Claudins and the kidney

Biophysics of claudin proteins in tight junction architecture: Three decades of progress

Tight junctions are cell-cell adhesion complexes that act as gatekeepers of the paracellular space. Formed by several transmembrane proteins, the claudin family performs the primary gate-keeping function. The claudin proteins form charge and size-selective diffusion barriers to maintain homeostasis across endothelial and epithelial tissue. Of the 27 known claudins in mammals, some are known to seal the paracellular space, while others provide selective permeability. The differences in permeability arise due to the varying expression levels of claudins in each tissue. The tight junctions are observed as strands in freeze-fracture electron monographs; however, at the molecular level, tight junction strands form when multiple claudin proteins assemble laterally (cis assembly) within a cell and head-on (trans assembly) with claudins of the adjacent cell in a zipper-like architecture, closing the gap between the neighboring cells. The disruption of tight junctions caused by changing claudin expression levels or mutations can lead to diseases. Therefore, knowledge of the molecular architecture of the tight junctions and how that is tied to tissue-specific function is critical for fighting diseases. Here, we review the current understanding of the tight junctions accrued over the last three decades from experimental and computational biophysics perspectives.

Navigating the multifaceted intricacies of the Na+-Cl- cotransporter, a highly regulated key effector in the control of hydromineral homeostasis

The Na+-Cl- cotransporter (NCC; SLC12A3) is a highly regulated integral membrane protein that is known to exist as three splice variants in primates. Its primary role in the kidney is to mediate the cosymport of Na+ and Cl- across the apical membrane of the distal convoluted tubule. Through this role and the involvement of other ion transport systems, NCC allows the systemic circulation to reclaim a fraction of the ultrafiltered Na+, K+, Cl-, and Mg+ loads in exchange for Ca2+ and [Formula: see text]. The physiological relevance of the Na+-Cl- cotransport mechanism in humans is illustrated by several abnormalities that result from NCC inactivation through the administration of thiazides or in the setting of hereditary disorders. The purpose of the present review is to discuss the molecular mechanisms and overall roles of Na+-Cl- cotransport as the main topics of interest. On reading the narrative proposed, one will realize that the knowledge gained in regard to these themes will continue to progress unrelentingly no matter how refined it has now become.

Autosomal dominant hypercalciuric hypocalcaemia: the calcium-sensing receptor in renal calcium homeostasis and the impact of renal transplantation

Calcium-sensing receptors (CaSRs) are G protein-coupled receptors that help maintain Ca2+ concentrations, modulating calciotropic hormone release (parathyroid hormone (PTH), calcitonin and 1,25-dihydroxyvitamin D) by direct actions in the kidneys, gastrointestinal tract and bone. Variability in population calcium levels has been attributed to single nucleotide polymorphisms in CaSR genes, and several conditions affecting calcium and phosphate homeostasis have been attributed to gain- or loss-of-function mutations. An example is autosomal dominant hypercalciuric hypocalcaemia, because of a missense mutation at codon 128 of chromosome 3, as reported in our specific case and her family. As a consequence of treating symptomatic hypocalcaemia as a child, this female subject slowly developed progressive end-stage kidney failure because of nephrocalcinosis and nephrolithiasis. After kidney transplantation, she remains asymptomatic, with decreased vitamin D and elemental calcium requirements, stable fluid and electrolyte homeostasis during intercurrent illnesses and has normalised urinary calcium and phosphate excretion, reducing the likelihood of hypercalciuria-induced graft impairment. We review the actions of the CaSR, its role in regulating renal Ca2+ homeostasis along with the impact of a proven gain-of-function mutation in the CaSR gene resulting in autosomal dominant hypercalciuric hypocalcaemia before and after kidney transplantation.

A short guide to the tight junction

Tight junctions (TJs) are specialized regions of contact between cells of epithelial and endothelial tissues that form selective semipermeable paracellular barriers that establish and maintain body compartments with different fluid compositions. As such, the formation of TJs represents a critical step in metazoan evolution, allowing the formation of multicompartmental organisms and true, barrier-forming epithelia and endothelia. In the six decades that have passed since the first observations of TJs by transmission electron microscopy, much progress has been made in understanding the structure, function, molecular composition and regulation of TJs. The goal of this Perspective is to highlight the key concepts that have emerged through this research and the future challenges that lie ahead for the field.

Convergent gene losses and pseudogenizations in multiple lineages of stomachless fishes

The regressive evolution of independent lineages often results in convergent phenotypes. Several teleost groups display secondary loss of the stomach, and four gastric genes, atp4a, atp4b, pgc, and pga2 have been co-deleted in agastric (stomachless) fish. Analyses of genotypic convergence among agastric fishes showed that four genes, slc26a9, kcne2, cldn18a, and vsig1, were co-deleted or pseudogenized in most agastric fishes of the four major groups. kcne2 and vsig1 were also deleted or pseudogenized in the agastric monotreme echidna and platypus, respectively. In the stomachs of sticklebacks, these genes are expressed in gastric gland cells or surface epithelial cells. An ohnolog of cldn18 was retained in some agastric teleosts but exhibited an increased non-synonymous substitution when compared with gastric species. These results revealed novel convergent gene losses at multiple loci among the four major groups of agastric fish, as well as a single gene loss in the echidna and platypus.

Sodium butyrate ameliorated diabetic nephropathy-associated tubulointerstitial inflammation by modulating the tight junctions of renal tubular epithelial cells

As one of the most significant pathological changes of diabetic nephropathy (DN), tubulointerstitial fibrosis (TIF) had a close relationship with tubulointerstitial inflammation (TI), and the occurrence of TI could have resulted from the disrupted tight junctions (TJs) of renal tubular epithelial cells (RTECs). Studies have demonstrated that sodium butyrate (NaB), a typical short chain fatty acid (SCFA), played an important regulatory role in intestinal TJs and inflammation. In this study, our in vivo and in vitro results showed that accompanied by TI, renal tubular TJs were gradually disrupted in the process of DN-related TIF. In HG and LPS co-cultured HK-2 cells and db/db mice, NaB treatment regained the TJs of RTECs via the sphingosine 1-phosphate receptor-1 (S1PR1)/AMPK signaling pathway, relieving inflammation. Small interfering RNA of S1PR1, S1PR1 antagonist W146 and agonist SEW2871, and AMPK agonist AICAR were all used to further confirm the essential role of the S1PR1/AMPK signaling pathway in NaB's TJ protection in RTECs in vitro. Finally, NaB administration not only improved the renal function and TIF, but also relieved the TI of db/db mice. These findings suggested that the use of NaB might be a potential adjuvant treatment strategy for DN-associated TIF, and this protective effect was linked to the TJ modulation of RTECs via the S1PR1/AMPK signaling pathway, leading to the improvement of TI.

Dual soluble epoxide hydrolase inhibitor - farnesoid X receptor agonist interventional treatment attenuates renal inflammation and fibrosis

Introduction

Renal fibrosis associated with inflammation is a critical pathophysiological event in chronic kidney disease (CKD). We have developed DM509 which acts concurrently as a farnesoid X receptor agonist and a soluble epoxide hydrolase inhibitor and investigated DM509 efficacy as an interventional treatment using the unilateral ureteral obstruction (UUO) mouse model.

Methods

Male mice went through either UUO or sham surgery. Interventional DM509 treatment (10mg/kg/d) was started three days after UUO induction and continued for 7 days. Plasma and kidney tissue were collected at the end of the experimental protocol.

Results

UUO mice demonstrated marked renal fibrosis with higher kidney hydroxyproline content and collagen positive area. Interventional DM509 treatment reduced hydroxyproline content by 41% and collagen positive area by 65%. Renal inflammation was evident in UUO mice with elevated MCP-1, CD45-positive immune cell positive infiltration, and profibrotic inflammatory gene expression. DM509 treatment reduced renal inflammation in UUO mice. Renal fibrosis in UUO was associated with epithelial-to-mesenchymal transition (EMT) and DM509 treatment reduced EMT. UUO mice also had tubular epithelial barrier injury with increased renal KIM-1, NGAL expression. DM509 reduced tubular injury markers by 25-50% and maintained tubular epithelial integrity in UUO mice. Vascular inflammation was evident in UUO mice with 9 to 20-fold higher ICAM and VCAM gene expression which was reduced by 40-50% with DM509 treatment. Peritubular vascular density was reduced by 35% in UUO mice and DM509 prevented vascular loss.

Discussion

Interventional treatment with DM509 reduced renal fibrosis and inflammation in UUO mice demonstrating that DM509 is a promising drug that combats renal epithelial and vascular pathological events associated with progression of CKD.

Claudin-2 Mediates the Proximal Tubular Epithelial Cell-Fibroblast Crosstalk via Paracrine CTGF

Purpose

Proximal tubular epithelial cell (PTEC) is vulnerable to injury in diabetic kidney disease (DKD) due to high energy expenditure. The injured PTECs-derived profibrotic factors are thought to be driving forces in tubulointerstitial fibrosis (TIF) as they activate surrounding fibroblasts. However, the mechanisms remain unclear.

Methods

The diabetes with uninephrectomy (DKD) rats were used to evaluated renal histological changes and the expression of Claudin-2 by immunofluorescence staining. Then, Claudin-2 expression in PTECs were modulated and subsequently determined the proliferation and activation of fibroblasts by building a transwell co-culture system in normal glucose (NG)or high glucose (HG) condition.

Results

Decreased expression of Claudin-2 in PTECs accompanied by tight junction disruption and increased interstitial fibrosis, were detected in DKD rats. In vitro, downregulated Claudin-2 in PTECs promoted proliferation and activation of fibroblasts, which coincided with elevated expression of profibrotic connective tissue growth factor (CTGF) in PTECs. Silenced CTGF inhibited the profibrotic of PTECs via Claudin-2 inhibition. Fibroblasts co-cultured with PTECs transitioned more to myofibroblasts and generated extracellular matrix (ECM) significantly in response to high glucose (HG) stimulation whereas overexpression of Claudin-2 in PTECs reversed the above results. Upregulating CTGF disrupted the beneficial anti-fibrosis effects obtained by overexpression of Claudin-2 in HG condition.

Conclusion

Our study suggested that Claudin-2 in PTECs, a key mediator of paracellular cation and water transport, promotes the activation and proliferation of surrounding fibroblasts significantly via CTGF in a paracrine manner.

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.

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

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

Angiotensin II hypertension along the female rat tubule: predicted impact on coupled transport of Na+ and K. Am J Physiol Renal Physiol 2023;325:F733-F749. [PMID: 37823196 PMCID: PMC10878725 DOI: 10.1152/ajprenal.00232.2023] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Chronic infusion of subpressor level of angiotensin II (ANG II) increases the abundance of Na+ transporters along the distal nephron, balanced by suppression of Na+ transporters along the proximal tubule and medullary thick ascending limb (defined as "proximal nephron"), which impacts K+ handling along the entire renal tubule. The objective of this study was to quantitatively assess the impact of chronic ANG II on the renal handling of Na+ and K+ in female rats, using a computational model of the female rat renal tubule. Our results indicate that the downregulation of proximal nephron Na+ reabsorption (TNa), which occurs in response to ANG II-triggered hypertension, involves changes in both transporter abundance and trafficking. Our model suggests that substantial (∼30%) downregulation of active NHE3 in proximal tubule (PT) microvilli is needed to reestablish the Na+ balance at 2 wk of ANG II infusion. The 35% decrease in SGLT2, a known NHE3 regulator, may contribute to this downregulation. Both depression of proximal nephron TNa and stimulation of distal ENaC raise urinary K+ excretion in ANG II-treated females, while K+ loss is slightly mitigated by cortical NKCC2 and NCC upregulation. Our model predicts that K+ excretion may be more significantly limited during ANG II infusion by ROMK inhibition in the distal nephron and/or KCC3 upregulation in the PT, which remain open questions for experimental validation. In summary, our analysis indicates that ANG II hypertension triggers a series of events from distal TNa stimulation followed by compensatory reduction in proximal nephron TNa and accompanying adjustments to limit excessive K+ secretion.NEW & NOTEWORTHY We used a computational model of the renal tubule to assess the impact of 2-wk angiotensin II (ANG II) infusion on the handling of Na+ and K+ in female rats. ANG II strongly stimulates distal Na+ reabsorption and K+ secretion. Simulations indicate that substantial downregulation of proximal tubule NHE3 is needed to reestablish Na+ balance at 2 wk. Proximal adaptations challenge K+ homeostasis, and regulation of distal NCC and specific K+ channels likely limit urinary K+ losses.

Claudin-19 localizes to the thick ascending limb where its expression is required for junctional claudin-16 localization

The kidney is critical for mineral homeostasis. Calcium and magnesium reabsorption in the renal thick ascending limb (TAL) involves claudin-16 (CLDN16) and claudin-19 (CLDN19) and pathogenic variants in either gene lead to familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC) with severe calcium and magnesium wasting. While both CLDN16 and CLDN19 localize to the TAL, varying expression patterns in the renal tubule have been reported using different antibodies. We, therefore, studied the localization of CLDN19 in the kidneys of wild-type and Cldn19-deleted mice using three anti-CLDN19 antibodies and examined the role of Cldn19 deletion on CLDN16 and CLDN10 localization. We find that CLDN19 localizes to basolateral membrane domains of the medullary and cortical TAL but only to the tight junction of TALs in the outer stripe of outer medulla and cortex, where it colocalizes with CLDN16. Furthermore, in TALs from Cldn19-deleted mice, CLDN16 is expressed in basolateral membrane domains but not at the tight junction. In contrast, Cldn19 ablation does not change CLDN10 localization. These findings directly implicate CLDN19 in regulating permeability in the TAL by allowing junctional insertion of CLDN16 and may explain the shared renal phenotypic characteristics in FHHNC patients.

Metabolic changes in kidney stone disease

Kidney stone disease (KSD) is one of the earliest medical diseases known, but the mechanism of its formation and metabolic changes remain unclear. The formation of kidney stones is a extensive and complicated process, which is regulated by metabolic changes in various substances. In this manuscript, we summarized the progress of research on metabolic changes in kidney stone disease and discuss the valuable role of some new potential targets. We reviewed the influence of metabolism of some common substances on stone formation, such as the regulation of oxalate, the release of reactive oxygen species (ROS), macrophage polarization, the levels of hormones, and the alternation of other substances. New insights into changes in substance metabolism changes in kidney stone disease, as well as emerging research techniques, will provide new directions in the treatment of stones. Reviewing the great progress that has been made in this field will help to improve the understanding by urologists, nephrologists, and health care providers of the metabolic changes in kidney stone disease, and contribute to explore new metabolic targets for clinical therapy.

Tight junctions and acute kidney injury

Acute kidney injury (AKI) is characterized by a rapid reduction in kidney function caused by various etiologies. Tubular epithelial cell dysregulation plays a pivotal role in the pathogenesis of AKI. Tight junction (TJ) is the major molecular structure that connects adjacent epithelial cells and is critical in maintaining barrier function and determining the permeability of epithelia. TJ proteins are dysregulated in various types of AKI, and some reno-protective drugs can reverse TJ changes caused by insult. An in-depth understanding of TJ regulation and its causality with AKI will provide more insight to the disease pathogenesis and will shed light on the potential role of TJs to serve as novel therapeutic targets in AKI.

The membrane-associated protein 17 (MAP17) is up-regulated in response to empagliflozin on top of RAS blockade in experimental diabetic nephropathy

Sodium-glucose cotransporter 2 inhibitors (SGLT2i) have proven to delay diabetic kidney disease (DKD) progression on top of the standard of care with the renin-angiotensin system (RAS) blockade. The molecular mechanisms underlying the synergistic effect of SGLT2i and RAS blockers is poorly understood. We gave a SGLT2i (empagliflozin), an angiotensin-converting enzyme inhibitor (ramipril), or a combination of both drugs for 8 weeks to diabetic (db/db) mice. Vehicle-treated db/db and db/m mice were used as controls. At the end of the experiment, mice were killed, and the kidneys were saved to perform a differential high-throughput proteomic analysis by mass spectrometry using isobaric tandem mass tags (TMT labeling) that allow relative quantification of the identified proteins. The differential proteomic analysis revealed 203 proteins differentially expressed in one or more experimental groups (false discovery rate < 0.05 and Log2 fold change ≥ ±1). Fourteen were differentially expressed in the kidneys from the db/db mice treated with empagliflozin with ramipril. Among them, MAP17 was up-regulated. These findings were subsequently validated by Western blot. The combined therapy of empagliflozin and ramipril up-regulated MAP17 in the kidney of a diabetic mice model. MAP17 is a major scaffolding protein of the proximal tubular cells that places transporters together, namely SGLT2 and NHE3. Our results suggest that SGLT2i on top of RAS blockade may protect the kidney by boosting the inactivation of NHE3 via the up-regulation of key scaffolder proteins such as MAP17.

Molecular mechanisms underlying paracellular calcium and magnesium reabsorption in the proximal tubule and thick ascending limb

Calcium and magnesium are the most abundant divalent cations in the body. The plasma level is controlled by coordinated interaction between intestinal absorption, reabsorption in the kidney, and, for calcium at least, bone storage and exchange. The kidney adjusts urinary excretion of these ions in response to alterations in their systemic concentration. Free ionized and anion-complexed calcium and magnesium are filtered at the glomerulus. The majority (i.e., >85%) of filtered divalent cations are reabsorbed via paracellular pathways from the proximal tubule and thick ascending limb (TAL) of the loop of Henle. Interestingly, the largest fraction of filtered calcium is reabsorbed from the proximal tubule (65%), while the largest fraction of filtered magnesium is reclaimed from the TAL (60%). The paracellular pathways mediating these fluxes are composed of tight junctional pores formed by claudins. In the proximal tubule, claudin-2 and claudin-12 confer calcium permeability, while the exact identity of the magnesium pore remains to be determined. Claudin-16 and claudin-19 contribute to the calcium and magnesium permeable pathway in the TAL. In this review, we discuss the data supporting these conclusions and speculate as to why there is greater fractional calcium reabsorption from the proximal tubule and greater fractional magnesium reabsorption from the TAL.

Unravelling the Complex Relationship between Diet and Nephrolithiasis: The Role of Nutrigenomics and Nutrigenetics

Nephrolithiasis is an increasingly prevalent condition, especially in high income countries, and is associated with high morbidity. Extraordinary progress in genetics made the identification of genetic forms of nephrolithiasis possible. These genetic diseases are usually rare and do not account for the most common forms of nephrolithiasis that are the result of several factors such as environment, dietary habits, and predisposing genes. This knowledge has shaped what we classify as nephrolithiasis, a condition that is now recognized as systemic. How and to what extent all these factors interact with one another and end in kidney stone formation, growth, and recurrence is not completely understood. Two new research fields have recently been trying to give some answers: nutrigenomics and nutrigenetics. These fields have the aim of understanding the intricate diet/genome interface that influences gene expression regulation mainly through epigenetic mechanisms and results in specific medical conditions such as cancer, metabolic syndrome, and cardiovascular diseases. Epigenetics seems to play a crucial role and could represent the link between environmental factors, that we are constantly exposed to, and risk factors for nephrolithiasis. In this systematic review, we summarize all the available evidence of proven or hypothesized epigenetic mechanisms related to nephrolithiasis.

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.

Role of claudins in idiopathic hypercalciuria and renal lithiasis

Paracellular transport in the kidney is mediated by a family of proteins located in the tight junctions called claudins which confers its ionic selectivity. Claudin-2 is highly expressed in the proximal tubule and descending limb of Henle and mediate paracellular reabsorption of sodium and calcium cations. In the thick ascending limb of Henle (TALH) calcium is reabsorbed by a paracellular channel formed by Claudin-16 and-19. Claudin-16 mediates cationic permeability while Claudin-19 increases the cationic selectivity of Claudin-16 by blocking anionic permeability. On the other hand, Claudin 14, that is also located in TALH, inhibits the paracellular permeability of Claudin-16 to calcium. Recent wide genomic association analysis studies have detected four common synonymous variants (genetic polymorphisms of a single nucleotide, SNPs) at the locus of Claudin-14 gene that were significantly associated with the presence of renal lithiasis. Another study of wide genomic association and nephrolithiasis was carried out in the general population but including chromosome X, where claudin-2 gene is located. They detected nine SNPs that had a significant association with renal lithiasis risk. A greater knowledge of the paracellular pathway controlled by claudins and its regulation will allow us to develop future new treatments for idiopathic hypercalciuria and renal lithiasis.

Physiology of Calcium Homeostasis: An Overview

Calcium plays a key role in skeletal mineralization and several intracellular and extracellular homeostatic networks. It is an essential element that is only available to the body through dietary sources. Daily acquisition of calcium depends, in addition to the actual intake, on the hormonally regulated state of calcium homeostasis through three main mechanisms: bone turnover, intestinal absorption, and renal reabsorption. These procedures are regulated by a group of interacting circulating hormones and their key receptors. This includes parathyroid hormone (PTH), PTH-related peptide, 1,25-dihydroxyvitamin D, calcitonin, fibroblast growth factor 23, the prevailing calcium concentration itself, the calcium-sensing receptor, as well as local processes in the bones, gut, and kidneys.

Claudin-1-Targeted Nanoparticles for Delivery to Aging-Induced Alterations in the Blood-Brain Barrier

Aging-induced alterations to the blood-brain barrier (BBB) are increasingly being seen as a primary event in chronic progressive neurological disorders that lead to cognitive decline. With the goal of increasing delivery into the brain in hopes of effectively treating these diseases, a large focus has been placed on developing BBB permeable materials. However, these strategies have suffered from a lack of specificity toward regions of disease progression. Here, we report on the development of a nanoparticle (C1C2-NP) that targets regions of increased claudin-1 expression that reduces BBB integrity. Using dynamic contrast enhanced magnetic resonance imaging, we find that C1C2-NP accumulation and retention is significantly increased in brains from 12 month-old mice as compared to nontargeted NPs and brains from 2 month-old mice. Furthermore, we find C1C2-NP accumulation in brain endothelial cells with high claudin-1 expression, suggesting target-specific binding of the NPs, which was validated through fluorescence imaging, in vitro testing, and biophysical analyses. Our results further suggest a role of claudin-1 in reducing BBB integrity during aging and show altered expression of claudin-1 can be actively targeted with NPs. These findings could help develop strategies for longitudinal monitoring of tight junction protein expression changes during aging as well as be used as a delivery strategy for site-specific delivery of therapeutics at these early stages of disease development.

Expression of claudin-8 is induced by aldosterone in renal collecting duct principal cells

Fine tuning of Na⁺ reabsorption takes place along the aldosterone-sensitive distal nephron, which includes the collecting duct (CD), where it is mainly regulated by aldosterone. In the CD, Na⁺ reabsorption is mediated by the epithelial Na⁺ channel and Na⁺ pump (Na⁺-K⁺-ATPase). Paracellular ion permeability is mainly dependent on tight junction permeability. Claudin-8 is one of the main tight junction proteins expressed along the aldosterone-sensitive distal nephron. We have previously shown a coupling between transcellular Na⁺ reabsorption and paracellular Na⁺ barrier. We hypothesized that aldosterone controls the expression levels of both transcellular Na⁺ transporters and paracellular claudin-8 in a coordinated manner. Here, we show that aldosterone increased mRNA and protein levels as well as lateral membrane localization of claudin-8 in cultured CD principal cells. The increase in claudin-8 mRNA levels in response to aldosterone was prevented by preincubation with 17-hydroxyprogesterone, a mineralocorticoid receptor antagonist, and by inhibition of transcription with actinomycin D. We also showed that a low-salt diet, which stimulated aldosterone secretion, was associated with increased claudin-8 abundance in the mouse kidney. Reciprocally, mice subjected to a high-salt diet, which inhibits aldosterone secretion, or treated with spironolactone, a mineralocorticoid receptor antagonist, displayed decreased claudin-8 expression. Inhibition of glycogen synthase kinase-3, Lyn, and Abl signaling pathways prevented the effect of aldosterone on claudin-8 mRNA and protein abundance, suggesting that signaling of protein kinases plays a permissive role on the transcriptional activity of the mineralocorticoid receptor. This study shows that signaling via multiple protein kinases working in concert mediates aldosterone-induced claudin-8 expression in the CD.NEW & NOTEWORTHY In this study, we showed that aldosterone modulates claudin-8 expression in cultured collecting duct principal cells and in the mouse kidney. The upregulation of claudin-8 expression in response to aldosterone is dependent on at least glycogen synthase kinase-3, Lyn, and Abl signaling pathways, indicating the participation of multiple protein kinases to the effect of aldosterone.

Sex-specific adaptations to high-salt diet preserve electrolyte homeostasis with distinct sodium transporter profiles

Kidneys continuously filter an enormous amount of sodium and adapt kidney Na+ reabsorption to match Na+ intake to maintain circulatory volume and electrolyte homeostasis. Males (M) respond to high-salt (HS) diet by translocating proximal tubule Na+/H+ exchanger isoform 3 (NHE3) to the base of the microvilli, reducing activated forms of the distal NaCl cotransporter (NCC) and epithelial Na+ channel (ENaC). Males (M) and females (F) on normal-salt (NS) diet present sex-specific profiles of "transporters" (cotransporters, channels, pumps, and claudins) along the nephron, e.g., F exhibit 40% lower NHE3 and 200% higher NCC abundance than M. We tested the hypothesis that adaptations to HS diet along the nephron will, likewise, exhibit sexual dimorphisms. C57BL/6J mice were fed for 15 days with 4% NaCl diet (HS) versus 0.26% NaCl diet (NS). On HS, M and F exhibited normal plasma [Na+] and [K+], similar urine volume, Na+, K+, and osmolal excretion rates normalized to body weight. In F, like M, HS lowered abundance of distal NCC, phosphorylated NCC, and cleaved (activated) forms of ENaC. The adaptations associated with achieving electrolyte homeostasis exhibit sex-dependent and independent mechanisms. Sex differences in baseline "transporters" abundance persist during HS diet, yet the fold changes during HS diet (normalized to NS) are similar along the distal nephron and collecting duct. Sex-dependent differences observed along the proximal tubule during HS show that female kidneys adapt differently from patterns reported in males, yet achieve and maintain fluid and electrolyte homeostasis.

Claudins as biomarkers of differential diagnosis and prognosis of tumors

Claudins are a superfamily of transmembrane proteins, the optimal expression and localization of which are important for the normal physiological function of the epithelium and any imbalance may have pathological consequences. Not only insufficient but also excessive production of claudins in cancer cells, as well as their aberrant localization, equally manifest the formation of a malignant phenotype. Many works are distinguished by contradictory data, which demonstrate the action of the same claudins both in the role of tumor-growth suppressors and promoters in the same cancers. The most important possible causes of significant discrepancies in the results of the works are a considerable variability of sampling and the absence of a consistent approach both to the assessment of the immune reactivity of claudins and to the differential analysis of their subcellular localization. Combined, these drawbacks hinder the histological assessment of the link between claudins and tumor progression. In particular, ambiguous expression of claudins in breast cancer subtypes, revealed by various authors in immunohistochemical analysis, not only fails to facilitate the identification of the claudin-low molecular subtype but rather complicates these efforts. Research into the role of claudins in carcinogenesis has undoubtedly confirmed the potential value of this class of proteins as significant biomarkers in some cancer types; however, the immunohistochemical approach to the assessment of claudins still has limitations, needs standardization, and, to date, has not reached a diagnostic or a prognostic value.

Differential localization patterns of claudin 10, 16, and 19 in human, mouse, and rat renal tubular epithelia

Functional properties of the paracellular pathway depend critically on the set of claudins (CLDN) expressed at the tight junction. Two syndromes are causally linked to loss-of-function mutations of claudins: hypohidrosis, electrolyte imbalance, lacrimal gland dysfunction, ichthyosis, and xerostomia (HELIX) syndrome caused by genetic variations in the CLDN10 gene and familial hypomagnesemia with hypercalciuria and nephrocalcinosis caused by genetic variations in the CLDN16 or CLDN19 genes. All three genes are expressed in the kidney, particularly in the thick ascending limb (TAL). However, localization of these claudins in humans and rodents remains to be delineated in detail. We studied the segmental and subcellular expression of CLDN10, CLDN16, and CLDN19 in both paraffin-embedded and frozen kidney sections from the adult human, mouse, and rat using immunohistochemistry and immunofluorescence, respectively. Here, CLDN10 was present in a subset of medullary and cortical TAL cells, localizing to basolateral domains and tight junctions in human and rodent kidneys. Weak expression was detected at the tight junction of proximal tubular cells. CLDN16 was primarily expressed in a subset of TAL cells in the cortex and outer stripe of outer medulla, restricted to basolateral domains and tight junctional structures in both human and rodent kidneys. CLDN19 predominantly colocalized with CLDN16 in tight junctions and basolateral domains of the TAL but was also found in basolateral and junctional domains in more distal sites. CLDN10 expression at tight junctions almost never overlapped with that of CLND16 and CLDN19, consistent with distinct junctional pathways with different permeation profiles in both human and rodent kidneys.NEW & NOTEWORTHY This study used immunohistochemistry and immunofluorescence to investigate the distribution of claudin 10, 16, and 19 in the human, mouse, and rat kidney. The findings showed distinct junctional pathways in both human and rodent kidneys, supporting the existence of different permeation profiles in all species investigated.

[Role of distal nephron in the control of extracellular volume in physiology and in nephrotic syndrome]

The kidney plays a major role to maintain the constancy of the "milieu intérieur" by adjusting the urinary excretion of water and solutes to the requirement of the body balance. This function is coordinated with elimination of waste products generated among others by the catabolism of proteins and nucleic acids. To cope with these two major functions, the human kidneys generate each day about 180 L of ultrafiltrate from plasma and reabsorbs the vast majority of filtered water and solutes to excrete daily about one-two liter(s) of urine containing concentrations of sodium, potassium and chloride ranging from 20 to 200 mM. The final adjustment of urine composition is finely tuned along the aldosterone-sensitive distal nephron (ASDN) which includes the distal convoluted tubule and the collecting system (connecting tubule and collecting duct). Sodium reabsorption is predominant along the distal tubule if potassium must be spared, or along the collecting system when large amounts of potassium must be secreted. Nephrotic syndrome is characterized by heavy proteinuria consecutive to a glomerular injury, associated with renal sodium and water retention taking initially place along ASDN and leading to edema.

Crosstalk between Renal and Vascular Calcium Signaling: The Link between Nephrolithiasis and Vascular Calcification

Calcium (Ca²⁺) is an important mediator of multicellular homeostasis and is involved in several diseases. The interplay among the kidney, bone, intestine, and parathyroid gland in Ca²⁺ homeostasis is strictly modulated by numerous hormones and signaling pathways. The calcium-sensing receptor (CaSR) is a G protein–coupled receptor, that is expressed in calcitropic tissues such as the parathyroid gland and the kidney, plays a pivotal role in Ca²⁺ regulation. CaSR is important for renal Ca²⁺, as a mutation in this receptor leads to hypercalciuria and calcium nephrolithiasis. In addition, CaSR is also widely expressed in the vascular system, including vascular endothelial cells (VECs) and vascular smooth muscle cells (VSMCs) and participates in the process of vascular calcification. Aberrant Ca²⁺ sensing by the kidney and VSMCs, owing to altered CaSR expression or function, is associated with the formation of nephrolithiasis and vascular calcification. Based on emerging epidemiological evidence, patients with nephrolithiasis have a higher risk of vascular calcification, but the exact mechanism linking the two conditions is unclear. However, a dysregulation in Ca²⁺ homeostasis and dysfunction in CaSR might be the connection between the two. This review summarizes renal calcium handling and calcium signaling in the vascular system, with a special focus on the link between nephrolithiasis and vascular calcification.

[Immunity and tubular dysfunction in case of systemic disease]

The immune renal tubular diseases are known since five decades, but their prevalence remains to be defined. They are caused by humoral and cellular effectors of innate and adaptative immunities on several targets of the renal tubule: protein channels, co or counter transporters, luminal or cytosolic enzymes, tight junctions. Genetic or epigenetic variations are also involved. Clinical manifestations are various and make the diagnosis difficult. They can precede the causal affection and they worsen the prognosis. The classical model consists in hypokalemic tubular distal acidosis observed in Sjögren's syndrome which illustrates the auto-immune epithelitis concept. Cellular immunity can act through other ways, like tertiary lymphoid neogenesis in systemic lupus. Humoral immunity through autoantibodies targets several membrane, cytosolic or nuclear proteins, causing specific tubular dysfonctions. It is also implied in the epithelial-mesenchymal transition of tubular cells. Innate immunity through cytokines may be involved. Treatment consists in electrolytic disorders correction and immunosupppressive medication: the choice should be guided at best by physiopathology.

Hyperglycemic levels in early stage of diabetic nephropathy affect differentially renal expression of claudins-2 and -5 by oxidative stress

AIMS

This study attempts to elicit whether the level of hyperglycemia in an early stage of diabetic nephropathy changes the renal expression of claudins-2 and -5 and to determine the involvement of glucose-induced oxidative stress.

MAIN METHODS

Streptozotocin-induced type-1 and type-2 diabetic (DM1, DM2)-rat models were used. At 14-week old, the rats were placed in metabolic cages to evaluate proteinuria, creatinine clearance, and electrolyte excretion. Proximal tubules and glomeruli were isolated and analyzed by Western blot and immunofluorescence. Renal oxidative stress and metalloproteinase activities were evaluated.

KEY FINDINGS

We found that claudin-5 expression in glomeruli and claudin-2 expression in proximal tubules were significantly reduced in DM1 versus DM2 model, paralleling with higher proteinuria and loss of sodium and potassium reabsorption, increased malondialdehyde levels, but lower antioxidant capacity in both models. Enzymatic activity of MMP-2 and-9 was increased in both diabetic groups versus control being higher in DM1 than DM2, suggesting higher claudin's degradation.

SIGNIFICANCE

The level of hyperglycemia determines the time-dependent progression to diabetic nephropathy; hyperglycemia-induced oxidative stress parallels an increase in metalloproteinases (MMPs) activities consequently affecting the integrity of claudin-2 and -5 in glomerulus and proximal tubule. Our results suggest that chronic high-glycemia levels in early stages of diabetic nephropathy decrease expression of claudins-2 and -5, increase oxidative stress, and induce MMP-activity faster than chronic middle-glycemia levels.

Integrative Omics Analyses Reveal Epigenetic Memory in Diabetic Renal Cells Regulating Genes Associated With Kidney Dysfunction

Diabetic kidney disease (DKD) is a major complication of diabetes and the leading cause of end-stage renal failure. Epigenetics has been associated with metabolic memory in which prior periods of hyperglycemia enhance the future risk of developing DKD despite subsequent glycemic control. To understand the mechanistic role of such epigenetic memory in human DKD and to identify new therapeutic targets, we profiled gene expression, DNA methylation, and chromatin accessibility in kidney proximal tubule epithelial cells (PTECs) derived from subjects with and without type 2 diabetes (T2D). T2D-PTECs displayed persistent gene expression and epigenetic changes with and without transforming growth factor-β1 treatment, even after culturing in vitro under similar conditions as nondiabetic PTECs, signified by deregulation of fibrotic and transport-associated genes (TAGs). Motif analysis of differential DNA methylation and chromatin accessibility regions associated with genes differentially regulated in T2D revealed enrichment for SMAD3, HNF4A, and CTCF transcription factor binding sites. Furthermore, the downregulation of several TAGs in T2D (including CLDN10, CLDN14, CLDN16, SLC16A2, and SLC16A5) was associated with promoter hypermethylation, decreased chromatin accessibility, and reduced enrichment of HNF4A, histone H3-lysine-27-acetylation, and CTCF. Together, these integrative analyses reveal epigenetic memory underlying the deregulation of key target genes in T2D-PTECs that may contribute to sustained renal dysfunction in DKD.

Clinical and Molecular Perspectives of Monogenic Hypertension

Advances in molecular research techniques have enabled a new frontier in discerning the mechanisms responsible for monogenic diseases. In this review, we discuss the current research on the molecular pathways governing blood pressure disorders with a Mendelian inheritance pattern, each presenting with a unique pathophysiology. Glucocorticoid Remediable Aldosteronism (GRA) and Apparent Mineralocorticoid Excess (AME) are caused by mutations in regulatory enzymes that induce increased production of mineralocorticoids or inhibit degradation of glucocorticoids, respectively. Geller syndrome is due to a point mutation in the hormone responsive element of the promotor for the mineralocorticoid receptor, rendering the receptor susceptible to activation by progesterone, leading to hypertension during pregnancy. Pseudohypoaldosteronism type II (PHA-II), also known as Gordon's syndrome or familial hyperkalemic hypertension, is a more variable disorder typically characterized by hypertension, high plasma potassium and metabolic acidosis. Mutations in a variety of intracellular enzymes that lead to enhanced sodium reabsorption have been identified. In contrast, hypertension in Liddle's syndrome, which results from mutations in the Epithelial sodium Channel (ENaC), is associated with low plasma potassium and metabolic alkalosis. In Liddle's syndrome, truncation of one the ENaC protein subunits removes a binding site necessary protein for ubiquitination and degradation, thereby promoting accumulation along the apical membrane and enhanced sodium reabsorption. The myriad effects due to mutation in phosphodiesterase 3A (PDE3A) lead to severe hypertension underlying sodium-independent autosomal dominant hypertension with brachydactyly. How mutations in PDE3A result in the phenotypic features of this disorder are discussed. Understanding the pathologies of these monogenic hypertensive disorders may provide insight into the causes of the more prevalent essential hypertension and new avenues to unravel the complexities of blood pressure regulation.

MMP7 damages the integrity of the renal tubule epithelium by activating MMP2/9 during ischemia-reperfusion injury

Renal ischemia-reperfusion (IR) injury is a common issue in urological surgery, and the renal tubules, particularly the proximal tubules, are extremely vulnerable to IR injury. In this work, we detected the differently expressed genes (DEGs) between normal rabbit kidneys and IR kidneys by RNA-sequencing, then identified that matrix metalloproteinase-7 (MMP7) played an important role in the progress of IR injury. Indeed, A time-dependent promotion of renal injury was detected in rabbit model, as demonstrated by the increased levels of MMP2/7/9, and the decreased of tight junction protein-1 (TJP1). Furtherly, similar results were confirmed in human renal proximal tubule epithelial (HK-2) cells model. Notably, downregulation of MMP7 affected the activity of MMP2/9 by suppressing expression of cleaved-MMP2/9 not the pro-MMP2/9 protein, which directly alleviated the degradation of TJP1 in HK-2 model. On the contrary, MMP7 had not been affected by inhibiting MMP2/9. In addition, coimmunoprecipitation assay showed that knockdown MMP7 restrained the interaction between MMP2/9 and TJP1. Collectively, this study suggested that MMP7 could serve as early biomarkers for renal tubular injury, and revealed that MMP7 could destroy the integrity of tubular epithelium through degrading TJP1 by activating MMP2/9.

Electrophoretic measurement of water charge density and ion hydration

Water exchange between bulk water and water-ion complexes will be at equilibrium when the charge density of the complex surface equals the charge density of bulk water, producing a constant radius water-ion complex. This complex will migrate in an electric field at a velocity proportional to the complex radius. CE velocity is the sum of the complex charge-dependent velocity and the buffer electro-osmotic flow. Simultaneous use of both a base (1.07 mM imidazole) and an acid (1.5 mM MOPS) buffer negates EOF at pH 7.4. Electric fields below 300 V/cm (potassium, calcium) and 400 V/cm (magnesium) yield migration velocities with no dehydration of the water-ion complexes. The number of waters per complex increase with the ion charge density: K⁺ 1.90, Ca⁺⁺ 5.90, Mg⁺⁺ 6.59 waters/ion. The charge densities of the complexes are similar: K⁺ 1.24, Ca⁺⁺ 1.43, Mg⁺⁺ 1.21 e/nm² , for an average bulk water charge density of 1.29 ± 0.11 (SD) e/nm² . The addition of 0.1% Triton increases the number of waters for Mg⁺⁺ to 25.33 and lowers the charge density to 0.497 e/nm² . High electric field dehydration shows that calcium will be fully dehydrated at 638.3 V/cm and magnesium fully dehydrated at 925.5 V/cm, which occur at 6.15 and 5.78 nm from the membrane. Dehydrated magnesium will then bind to calcium channels leading to decreased smooth muscle activation.

Interaction between Epithelial Sodium Channel γ-Subunit and Claudin-8 Modulates Paracellular Sodium Permeability in Renal Collecting Duct

BACKGROUND

Water and solute transport across epithelia can occur via the transcellular or paracellular pathways. Tight junctions play a key role in mediating paracellular ion reabsorption in the kidney. In the renal collecting duct, which is a typical absorptive tight epithelium, coordination between transcellular sodium reabsorption and paracellular permeability may prevent the backflow of reabsorbed sodium to the tubular lumen along a steep electrochemical gradient.

METHODS

To investigate whether transcellular sodium transport controls tight-junction composition and paracellular permeability via modulating expression of the transmembrane protein claudin-8, we used cultured mouse cortical collecting duct cells to see how overexpression or silencing of epithelial sodium channel (ENaC) subunits and claudin-8 affect paracellular permeability. We also used conditional kidney tubule-specific knockout mice lacking ENaC subunits to assess the ENaC's effect on claudin-8 expression.

RESULTS

Overexpression or silencing of the ENaC γ-subunit was associated with parallel and specific changes in claudin-8 abundance. Increased claudin-8 abundance was associated with a reduction in paracellular permeability to sodium, whereas decreased claudin-8 abundance was associated with the opposite effect. Claudin-8 overexpression and silencing reproduced these functional effects on paracellular ion permeability. Conditional kidney tubule-specific ENaC γ-subunit knockout mice displayed decreased claudin-8 expression, confirming the cell culture experiments' findings. Importantly, ENaC β-subunit or α-subunit silencing or kidney tubule-specific β-ENaC or α-ENaC knockout mice did not alter claudin-8 abundance.

CONCLUSIONS

Our data reveal the specific coupling between ENaC γ-subunit and claudin-8 expression. This coupling may play an important role in preventing the backflow of reabsorbed solutes and water to the tubular lumen, as well as in coupling paracellular and transcellular sodium permeability.

Claudin-12 Knockout Mice Demonstrate Reduced Proximal Tubule Calcium Permeability

The renal proximal tubule (PT) is responsible for the reabsorption of approximately 65% of filtered calcium, primarily via a paracellular pathway. However, which protein(s) contribute this paracellular calcium pore is not known. The claudin family of tight junction proteins confers permeability properties to an epithelium. Claudin-12 is expressed in the kidney and when overexpressed in cell culture contributes paracellular calcium permeability (P_Ca). We therefore examined claudin-12 renal localization and its contribution to tubular paracellular calcium permeability. Claudin-12 null mice (KO) were generated by replacing the single coding exon with β-galactosidase from Escherichia coli. X-gal staining revealed that claudin-12 promoter activity colocalized with aquaporin-1, consistent with the expression in the PT. PTs were microperfused ex vivo and P_Ca was measured. P_Ca in PTs from KO mice was significantly reduced compared with WT mice. However, urinary calcium excretion was not different between genotypes, including those on different calcium containing diets. To assess downstream compensation, we examined renal mRNA expression. Claudin-14 expression, a blocker of P_Ca in the thick ascending limb (TAL), was reduced in the kidney of KO animals. Thus, claudin-12 is expressed in the PT, where it confers paracellular calcium permeability. In the absence of claudin-12, reduced claudin-14 expression in the TAL may compensate for reduced PT calcium reabsorption.

Recent Insights Into SREBP as a Direct Mediator of Kidney Fibrosis via Lipid-Independent Pathways

Sterol regulatory-element binding proteins (SREBPs) are classical regulators of cellular lipid metabolism in the kidney and other tissues. SREBPs are currently recognized as versatile transcription factors involved in a myriad of cellular processes. Meanwhile, SREBPs have been recognized to mediate lipotoxicity, contributing to the progression of kidney diseases. SREBP1 has been shown to bind to the promoter region of TGFβ, a major pro-fibrotic signaling mechanism in the kidney. Conversely, TGFβ activates SREBP1 transcriptional activity suggesting a positive feedback loop of SREBP1 in TGFβ signaling. Public ChIP-seq data revealed numerous non-lipid transcriptional targets of SREBPs that plausibly play roles in progressive kidney disease and fibrosis. This review provides new insights into SREBP as a mediator of kidney fibrosis via lipid-independent pathways.

Claudins in Renal Physiology and Pathology

Claudins are integral proteins expressed at the tight junctions of epithelial and endothelial cells. In the mammalian kidney, every tubular segment express a specific set of claudins that give to that segment unique properties regarding permeability and selectivity of the paracellular pathway. So far, 3 claudins (10b, 16 and 19) have been causally traced to rare human syndromes: variants of CLDN10b cause HELIX syndrome and variants of CLDN16 or CLDN19 cause familial hypomagnesemia with hypercalciuria and nephrocalcinosis. The review summarizes our current knowledge on the physiology of mammalian tight junctions and paracellular ion transport, as well as on the role of the 3 above-mentioned claudins in health and disease. Claudin 14, although not having been causally linked to any rare renal disease, is also considered, because available evidence suggests that it may interact with claudin 16. Some single-nucleotide polymorphisms of CLDN14 are associated with urinary calcium excretion and/or kidney stones. For each claudin considered, the pattern of expression, the function and the human syndrome caused by pathogenic variants are described.

Cell junction proteins: Crossing the glomerular filtration barrier in diabetic nephropathy

Diabetic nephropathy as a deleterious complication of diabetes mellitus and an important cause of end-stage renal failure is characterized by changes in the molecular and cellular levels. Cell-cell communication via the gap and tight junctions are involved in the pathogenesis of diseases such as diabetes and kidney failure. Studying cell junctions including gap junctions, tight junctions, and anchoring junctions within the nephron can be used as an early sign of diabetic nephropathy. Furthermore, cell junctions may be an upcoming target by pharmacological methods to improve treatments of diabetic nephropathy and pave the way to introduce promising therapeutic strategies based on cell-cell communications effects and its translation into clinical studies for the treatment of diabetic nephropathy.

Claudins in the Renal Collecting Duct

The renal collecting duct fine-tunes urinary composition, and thereby, coordinates key physiological processes, such as volume/blood pressure regulation, electrolyte-free water reabsorption, and acid-base homeostasis. The collecting duct epithelium is comprised of a tight epithelial barrier resulting in a strict separation of intraluminal urine and the interstitium. Tight junctions are key players in enforcing this barrier and in regulating paracellular transport of solutes across the epithelium. The features of tight junctions across different epithelia are strongly determined by their molecular composition. Claudins are particularly important structural components of tight junctions because they confer barrier and transport properties. In the collecting duct, a specific set of claudins (Cldn-3, Cldn-4, Cldn-7, Cldn-8) is expressed, and each of these claudins has been implicated in mediating aspects of the specific properties of its tight junction. The functional disruption of individual claudins or of the overall barrier function results in defects of blood pressure and water homeostasis. In this concise review, we provide an overview of the current knowledge on the role of the collecting duct epithelial barrier and of claudins in collecting duct function and pathophysiology.

Temporal Effects of Quercetin on Tight Junction Barrier Properties and Claudin Expression and Localization in MDCK II Cells

: Kidney stones affect 10% of the population. Yet, there is relatively little known about how they form or how to prevent and treat them. The claudin family of tight junction proteins has been linked to the formation of kidney stones. The flavonoid quercetin has been shown to prevent kidney stone formation and to modify claudin expression in different models. Here we investigate the effect of quercetin on claudin expression and localization in MDCK II cells, a cation-selective cell line, derived from the proximal tubule. For this study, we focused our analyses on claudin family members that confer different tight junction properties: barrier-sealing (Cldn1, -3, and -7), cation-selective (Cldn2) or anion-selective (Cldn4). Our data revealed that quercetin's effects on the expression and localization of different claudins over time corresponded with changes in transepithelial resistance, which was measured continuously throughout the treatment. In addition, these effects appear to be independent of PI3K/AKT signaling, one of the pathways that is known to act downstream of quercetin. In conclusion, our data suggest that quercetin's effects on claudins result in a tighter epithelial barrier, which may reduce the reabsorption of sodium, calcium and water, thereby preventing the formation of a kidney stone.

Ectopic expression of CLDN2 in podocytes is associated with childhood onset nephrotic syndrome

BACKGROUND

Animal models of nephrotic syndrome (NS) revealed that tight junction (TJ)-like structures are generated together with a concomitant decrease in slit diaphragms (SDs). Claudins (CLDNs) are capable of forming TJ strands and thereby the backbone of TJs. We showed the ectopic expression of CLDN2 in podocytes in pediatric NS, and detected its localization.

METHODS

Renal frozen specimens were obtained by biopsy from 49 pediatric patients: 21 subjects with MCD, 18 with FSGS, and 10 with IgA nephritis (IgA-N). CLDN2 expression was observed by immunohistochemistry and the CLDN2-positive area was calculated. Moreover, its localization was detected using immunoelectron microscopy.

RESULTS

CLDN2 is ectopically detected in cases with MCD and FSGS before remission. The CLDN2-stained region in MCD and FSGS glomeruli before remission was significantly greater than that after remission as well as in IgA-N patients. Immunoelectron microscopy revealed that CLDN2 was concentrated along newly formed TJs in podocytes.

CONCLUSION

The same pathological findings in terms of ectopic CLDN2 expression in podocytes were shown in cases with MCD and FSGS before remission. Immunofluorescence and immunoelectron studies of CLDN2 appear to afford a powerful tool for the diagnosis of primary NS. In addition, CLDN2 expression level may be related to disease status.

Spatiotemporal immune zonation of the human kidney

Tissue-resident immune cells are important for organ homeostasis and defense. The epithelium may contribute to these functions directly or by cross-talk with immune cells. We used single-cell RNA sequencing to resolve the spatiotemporal immune topology of the human kidney. We reveal anatomically defined expression patterns of immune genes within the epithelial compartment, with antimicrobial peptide transcripts evident in pelvic epithelium in the mature, but not fetal, kidney. A network of tissue-resident myeloid and lymphoid immune cells was evident in both fetal and mature kidney, with postnatal acquisition of transcriptional programs that promote infection-defense capabilities. Epithelial-immune cross-talk orchestrated localization of antibacterial macrophages and neutrophils to the regions of the kidney most susceptible to infection. Overall, our study provides a global overview of how the immune landscape of the human kidney is zonated to counter the dominant immunological challenge.

Claudin 19 Is Regulated by Extracellular Osmolality in Rat Kidney Inner Medullary Collecting Duct Cells

The inner medullary collecting duct (IMCD) is subject to severe changes in ambient osmolality and must either allow water transport or be able to seal the lumen against a very high osmotic pressure. We postulate that the tight junction protein claudin-19 is expressed in IMCD and that it takes part in epithelial adaptation to changing osmolality at different functional states. Presence of claudin-19 in rat IMCD was investigated by Western blotting and immunofluorescence. Primary cell culture of rat IMCD cells on permeable filter supports was performed under different osmotic culture conditions and after stimulation by antidiuretic hormone (AVP). Electrogenic transepithelial transport properties were measured in Ussing chambers. IMCD cells cultivated at 300 mosm/kg showed high transepithelial resistance, a cation selective paracellular pathway and claudin-19 was mainly located in the tight junction. Treatment by AVP increased cation selectivity but did not alter transepithelial resistance or claudin-19 subcellular localization. In contrast, IMCD cells cultivated at 900 mosm/kg had low transepithelial resistance, anion selectivity, and claudin-19 was relocated from the tight junctions to intracellular vesicles. The data shows osmolality-dependent transformation of IMCD epithelium from tight and sodium-transporting to leaky, with claudin-19 expression in the tight junction associated to tightness and cation selectivity under low osmolality.

Dietary reference values for sodium

Following a request from the European Commission, the EFSA Panel on Nutrition, Novel Foods and Food Allergens (NDA) derived dietary reference values (DRVs) for sodium. Evidence from balance studies on sodium and on the relationship between sodium intake and health outcomes, in particular cardiovascular disease (CVD)-related endpoints and bone health, was reviewed. The data were not sufficient to enable an average requirement (AR) or population reference intake (PRI) to be derived. However, by integrating the available evidence and associated uncertainties, the Panel considers that a sodium intake of 2.0 g/day represents a level of sodium for which there is sufficient confidence in a reduced risk of CVD in the general adult population. In addition, a sodium intake of 2.0 g/day is likely to allow most of the general adult population to maintain sodium balance. Therefore, the Panel considers that 2.0 g sodium/day is a safe and adequate intake for the general EU population of adults. The same value applies to pregnant and lactating women. Sodium intakes that are considered safe and adequate for children are extrapolated from the value for adults, adjusting for their respective energy requirement and including a growth factor, and are as follows: 1.1 g/day for children aged 1-3 years, 1.3 g/day for children aged 4-6 years, 1.7 g/day for children aged 7-10 years and 2.0 g/day for children aged 11-17 years, respectively. For infants aged 7-11 months, an Adequate Intake (AI) of 0.2 g/day is proposed based on upwards extrapolation of the estimated sodium intake in exclusively breast-fed infants aged 0-6 months.

Disease-associated mutations of claudin-19 disrupt retinal neurogenesis and visual function

Mutations of claudin-19 cause Familial Hypomagnesaemia and Hypercalciuria, Nephrocalcinosis with Ocular Involvement. To study the ocular disease without the complications of the kidney disease, naturally occurring point mutations of human CLDN19 were recreated in human induced pluripotent cells or overexpressed in the retinae of newborn mice. In human induced pluripotent cells, we show that the mutation affects retinal neurogenesis and maturation of retinal pigment epithelium (RPE). In mice, the mutations diminish the P1 wave of the electroretinogram, activate apoptosis in the outer nuclear layer, and alter the morphology of bipolar cells. If mice are given 9-cis-retinal to counter the loss of retinal isomerase, the P1 wave is partially restored. The ARPE19 cell line fails to express claudin-19. Exogenous expression of wild type, but not mutant claudin-19, increases the expression of RPE signature genes. Mutated claudin-19 affects multiple stages of RPE and retinal differentiation through its effects on multiple functions of the RPE.

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.

Urate Transport via Paracellular Route across Epithelial Cells

Urate is the final oxidation product of purine metabolism in humans. We have recently reported that the paracellular route is the major urate transport pathway across the blood-placental barrier. In this study, the mechanism of urate paracellular transport was investigated in several epithelial cell lines including Madin-Darby canine kidney (MDCK) type I, Lilly Laboratories cell-porcine kidney 1 (LLC-PK1) and Caco-2 cells. Very little urate passed through MDCK and LLC-PK1 cell layers. In contrast, one of the Caco-2 cell lines was found to be urate-permeable. This urate paracellular movement across Caco-2 cell layer was not inhibited by the urate transporter inhibitor benzbromarone but was partially inhibited by 4,4'-diisothiocyanato-2,2'-stilbenedisulfonic acid (DIDS), which inhibits chloride transport. Detection and quantification of claudin proteins that are important for paracellular transport of ions were performed by LC/MS. Claudins 1, 3, 4, 6, 7 and 12 were detected in urate-permeable cell lines, BeWo cells and Caco-2 cells. We compared claudin expression patterns in urate-permeable and urate-non-permeable Caco-2 cells by LC/MS and found that claudin 12 had a higher expression level in urate-permeable Caco-2 cells. Overexpression of these claudins in MDCK cells did not increase urate paracellular transport. Although there were differences in claudin expression pattern between urate-permeable and non-permeable cells, increased expression of single claudin alone did not explain paracellular permeability of urate.