Claudins and renal salt transport

Tight junctions (TJs) are the most apical component of junctional complexes and regulate the movement of electrolytes and solutes by the paracellular pathway across epithelia. The defining ultrastructural features of TJs are strands of transmembrane protein particles that adhere to similar strands on adjacent cells. These strands are mainly composed of linearly polymerized integral membrane proteins called claudins. Claudins comprise a multigene family consisting of more than 20 members in mammals. Recent work has shown that claudins form barriers, determined by the paracellular electrical resistance and charge selectivity, and pores in the TJ strands. The paracellular pathways in renal tubular epithelia such as the proximal tubule, which reabsorbs the largest fraction of filtered NaCl and water, are important routes for the transport of electrolytes and water. Their transport characteristics vary among different nephron segments. Multiple claudins are expressed at TJs of individual nephron segments in a nephron segment-specific manner. Among them, claudin-2 is highly expressed at TJs of proximal tubules, which are leaky epithelia. Overexpression and knockdown of claudin-2 in epithelial cell lines, and knockout of the claudin-2 gene in mice, have demonstrated that claudin-2 forms high-conductance cation-selective pores in the proximal tubule. Here, we review the renal physiology of paracellular transport and the physiological roles of claudins in kidney function, especially claudin-2 and proximal tubule paracellular NaCl transport.     

Familial hypomagnesemia with hypercalciuria and nephrocalcinosis: report of three Turkish siblings

Familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC), an autosomal recessive renal tubular disorder is characterized by the impaired tubular reabsorption of magnesium and calcium in the thick ascending limb of the loop of Henle. This disease is caused by mutations in the claudin-16 gene (CLDN16), which encodes the tight junction protein, claudin-16. Claudin-16 belongs to the claudin family and regulates the paracellular transport of magnesium and calcium. Here, we report on three Turkish siblings with typical clinical features of FHHNC in association with the homozygous mutation Leu151Phe.     

Claudins and the Kidney

Claudins are tight-junction membrane proteins that function as both pores and barriers in the paracellular pathway in epithelial cells. In the kidney, claudins determine the permeability and selectivity of different nephron segments along the renal tubule. In the proximal tubule, claudins have a role in the bulk reabsorption of salt and water. In the thick ascending limb, claudins are important for the reabsorption of calcium and magnesium and are tightly regulated by the calcium-sensing receptor. In the distal nephron, claudins need to form cation barriers and chloride pores to facilitate electrogenic sodium reabsorption and potassium and acid secretion. Aldosterone and the with-no-lysine (WNK) proteins likely regulate claudins to fine-tune distal nephron salt transport. Genetic mutations in claudin-16 and -19 cause familial hypomagnesemic hypercalciuria with nephrocalcinosis, whereas polymorphisms in claudin-14 are associated with kidney stone risk. It is likely that additional roles for claudins in the pathogenesis of other types of kidney diseases have yet to be uncovered.     

MicroRNA-196a靶向调节HDAC9对MC3T3-E1细胞成骨分化的影响

目的 探究微小RNA（miR）-196a靶向调节组蛋白去乙酰化酶9（HDAC9）对MC3T3-E1细胞成骨分化的影响。方法 将MC3T3-E1细胞分为对照组（Cont）组、诱导组、miR-196a-mimics-NC组、miR-196a-mimics组、miR-196a-inhibitor-NC组、miR-196a-inhibitor组、miR-196a-mimics+pCMV-HDAC9-NC组、miR-196a-mimics+pCMV-HDAC9组,根据分组转染后进行成骨诱导。定量荧光PCR检测MC3T3-E1细胞中miR-196a、HDAC9表达量;试剂盒检测碱性磷酸酶（ALP）活性;茜素红染色观察矿化程度;Western blot检测HDAC9、ALP、Runt相关转录因子2（Runx2）、胶原蛋白I（COL1）、骨桥蛋白（OPN）、Histone H3、Histone H3（acetyl K9、K14和K23）表达量。结果 与Cont组相比,诱导组MC3T3-E1细胞中miR-196a表达、ALP、Runx2、COL1、OPN蛋白表达、ALP活性、矿化程度及Histone H3 K9、K14、K23位点乙酰化水平增高（P＜0.05）,HDAC9 mRNA和蛋白表达降低（P＜0.05）。转染miR-196a-mimics可明显增加miR-196a表达,降低HDAC9表达,并增加ALP、Runx2、COL1、OPN蛋白表达、ALP活性、矿化程度及Histone H3乙酰化,转染miR-196a-inhibitor则作用相反。miR-196a可靶向下调HDAC9表达,过表达HDAC9可部分逆转miR-196a mimics对MC3T3-E1细胞成骨分化的促进效应。结论 miR-196a可靶向下调HDAC9表达,增加组蛋白乙酰化水平,促进MC3T3-E1细胞成骨分化。     

MicroRNAs 29b, 133b,and 211 Regulate Vascular Smooth Muscle Calcification Mediated by High Phosphorus

Vascular calcification is a frequent cause of morbidity and mortality in patients with CKD and the general population. The common association between vascular calcification and osteoporosis suggests a link between bone and vascular disorders. Because microRNAs (miRs) are involved in the transdifferentiation of vascular smooth muscle cells into osteoblast-like cells, we investigated whether miRs implicated in osteoblast differentiation and bone formation are involved in vascular calcification. Different levels of uremia, hyperphosphatemia, and aortic calcification were induced by feeding nephrectomized rats a normal or high-phosphorus diet for 12 or 20 weeks, at which times the levels of eight miRs (miR-29b, miR-125, miR-133b, miR-135, miR-141, miR-200a, miR-204, and miR-211) in the aorta were analyzed. Compared with controls and uremic rats fed a normal diet, uremic rats fed a high-phosphorous diet had lower levels of miR-133b and miR-211 and higher levels of miR-29b that correlated respectively with greater expression of osteogenic RUNX2 and with lower expression of several inhibitors of osteoblastic differentiation. Uremia per se mildly reduced miR-133b levels only. Similar results were obtained in two in vitro models of vascular calcification (uremic serum and high–calcium and –phosphorus medium), and experiments using antagomirs and mimics to modify miR-29b, miR-133b, and miR-211 expression levels in these models confirmed that these miRs regulate the calcification process. We conclude that miR-29b, miR-133b, and miR-211 have direct roles in the vascular smooth muscle calcification induced by high phosphorus and may be new therapeutic targets in the management of vascular calcification.     

miR-150 Promotes Renal Fibrosis in Lupus Nephritis by Downregulating SOCS1

MicroRNAs (miRs) seem to mediate renal fibrosis in several renal diseases, with some miRs having profibrotic effects and others having opposing effects. Although differential expression of certain miRs has been described in lupus nephritis, it is unknown whether miRs contribute to fibrosis or could serve as biomarkers of specific histologic manifestations of lupus nephritis. Here, we compared miR expression in kidney biopsies from patients with lupus nephritis and identified miR-150 as the most differentially expressed miR in kidneys with high chronicity (chronicity index [CI] ≥4); miR-150 positively correlated with chronicity scores and the expression of profibrotic proteins. Overexpression of miR-150 significantly reduced expression of the antifibrotic protein suppressor of cytokine signaling 1 (SOCS1) and upregulated profibrotic proteins in both proximal tubular and mesangial cells. Directly targeting SOCS1 with a small interfering RNA produced similar results. Furthermore, TGF-β1 induced miR-150 expression, decreased SOCS1, and increased profibrotic proteins in proximal tubular cells and podocytes; a miR-150 inhibitor reversed these changes, suggesting that the profibrotic effects of TGF-β1 are, at least in part, mediated by miR-150. Consistent with these in vitro observations, biopsies with high miR-150 and high CI exhibited substantial expression of TGF-β1, reduced SOCS1, and an increase in profibrotic proteins. In summary, miR-150 is a promising quantitative renal biomarker of kidney injury in lupus nephritis. Our results suggest that miR-150 promotes renal fibrosis by increasing profibrotic molecules through downregulation of SOCS1.Despite improvements in renal outcomes in lupus nephritis (LN), a significant proportion of patients still progress to ESRD.^1,2 Fibrosis is the main pathologic feature in progressive LN and is captured by the chronicity index (CI), a semiquantitative score of chronic kidney injury, strongly associated with progression to ESRD.³MicroRNAs (miRs) are involved in the pathogenesis of CKD and renal fibrosis,^4–6 with some miRs showing profibrotic and others antifibrotic effects.⁷ miR-192, miR-141, miR-205, miR-377, and miR-21 are increased, whereas miR-29 and miR-200 are decreased in patients or animal models with renal fibrosis due to diabetic nephropathy (DN),^8–10 obstructive nephropathy,^11–13 IgA nephropathy,¹⁴ and hypertensive nephrosclerosis.¹⁵ Moreover, inhibition of miR-192 ameliorated renal fibrosis in diabetic mice.¹⁰ These findings suggest that miRs are important mediators in renal fibrosis and might be potential therapeutic targets to prevent ESRD.There are few studies of miRs in LN. One study identified 66 differentially expressed miRs in renal biopsies from LN patients compared with normal controls.¹⁶ Another study found that the intrarenal expression of miR-638, miR-198, and miR-146a was different between LN and normal controls.¹⁷ No study explored miRs as biomarkers of any specific histologic manifestation or their potential role in renal fibrosis in LN.In this study, we aimed to identify miR biomarkers reflective of CI in kidney biopsies from LN patients and to explore the potential pathogenic role of differentially expressed miRs in renal fibrosis. We show that miR-150 is significantly increased in renal biopsies with high CI and that increased miR-150 levels lead to increased production of profibrotic molecules through downregulation of suppressor of cytokine signaling 1 (SOCS1), a negative regulator of fibrosis.^18–20     

Calcilytic Ameliorates Abnormalities of Mutant Calcium‐Sensing Receptor (CaSR) Knock‐In Mice Mimicking Autosomal Dominant Hypocalcemia (ADH)

Activating mutations of calcium‐sensing receptor (CaSR) cause autosomal dominant hypocalcemia (ADH). ADH patients develop hypocalcemia, hyperphosphatemia, and hypercalciuria, similar to the clinical features of hypoparathyroidism. The current treatment of ADH is similar to the other forms of hypoparathyroidism, using active vitamin D₃ or parathyroid hormone (PTH). However, these treatments aggravate hypercalciuria and renal calcification. Thus, new therapeutic strategies for ADH are needed. Calcilytics are allosteric antagonists of CaSR, and may be effective for the treatment of ADH caused by activating mutations of CaSR. In order to examine the effect of calcilytic JTT‐305/MK‐5442 on CaSR harboring activating mutations in the extracellular and transmembrane domains in vitro, we first transfected a mutated CaSR gene into HEK cells. JTT‐305/MK‐5442 suppressed the hypersensitivity to extracellular Ca²⁺ of HEK cells transfected with the CaSR gene with activating mutations in the extracellular and transmembrane domains. We then selected two activating mutations locating in the extracellular (C129S) and transmembrane (A843E) domains, and generated two strains of CaSR knock‐in mice to build an ADH mouse model. Both mutant mice mimicked almost all the clinical features of human ADH. JTT‐305/MK‐5442 treatment in vivo increased urinary cAMP excretion, improved serum and urinary calcium and phosphate levels by stimulating endogenous PTH secretion, and prevented renal calcification. In contrast, PTH(1‐34) treatment normalized serum calcium and phosphate but could not reduce hypercalciuria or renal calcification. CaSR knock‐in mice exhibited low bone turnover due to the deficiency of PTH, and JTT‐305/MK‐5442 as well as PTH(1‐34) increased bone turnover and bone mineral density (BMD) in these mice. These results demonstrate that calcilytics can reverse almost all the phenotypes of ADH including hypercalciuria and renal calcification, and suggest that calcilytics can become a novel therapeutic agent for ADH. © 2015 American Society for Bone and Mineral Research.     

Phenotype of a calbindin-D9k gene knockout is compensated for by the induction of other calcium transporter genes in a mouse model.

CaBP-9k may be involved in the active calcium absorption and embryo implantation. Although we generated CaBP-9k KO mice to explore its function, no distinct phenotypes were observed in these KO mice. It can be hypothesized that TRPV5 and 6 and plasma membrane calcium ATPase 1b may play a role in the regulation of calcium transport to compensate CaBP-9k deficiency in its KO model. INTRODUCTION: Active calcium transport in the duodenum and kidney is carried in three steps: calcium entry through epithelial Ca2+ channels (TRPV5 and TRPV6), buffering and/or transport by calbindin-D9k (CaBP-9k) and -D28k (CaBP-28k), and extrusion through the plasma membrane calcium ATPase 1b (PMCA1b) and sodium/calcium exchanger 1. Although the molecular mechanism of calcium absorption has been studied using knockouts (KOs) of the vitamin D receptor and CaBP-28k in animals, the process is not fully understood. MATERIALS AND METHODS: We generated CaBP-9k KO mice and assessed the phenotypic characterization and the molecular regulation of active calcium transporting genes when the mice were fed different calcium diets during growth. RESULTS: General phenotypes showed no distinct abnormalities. Thus, the active calcium transport of CaBP-9k-null mice proceeded normally in this study. Therefore, the compensatory molecular regulation of this mechanism was elucidated. Duodenal TRPV6 and CaBP-9k mRNA of wildtype (WT) mice increased gradually during preweaning. CaBP-9k is supposed to be an important factor in active calcium transport, but its role is probably compensated for by other calcium transporter genes (i.e., intestinal TRPV6 and PMCA1b) during preweaning and renal calcium transporters in adult mice. CONCLUSIONS: Depletion of the CaBP-9k gene in a KO mouse model had little phenotypic effect, suggesting that its depletion may be compensated for by calcium transporter genes in the intestine of young mice and in the kidney of adult mice.     

The Role of Claudin in Hypercalciuric Nephrolithiasis

Calcium nephrolithiasis is a common condition. Family-based genetic linkage studies and genome-wide association studies (GWASs) have uncovered a run of important candidate genes involved in renal Ca⁺⁺ disorders and kidney stone diseases. The susceptible genes include NKCC2, ROMK and ClCkb/Barttin that underlie renal salt excretion; claudin-14, -16 and -19 that underlie renal Ca⁺⁺ excretion; and CaSR that provides a sensing mechanism for the kidney to regulate salt, water and Ca⁺⁺ homeostasis. Biological and physiological analyses have revealed the cellular mechanism for transepithelial Ca⁺⁺ transport in the kidney that depends on the concerted action of these gene products. Although the individual pathogenic weight of the susceptible genes in nephrolithiasis remains unclear, perturbation of their expression or function compromises the different steps within the integrated pathway for Ca⁺⁺ reabsorption, providing a physiological basis for diagnosing and managing kidney stone diseases.     

Sustained increase of microRNA-21 abundance drives aristolochic acid-induced acute kidney injury to renal tubulointerstitial fibrosis

Objective To investigate the role of increased microRNA-21 (miR-21) in the development of renal tubulointerstitial fibrosis secondary to aristolochic acid induced acute kidney injury. Methods C57BL/6J male mice were intraperitoneally injected with aristolochic acid at a dose of 10 mg/kg. Blood samples and kidneys were harvested at day 1, 3, 7, 14, 28 after aristolochic acid treatment. To assess the role of miR-21 in aristolochic acid induced acute kidney injury to chronic kidney disease progression, mice were intravenously injected with anti-miR-21 or anti-scramble (10 mg/kg) at 1 h before aristolochic acid dosing, as well as d5 and d10 after aristolochic acid dosing. Results Increased serum creatinine and severe kidney injury were found at d3 after aristolochic acid treatment. Renal tubulointerstitial fibrosis was developed at d14 after aristolochic acid treatment. Protein expression of α-SMA, vimentin and collagen I were significantly up-regulated at d7 and peaked at d14 (P＜0.01), while protein abundance of E-Cadherin decreased at d14 and lasted until d28 (P＜0.01). The abundance of miR-21 increased at d7 after aristolochic acid dosing, peaking at d14 and thereafter maintaining at a high level. Anti-miR-21 intervention relieved renal injury with reduced serum creatinine (P＜0.05) and attenuation of renal tubulointerstitial fibrosis. Besides, the protein expression of α-SMA, vimentin, and collagen I/IV was all down-regulated after anti-miR-21 treatment (P＜0.05). PTEN was up-regulated and the ratio of its downstream genes p-AKT/AKT was decreased. (P＜0.05) Conclusions A single high dose of aristolochic acid leads to acute kidney injury and the development of renal tubulointerstitial fibrosis secondary to AKI. Renal tubulointerstitial fibrosis could be partially reversed by inhibiting miR-21 via PTEN/ p-AKT pathway.     

MicroRNA-24 Antagonism Prevents Renal Ischemia Reperfusion Injury

Ischemia-reperfusion (I/R) injury of the kidney is a major cause of AKI. MicroRNAs (miRs) are powerful regulators of various diseases. We investigated the role of apoptosis-associated miR-24 in renal I/R injury. miR-24 was upregulated in the kidney after I/R injury of mice and in patients after kidney transplantation. Cell-sorting experiments revealed a specific miR-24 enrichment in renal endothelial and tubular epithelial cells after I/R induction. In vitro, anoxia/hypoxia induced an enrichment of miR-24 in endothelial and tubular epithelial cells. Transient overexpression of miR-24 alone induced apoptosis and altered functional parameters in these cells, whereas silencing of miR-24 ameliorated apoptotic responses and rescued functional parameters in hypoxic conditions. miR-24 effects were mediated through regulation of H2A histone family, member X, and heme oxygenase 1, which were experimentally validated as direct miR-24 targets through luciferase reporter assays. In vitro, adenoviral overexpression of miR-24 targets lacking miR-24 binding sites along with miR-24 precursors rescued various functional parameters in endothelial and tubular epithelial cells. In vivo, silencing of miR-24 in mice before I/R injury resulted in a significant improvement in survival and kidney function, a reduction of apoptosis, improved histologic tubular epithelial injury, and less infiltration of inflammatory cells. miR-24 also regulated heme oxygenase 1 and H2A histone family, member X, in vivo. Overall, these results indicate miR-24 promotes renal ischemic injury by stimulating apoptosis in endothelial and tubular epithelial cell. Therefore, miR-24 inhibition may be a promising future therapeutic option in the treatment of patients with ischemic AKI.     

Oxalate transport and calcium oxalate renal stone disease

Hyperoxaluria is considered to play a crucial role in calcium oxalate (CaOx) renal stone disease. The amount of oxalate excreted into the urine depends on intestinal absorption, endogenous production, renal clearance and renal tubular transport. Since a primary disorder has not been found so far in most CaOx stone formers and since oxalate is freely filtered at the glomerulus, most studies are presently focussed on alterations in epithelial oxalate transport pathways. Oxalate can be transported across an epithelium by the paracellular (passive) and transcellular (active) pathway. Oxalate transport across cellular membranes is mediated by anion-exchange transport proteins. A defect in the structure of these transport proteins could explain augmented transcellular oxalate transport. Little is known about the physiological regulation of oxalate transport. In this review cellular transport systems for oxalate will be summarized with special attention for the progress that has been made to study oxalate transport in a model of cultured renal tubule cells. Better understanding of the physiological processes that are involved in oxalate transport could yield information on the basis of which it might be possible to design new approaches for an effective treatment of CaOx stone disease.     

Hypercalciuria revisited: one or many conditions?

Idiopathic hypercalciuria is a defect occurring in 5–10% of the general population and most commonly detected in patients with calcium kidney stones or osteoporosis. Although high-penetrance autosomal dominant inheritance cannot be ruled out, hypercalciuria is probably a polygenic phenomenon. Findings obtained in monogenic disorders characterized by renal calcium stones, and/or hypercalciuria, and/or nephrocalcinosis, have suggested a number of genes as candidate genes in the pathogenesis of idiopathic hypercalciuria, i.e. soluble adenylate cyclase, calcium sensing receptor, vitamin D receptor and 1-alpha hydroxylase, sodium-phosphate co-transporter-2, claudin-16, chloride channel 5, etc. All the genetic findings obtained so far do not support the idea of different types of idiopathic hypercalciuria, i.e. absorptive, renal, and resorptive. On the contrary, they support clinical observations, which suggest idiopathic hypercalciuria as a single disorder characterized by altered calcium transport in the intestine, kidney and bone, due to various different combinations of multiple genetic and dietary players.     

The tight junction proteins claudin-7 and -8 display a different subcellular localization at Henle's loops and collecting ducts of rabbit kidney.

BACKGROUND: The tight junction (TJ) regulates the passage of ions and molecules through the paracellular pathway. In multicellular organisms, epithelial sheets function as a barrier between a variety of environments and the internal media. Therefore, TJs are required to control the passage of diverse molecules in different epithelia. The mammalian nephron constitutes a particularly relevant model of this diversity, since the paracellular transport in this organ is significantly different along the various tubular segments. Here, we have analysed the distribution of claudins-7 and -8 in Henle's loops and collecting ducts isolated from rabbit kidneys. METHODS: Renal segments were manually isolated from newborn and adult rabbit kidneys and processed for immunofluorescence. The distribution of claudins-7 and -8 was studied by confocal microscopy. RESULTS: The localization of claudins-7 and -8 along Henle's loops and collecting ducts is remarkably different. While claudin-8 displays a clear cell border distribution in Henle's segment, claudin-7 shows a non-specific cytosolic staining. Moreover, in the collecting ducts, claudin-8 localizes at the TJ region, while claudin-7 shows a basolateral staining. This pattern is present from the newborn stage. The distribution of claudins along the mammalian kidney has been found to vary in different mammalian species. Accordingly, in the rabbit, we have found the expression of claudin-8 at the descending and ascending thin limbs of Henle, a distribution that differs from that found in the mouse by others. CONCLUSION: In the rabbit Henle's loop, claudin-8 is present at the cellular borders of the descending and ascending thin limbs, while claudin-7 displays no specific labelling. Instead, at the collecting duct, both claudins are present but exhibit a different subcellular distribution.