urinary calculus. Fecal microbiome transplantation (FMT) has been reported to be effective in restoring the gastrointestinal microbial community in different conditions. The transplantation of whole communities that have oxalate-degrading function may be a more effective strategy than the transplantation of isolated strains.

Methods

FMT was carried out in male guinea pigs and male Sprague-Dawley laboratory rats (SDRs). Fresh feces were collected from guinea pigs housed in metabolic cages. SDRs were divided into four groups: two groups received standard rat chow (SC) (groups SC and SC + FMT), and two groups were fed a 5% potassium oxalate diet (OD) (groups OD + phosphate-buffered saline (PBS) and OD + FMT). On day 14, groups OD + PBS, OD + FMT, and SC + FMT received either PBS or guinea pig feces by esophageal gavage. The composition of the microbiota of guinea pigs and SDRs was analyzed using a 16S rRNA gene sequencing approach. Biochemical analysis of urine samples from SDRs revealed the presence of calcium oxalate (CaOx) crystals, which were presumed to originate from kidney stones. Renal function was examined using real-time PCR analysis and immunohistochemical staining for renin, angiotensin-converting enzyme, and osteopontin (OPN) expression.

Results

FMT resulted in a gut microbiota that was a mixture of guinea pig and SDR bacteria. A microbial network involving Muribaculaceae, Lactobacillus, and Bifidobacterium was activated by FMT in group OD + FMT. As a result, urinary oxalate, calcium, uric acid, creatinine and urea in urine samples were reduced significantly. Similarly, significant reduction of uric acid and blood urea nitrogen to creatinine ratio in serum samples was observed (p < 0.05). Microscopic observations revealed a high CaOx crystal score (4+) in the kidneys of rats in group OD + PBS, whereas a lower score (2+) was observed in the rats in group OD + FMT. Up-regulation of OPN and down-regulation of renin were also associated with FMT.

Conclusion

A microbial network involving Muribaculaceae and other oxalate-degrading bacteria achieved by FMT was capable of reducing urinary oxalate excretion and CaOx crystal deposition in the kidney through increasing intestinal oxalate degradation. FMT may exert a renoprotective function in oxalate-related kidney stones.

Oxalate secretion is stimulated by a cAMP-dependent pathway in the mouse cecum

Elevated levels of the intracellular second messenger cAMP can stimulate intestinal oxalate secretion however the membrane transporters responsible are unclear. Oxalate transport by the chloride/bicarbonate (Cl-/HCO3-) exchanger Slc26a6 or PAT-1 (Putative Anion Transporter 1), is regulated via cAMP when expressed in Xenopus oocytes and cultured cells but whether this translates to the native epithelia is unknown. This study investigated the regulation of oxalate transport by the mouse intestine focusing on transport at the apical membrane hypothesizing PAT-1 is the target of a cAMP-dependent signaling pathway. Adopting the Ussing chamber technique we measured unidirectional 14C-oxalate and 36Cl- flux ([Formula: see text] and [Formula: see text]) across distal ileum, cecum and distal colon, employing forskolin (FSK) and 3-isobutyl-1-methylxanthine (IBMX) to trigger cAMP production. FSK/IBMX initiated a robust secretory response by all segments but the stimulation of net oxalate secretion was confined to the cecum only involving activation of [Formula: see text] and distinct from net Cl- secretion produced by inhibiting [Formula: see text]. Using the PAT-1 knockout (KO) mouse we determined cAMP-stimulated [Formula: see text] was not directly dependent on PAT-1, but it was sensitive to mucosal DIDS (4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid), although unlikely to be another Cl-/HCO3- exchanger given the lack of trans-stimulation or cis-inhibition by luminal Cl- or HCO3-. The cAMP-activated oxalate efflux was reliant on CFTR (Cystic Fibrosis Transmembrane conductance Regulator) activity, but only in the presence of PAT-1, leading to speculation on the involvement of a multi-transporter regulatory complex. Further investigations at the cellular and molecular level are necessary to define the mechanism and transporter(s) responsible.

Improving Treatment Options for Primary Hyperoxaluria

The primary hyperoxalurias are three rare inborn errors of the glyoxylate metabolism in the liver, which lead to massively increased endogenous oxalate production, thus elevating urinary oxalate excretion and, based on that, recurrent urolithiasis and/or progressive nephrocalcinosis. Frequently, especially in type 1 primary hyperoxaluria, early end-stage renal failure occurs. Treatment possibilities are scare, namely, hyperhydration and alkaline citrate medication. In type 1 primary hyperoxaluria, vitamin B6, though, is helpful in patients with specific missense or mistargeting mutations. In those vitamin B6 responsive, urinary oxalate excretion and concomitantly urinary glycolate is significantly decreased, or even normalized. In patients non-responsive to vitamin B6, RNA interference medication is now available. Lumasiran® is already available on prescription and targets the messenger RNA of glycolate oxidase, thus blocking the conversion of glycolate into glyoxylate, hence decreasing oxalate, but increasing glycolate production. Nedosiran blocks liver-specific lactate dehydrogenase A and thus the final step of oxalate production. Similar to vitamin B6 treatment, where both RNA interference urinary oxalate excretion can be (near) normalized and plasma oxalate decreases, however, urinary and plasma glycolate increases with lumasiran treatment. Future treatment possibilities are on the horizon, for example, substrate reduction therapy with small molecules or gene editing, induced pluripotent stem cell-derived autologous hepatocyte-like cell transplantation, or gene therapy with newly developed vector technologies. This review provides an overview of current and especially new and future treatment options.

High and low oxalate content in spinach: an investigation of accumulation patterns

BACKGROUND

Oxalic acid is a common antinutrient in the human diet, found in large quantities in spinach. However, spinach is highly regarded by vegetable producers because of its nutritional content and economic value. One of the primary purposes of spinach-breeding programs is to improve the nutritional value of spinach by adjusting oxalate accumulation. Knowledge of the biosynthetic patterns of oxalic acid, and its different forms, is important for a better understanding of this process.

RESULTS

We found three biosynthetic patterns of accumulation and concentration of oxalates. Two of them are related to the maximum type and one is related to the minimum type. We also developed a general model of variations in these compounds in the genotypes that were studied.

CONCLUSION

This study introduced a unique type of spinach with high oxalate accumulation, which could be particularly suitable for consumption. This had the highest ratio of insoluble oxalate to soluble oxalate. It also accumulated more ascorbic acid (AA) than other types. Our findings in this study also indicate a small role for AA as a precursor to oxalate production in spinach, possibly confirming the significant role of glyoxylate as the most critical precursor in this plant. © 2021 Society of Chemical Industry.

Effect of Brine fermented Pickling to Physicochemical, Anti-nutritional, and Microbiological Attributes of Pickled gboma Eggplant (Solanum macrocarpon)

Gboma eggplant (Solanum macrocarpon) has low calorie, high phenolic and antioxidant, excellent dietary fibre and mineral content. Besides healthy effects, gboma eggplant also contained numerous anti-nutrients unbeneficial for human health. Due to high moisture content, gboma eggplant was highly perishable during post-harvest. This research observed the changes of nutritional proximate, mineral contents, acidification indexes, anti-nutrients, phytochemical and antioxidant properties, texture profiles, microbiological characteristics of both raw and pickled gboma eggplant. Gboma eggplant fruits were soaked in clean water for 2 minutes before cutting their calyx lobes. The pre-treated fruits were submerged in sterilized brine (5% salt) for 8 days. Periodically, pickled samples were taken to examine physicochemical, anti-nutritional, and microbiological attributes of pickled gboma eggplant. Results showed that nutritional proximate of moisture, ash, protein, fat, fibre, carbohydrate was varied slightly during fermentation. There was a decreasing trend of moisture and carbohydrate; meanwhile, there was an increasing trend of ash, protein, fibre during 8 days of fermentation. There was no significant difference of fat during pickling. There was a minor increment of mineral contents in all samples. There was accumulation of phenyllactic acid, ascorbic acid content and titratable acidity with exception of pH. Anti-nutrient contents like tannin, phytate, oxalate, steroidal glycoalkaloid greatly decreased in raw and pickled eggplant. Remarkable increments of total phenolic, flavonoid, DPPH free radical scavenging and FRAP ferric reducing antioxidant of raw and pickled eggplant was presented. Texture profiles of hardness, crispness, fracturability, crunchiness revealed a minor reduction of sensory scores during 8 days of fermentation. Lactic acid bacteria, Bacillus significantly proliferated; meanwhile, Micrococcus and S. aureus were absolutely retarded in pickled eggplant. Yeast and fungi increased in the first 4 days and decreased afterwards. There was no significant difference of proximate compositions; mineral contents; phenyllactic acid, ascorbic acid content, pH and titratable acidity; anti-nutritional contents; phytochemical and antioxidant properties; texture profile (sensory score); microbiological load (with exception of S. aureus) between pickled gloma eggplants at day 6th and day 8th of fermentation. At a quick glance, the length of fermentation could be shorted to 6 days instead of 8 days. However, at the 6th day, S. aureus load was still presented at 0.33±0.01 log CFU/g. The fermentation should be lasted to 8th day so that S. aureus load could be dropped down to zero to ensure microbial food safety. Raw gboma eggplant should be fermented in 8% brine solution for 8 days to obtain the best physicochemical, anti-nutritional, and microbiological properties of pickle. Findings of this research suggested that fermentation pickling would be an appropriate approach to improve nutritional, physicochemical and functional criteria while controlling toxic chemical residues, retarding the growth and proliferation of spoilage and pathogen microorganisms.

Probiotic Lactiplantibacillus plantarum N-1 could prevent ethylene glycol-induced kidney stones by regulating gut microbiota and enhancing intestinal barrier function

Defective permeability barrier is considered to be an incentive of hyperuricemia, however, the link between them has not been proven. Here, we evaluated the potential preventive effects of Lactiplantibacillus plantarum N-1 (LPN1) on gut microbiota and intestinal barrier function in rats with hyperoxaluria-induced kidney stones. Male rats were supplied with 1% ethylene glycol (EG) dissolved in drinking water for 4 weeks to develop hyperoxaluria, and some of them were administered with LPN1 for 4 weeks before EG treatment as a preventive intervention. We found that EG not only resulted hyperoxaluria and kidney stone formation, but also promoted the intestinal inflammation, elevated intestinal permeability, and gut microbiota disorders. Supplementation of LPN1 inhibited the renal crystalline deposits through reducing urinary oxalic acid and renal osteopontin and CD44 expression and improved EG-induced intestinal inflammation and barrier function by decreasing the serum LPS and TLR4/NF-κB signaling and up-regulating tight junction Claudin-2 in the colon, as well as increasing the production of short-chain fatty acid (SCFAs) and the abundance of beneficial SCFAs-producing bacteria, mainly from the families of Lachnospiraceae and Ruminococcaceae. Probiotic LPN1 could prevent EG-induced hyperoxaluria by regulating gut microbiota and enhancing intestinal barrier function.

Forty Years of Oxalobacter formigenes, a Gutsy Oxalate-Degrading Specialist

Oxalobacter formigenes, a unique anaerobic bacterium that relies solely on oxalate for growth, is a key oxalate-degrading bacterium in the mammalian intestinal tract. Degradation of oxalate in the gut by O. formigenes plays a critical role in preventing renal toxicity in animals that feed on oxalate-rich plants. The role of O. formigenes in reducing the risk of calcium oxalate kidney stone disease and oxalate nephropathy in humans is less clear, in part due to difficulties in culturing this organism and the lack of studies which have utilized diets in which the oxalate content is controlled. Herein, we review the literature on the 40th anniversary of the discovery of O. formigenes, with a focus on its biology, its role in gut oxalate metabolism and calcium oxalate kidney stone disease, and potential areas of future research. Results from ongoing clinical trials utilizing O. formigenes in healthy volunteers and in patients with primary hyperoxaluria type 1 (PH1), a rare but severe form of calcium oxalate kidney stone disease, are also discussed. Information has been consolidated on O. formigenes strains and best practices to culture this bacterium, which should serve as a good resource for researchers.

A hidden cause of oxalate nephropathy: a case report

Background

Oxalate nephropathy is a rare disorder that can result in acute kidney injury (AKI) and progresses to end-stage kidney disease (ESKD). The causes can be either primary or secondary. Primary hyperoxaluria includes a group of hereditary disorders with enzymatic defects in the glyoxylate pathway, resulting in decreased oxalate metabolism. Secondary hyperoxaluria, often overlooked can result from increased intestinal absorption, nutritional deficiencies, decreased fluid intake, impaired excretion, and increased dietary consumption of oxalate.

Case presentation

We present a Caucasian case of acute oxalate induced nephropathy associated with consumption of large quantities of green vegetables in a patient with chronic kidney disease (CKD). Imaging study showed no evidence of kidney stone, but a kidney biopsy revealed acute tubular injury, tubular atrophy, interstitial fibrosis, and dense tubular deposition of calcium oxalate crystals. Upon further questioning the patient, we learned that in the months prior to presentation, he had very significantly increased his consumption of green vegetables. Because of no clinical improvement, the patient was initiated and maintained on hemodialysis.

Conclusion

This report illustrates a case of acute oxalate nephropathy in the setting of very high dietary consumption of oxalate-rich foods in a patient with advanced CKD. Special attention should be given to the secondary causes of hyperoxaluria in patients with predisposing conditions such as CKD.

Gut-kidney axis in oxalate homeostasis

PURPOSE OF REVIEW

The gut-kidney axis plays a critical role in oxalate homeostasis, and better understanding of oxalate transport regulatory mechanisms is essential for developing novel therapies.

RECENT FINDINGS

Oxalate potentially contributes to chronic kidney disease (CKD) progression, CKD - and end stage renal disease (ESRD)-associated cardiovascular diseases, polycystic kidney disease (PKD) progression, and/or poor renal allograft survival, emphasizing the need for plasma and urinary oxalate lowering therapies. One promising strategy would be to enhance the bowel's ability to secrete oxalate, which might be facilitated by the following findings. Oxalobacter formigenes (O. formigenes)-derived factors recapitulate O. formigenes colonization effects by reducing urinary oxalate excretion in hyperoxaluric mice by inducing colonic oxalate secretion. Protein kinase A activation stimulates intestinal oxalate transport by enhancing the surface expression of the oxalate transporter SLC26A6 (A6). Glycosylation also stimulates A6-mediated oxalate transport. The colon adapts to chronic acidosis in rats through increased colonic oxalate secretion as previously reported in CKD rats, and A6-mediated enteric oxalate secretion is critical in reducing the body oxalate burden in CKD mice. Intestinal oxalate transport is negatively regulated by proinflammatory cytokines and cholinergic, purinergic, and adenosinergic signaling.

SUMMARY

These findings could facilitate the development of novel therapeutics for hyperoxalemia, hyperoxaluria, and related disorders if similar regulatory mechanisms are confirmed in humans.

Physiological and Pathological Functions of SLC26A6

Solute Carrier Family 26 (SLC26) is a conserved anion transporter family with 10 members in human (SLC26A1-A11, A10 being a pseudogene). All SLC26 genes except for SLC26A5 (prestin) are versatile anion exchangers with notable ability to transport a variety of anions. SLC26A6 has the most extensive exchange functions in the SLC26 family and is widely expressed in various organs and tissues of mammals. SLC26A6 has some special properties that make it play a particularly important role in ion homeostasis and acid-base balance. In the past few years, the function of SLC26A6 in the diseases has received increasing attention. SLC26A6 not only participates in the development of intestinal and pancreatic diseases but also serves a significant role in mediating nephrolithiasis, fetal skeletal dysplasia and arrhythmia. This review aims to explore the role of SLC26A6 in physiology and pathophysiology of relative mammalian organs to guide in-depth studies about related diseases of human.

Contribution of Dietary Oxalate and Oxalate Precursors to Urinary Oxalate Excretion

Kidney stone disease is increasing in prevalence, and the most common stone composition is calcium oxalate. Dietary oxalate intake and endogenous production of oxalate are important in the pathophysiology of calcium oxalate stone disease. The impact of dietary oxalate intake on urinary oxalate excretion and kidney stone disease risk has been assessed through large cohort studies as well as smaller studies with dietary control. Net gastrointestinal oxalate absorption influences urinary oxalate excretion. Oxalate-degrading bacteria in the gut microbiome, especially Oxalobacter formigenes, may mitigate stone risk through reducing net oxalate absorption. Ascorbic acid (vitamin C) is the main dietary precursor for endogenous production of oxalate with several other compounds playing a lesser role. Renal handling of oxalate and, potentially, renal synthesis of oxalate may contribute to stone formation. In this review, we discuss dietary oxalate and precursors of oxalate, their pertinent physiology in humans, and what is known about their role in kidney stone disease.

The anion exchanger PAT-1 (Slc26a6) does not participate in oxalate or chloride transport by mouse large intestine

The membrane-bound transport proteins responsible for oxalate secretion across the large intestine remain unidentified. The apical chloride/bicarbonate (Cl^-/HCO₃^-) exchanger encoded by Slc26a6, known as PAT-1 (putative anion transporter 1), is a potential candidate. In the small intestine, PAT-1 makes a major contribution to oxalate secretion but whether this role extends into the large intestine has not been directly tested. Using the PAT-1 knockout (KO) mouse, we compared the unidirectional absorptive ([Formula: see text]) and secretory ([Formula: see text]) flux of oxalate and Cl^- across cecum, proximal colon, and distal colon from wild-type (WT) and KO mice in vitro. We also utilized the non-specific inhibitor DIDS (4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid) to confirm a role for PAT-1 in WT large intestine and (in KO tissues) highlight any other apical anion exchangers involved. Under symmetrical, short-circuit conditions the cecum and proximal colon did not transport oxalate on a net basis, whereas the distal colon supported net secretion. We found no evidence for the participation of PAT-1, or indeed any other DIDS-sensitive transport mechanism, in oxalate or Cl^- by the large intestine. Most unexpectedly, mucosal DIDS concurrently stimulated [Formula: see text] and [Formula: see text] by 25-68% across each segment without impacting net transport. For the colon, these changes were directly proportional to increased transepithelial conductance suggesting this response was the result of bidirectional paracellular flux. In conclusion, PAT-1 does not contribute to oxalate or Cl^- transport by the large intestine, and we urge caution when using DIDS with mouse colonic epithelium.

Urinary oxalate as a potential mediator of kidney disease in diabetes mellitus and obesity

PURPOSE OF REVIEW

Hyperoxaluria can cause kidney disease through multiple mechanisms, including tubular obstruction from calcium oxalate crystals, sterile inflammation, and tubular epithelial cell injury. Hyperoxaluria is also observed in individuals with diabetes mellitus and obesity, which are in turn risk factors for chronic kidney disease (CKD). Whether hyperoxaluria is a potential mediator of increased risk of CKD in diabetes mellitus and obesity is unknown.

RECENT FINDINGS

Individuals with diabetes have increased levels of plasma glyoxal (a protein glycation product) and glyoxylate, both of which are precursors for oxalate. Increased gut absorption of oxalate in obesity may be because of obesity-associated inflammation. A recent study in individuals with CKD found that higher 24 h urinary oxalate excretion was independently associated with increased risk of kidney disease progression, especially in individuals with diabetes and obesity.

SUMMARY

Both diabetes mellitus and obesity are associated with higher urinary oxalate excretion through distinct mechanisms. Hyperoxaluria could be a mechanism by which kidney disease develops in individuals with diabetes mellitus or obesity and could also contribute to progressive loss of renal function. Future research on pharmacologic or dietary measures to limit oxalate absorption or generation are required to test whether lowering urinary oxalate excretion is beneficial in preventing kidney disease development and progression in diabetes mellitus and obesity.

Activation of the PKA signaling pathway stimulates oxalate transport by human intestinal Caco2-BBE cells

Most kidney stones are composed of calcium oxalate, and small increases in urine oxalate enhance the stone risk. The mammalian intestine plays a crucial role in oxalate homeostasis, and we had recently reported that Oxalobacter-derived factors stimulate oxalate transport by human intestinal Caco2-BBE (C2) cells through PKA activation. We therefore evaluated whether intestinal oxalate transport is directly regulated by activation of the PKA signaling pathway. To this end, PKA was activated with forskolin and IBMX (F/I). F/I significantly stimulated (3.7-fold) [14C]oxalate transport by C2 cells [≥49% of which is mediated by the oxalate transporter SLC26A6 (A6)], an effect completely blocked by the PKA inhibitor H89, indicating that it is PKA dependent. PKA stimulation of intestinal oxalate transport is not cell line specific, since F/I similarly stimulated oxalate transport by the human intestinal T84 cells. F/I significantly increased (2.5-fold) A6 surface protein expression by use of immunocytochemistry. Assessing [14C]oxalate transport as a function of increasing [14C]oxalate concentration in the flux medium showed that the observed stimulation is due to a F/I-induced increase (1.8-fold) in Vmax and reduction (2-fold) in Km. siRNA knockdown studies showed that significant components of the observed stimulation are mediated by A6 and SLC26A2 (A2). Besides enhancing A6 surface protein expression, it is also possible that the observed stimulation is due to PKA-induced enhanced A6 and/or A2 transport activity in view of the reduced Km. We conclude that PKA activation positively regulates oxalate transport by intestinal epithelial cells and that PKA agonists might therapeutically impact hyperoxalemia, hyperoxaluria, and related kidney stones.

Absence of the sulfate transporter SAT-1 has no impact on oxalate handling by mouse intestine and does not cause hyperoxaluria or hyperoxalemia

The anion exchanger SAT-1 [sulfate anion transporter 1 (Slc26a1)] is considered an important regulator of oxalate and sulfate homeostasis, but the mechanistic basis of these critical roles remain undetermined. Previously, characterization of the SAT-1-knockout (KO) mouse suggested that the loss of SAT-1-mediated oxalate secretion by the intestine was responsible for the hyperoxaluria, hyperoxalemia, and calcium oxalate urolithiasis reportedly displayed by this model. To test this hypothesis, we compared the transepithelial fluxes of ¹⁴C-oxalate, ³⁵SO42- , and ³⁶Cl^- across isolated, short-circuited segments of the distal ileum, cecum, and distal colon from wild-type (WT) and SAT-1-KO mice. The absence of SAT-1 did not impact the transport of these anions by any part of the intestine examined. Additionally, SAT-1-KO mice were neither hyperoxaluric nor hyperoxalemic. Instead, 24-h urinary oxalate excretion was almost 50% lower than in WT mice. With no contribution from the intestine, we suggest that this may reflect the loss of SAT-1-mediated oxalate efflux from the liver. SAT-1-KO mice were, however, profoundly hyposulfatemic, even though there were no changes to intestinal sulfate handling, and the renal clearances of sulfate and creatinine indicated diminished rates of sulfate reabsorption by the proximal tubule. Aside from this distinct sulfate phenotype, we were unable to reproduce the hyperoxaluria, hyperoxalemia, and urolithiasis of the original SAT-1-KO model. In conclusion, oxalate and sulfate transport by the intestine were not dependent on SAT-1, and we found no evidence supporting the long-standing hypothesis that intestinal SAT-1 contributes to oxalate and sulfate homeostasis. NEW & NOTEWORTHY SAT-1 is a membrane-bound transport protein expressed in the intestine, liver, and kidney, where it is widely considered essential for the excretion of oxalate, a potentially toxic waste metabolite. Previously, calcium oxalate kidney stone formation by the SAT-1-knockout mouse generated the hypothesis that SAT-1 has a major role in oxalate excretion via the intestine. We definitively tested this proposal and found no evidence for SAT-1 as an intestinal anion transporter contributing to oxalate homeostasis.

Adenosinergic signaling inhibits oxalate transport by human intestinal Caco2-BBE cells through the A2B adenosine receptor

Most kidney stones (KS) are composed of calcium oxalate, and small increases in urine oxalate affect the stone risk. Intestinal oxalate secretion mediated by anion exchanger SLC26A6 (PAT1) plays a crucial role in limiting net absorption of ingested oxalate, thereby preventing hyperoxaluria and related KS, reflecting the importance of understanding regulation of intestinal oxalate transport. We previously showed that ATP and UTP inhibit oxalate transport by human intestinal Caco2-BBE cells (C2). Since ATP is rapidly degraded to adenosine (ADO), we examined whether intestinal oxalate transport is regulated by ADO. We measured [¹⁴C]oxalate uptake in the presence of an outward Cl gradient as an assay of Cl-oxalate exchange activity, ≥49% of which is PAT1-mediated in C2 cells. We found that ADO significantly inhibited oxalate transport by C2 cells, an effect completely blocked by the nonselective ADO receptor antagonist 8- p-sulfophenyltheophylline. ADO also significantly inhibited oxalate efflux by C2 cells, which is important since PAT1 mediates oxalate efflux in vivo. Using pharmacological antagonists and A_2B adenosine receptor (A_2B AR) siRNA knockdown studies, we observed that ADO inhibits oxalate transport through the A_2B AR, phospholipase C, and PKC. ADO inhibits oxalate transport by reducing PAT1 surface expression as shown by biotinylation studies. We conclude that ADO inhibits oxalate transport by lowering PAT1 surface expression in C2 cells through signaling pathways including the A_2B AR, PKC, and phospholipase C. Given higher ADO levels and overexpression of the A_2B AR in inflammatory bowel disease (IBD), our findings have potential relevance to pathophysiology of IBD-associated hyperoxaluria and related KS.

High expression of SLC26A6 in the kidney may contribute to renal calcification via an SLC26A6-dependent mechanism

Background

Solute-linked carrier 26 gene family 6 (SLC26A6), which is mainly expressed in intestines and kidneys, is a multifunctional anion transporter crucial in the transport of oxalate anions. This study aimed to investigate the role of kidney SLC26A6 in urolithiasis.

Methods

Patients were divided into two groups: stone formers and nonstone formers. Samples were collected from patients following nephrectomy. Lentivirus with Slc26a6 (lentivirus-Slc26a6) sequence and lentivirus with siRNA-Slc26a6 (lentivirus-siRNA-Slc26a6) sequence were transfected into rats’ kidneys respectively and Slc26a6 expression was detected using Western blot and immunohistochemical analyses. After administering ethylene glycol, oxalate concentration and prevalence of stone formation between the transgenic and control groups were measured using 24-h urine analysis and Von Kossa staining, respectively.

Results

Immunohistochemical and Western blot analyses indicated that stone formers had a significantly higher level of expression of SLC26A6 in the kidney compared with the control group. After lentivirus infection, the urinary oxalate concentration and rate of stone formation in lentivirus-Slc26a6-tranfected rats increased remarkably, while lentivirus-siRNA-Slc26a6-transfected rats showed few crystals.

Conclusion

The results showed that high expression levels of renal SLC26A6 may account for kidney stone formation. Downregulating the expression of SLC26A6 in the kidney may be a potential therapeutic target to prevent or treat urolithiasis.

Probiotics for prevention of urinary stones

BACKGROUND

Urinary supersaturation is one key determinant of calcium oxalate (CaOx) urinary stone formation, and urinary excretions of oxalate and citrate are two key determinants. Each is influenced by gastrointestinal processes.

METHODS

Open label and randomized placebo studies have examined the effect of oral probiotic preparations on urinary supersaturation and oxalate excretion. Cross sectional studies in humans have studied the association of Oxalobacter formigenes colonization status and urinary oxalate excretion and prevalence of urinary stones. The intestinal microbiome of representative animals adapted to a high oxalate diet has been defined.

RESULTS

The fecal content of O. formigenes, the best studied oxalate-degrader, varies depending on stone status. However, trials with probiotics designed to degrade oxalate including those containing O. formigenes, Lactobacillus, and/or Bifidobacterium spp., have been disappointing. Multiple intestinal segments of animals on a high oxalate diet contains diverse communities of microorganisms that can function together to degrade and detoxify a large oxalate load.

CONCLUSIONS

Although the intestinal microbiome seems likely to play a role to modify gastrointestinal absorption of lithogenic substances and hence urinary stone risk, whether we can develop tools to manipulate it and decrease this kidney stone risk remains to be determined.

The role of intestinal oxalate transport in hyperoxaluria and the formation of kidney stones in animals and man

The intestine exerts a considerable influence over urinary oxalate in two ways, through the absorption of dietary oxalate and by serving as an adaptive extra-renal pathway for elimination of this waste metabolite. Knowledge of the mechanisms responsible for oxalate absorption and secretion by the intestine therefore have significant implications for understanding the etiology of hyperoxaluria, as well as offering potential targets for future treatment strategies for calcium oxalate kidney stone disease. In this review, we present the recent developments and advances in this area over the past 10 years, and put to the test some of the new ideas that have emerged during this time, using human and mouse models. A key focus for our discussion are the membrane-bound anion exchangers, belonging to the SLC26 gene family, some of which have been shown to participate in transcellular oxalate absorption and secretion. This has offered the opportunity to not only examine the roles of these specific transporters, revealing their importance to oxalate homeostasis, but to also probe the relative contributions made by the active transcellular and passive paracellular components of oxalate transport across the intestine. We also discuss some of the various physiological stimuli and signaling pathways which have been suggested to participate in the adaptation and regulation of intestinal oxalate transport. Finally, we offer an update on research into Oxalobacter formigenes, alongside recent investigations of other oxalate-degrading gut bacteria, in both laboratory animals and humans.

Oxalate, inflammasome, and progression of kidney disease

PURPOSE OF REVIEW

Oxalate is an end product of metabolism excreted via the kidney. Excess urinary oxalate, whether from primary or enteric hyperoxaluria, can lead to oxalate deposition in the kidney. Oxalate crystals are associated with renal inflammation, fibrosis, and progressive renal failure. It has long been known that as the glomerular filtration rate becomes reduced in chronic kidney disease (CKD), there is striking elevation of plasma oxalate. Taken together, these findings raise the possibility that elevation of plasma oxalate in CKD may promote renal inflammation and more rapid progression of CKD independent of primary cause.

RECENT FINDINGS

The inflammasome has recently been identified to play a critical role in oxalate-induced renal inflammation. Oxalate crystals have been shown to activate the NOD-like receptor family, pyrin domain containing 3 inflammasome (also known as NALP3, NLRP3, or cryopyrin), resulting in release of IL-1β and macrophage infiltration. Deletion of inflammasome proteins in mice protects from oxalate-induced renal inflammation and progressive renal failure.

SUMMARY

The findings reviewed in this article expand our understanding of the relevance of elevated plasma oxalate levels leading to inflammasome activation. We propose that inhibiting oxalate-induced inflammasome activation, or lowering plasma oxalate, may prevent or mitigate progressive renal damage in CKD, and warrants clinical trials.

Physiology of Intestinal Absorption and Secretion

Virtually all nutrients from the diet are absorbed into blood across the highly polarized epithelial cell layer forming the small and large intestinal mucosa. Anatomical, histological, and functional specializations along the gastrointestinal tract are responsible for the effective and regulated nutrient transport via both passive and active mechanisms. In this chapter, we summarize the current state of knowledge regarding the mechanism of intestinal absorption of key nutrients such as sodium, anions (chloride, sulfate, oxalate), carbohydrates, amino acids and peptides, lipids, lipid- and water-soluble vitamins, as well as the major minerals and micronutrients. This outline, including the molecular identity, specificity, and coordinated activities of key transport proteins and genes involved, serves as the background for the following chapters focused on the pathophysiology of acquired and congenital intestinal malabsorption, as well as clinical tools to test and treat malabsorptive symptoms.

Oxalate Concentrations in Human Gastrointestinal Fluid

BACKGROUND AND PURPOSE

Urinary oxalate excretion is a risk factor for nephrolithiasis and is a result of endogenous metabolism and gastrointestinal processes. Gastrointestinal absorption of oxalate has been well demonstrated but to our knowledge evidence for secretion of oxalate is absent in humans. The objective of this study was to measure the amount and conformation of oxalate in the stomach and small intestine of adult subjects undergoing gastrointestinal endoscopy.

MATERIALS AND METHODS

Eleven adults participated in this study. Gastrointestinal fluid was collected from the stomach and small intestine during endoscopy. A determination of the soluble and insoluble components of oxalate was made by centrifugation of the sample and subsequent acidification of the resultant pellet and supernatant. Samples were processed and the amount of oxalate was measured by ion chromatography, the limit of which is 1.6 μM.

RESULTS

The majority of small intestinal samples contained some degree of oxalate. This is in contrast to the stomach where minimal oxalate was detected. There was a wide range of oxalate concentrations and a greater degree of insoluble oxalate in small intestinal samples.

CONCLUSIONS

Our results suggest that some degree of oxalate secretion in the small intestine may occur in the fasted state while this is less likely in the stomach. Further studies are warranted to provide definitive evidence of gastrointestinal secretion of oxalate.

Effects of acid-base variables and the role of carbonic anhydrase on oxalate secretion by the mouse intestine in vitro

Hyperoxaluria is a major risk factor for calcium oxalate kidney stones and the intestine is recognized as an important extra-renal pathway for eliminating oxalate. The membrane-bound chloride/bicarbonate (Cl(-)/) exchangers are involved in the transcellular movement of oxalate, but little is understood about how they might be regulated. , CO2, and pH are established modulators of intestinal NaCl cotransport, involving Na(+)/H(+) and Cl(-)/ exchange, but their influence on oxalate transport is unknown. Measuring (14)C-oxalate and (36)Cl fluxes across isolated, short-circuited segments of the mouse distal ileum and distal colon we examined the role of these acid-base variables and carbonic anhydrase (CA) in oxalate and Cl(-) transport. In standard buffer both segments performed net oxalate secretion (and Cl(-) absorption), but only the colon, and the secretory pathway were responsive to and CO2. Ethoxzolamide abolished net oxalate secretion by the distal colon, and when used in tandem with an impermeant CA inhibitor, signaled an intracellular CA isozyme was required for secretion. There was a clear dependence on as their removal eliminated secretion, while at 42 mmol/L was also decreased and eradicated. Independent of pH, raising Pco2 from 28 to 64 mmHg acutely stimulated net oxalate secretion 41%. In summary, oxalate secretion by the distal colon was dependent on , CA and specifically modulated by CO2, whereas the ileum was remarkably unresponsive. These findings highlight the distinct segmental heterogeneity along the intestine, providing new insights into the oxalate transport mechanism and how it might be regulated.

Update on oxalate crystal disease

Oxalate arthropathy is a rare cause of arthritis characterized by deposition of calcium oxalate crystals within synovial fluid. This condition typically occurs in patients with underlying primary or secondary hyperoxaluria. Primary hyperoxaluria constitutes a group of genetic disorders resulting in endogenous overproduction of oxalate, whereas secondary hyperoxaluria results from gastrointestinal disorders associated with fat malabsorption and increased absorption of dietary oxalate. In both conditions, oxalate crystals can deposit in the kidney leading to renal failure. Since oxalate is primarily renally eliminated, it accumulates throughout the body in renal failure, a state termed oxalosis. Affected organs can include bones, joints, heart, eyes, and skin. Since patients can present with renal failure and oxalosis before the underlying diagnosis of hyperoxaluria has been made, it is important to consider hyperoxaluria in patients who present with unexplained soft tissue crystal deposition. The best treatment of oxalosis is prevention. If patients present with advanced disease, treatment of oxalate arthritis consists of symptom management and control of the underlying disease process.

Human SLC26A1 gene variants: a pilot study

Kidney stones are a global health problem, incurring massive health costs annually. Why stones recur in many patients remains unknown but likely involves environmental, physiological, and genetic factors. The solute linked carrier (SLC) 26A1 gene has previously been linked to kidney stones in mice. SLC26A1 encodes the sulfate anion transporter 1 (SAT1) protein, and its loss in mice leads to hyperoxaluria and calcium oxalate renal stones. To investigate the possible involvement of SAT1 in human urolithiasis, we screened the SLC26A1 gene in a cohort of 13 individuals with recurrent calcium oxalate urolithiasis, which is the commonest type. DNA sequence analyses showed missense mutations in seven patients: one individual was heterozygous R372H; 4 individuals were heterozygous Q556R; one patient was homozygous Q556R; and one patient with severe nephrocalcinosis (requiring nephrectomy) was homozygous Q556R and heterozygous M132T. The M132 amino acid in human SAT1 is conserved with 15 other species and is located within the third transmembrane domain of the predicted SAT1 protein structure, suggesting that this amino acid may be important for SAT1 function. These initial findings demonstrate genetic variants in SLC26A1 of recurrent stone formers and warrant wider independent studies of SLC26A1 in humans with recurrent calcium oxalate stones.

Transcellular oxalate and Cl- absorption in mouse intestine is mediated by the DRA anion exchanger Slc26a3, and DRA deletion decreases urinary oxalate

Active transcellular oxalate transport in the mammalian intestine contributes to the homeostasis of this important lithogenic anion. Several members of the Slc26a gene family of anion exchangers have a measurable oxalate affinity and are expressed along the gut, apically and basolaterally. Mouse Slc26a6 (PAT1) targets to the apical membrane of enterocytes in the small intestine, and its deletion results in net oxalate absorption and hyperoxaluria. Apical exchangers of the Slc26a family that mediate oxalate absorption have not been established, yet the Slc26a3 [downregulated in adenoma (DRA)] protein is a candidate mediator of oxalate uptake. We evaluated the role of DRA in intestinal oxalate and Cl(-) transport by comparing unidirectional and net ion fluxes across short-circuited segments of small (ileum) and large (cecum and distal colon) intestine from wild-type (WT) and DRA knockout (KO) mice. In WT mice, all segments demonstrated net oxalate and Cl(-) absorption to varying degrees. In KO mice, however, all segments exhibited net anion secretion, which was consistently, and solely, due to a significant reduction in the absorptive unidirectional fluxes. In KO mice, daily urinary oxalate excretion was reduced 66% compared with that in WT mice, while urinary creatinine excretion was unchanged. We conclude that DRA mediates a predominance of the apical uptake of oxalate and Cl(-) absorbed in the small and large intestine of mice under short-circuit conditions. The large reductions in urinary oxalate excretion underscore the importance of transcellular intestinal oxalate absorption, in general, and, more specifically, the importance of the DRA exchanger in oxalate homeostasis.

The trigger-maintenance model of persistent mild to moderate hyperoxaluria induces oxalate accumulation in non-renal tissues

Persistent mild to moderate hyperoxaluria (PMMH) is a common side effect of bariatric surgery. However, PMMH's role in the progression to calcium oxalate (CaOx) urolithiasis and its potential effects on non-renal tissues are unknown. To address these points, a trigger + maintenance (T + Mt) model of PMMH was developed in rats (Experiment 1). The trigger was an i.p. injection of PBS (TPBS) or 288 μmol sodium oxalate (T288). Maintenance (Mt) was given via minipumps dispensing PBS or 7.5-30 μmol potassium oxalate/day for 28 days. Urinary oxalate ranged from 7.7 ± 0.8 μmol/day for TPBS + MtPBS to 18.2 ± 1.5 μmol/day for T288 + Mt30 (p ≤ 0.0005). All rats receiving T288 developed CaOx nephrocalcinosis, and many developed 'stones'. This was also true for Mt doses that did not elevate urinary oxalate above that of TPBS + MtPBS (p > 0.1) and for rats that did not have a detectable surge in urinary oxalate post T288. When TPBS was administered, CaOx nephrocalcinosis did not develop regardless of the Mt dose even if urinary oxalate was elevated compared to TPBS + MtPBS (p ≤ 0.0005). One of the risks associated with PMMH is oxalate accumulation within tissues. Hence, in a second set of experiments (Experiment 2) different doses of oxalate (Mt0.05, Mt15, Mt30) labeled with (14)C-oxalate ((14)C-Ox) were administered by minipump for 13 days. Tissues were harvested and (14)C-Ox accumulation assessed by scintillation counting. (14)C-Ox accumulated in a dose dependent manner (p ≤ 0.004) in bone, kidney, muscle, liver, heart, kidney, lungs, spleen, and testis. All these tissues exhibited (14)C-Ox concentrations higher (p ≤ 0.05) than the plasma. Extrapolation of our results to patients suggests that PMMH patients should take extra care to avoid dietary-induced spikes in oxalate excretion to help prevent CaOx nephrocalcinosis or stone development. Monitoring for oxalate accumulation within tissues susceptible to damage by oxalate or CaOx crystals may also be required.

Extracellular nucleotides inhibit oxalate transport by human intestinal Caco-2-BBe cells through PKC-δ activation

Nephrolithiasis remains a major health problem in Western countries. Seventy to 80% of kidney stones are composed of calcium oxalate, and small changes in urinary oxalate affect risk of kidney stone formation. Intestinal oxalate secretion mediated by the anion exchanger SLC26A6 plays an essential role in preventing hyperoxaluria and calcium oxalate nephrolithiasis, indicating that understanding the mechanisms regulating intestinal oxalate transport is critical for management of hyperoxaluria. Purinergic signaling modulates several intestinal processes through pathways including PKC activation, which we previously found to inhibit Slc26a6 activity in mouse duodenal tissue. We therefore examined whether purinergic stimulation with ATP and UTP affects oxalate transport by human intestinal Caco-2-BBe (C2) cells. We measured [¹⁴C]oxalate uptake in the presence of an outward Cl⁻ gradient as an assay of Cl⁻/oxalate exchange activity, ≥50% of which is mediated by SLC26A6. We found that ATP and UTP significantly inhibited oxalate transport by C2 cells, an effect blocked by the PKC inhibitor Gö-6983. Utilizing pharmacological agonists and antagonists, as well as PKC-δ knockdown studies, we observed that ATP inhibits oxalate transport through the P2Y₂ receptor, PLC, and PKC-δ. Biotinylation studies showed that ATP inhibits oxalate transport by lowering SLC26A6 surface expression. These findings are of potential relevance to pathophysiology of inflammatory bowel disease-associated hyperoxaluria, where supraphysiological levels of ATP/UTP are expected and overexpression of the P2Y₂ receptor has been reported. We conclude that ATP and UTP inhibit oxalate transport by lowering SLC26A6 surface expression in C2 cells through signaling pathways including the P2Y₂ purinergic receptor, PLC, and PKC-δ.

Oxalate nephropathy and intravenous vitamin C

Oxalate nephropathy is a rare condition characterized by extensive calcium oxalate deposition in the renal tubules, resulting in kidney injury. There are primary forms of the disease that arise from genetic mutation causing overproduction of oxalate. More commonly, this condition is seen as a secondary phenomenon. The clinical presentation is nonspecific, with acute kidney injury and normal serologic study results. The characteristic finding on kidney biopsy is the presence of acute tubular injury associated with polarizable crystals in the tubular lumen and epithelial cytoplasm. We present a case of acute oxalate nephropathy in a patient with underlying systemic lupus erythematosus who recently received intravenous vitamin C.

Leaf oxalate content of Eucalyptus spp. and its implications for koalas (Phascolarctos cinereus) with oxalate nephrosis

Oxalate nephrosis is a leading disease of the Mount Lofty Ranges koala population in South Australia, but the cause is unclear. In other herbivorous species, a common cause is high dietary oxalate; therefore this study aimed to determine the oxalate content of eucalypt leaves. Juvenile, semimature and mature leaves were collected during spring from eucalypt species eaten by koalas in the Mount Lofty Ranges and compared with those from Moggill, Queensland, where oxalate nephrosis has lower prevalence. Total oxalate was measured as oxalic acid by high-performance liquid chromatography. The oxalate content of eucalypts was low (<1% dry weight), but occasional Mount Lofty leaf samples had oxalate levels of 4.68–7.51% dry weight. Mount Lofty eucalypts were found to be higher in oxalate than those from Queensland (P < 0.001). In conclusion, dietary oxalate in eucalypt leaves is unlikely to be the primary cause of oxalate nephrosis in the Mount Lofty koala population. However, occasional higher oxalate levels could cause oxalate nephrosis in individual koalas or worsen disease in those already affected. Further studies on the seasonal variation of eucalypt leaf oxalate are needed to determine its role in the pathogenesis of oxalate nephrosis in koalas.

Hyperoxaluria and systemic oxalosis: an update on current therapy and future directions

INTRODUCTION

The primary hyperoxalurias (PH) are rare, but underdiagnosed disorders where the loss of enzymatic activity in key compounds of glyoxylate metabolism results in excessive endogenous oxalate generation. Clinically, they are characterized by recurrent urolithiasis and/or nephrocalcinosis. PH type I is the most frequent and most devastating subtype often leading to early end-stage renal failure.

AREAS COVERED

Profound overview of clinical, diagnostic, and currently available treatment options with a focus on PH I at different stages of the disease. Discussion of future therapeutic avenues including pharmacological chaperones (small molecules rescuing protein function), gene therapy with safer adenoviral vectors, and potential application of cell-based transplantation strategies is provided.

EXPERT OPINION

Due to lack of familiarity with PH and its heterogeneous clinical expression, diagnosis is often delayed until advanced disease is present, a condition, requiring intensive hemodialysis and timely transplantation. Achieving the most beneficial outcome largely depends on the knowledge of the clinical spectrum, early diagnosis, and initiation of treatment before renal failure ensues. A number of preconditions required for substantial improvement in the care of orphan disease like PH have now been achieved or soon will come within reach, so new treatment options can be expected in the near future.

An update on primary hyperoxaluria

The autosomal recessive inherited primary hyperoxalurias types I, II and III are caused by defects in glyoxylate metabolism that lead to the endogenous overproduction of oxalate. Type III primary hyperoxaluria was first described in 2010 and further types are likely to exist. In all forms, urinary excretion of oxalate is strongly elevated (>1 mmol/1.73 m(2) body surface area per day; normal <0.5 mmol/1.73 m(2) body surface area per day), which results in recurrent urolithiasis and/or progressive nephrocalcinosis. All entities can induce kidney damage, which is followed by reduced oxalate elimination and consequent systemic deposition of calcium oxalate crystals. Systemic oxalosis should be prevented, but diagnosis is all too often missed or delayed until end-stage renal disease (ESRD) occurs; this outcome occurs in >30% of patients with primary hyperoxaluria type I. The fact that such a large proportion of patients have such poor outcomes is particularly unfortunate as ESRD can be delayed or even prevented by early intervention. Treatment options for primary hyperoxaluria include alkaline citrate, orthophosphate, or magnesium. In addition, pyridoxine treatment can be used to normalize or reduce oxalate excretion in about 30% of patients with primary hyperoxaluria type I. Time on dialysis should be short to avoid overt systemic oxalosis. Transplantation methods depend on the type of primary hyperoxaluria and on the particular patient, but combined liver and kidney transplantation is the method of choice in patients with primary hyperoxaluria type I and isolated kidney transplantation is the preferred method in those with primary hyperoxaluria type II. To the best of our knowledge, progression to ESRD has not yet been reported in any patient with primary hyperoxaluria type III.

Hyperoxaluric rats do not exhibit alterations in renal expression patterns of Slc26a1 (SAT1) mRNA or protein

Little is known about oxalate transport in renal epithelia under basal conditions, let alone in hyperoxaluria when the capacity for renal oxalate excretion is increased. Sulfate anion transporter 1 (SAT1, Slc26a1) is considered to be a major basolateral anion-oxalate exchanger in the proximal tubule and we hypothesized its expression may correlate with urinary oxalate excretion. We quantified changes in the renal expression of SAT1 mRNA and protein in two rat models, one with hyperoxaluria (HYP) and one with renal insufficiency (HRF) induced by hyperoxaluria. The hyperoxaluria observed in the HYP group could not simply be ascribed to changes in SAT1 mRNA or protein abundance. However, when hyperoxaluria was accompanied by renal insufficiency, significant reductions in SAT1 mRNA and protein were detected in medullary and papillary tissue. Together, the results indicate that transcriptional modulation of the SAT1 gene is not a significant component of the hyperoxaluria observed in these rat models.

Net intestinal transport of oxalate reflects passive absorption and SLC26A6-mediated secretion

Mice lacking the oxalate transporter SLC26A6 develop hyperoxalemia, hyperoxaluria, and calcium-oxalate stones as a result of a defect in intestinal oxalate secretion, but what accounts for the absorptive oxalate flux remains unknown. We measured transepithelial absorption of [(14)C]oxalate simultaneously with the flux of [(3)H]mannitol, a marker of the paracellular pathway, across intestine from wild-type and Slc26a6-null mice. We used the anion transport inhibitor DIDS to investigate other members of the SLC26 family that may mediate transcellular oxalate absorption. Absorptive flux of oxalate in duodenum was similar to mannitol, insensitive to DIDS, and nonsaturable, indicating that it is predominantly passive and paracellular. In contrast, in wild-type mice, secretory flux of oxalate in duodenum exceeded that of mannitol, was sensitive to DIDS, and saturable, indicating transcellular secretion of oxalate. In Slc26a6-null mice, secretory flux of oxalate was similar to mannitol, and no net flux of oxalate occurred. Absorptive fluxes of both oxalate and mannitol varied in parallel in different segments of small and large intestine. In epithelial cell lines, modulation of the charge selectivity of the claudin-based pore pathway did not affect oxalate permeability, but knockdown of the tight-junction protein ZO-1 enhanced permeability to oxalate and mannitol in parallel. Moreover, formation of soluble complexes with cations did not affect oxalate absorption. In conclusion, absorptive oxalate flux occurs through the paracellular "leak" pathway, and net absorption of dietary oxalate depends on the relative balance between absorption and SLC26A6-dependent transcellular secretion.

Net intestinal transport of oxalate reflects passive absorption and SLC26A6-mediated secretion

Mice lacking the oxalate transporter SLC26A6 develop hyperoxalemia, hyperoxaluria, and calcium-oxalate stones as a result of a defect in intestinal oxalate secretion, but what accounts for the absorptive oxalate flux remains unknown. We measured transepithelial absorption of [(14)C]oxalate simultaneously with the flux of [(3)H]mannitol, a marker of the paracellular pathway, across intestine from wild-type and Slc26a6-null mice. We used the anion transport inhibitor DIDS to investigate other members of the SLC26 family that may mediate transcellular oxalate absorption. Absorptive flux of oxalate in duodenum was similar to mannitol, insensitive to DIDS, and nonsaturable, indicating that it is predominantly passive and paracellular. In contrast, in wild-type mice, secretory flux of oxalate in duodenum exceeded that of mannitol, was sensitive to DIDS, and saturable, indicating transcellular secretion of oxalate. In Slc26a6-null mice, secretory flux of oxalate was similar to mannitol, and no net flux of oxalate occurred. Absorptive fluxes of both oxalate and mannitol varied in parallel in different segments of small and large intestine. In epithelial cell lines, modulation of the charge selectivity of the claudin-based pore pathway did not affect oxalate permeability, but knockdown of the tight-junction protein ZO-1 enhanced permeability to oxalate and mannitol in parallel. Moreover, formation of soluble complexes with cations did not affect oxalate absorption. In conclusion, absorptive oxalate flux occurs through the paracellular "leak" pathway, and net absorption of dietary oxalate depends on the relative balance between absorption and SLC26A6-dependent transcellular secretion.

Cholinergic signaling inhibits oxalate transport by human intestinal T84 cells

Urolithiasis remains a very common disease in Western countries. Seventy to eighty percent of kidney stones are composed of calcium oxalate, and minor changes in urinary oxalate affect stone risk. Intestinal oxalate secretion mediated by anion exchanger SLC26A6 plays a major constitutive role in limiting net absorption of ingested oxalate, thereby preventing hyperoxaluria and calcium oxalate urolithiasis. Using the relatively selective PKC-δ inhibitor rottlerin, we had previously found that PKC-δ activation inhibits Slc26a6 activity in mouse duodenal tissue. To identify a model system to study physiologic agonists upstream of PKC-δ, we characterized the human intestinal cell line T84. Knockdown studies demonstrated that endogenous SLC26A6 mediates most of the oxalate transport by T84 cells. Cholinergic stimulation with carbachol modulates intestinal ion transport through signaling pathways including PKC activation. We therefore examined whether carbachol affects oxalate transport in T84 cells. We found that carbachol significantly inhibited oxalate transport by T84 cells, an effect blocked by rottlerin. Carbachol also led to significant translocation of PKC-δ from the cytosol to the membrane of T84 cells. Using pharmacological inhibitors, we observed that carbachol inhibits oxalate transport through the M(3) muscarinic receptor and phospholipase C. Utilizing the Src inhibitor PP2 and phosphorylation studies, we found that the observed regulation downstream of PKC-δ is partially mediated by c-Src. Biotinylation studies revealed that carbachol inhibits oxalate transport by reducing SLC26A6 surface expression. We conclude that carbachol negatively regulates oxalate transport by reducing SLC26A6 surface expression in T84 cells through signaling pathways including the M(3) muscarinic receptor, phospholipase C, PKC-δ, and c-Src.

Acute probiotic ingestion reduces gastrointestinal oxalate absorption in healthy subjects

Both a high dietary oxalate intake and increased intestinal absorption appear to be major causes of elevated urine oxalate, a risk factor for kidney stone formation. A number of recent studies have assessed whether daily ingestion of a probiotic containing oxalate-degrading bacteria could lead to sufficient gut colonization to increase oxalate degradation, thereby reducing urinary oxalate. In contrast, the present study assessed whether simultaneous ingestion of oxalate-degrading probiotic bacteria with a 176 mg oxalate load could lead to decreased urinary oxalate in a population of 11 healthy non-stone formers (8 females, 3 males), aged 21-45 years. The results indicated that both the single and double doses of VSL#3(®) probiotic solutions were effective in reducing urinary oxalate and estimated oxalate absorption with no significant difference between the two probiotic doses. The timing of the reduction in urinary oxalate suggested a small intestinal and possibly gastric reduction in oxalate absorption. Similar to what had been reported for chronic or daily probiotic ingestion, individuals characterized by high oxalate absorption were most likely to experience clinically significant reductions in urinary oxalate in response to acute probiotic ingestion.