Loss of Cystic Fibrosis Transmembrane Regulator Impairs Intestinal Oxalate Secretion

Recurrent symptomatic urolithiasis in a patient with cystic fibrosis

A 6-month-old girl, previously diagnosed with cystic fibrosis (CF), was admitted to hospital for nephrolithiasis. Her parents were first-degree cousins. The patient underwent endoscopic stone management. Despite no family history of stones and medical treatment with potassium citrate, the patient developed recurrent renal stones and atypical urinary tract infections during follow-up. Basic investigations were all normal. Due to consanguinity and early presentation of nephrolithiasis, metabolic causes such as cystinuria and hyperoxaluria were considered. Cystinuria was excluded due to normal cystine levels. High urinary oxalate excretion was found as expected due to absorptive (secondary) hyperoxaluria in CF patients. An early stone burden in the patient with a history of medical treatment and consanguinity led us to perform a genetic testing. Genetic testing revealed a missense homozygous variant in exon 1 of the AGXT gene. The patient was diagnosed with primary hyperoxaluria type 1. Two rare life-threatening genetic diseases were found together in the same child.

Beneficial roles of gastrointestinal and urinary microbiomes in kidney stone prevention via their oxalate-degrading ability and beyond

Formation of calcium oxalate (CaOx) crystal, the most common composition in kidney stones, occurs following supersaturation of calcium and oxalate ions in the urine. In addition to endogenous source, another main source of calcium and oxalate ions is dietary intake. In the intestinal lumen, calcium can bind with oxalate to form precipitates to be eliminated with feces. High intake of oxalate-rich foods, inappropriate amount of daily calcium intake, defective intestinal transporters for oxalate secretion and absorption, and gastrointestinal (GI) malabsorption (i.e., from gastric bypass surgery) can enhance intestinal oxalate absorption, thereby increasing urinary oxalate level and risk of kidney stone disease (KSD). The GI microbiome rich with oxalate-degrading bacteria can reduce intestinal oxalate absorption and urinary oxalate level. In addition to the oxalate-degrading ability, the GI microbiome also affects expression of oxalate transporters and net intestinal oxalate transport, cholesterol level, and short-chain fatty acids (SCFAs) production, leading to lower KSD risk. Recent evidence also shows beneficial effects of urinary microbiome in KSD prevention. This review summarizes the current knowledge on the aforementioned aspects. Potential benefits of the GI and urinary microbiomes as probiotics for KSD prevention are emphasized. Finally, challenges and future perspectives of probiotic treatment in KSD are discussed.

Oxalate as a potent promoter of kidney stone formation

Kidney stones are among the most prevalent urological diseases, with a high incidence and recurrence rate. Treating kidney stones has been greatly improved by the development of various minimally invasive techniques. Currently, stone treatment is relatively mature. However, most current treatment methods are limited to stones and cannot effectively reduce their incidence and recurrence. Therefore, preventing disease occurrence, development, and recurrence after treatment, has become an urgent issue. The etiology and pathogenesis of stone formation are key factors in resolving this issue. More than 80% of kidney stones are calcium oxalate stones. Several studies have studied the formation mechanism of stones from the metabolism of urinary calcium, but there are few studies on oxalate, which plays an equally important role in stone formation. Oxalate and calcium play equally important roles in calcium oxalate stones, whereas the metabolism and excretion disorders of oxalate play a crucial role in their occurrence. Therefore, starting from the relationship between renal calculi and oxalate metabolism, this work reviews the occurrence of renal calculi, oxalate absorption, metabolism, and excretion mechanisms, focusing on the key role of SLC26A6 in oxalate excretion and the regulatory mechanism of SLC26A6 in oxalate transport. This review provides some new clues for the mechanism of kidney stones from the perspective of oxalate to improve the understanding of the role of oxalate in the formation of kidney stones and to provide suggestions for reducing the incidence and recurrence rate of 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.

Role of SLC4 and SLC26 solute carriers during oxidative stress

Bicarbonate is one of the major anions in mammalian tissues and fluids, is utilized by various exchangers to transport other ions and organic substrates across cell membranes and plays a critical role in cell and systemic pH homoeostasis. Chloride/bicarbonate (Cl⁻/HCO₃⁻) exchangers are abundantly expressed in erythrocytes and epithelial cells and, as a consequence, are particularly exposed to oxidants in the systemic circulation and at the interface with the external environment. Here, we review the physiological functions and pathophysiological alterations of Cl⁻/HCO₃⁻ exchangers belonging to the solute carriers SLC4 and SLC26 superfamilies in relation to oxidative stress. Particularly well studied is the impact of oxidative stress on the red blood cell SLC4A1/AE1 (Band 3 protein), of which the function seems to be directly affected by oxidative stress and possibly involves oxidation of the transporter itself or its interacting proteins, with detrimental consequences in oxidative stress‐related diseases including inflammation, metabolic dysfunctions and ageing. The effect of oxidative stress on SLC26 members was less extensively explored. Indirect evidence suggests that SLC26 transporters can be target as well as determinants of oxidative stress, especially when their expression is abolished or dysregulated.

Oxalate Flux Across the Intestine: Contributions from Membrane Transporters

Epithelial oxalate transport is fundamental to the role occupied by the gastrointestinal (GI) tract in oxalate homeostasis. The absorption of dietary oxalate, together with its secretion into the intestine, and degradation by the gut microbiota, can all influence the excretion of this nonfunctional terminal metabolite in the urine. Knowledge of the transport mechanisms is relevant to understanding the pathophysiology of hyperoxaluria, a risk factor in kidney stone formation, for which the intestine also offers a potential means of treatment. The following discussion presents an expansive review of intestinal oxalate transport. We begin with an overview of the fate of oxalate, focusing on the sources, rates, and locations of absorption and secretion along the GI tract. We then consider the mechanisms and pathways of transport across the epithelial barrier, discussing the transcellular, and paracellular components. There is an emphasis on the membrane-bound anion transporters, in particular, those belonging to the large multifunctional Slc26 gene family, many of which are expressed throughout the GI tract, and we summarize what is currently known about their participation in oxalate transport. In the final section, we examine the physiological stimuli proposed to be involved in regulating some of these pathways, encompassing intestinal adaptations in response to chronic kidney disease, metabolic acid-base disorders, obesity, and following gastric bypass surgery. There is also an update on research into the probiotic, Oxalobacter formigenes, and the basis of its unique interaction with the gut epithelium. © 2021 American Physiological Society. Compr Physiol 11:1-41, 2021.

Short-Chain Fatty Acids Reduced Renal Calcium Oxalate Stones by Regulating the Expression of Intestinal Oxalate Transporter SLC26A6

Renal calcium oxalate (CaOx) stone is a common urologic disease with a high prevalence and recurrence rate. However, short-chain fatty acids (SCFAs) are less often reported in the prevention of urolithiasis. This study aimed to explore the effect of SCFAs on the renal CaOx stone formation and the underlying mechanisms. Ethylene glycol was used to induce renal CaOx crystals in rats. SCFAs (acetate, propionate, or butyrate) were added as supplements to the drinking water with or without antibiotics. Because intestinal oxalate transporters SLC26A6 and SLC26A3 regulate the excretion and absorption of oxalate in the intestine, we injected adeno-associated virus 9 (AAV9)-SLC26A6-shRNA (short hairpin RNA) and AAV9-SLC26A3 into the tail vein of rats to suppress SLC26A6 and overexpress SLC26A3 expression in the intestine, respectively, to explore the role of SLC26A3 and SLC26A6 (SLC26A3/6) in the reduction of renal CaOx crystals induced by SCFAs. Results showed that SCFAs reduced renal CaOx crystals and urinary oxalate levels but, however, increased the abundance of SCFA-producing bacteria and cecum SCFA levels. SCFA supplements still reduced renal crystals and urinary oxalate after gut microbiota depletion. Propionate and butyrate downregulated intestinal oxalate transporter SLC26A3 expression, while acetate and propionate upregulated SLC26A6 expression, both in vivo and in vitro. AAV9-SLC26A3 exerted a protective effect against renal crystals, while AAV9-SLC26A6-shRNA contributed to the renal crystal formation even though the SCFAs were supplemented. In conclusion, SCFAs could reduce urinary oxalate and renal CaOx stones through the oxalate transporter SLC26A6 in the intestine. SCFAs may be new supplements for preventing the formation of renal CaOx stones. IMPORTANCE Some studies found that the relative abundances of short-chain-fatty-acid (SCFA)-producing bacteria were lower in the gut microbiota of renal stone patients than healthy controls. Our previous study demonstrated that SCFAs could reduce the formation of renal calcium oxalate (CaOx) stones, but the mechanism is still unknown. In this study, we found that SCFAs (acetate, propionate, and butyrate) reduced the formation of renal calcium oxalate (CaOx) crystals and the level of urinary oxalate. Depleting gut microbiota increased the amount of renal crystals in model rats, and SCFA supplements reduced renal crystals and urinary oxalate after gut microbiota depletion. Intestinal oxalate transporter SLC26A6 was a direct target of SCFAs. Our findings suggested that SCFAs could reduce urinary oxalate and renal CaOx stones through the oxalate transporter SLC26A6 in the intestine. SCFAs may be new supplements for preventing the formation of renal CaOx stones.

SLC26A6-selective inhibitor identified in a small-molecule screen blocks fluid absorption in small intestine

SLC26A6 (also known as putative anion transporter 1 [PAT1]) is a Cl^–/HCO₃^– exchanger expressed at the luminal membrane of enterocytes where it facilitates intestinal Cl^– and fluid absorption. Here, high-throughput screening of 50,000 synthetic small molecules in cells expressing PAT1 and a halide-sensing fluorescent protein identified several classes of inhibitors. The most potent compound, the pyrazolo-pyrido-pyrimidinone PAT1_inh-B01, fully inhibited PAT1-mediated anion exchange (IC₅₀ ~350 nM), without inhibition of the related intestinal transporter SLC26A3 (also known as DRA). In closed midjejunal loops in mice, PAT1_inh-B01 inhibited fluid absorption by 50%, which increased to >90% when coadministered with DRA inhibitor DRA_inh-A270. In ileal loops, PAT1_inh-B01 blocked fluid absorption by >80%, whereas DRA_inh-A270 was without effect. In colonic loops, PAT1_inh-B01 was without effect, whereas DRA_inh-A270 completely blocked fluid absorption. In a loperamide constipation model, coadministration of PAT1_inh-B01 with DRA_inh-A270 increased stool output compared with DRA_inh-A270 alone. These results provide functional evidence for complementary and region-specific roles of PAT1 and DRA in intestinal fluid absorption, with PAT1 as the predominant anion exchanger in mouse ileum. We believe that PAT1_inh-B01 is a novel tool to study intestinal ion and fluid transport and perhaps a drug candidate for small intestinal hyposecretory disorders such as cystic fibrosis–related meconium ileus and distal intestinal obstruction syndrome.

The role of NHE3 (Slc9a3) in oxalate and sodium transport by mouse intestine and regulation by cAMP

Intestinal oxalate transport involves Cl⁻/HCO₃⁻ exchangers but how this transport is regulated is not currently known. NHE3 (Slc9a3), an apical Na⁺/H⁺ exchanger, is an established target for regulation of electroneutral NaCl absorption working in concert with Cl⁻/HCO₃⁻ exchangers. To test whether NHE3 could be involved in regulation of intestinal oxalate transport and renal oxalate handling we compared urinary oxalate excretion rates and intestinal transepithelial fluxes of ¹⁴C‐oxalate and ²²Na⁺ between NHE3 KO and wild‐type (WT) mice. NHE3 KO kidneys had lower creatinine clearance suggesting reduced GFR, but urinary oxalate excretion rates (µmol/24 h) were similar compared to the WT but doubled when expressed as a ratio of creatinine. Intestinal transepithelial fluxes of ¹⁴C‐oxalate and ²²Na⁺ were measured in the distal ileum, cecum, and distal colon. The absence of NHE3 did not affect basal net transport rates of oxalate or sodium across any intestinal section examined. Stimulation of intracellular cAMP with forskolin (FSK) and 3‐isobutyl‐1‐methylxanthine (IBMX) led to an increase in net oxalate secretion in the WT distal ileum and cecum and inhibition of sodium absorption in the cecum and distal colon. In NHE3 KO cecum, cAMP stimulation of oxalate secretion was impaired suggesting the possibility of a role for NHE3 in this process. Although, there is little evidence for a role of NHE3 in basal intestinal oxalate fluxes, NHE3 may be important for cAMP stimulation of oxalate in the cecum and for renal handling of oxalate.

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.

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.

Impaired renal HCO3 - secretion in CFTR deficient mice causes metabolic alkalosis during chronic base-loading

AIM

Cystic fibrosis patients have an increased risk of developing metabolic alkalosis presumably as a result of altered renal HCO₃ ^- handling. In this study, we directly assess the kidneys' ability to compensate for a chronic base-load in the absence of functional CFTR.

METHODS

Comprehensive urine and blood acid-base analyses were done in anaesthetized WT mice or mice lacking either CFTR or pendrin, with or without 7 days of oral NaHCO₃ loading. The in vivo experiments were complemented by a combination of immunoblotting and experiments with perfused isolated mouse cortical collecting ducts (CCD).

RESULTS

Base-loaded WT mice maintained acid-base homeostasis by elevating urinary pH and HCO₃ ^- excretion and decreasing urinary net acid excretion. In contrast, pendrin KO mice and CFTR KO mice were unable to increase urinary pH and HCO₃ ^- excretion and unable to decrease urinary net acid excretion sufficiently and thus developed metabolic alkalosis in response to the same base-load. The expression of pendrin was increased in response to the base-load in WT mice with a paralleled increased pendrin function in the perfused CCD. In CFTR KO mice, 7 days of base-loading did not upregulate pendrin expression and apical Cl^- /HCO₃ ^- exchange function was strongly blunted in the CCD.

CONCLUSION

CFTR KO mice develop metabolic alkalosis during a chronic base-load because they are unable to sufficiently elevate renal HCO₃ ^- excretion. This can be explained by markedly reduced pendrin function in the absence of CFTR.

Urinary stone disease prevalence and associations in cystic fibrosis

Cystic fibrosis (CF) may predispose patients to urinary stone disease (USD), but reported prevalence of USD in patients with CF in previous small studies is variable. To date, analysis of risk factors for USD within the CF population has been limited. We studied 29,396 patients in the Cystic Fibrosis Foundation Patient Registry to calculate age and sex-stratified prevalence of USD. For adult patients, we examined age and multivariable-adjusted cross-sectional associations between demographic and clinical factors, CFTR mutation class, and prevalent USD. Prevalence of USD was 0.4% (95% CI 0.3-0.5%) under age 18 years, 3.1% (2.7-3.6%) at 18-24 years, 6.4% (5.8-7.1%) at 25-34 years, 7.5% (6.5-8.5%) at 35-44 years, and 6.7% (5.8-7.8%) at 45 years and older. Prevalence for women was higher than men at younger (< 45 years) but not older ages (P value for interaction < 0.0005). Multivariable odds of prevalent USD were significantly increased for severe CFTR mutations, OR 1.53 (1.14-2.06), diabetes, OR 1.24 (1.03-1.50), hypertension, OR 1.58 (1.29-1.93), and chronic macrolide therapy, OR 1.27 (1.07-1.52). BMI was not associated with USD. USD prevalence in CF is similar to that in the general population. With the exception of BMI, known risk factors for USD in the general population also appear to be important for patients with CF. We identified several novel associations in CF patients, including greater prevalence of USD in individuals with severe CFTR mutations and among young women.

Chronic kidney disease and kidney stones

PURPOSE OF REVIEW

Both chronic kidney disease (CKD) and kidney stones are major public health problems, which are closely interrelated. Recurrent kidney stones predispose to CKD although CKD seems to decrease risk of further kidney stone formation. Herein, we review new information of this interrelationship.

RECENT FINDINGS

Several epidemiological studies in the past have shown an association between history of kidney stones and risk for CKD and CKD progression. Recent literature supports this concept and it is reviewed in this article. The issue of whether CKD protects against new kidney stone formation remains unsettled and there is no recent literature addressing it. In relation to stone risk factors in CKD, there are several interesting new articles that discuss mechanisms of hypocitraturia in early CKD before overt metabolic acidosis. Since hypocitraturia is an important risk factor for kidney stone formation we addressed these new data in detail. There are also new data supporting urinary oxalate excretion as a predictor of CKD progression.

SUMMARY

It seems clear that recurrent kidney stones should be avoided not only because of their immediate clinical manifestations but also because of their long-term predisposition to CKD progression. Mechanisms leading to hypocitraturia in early CKD still remain controversial.

Enteric Oxalate Secretion Mediated by Slc26a6 Defends against Hyperoxalemia in Murine Models of Chronic Kidney Disease

BACKGROUND

A state of oxalate homeostasis is maintained in patients with healthy kidney function. However, as GFR declines, plasma oxalate (P_ox) concentrations start to rise. Several groups of researchers have described augmentation of oxalate secretion in the colon in models of CKD, but the oxalate transporters remain unidentified. The oxalate transporter Slc26a6 is a candidate for contributing to the extrarenal clearance of oxalate via the gut in CKD.

METHODS

Feeding a diet high in soluble oxalate or weekly injections of aristolochic acid induced CKD in age- and sex-matched wild-type and Slc26a6 ^-/- mice. qPCR, immunohistochemistry, and western blot analysis assessed intestinal Slc26a6 expression. An oxalate oxidase assay measured fecal and P_ox concentrations.

RESULTS

Fecal oxalate excretion was enhanced in wild-type mice with CKD. This increase was abrogated in Slc26a6 ^-/- mice associated with a significant elevation in plasma oxalate concentration. Slc26a6 mRNA and protein expression were greatly increased in the intestine of mice with CKD. Raising P_ox without inducing kidney injury did not alter intestinal Slc26a6 expression, suggesting that changes associated with CKD regulate transporter expression rather than elevations in P_ox.

CONCLUSIONS

Slc26a6-mediated enteric oxalate secretion is critical in decreasing the body burden of oxalate in murine CKD models. Future studies are needed to address whether similar mechanisms contribute to intestinal oxalate elimination in humans to enhance extrarenal oxalate clearance.

Dried Blood Spot-Based Metabolomic Profiling in Adults with Cystic Fibrosis

Mucoviscidosis of the respiratory, gastrointestinal, and genitourinary tracts is the major pathology in patients with cystic fibrosis (CF), a lethal monogenic panethnic and multisystemic disease most commonly identified in Caucasians. Currently, the measurement of immuno reactive trypsinogen in dry blood spots (DBSs) is the gold-standard method for initial newborn screening for CF, followed by targeted CF transmembrane regulator (CFTR) mutation analysis, and ultimate confirmation with abnormally elevated sweat chloride. Previous metabolomics studies in patients with CF reported on different biomarkers such as breath 2-aminoacetophenone produced during acute and chronic infection in human tissues, including the lungs of CF patients. Herein, we used liquid and gas chromatography-mass spectrometry-based targeted metabolomics profiling to identify potentially reliable, sensitive, and specific biomarkers in DBSs collected from 69 young and adult people including CF patients (n = 39) and healthy control (n = 30). A distinctive metabolic profile including 26 significantly differentially expressed metabolites involving amino acids, glycolysis, mitochondrial and peroxisomal metabolism, and sorbitol pathways was identified. Specifically, the osmolyte (sorbitol) was remarkably downregulated in CF patients compared to healthy controls indicating perturbation in the sorbitol pathway, which may be responsible for the mucoviscidosis seen in patients with CF. The significance of our findings is supported by the clinical utility of inhaled mannitol and hypertonic saline in patients with CF. The systemic administration of sorbitol in such patients may confer additional benefits beyond the respiratory system, especially in those with misfolded CFTR proteins.

Physiology of Electrolyte Transport in the Gut: Implications for Disease

We now have an increased understanding of the genetics, cell biology, and physiology of electrolyte transport processes in the mammalian intestine, due to the availability of sophisticated methodologies ranging from genome wide association studies to CRISPR-CAS technology, stem cell-derived organoids, 3D microscopy, electron cryomicroscopy, single cell RNA sequencing, transgenic methodologies, and tools to manipulate cellular processes at a molecular level. This knowledge has simultaneously underscored the complexity of biological systems and the interdependence of multiple regulatory systems. In addition to the plethora of mammalian neurohumoral factors and their cross talk, advances in pyrosequencing and metagenomic analyses have highlighted the relevance of the microbiome to intestinal regulation. This article provides an overview of our current understanding of electrolyte transport processes in the small and large intestine, their regulation in health and how dysregulation at multiple levels can result in disease. Intestinal electrolyte transport is a balance of ion secretory and ion absorptive processes, all exquisitely dependent on the basolateral Na⁺ /K⁺ ATPase; when this balance goes awry, it can result in diarrhea or in constipation. The key transporters involved in secretion are the apical membrane Cl^- channels and the basolateral Na⁺ -K⁺ -2Cl^- cotransporter, NKCC1 and K⁺ channels. Absorption chiefly involves apical membrane Na⁺ /H⁺ exchangers and Cl^- /HCO₃ ^- exchangers in the small intestine and proximal colon and Na⁺ channels in the distal colon. Key examples of our current understanding of infectious, inflammatory, and genetic diarrheal diseases and of constipation are provided. © 2019 American Physiological Society. Compr Physiol 9:947-1023, 2019.

miR-125a-5p: a novel regulator of SLC26A6 expression in intestinal epithelial cells

Putative anion transporter 1 (PAT1, SLC26A6), an intestinal epithelial Cl^-/ HCO3- exchanger, also plays a key role in oxalate homeostasis via mediating intestinal oxalate secretion. Indeed, Slc26a6-null mice showed defect in intestinal oxalate secretion and high incidence of kidney stones. Recent emergence of PAT-1 as a novel therapeutic target for nephrolithiasis warrants detailed understanding of the mechanisms of PAT-1 regulation in health and disease. Therefore, we investigated the regulation of PAT-1 expression by microRNAs (miRNA), as they have been shown to play key role in modulating expression of other ion transporters. In silico analysis of PAT-1 3'-untranslated region (UTR) revealed potential binding sites for several miRNAs, suggesting the role of miRNAs in modulating PAT1 expression. miRNAs showing highest context scores (125a-5p, 339-5p, 423-5p, 485-5p, and 501-3p) were selected as candidates for their effects on the activity of a 263-bp PAT-1 3'-untranslated region (UTR) fragment cloned into pmirGLO vector upstream of luciferase. The 3'-UTR activity was measured by dual luciferase reporter assay in Caco-2, T-84, HT-29, and SK-CO15 cells. Transient transfection of PAT-1 3'-UTR significantly decreased the relative luciferase activity compared with the empty vector suggesting binding of potential miRNA(s) to the PAT-1 3'-UTR. Among all the selected candidates, cotransfection with miRNA mimics 125a-5p and 423-5p further decreased PAT-1 3'-UTR activity. Furthermore, increasing miR-125a-5p abundance via mimic transfection in Caco-2 cells decreased both mRNA and protein levels of PAT-1. Our results demonstrate a novel regulatory mechanism of intestinal PAT-1 expression via miR-125a-5p that could be of therapeutic importance in disorders associated with decreased PAT-1 expression and function.

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.

The Clinical Biology of Cystic Fibrosis Transmembrane Regulator Protein: Its Role and Function in Extrapulmonary Disease

Normal cystic fibrosis (CF) transmembrane regulator (CFTR) protein has multiple functions in health and disease. Many mutations in the CFTR gene produce abnormal or absent protein. CFTR protein dysfunction underlies the classic CF phenotype of progressive pulmonary and GI pathology but may underlie diseases not usually associated with CF. This review highlights selected extrapulmonary disease that may be associated with abnormal CFTR. Increasing survival in CF is associated with increasing incidence of diseases associated with aging. CFTR dysfunction in older individuals may have novel effects on glucose metabolism, control of insulin release, regulation of circadian rhythm, and cancer cell pathophysiology. In individuals who have cancers with acquired CFTR suppression, their tumors may more likely exhibit rapid expansion, epithelial-to-mesenchymal transformation, abnormally reduced apoptosis, and increased metastatic potential. The new modulators of CFTR protein synthesis could facilitate the additional exploration needed to better understand the unfolding clinical biology of CFTR in human disease, even as they revolutionize treatment of patients with CF.

Secondary Oxalate Nephropathy: A Systematic Review

Introduction

Little is known of the clinical outcomes of secondary oxalate nephropathy. To inform clinical practice, we performed a systematic review of case reports and case series to examine the clinical characteristics and outcomes of patients with secondary oxalate nephropathy.

Methods

Electronic databases were searched for case reports and case series of individual cases or cohorts of patients with biopsy-proven oxalate nephropathy in native or transplanted kidneys from 1950 until January 2018.

Results

Fifty-seven case reports and 10 case series met the inclusion criteria, totaling 108 patients. The case series were meta-analyzed. Mean age was 56.4 years old, 59% were men, and 15% were kidney transplant recipients. Fat malabsorption (88%) was the most commonly attributed cause of oxalate nephropathy, followed by excessive dietary oxalate consumption (20%). The mean baseline serum creatinine was 1.3 mg/dl and peaked at 4.6 mg/dl. Proteinuria, hematuria, and urinary crystals was reported in 69%, 32%, and 26% of patients, respectively. Mean 24-hour urinary oxalate excretion was 85.4 mg/d. In addition to universal oxalate crystal deposition in tubules and/or interstitium, kidney biopsy findings included acute tubular injury (71%), tubular damage and atrophy (69%), and interstitial mononuclear cell infiltration (72%); 55% of patients required dialysis. None had complete recovery, 42% had partial recovery, and 58% remained dialysis-dependent. Thirty-three percent of patients died.

Conclusion

Secondary oxalate nephropathy is a rare but potentially devastating condition. Renal replacement therapy is required in >50% of patients, and most patients remain dialysis-dependent. Studies are needed for effective preventive and treatment strategies in high-risk patients with hyperoxaluria-enabling conditions.

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.

Drug-Induced Kidney Stones and Crystalline Nephropathy: Pathophysiology, Prevention and Treatment

Drug-induced calculi represent 1-2% of all renal calculi. The drugs reported to produce calculi may be divided into two groups. The first one includes poorly soluble drugs with high urine excretion that favour crystallisation in the urine. Among them, drugs used for the treatment of patients with human immunodeficiency, namely atazanavir and other protease inhibitors, and sulphadiazine used for the treatment of cerebral toxoplasmosis, are the most frequent causes. Besides these drugs, about 20 other molecules may induce nephrolithiasis, such as ceftriaxone or ephedrine-containing preparations in subjects receiving high doses or long-term treatment. Calculi analysis by physical methods including infrared spectroscopy or X-ray diffraction is needed to demonstrate the presence of the drug or its metabolites within the calculi. Some drugs may also provoke heavy intra-tubular crystal precipitation causing acute renal failure. Here, the identification of crystalluria or crystals within the kidney tissue in the case of renal biopsy is of major diagnostic value. The second group includes drugs that provoke the formation of urinary calculi as a consequence of their metabolic effects on urinary pH and/or the excretion of calcium, phosphate, oxalate, citrate, uric acid or other purines. Among such metabolically induced calculi are those formed in patients taking uncontrolled calcium/vitamin D supplements, or being treated with carbonic anhydrase inhibitors such as acetazolamide or topiramate. Here, diagnosis relies on a careful clinical inquiry to differentiate between common calculi and metabolically induced calculi, of which the incidence is probably underestimated. Specific patient-dependent risk factors also exist in relation to urine pH, volume of diuresis and other factors, thus providing a basis for preventive or curative measures against stone formation.

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.