Claudins and mineral metabolism

Non-Coding RNAs in Kidney Stones

Nephrolithiasis is a major public health concern associated with high morbidity and recurrence. Despite decades of research, the pathogenesis of nephrolithiasis remains incompletely understood, and effective prevention is lacking. An increasing body of evidence suggests that non-coding RNAs, especially microRNAs (miRNAs) and long non-coding RNAs (lncRNAs), play a role in stone formation and stone-related kidney injury. MiRNAs have been studied quite extensively in nephrolithiasis, and a plethora of specific miRNAs have been implicated in the pathogenesis of nephrolithiasis, involving remarkable changes in calcium metabolism, oxalate metabolism, oxidative stress, cell-crystal adhesion, cellular autophagy, apoptosis, and macrophage (Mp) polarization and metabolism. Emerging evidence suggests a potential for miRNAs as novel diagnostic biomarkers of nephrolithiasis. LncRNAs act as competing endogenous RNAs (ceRNAs) to bind miRNAs, thereby modulating mRNA expression to participate in the regulation of physiological mechanisms in kidney stones. Small interfering RNAs (siRNAs) may provide a novel approach to kidney stone prevention and treatment by treating related metabolic conditions that cause kidney stones. Further investigation into these non-coding RNAs will generate novel insights into the mechanisms of renal stone formation and stone-related renal injury and might lead to new strategies for diagnosing and treating this disease.

Tight junction and kidney stone disease

Defects of tight junction (TJ) are involved in many diseases related to epithelial cell functions, including kidney stone disease (KSD), which is a common disease affecting humans for over a thousand years. This review provides brief overviews of KSD and TJ, and summarizes the knowledge on crystal-induced defects of TJ in renal tubular epithelial cells (RTECs) in KSD. Calcium oxalate (CaOx) crystals, particularly COM, disrupt TJ via p38 MAPK and ROS/Akt/p38 MAPK signaling pathways, filamentous actin (F-actin) reorganization and α-tubulin relocalization. Stabilizing p38 MAPK signaling, reactive oxygen species (ROS) production, F-actin and α-tubulin by using SB239063, N-acetyl-L-cysteine (NAC), phalloidin and docetaxel, respectively, successfully prevent the COM-induced TJ disruption and malfunction. Additionally, genetic disorders of renal TJ, including mutations and single nucleotide polymorphisms (SNPs) of CLDN2, CLDN10b, CLDN14, CLDN16 and CLDN19, also affect KSD. Finally, the role of TJ as a potential target for KSD therapeutics and prevention is also discussed.

Hypomagnesemia: a potential underlooked cause of persistent vitamin D deficiency in chronic kidney disease

Magnesium and vitamin D play important roles in most cells of the body. These nutrients act in a coordinated fashion to maintain physiologic functions of various organs, and their abnormal balance could adversely affect these functions. Therefore, deficient states of both nutrients may lead to several chronic medical conditions and increased cardiovascular and all-cause mortality. Chronic kidney disease (CKD) patients have altered metabolism of both magnesium and vitamin D. Some studies indicate that magnesium could have a role in the synthesis and metabolism of vitamin D, and that magnesium supplementation substantially reversed the resistance to vitamin D treatment in some clinical situations. Recent observational studies also found that magnesium intake significantly interacted with vitamin D status and, particularly with the risk of cardiovascular mortality. It is therefore essential to ensure adequate levels of magnesium to obtain the optimal benefits of vitamin D supplementation in CKD patients. In this review, we discuss magnesium physiology, magnesium and vitamin D metabolism in CKD, potential metabolic interactions between magnesium and vitamin D and its clinical relevance, as well as the possible role of magnesium supplementation to assure adequate vitamin D levels.

Mutations in CLDN2 Are Not a Common Cause of Pediatric Idiopathic Hypercalciuria in Canada

Background:

Hypercalciuria is the most common risk factor for kidney stone formation, including in pediatric patients. However, the etiology is often unknown and children are frequently diagnosed with idiopathic hypercalciuria. Nearly 50% of children with hypercalciuria have a first-degree relative with kidney stones, suggesting a strong genetic basis for this disease. A failure of calcium reabsorption from the proximal nephron is implicated in the pathogenesis of hypercalciuria. Claudin-2 is a tight junction protein abundantly expressed in the proximal tubule. It confers paracellular permeability to calcium that is essential for transport across the proximal tubule where the majority of filtered calcium is reabsorbed.

Objective:

Our objective was to examine the frequency of coding variations in CLDN2 in a cohort of children with idiopathic hypercalciuria.

Design:

Mixed method including retrospective chart review and patient interview, followed by genetic sequencing.

Setting:

Three tertiary care centers in Canada.

Patients:

Children (age 1-18 years) with idiopathic hypercalciuria. Patients with other causes of hypercalciuria were excluded.

Methods:

Data were collected from 40 patients with idiopathic hypercalciuria. Informed consent to collect DNA was obtained from 13 patients, and the final and only coding exon of CLDN2 was sequenced.

Results:

The majority of patients were male, white, and had a positive family history of kidney stones. Parathyroid hormone levels were significantly lower than the reference range (P < .001). The levels of 1,25-dihydroxyvitamin D were also significantly higher in our patient cohort, relative to the reference range (P < .001). Sequence analysis of CLDN2 did not identify any coding variations.

Limitations:

Sequencing analysis was limited to the final coding exon and small sample size.

Conclusions:

CLDN2 coding variations are not a common cause of idiopathic hypercalciuria in Canadian children. Further study is needed to determine the causes of hypercalciuria in pediatric patients and develop targeted therapies.

Claudins in kidney health and disease

Claudins are strategically located to exert their physiologic actions along with the nephron segments from the glomerulus. Claudin-1 is normally located in the Bowman’s capsule, but its overexpression can reach the podocytes and lead to albuminuria. In the proximal tubule (PT), claudin-2 forms paracellular channels selective for water, Na⁺, K⁺, and Ca²⁺. Claudin-2 gene mutations are associated with hypercalciuria and kidney stones. Claudin-10 has two splice variants, -10a and -10b; Claudin-10a acts as an anion-selective channel in the PT, and claudin-10b functions as a cation-selective pore in the thick ascending limb (TAL). Claudin-16 and claudin-19 mediate paracellular transport of Na⁺, Ca²⁺, and Mg²⁺ in the TAL, where the expression of claudin-3/16/19 and claudin-10b are mutually exclusive. The claudin-16 or -19 mutation causes familial hypomagnesemia with hypercalciuria and nephrocalcinosis. Claudin-14 polymorphisms have been linked to increased risk of hypercalciuria. Claudin-10b mutations produce HELIX syndrome, which encompasses hypohidrosis, electrolyte imbalance, lacrimal gland dysfunction, ichthyosis, and xerostomia. Hypercalciuria and magnesuria in metabolic acidosis are related to downregulation of PT and TAL claudins. In the TAL, stimulation of calcium-sensing receptors upregulates claudin-14 and negatively acts on the claudin-16/19 complex. Claudin-3 acts as a general barrier to ions in the collecting duct. If this barrier is disturbed, urine acidification might be impaired. Claudin-7 forms a nonselective paracellular channel facilitating Cl^– and Na⁺ reabsorption in the collecting ducts. Claudin-4 and -8 serve as anion channels and mediate paracellular Cl^– transport; their upregulation may contribute to pseudohypoaldosteronism II and salt-sensitive hypertension.

Hypercalciuria in Postmenopausal Women With Reduced Bone Mineral Density Is Associated With Different Mineral Metabolic Profiles: Effects of Treatment With Thiazides and Anti-resorptives

Hypercalciuria may represent a challenge during the workup for osteoporosis management. The present study aimed: (1) to describe the phenotype associated with hypercalciuria in vitamin D-sufficient (serum 25 hydroxyvitamin D (25OHD) > 20 ng/ml) patients with osteopenia/osteoporosis; (2) to analyze the effects of thiazides and anti-resorptive drugs on urine calcium excretion (UCa), mineral metabolic markers, and bone mineral density. Seventy-seven postmenopausal women with hypercalciuria (Uca > 4.0 mg/kg body weight/24 h on two determinations) were retrospectively evaluated in a real-life setting. Median UCa was 5.39 (4.75–6.70) mg/kg/24 h. Kidney stones occurred in 32.9% of patients, who had median UCa similar to that of patients without kidney stones. Clustering analysis considering the three variables, such as serum calcium, phosphate, and parathormone (PTH), identified two main clusters of hypercalciuric patients. Cluster 1 (n = 13) included patients with a primary hyperparathyroidism-like profile, suggesting a certain degree of autonomous PTH secretion from parathyroid glands. Within cluster 2 (n = 61), two subgroups were recognized, cluster 2A (n = 18) that included patients with relatively increased PTH and normophosphatemia, and cluster 2B (n = 43) that included patients with the normal mineral profile. After a follow-up of 33.4 ± 19.6 months, 49 patients treated with thiazidic diuretics (TZD) were reevaluated; 20 patients were treated with hydrochlorothiazide (HCT; 12.5–37.5 mg/day), 29 with indapamide (IND; 1.50–3.75 mg/day). Any significant difference could be detected in all the parameters both basal and treated conditions between patients treated with HCT or IND. TZD induced a mean 39% reduction in UCa and 63.3% of patients obtained Uca < 4.0 mg/kg/24 h, independent of their mineral metabolic profile. Moreover, TZD induced a significant decrease in PTH levels. TZD-treated patients normalizing UCa experienced an increase in bone mineral densities when concomitantly treated with anti-resorptives, while any gain could be observed in TZD-treated patients with persistent hypercalciuria. Finally, multiple regression analysis showed that UCa reduction was at least in part related to denosumab treatment. In conclusion, in postmenopausal osteoporotic women, hypercalciuria is associated with kidney stones in about one-third of patients and with a wide range of impaired PTH secretion, determining a diagnostic challenge. TZD efficiently reduces UCa and normalization contributes to increasing anti-resorptives positive effect on bone mineral density.

Genetics of kidney stone disease-Polygenic meets monogenic

Kidney stone disease comprising nephrolithiasis and nephrocalcinosis is a clinical syndrome of increasing prevalence with remarkable heterogeneity. Stone composition, age of manifestation, rate of recurrence, and impairment of kidney function varies with underlying etiologies. While calcium-based kidney stones account for the vast majority their etiology is still poorly understood. Recent studies underline the notion that genetic susceptibility together with dietary habits constitutes the major driver of kidney stone formation. In addition to single gene (Mendelian) disorders, which are most likely underestimated in the adult population, common risk alleles explain part of the observed heritability. Interestingly, identified GWAS loci often match those of Mendelian disease genes and vice versa (CASR, SLC34A1, CYP24A1). These findings provide mechanistic links related to renal calcium homeostasis, vitamin D metabolism, and CaSR-signaling regulated by the CaSR-CLDN14-CLDN16/19 axis (paracellular Ca²⁺ reabsorption) and TRPV5 (transcellular Ca²⁺ reabsorption). Recent identification of new single gene disorders of calcium-oxalate-nephrolithiasis (SLC26A1, CLDN2) and distal renal tubular acidosis with nephrocalcinosis (FOXI1, WDR72, ATP6V1C2) enabled additional insights into the kidney-gut axis and molecular prerequisites of proper urinary acidification. Implementation of centralized patient registries on hereditary kidney stone diseases are necessary to build up well characterized cohorts for urgently needed clinical studies.

Tight junctions and their regulation by non-coding RNAs

Tight junction (TJ) is a “zippering up” junction structure located at the uppermost portion of adjacent epithelial/endothelial cells in organs and tissues. TJs maintain the relative stability of intracellular substances and functions by closing or opening intercellular pathways, coordinating the entry and exit of molecules of different sizes and charges, and regulating the permeability of paracellular barrier. TJs also prevent microbial invasion, maintain epithelial/endothelial cell polarity, and regulate cell proliferation. TJs are widely present in the skin and mucosal epithelial barriers, intestinal epithelial barrier, glomerular filtration barrier, bladder epithelial barrier, blood-brain barrier, brain-blood tumor barrier, and blood-testis barrier. TJ dysfunction in different organs can lead to a variety of diseases. In addition to signal pathways, transcription factors, DNA methylation, histone modification, TJ proteins can also be regulated by a variety of non-coding RNAs, such as micro-RNAs, long-noncoding RNAs, and circular RNAs, directly or indirectly. This review summarizes the structure of TJs and introduces the functions and regulatory mechanisms of TJs in different organs and tissues. The roles and mechanisms of non-coding RNAs in the regulation of TJs are also highlighted in this review.

Differential diagnosis of perinatal Bartter, Bartter and Gitelman syndromes

The common finding of hypokalemic alkalosis in several unrelated disorders may confound the early diagnosis of salt-losing tubulopathy (SLT). Antenatal Bartter syndrome (BS) must be considered in idiopathic early-onset polyhydramnios. Fetal megabladder in BS may allow its distinction from third-trimester polyhydramnios that occurs in congenital chloride diarrhea (CCD). Fetal megacolon occurs in CCD while fecal chloride >90 mEq/L in infants is diagnostic. Failure-to-thrive, polydipsia and polyuria in early childhood are the hallmarks of classic BS. Unlike BS, there is low urinary chloride in hypokalemic alkalosis of intractable emesis and cystic fibrosis. Rarely, renal salt wasting may result from cystinosis, Dent disease, disorders of paracellular claudin-10b and Kir4.1 potassium-channel deficiency. Acquired BS may result from calcimimetic up-regulation of a calcium-sensing receptor or autoantibody inactivation of sodium chloride co-transporters in Sjögren syndrome. A relatively common event of heterozygous gene mutations for Gitelman syndrome increases the likelihood of its random occurrence in certain diseases of adult onset. Finally, diuretic abuse is the most common differential diagnosis of SLT. Unlike the persistent elevation in BS, urinary chloride concentration losses waxes and wanes on day-to-day assessment in patients with diuretic misuse.

Mouse Models of Human Claudin-Associated Disorders: Benefits and Limitations

In higher organisms, epithelia separate compartments in order to guarantee their proper function. Such structures are able to seal but also to allow substances to pass. Within the paracellular pathway, a supramolecular structure, the tight junction transport is largely controlled by the temporospatial regulation of its major protein family called claudins. Besides the fact that the expression of claudins has been identified in different forms of human diseases like cancer, clearly defined mutations in the corresponding claudin genes have been shown to cause distinct human disorders. Such disorders comprise the skin and its adjacent structures, liver, kidney, the inner ear, and the eye. From the phenotype analysis, it has also become clear that different claudins can cause a complex phenotype when expressed in different organs. To gain deeper insights into the physiology and pathophysiology of claudin-associated disorders, several mouse models have been generated. In order to model human disorders in detail, they have been designed either as full knockouts, knock-downs or knock-ins by a variety of techniques. Here, we review human disorders caused by CLDN mutations and their corresponding mouse models that have been generated thus far and assess their usefulness as a model for the corresponding human disorder.

Renal Ca2+ and Water Handling in Response to Calcium Sensing Receptor Signaling: Physiopathological Aspects and Role of CaSR-Regulated microRNAs

Calcium (Ca²⁺) is a universal and vital intracellular messenger involved in a diverse range of cellular and biological processes. Changes in the concentration of extracellular Ca²⁺ can disrupt the normal cellular activities and the physiological function of these systems. The calcium sensing receptor (CaSR) is a unique G protein-coupled receptor (GPCR) activated by extracellular Ca²⁺ and by other physiological cations, aminoacids, and polyamines. CaSR is the main controller of the extracellular Ca²⁺ homeostatic system by regulating parathyroid hormone (PTH) secretion and, in turn, Ca²⁺ absorption and resorption. Recent advances highlight novel signaling pathways activated by CaSR signaling involving the regulation of microRNAs (miRNAs). miRNAs are naturally-occurring small non-coding RNAs that regulate post-transcriptional gene expression and are involved in several diseases. We previously described that high luminal Ca²⁺ in the renal collecting duct attenuates short-term vasopressin-induced aquaporin-2 (AQP2) trafficking through CaSR activation. Moreover, we demonstrated that CaSR signaling reduces AQP2 abundance via AQP2-targeting miRNA-137. This review summarizes the recent data related to CaSR-regulated miRNAs signaling pathways in the kidney.

Role of Citrate in Pathophysiology and Medical Management of Bone Diseases

Citrate is an intermediate in the “Tricarboxylic Acid Cycle” and is used by all aerobic organisms to produce usable chemical energy. It is a derivative of citric acid, a weak organic acid which can be introduced with diet since it naturally exists in a variety of fruits and vegetables, and can be consumed as a dietary supplement. The close association between this compound and bone was pointed out for the first time by Dickens in 1941, who showed that approximately 90% of the citrate bulk of the human body resides in mineralised tissues. Since then, the number of published articles has increased exponentially, and considerable progress in understanding how citrate is involved in bone metabolism has been made. This review summarises current knowledge regarding the role of citrate in the pathophysiology and medical management of bone disorders.

Gushukang exerts osteopreserve effects by regulating vitamin D and calcium metabolism in ovariectomized mice

Calcium homeostasis plays vital roles in the management of bone health. Traditional herbal formula Gushukang (GSK) was clinically applied to treat primary osteoporosis. This study aimed to explore the osteoprotective effects of GSK and its roles in maintaining calcium homeostasis in ovariectomized (OVX) mice. The OVX mice were orally treated with low (0.38 g/kg), middle (0.76 g/kg) and high (1.52 g/kg) dose of GSK for 8 weeks. GSK treatment dramatically increased serum calcium level and decreased urinary calcium excretion as well as enhanced calcium content in bone of OVX mice. Serum level of 25-hydroxyvitamin D was significantly increased in OVX mice with exposure to GSK. Treatment with GSK improved bone mass and micro-structure of trabecular bone at distal metaphysis of femur and proximal metaphysis of tibia in OVX mice shown by safranin O staining and micro-CT measurement. GSK treatment at all doses up-regulated mRNA expression of calcium-binding protein-28k and vitamin D receptor in kidney of OVX mice, and dose-dependently decreased mRNA expression of claudin-14 and elevated mRNA expression of claudin-16 in duodenum of OVX mice. Taken together, GSK exerted beneficial effects on trabecular bone of OVX mice by improving calcium homeostasis via regulating paracellular calcium absorption in duodenum and transcellular calcium reabsorption in kidney.

Diabetes-induced hypomagnesemia is not modulated by metformin treatment in mice

Approximately 30% of patients with type 2 diabetes mellitus (T2D) have hypomagnesemia (blood magnesium (Mg²⁺) concentration <0.7 mmol/L). In T2D patients, treatment with metformin is associated with reduced blood Mg²⁺ levels. To investigate how T2D and metformin affect Mg²⁺ homeostasis db/m and db/db mice were treated with metformin or placebo. Mice were housed in metabolic cages to measure food and water intake, and to collect urine and feces. Serum and urinary Mg²⁺ concentrations were determined and mRNA expression of magnesiotropic genes was determined in kidney and distal colon using RT-qPCR. Db/db mice had significantly lower serum Mg²⁺ levels than db/m mice. Mild hypermagnesuria was observed in the db/db mice at two weeks, but not at four weeks. Metformin-treatment had no effect on the serum Mg²⁺ concentration and on the urinary Mg²⁺ excretion. Both in kidney and distal colon of db/db mice, there was a compensatory upregulation in the mRNA expression of magnesiotropic genes, such as transient receptor potential melastatin 6 (Trpm6), whereas metformin treatment did not affect gene expression levels. In conclusion, we show that T2D causes hypomagnesemia and that metformin treatment has no effect on Mg²⁺ homeostasis in mice.

Exome sequencing identifies a novel frameshift variant causing hypomagnesemia with secondary hypocalcemia

Hypomagnesemia with secondary hypocalcemia is a rare autosomal-recessive disorder characterized by intense hypomagnesemia associated with hypocalcemia (HSH). Mutations in the TRPM6 gene, encoding the epithelial Mg²⁺ channel TRPM6, have been proven to be the molecular cause of this disease. This study identified causal mutations in a 2-month-old male patient of hypomagnesemia from a consanguineous marriage. Biochemical analyses indicated the diagnosis of HSH due to primary gastrointestinal loss of magnesium. Whole exome sequencing of the trio (i.e. proband and both parents) was carried out with mean coverage of > 150×. ANNOVAR was used to annotate functional consequences of genetic variation from exome sequencing data. After variant filtering and annotation, a number of single nucleotide variants (SNVs) and 2 bp deletion at exon26:c.4402_4403delCT in TRPM6 gene were identified. This deletion which resulted in a novel frameshift mutation in exon 26 of this gene was confirmed by Sanger sequencing. With these investigations in hand, the patient was managed with magnesium sulphate. The patient remained asymptomatic and was developmentally and neurologically normal till his last follow up.

Personalized Intervention in Monogenic Stone Formers

PURPOSE

Treatment of a first-time renal stone consists of acute management followed by medical efforts to prevent stone recurrence. Although nephrolithiasis is roughly 50% heritable, the presence of a family history usually does not affect treatment since most stone disease is regarded as polygenic, ie not attributable to a single gene. Recent evidence has suggested that single mutations could be responsible for a larger proportion of renal stones than previously thought. This intriguing possibility holds the potential to change the management paradigm in stone prevention from metabolically directed therapy to more specific approaches informed by genetic screening and testing. This review synthesizes new findings concerning monogenic kidney stone disease, and provides a concise and clinically useful reference for monogenic causes. It is expected that increased awareness of these etiologies will lead to increased use of genetic testing in recurrent stone formers and further research into the prevalence of monogenic stone disease.

MATERIALS AND METHODS

We assembled a complete list of genes known to cause or influence nephrolithiasis based on recent reviews and commentaries. We then comprehensively searched PubMed® and Google Scholar™ for all research on each gene having a pertinent role in nephrolithiasis. We determined which genes could be considered monogenic causes of nephrolithiasis. One gene, ALPL, was excluded since nephrolithiasis is a relatively minor aspect of the disorder associated with the gene (hypophosphatasia). We summarized selected studies and assembled clinically relevant details.

RESULTS

A total of 27 genes were reviewed in terms of recent findings, mode of inheritance of stone disease, known or supposed prevalence of mutations in the general population of stone patients and specific therapies or considerations.

CONCLUSIONS

There is a distinct opportunity for increased use of genetic testing to improve the lives of pediatric and adult stone patients. Several genes first reported in association with rare disease may be loci for novel mutations, heterozygous disease and forme frustes as causes of stones in the broader population. Cases of idiopathic nephrolithiasis should be considered as potentially having a monogenic basis.