Unrecognized role of claudin-10b in basolateral membrane infoldings of the thick ascending limb.

Claudin-10b is an important component of the tight junction in the thick ascending limb (TAL) of Henle's loop and allows paracellular sodium transport. In immunofluorescence stainings, claudin-10b-positive cells exhibited extensive extra staining of basolateral, column-like structures. The precise localization and function have so far remained elusive. In isolated cortical TAL segments from C57BL/6J mice, kidney-specific claudin-10 knockout mice (cKO), and respective litter mates (WT), we investigated the localization and protein expression and function by fluorescence microscopy and electrophysiological measurements. Ultrastructural analysis of TAL in kidney sections was performed by electron microscopy. Claudin-10b colocalized with the basolateral Na+ -K+ ATPase and the Cl- channel subunit barttin, but the lack of claudin-10b did not influence the localization or abundance of these proteins. However, the accessibility of the basolateral infolded extracellular space to ouabain or fluorescein was increased by basolateral Ca2+ removal and in the absence of claudin-10b. Ultrastructural analysis by electron microscopy revealed a widening of basolateral membrane infoldings in cKO in comparison to WT. We hypothesize that claudin-10b shapes neighboring membrane invaginations by trans interaction to stabilize and facilitate high-flux salt transport in a water-tight epithelium.

Claudin-10a Deficiency Shifts Proximal Tubular Cl- Permeability to Cation Selectivity via Claudin-2 Redistribution.

BACKGROUND: The tight junction proteins claudin-2 and claudin-10a form paracellular cation and anion channels, respectively, and are expressed in the proximal tubule. However, the physiologic role of claudin-10a in the kidney has been unclear. METHODS: To investigate the physiologic role of claudin-10a, we generated claudin-10a-deficient mice, confirmed successful knockout by Southern blot, Western blot, and immunofluorescence staining, and analyzed urine and serum of knockout and wild-type animals. We also used electrophysiologic studies to investigate the functionality of isolated proximal tubules, and studied compensatory regulation by pharmacologic intervention, RNA sequencing analysis, Western blot, immunofluorescence staining, and respirometry. RESULTS: Mice deficient in claudin-10a were fertile and without overt phenotypes. On knockout, claudin-10a was replaced by claudin-2 in all proximal tubule segments. Electrophysiology showed conversion from paracellular anion preference to cation preference and a loss of paracellular Cl- over HCO3- preference. As a result, there was tubular retention of calcium and magnesium, higher urine pH, and mild hypermagnesemia. A comparison with other urine and serum parameters under control conditions and sequential pharmacologic transport inhibition, and unchanged fractional lithium excretion, suggested compensative measures in proximal and distal tubular segments. Changes in proximal tubular oxygen handling and differential expression of genes regulating fatty acid metabolism indicated proximal tubular adaptation. Western blot and immunofluorescence revealed alterations in distal tubular transport. CONCLUSIONS: Claudin-10a is the major paracellular anion channel in the proximal tubule and its deletion causes calcium and magnesium hyper-reabsorption by claudin-2 redistribution. Transcellular transport in proximal and distal segments and proximal tubular metabolic adaptation compensate for loss of paracellular anion permeability.

Viewing Cortical Collecting Duct Function Through Phenotype-guided Single-Tubule Proteomics.

The revolution of the omics technologies has enabled profiling of the molecules of any sample. However, the heterogeneity of the kidney with highly specialized nephron segments like the cortical collecting duct (CCD) poses a challenge regarding integration of omics data and functional analysis. We examined function and proteome from the same single CCDs of C57Bl6 mice by investigating them in a double-barreled perfusion system before targeted mass spectrometry. Transepithelial voltage (Vte), transepithelial resistance, as well as amiloride-sensitive voltage (ΔVteamil) were recorded. CCDs were of 400-600 µm of length, showed lumen negative Vte between -8.5 and -32.5 mV and an equivalent short circuit current I'sc between 54 and 192 µA/cm2. On a single-tubule proteome level, intercalated cell (IC) markers strongly correlated with other intercalated cell markers and negatively with principal cell markers. Integration of proteome data with phenotype data revealed that tubular length correlated with actin and Na+-K+-ATPase expression. ΔVte(amil) reflected the expression level of the ß-subunit of the epithelial sodium channel. Intriguingly, ΔVte(amil) correlated inversely with the water channel AQP2 and the negative regulator protein NEDD4L (NEDD4-2). In pendrin knockout (KO) mice, the CCD proteome was accompanied by strong downregulation of other IC markers like CLCNKB, BSND (Barttin), and VAA (vH+-ATPase), a configuration that may contribute to the salt-losing phenotype of Pendred syndrome. Proteins normally coexpressed with pendrin were decreased in pendrin KO CCDs. In conclusion, we show that functional proteomics on a single nephron segment scale allows function-proteome correlations, and may potentially help predicting function from omics data.

Phosphorylated claudin-16 interacts with Trpv5 and regulates transcellular calcium transport in the kidney.

Familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC) was previously considered to be a paracellular channelopathy caused by mutations in the claudin-16 and claudin-19 genes. Here, we provide evidence that a missense FHHNC mutation c.908C>G (p.T303R) in the claudin-16 gene interferes with the phosphorylation in the claudin-16 protein. The claudin-16 protein carrying phosphorylation at residue T303 is localized in the distal convoluted tubule (DCT) but not in the thick ascending limb (TAL) of the mouse kidney. The phosphomimetic claudin-16 protein carrying the T303E mutation but not the wildtype claudin-16 or the T303R mutant protein increases the Trpv5 channel conductance and membrane abundance in human kidney cells. Phosphorylated claudin-16 and Trpv5 are colocalized in the luminal membrane of the mouse DCT tubule; phosphomimetic claudin-16 and Trpv5 interact in the yeast and mammalian cell membranes. Knockdown of claudin-16 gene expression in transgenic mouse kidney delocalizes Trpv5 from the luminal membrane in the DCT. Unlike wildtype claudin-16, phosphomimetic claudin-16 is delocalized from the tight junction but relocated to the apical membrane in renal epithelial cells because of diminished binding affinity to ZO-1. High-Ca2+ diet reduces the phosphorylation of claudin-16 protein at T303 in the DCT of mouse kidney via the PTH signaling cascade. Knockout of the PTH receptor, PTH1R, from the mouse kidney abrogates the claudin-16 phosphorylation at T303. Together, these results suggest a pathogenic mechanism for FHHNC involving transcellular Ca2+ pathway in the DCT and identify a molecular component in renal Ca2+ homeostasis under direct regulation of PTH.

Fluconazole Increases Osmotic Water Transport in Renal Collecting Duct through Effects on Aquaporin-2 Trafficking.

BACKGROUND: Arginine-vasopressin (AVP) binding to vasopressin V2 receptors promotes redistribution of the water channel aquaporin-2 (AQP2) from intracellular vesicles into the plasma membrane of renal collecting duct principal cells. This pathway fine-tunes renal water reabsorption and urinary concentration, and its perturbation is associated with diabetes insipidus. Previously, we identified the antimycotic drug fluconazole as a potential modulator of AQP2 localization. METHODS: We assessed the influence of fluconazole on AQP2 localization in vitro and in vivo as well as the drug's effects on AQP2 phosphorylation and RhoA (a small GTPase, which under resting conditions, maintains F-actin to block AQP2-bearing vesicles from reaching the plasma membrane). We also tested fluconazole's effects on water flow across epithelia of isolated mouse collecting ducts and on urine output in mice treated with tolvaptan, a VR2 blocker that causes a nephrogenic diabetes insipidus-like excessive loss of hypotonic urine. RESULTS: Fluconazole increased plasma membrane localization of AQP2 in principal cells independent of AVP. It also led to an increased AQP2 abundance associated with alterations in phosphorylation status and ubiquitination as well as inhibition of RhoA. In isolated mouse collecting ducts, fluconazole increased transepithelial water reabsorption. In mice, fluconazole increased collecting duct AQP2 plasma membrane localization and reduced urinary output. Fluconazole also reduced urinary output in tolvaptan-treated mice. CONCLUSIONS: Fluconazole promotes collecting duct AQP2 plasma membrane localization in the absence of AVP. Therefore, it might have utility in treating forms of diabetes insipidus (e.g., X-linked nephrogenic diabetes insipidus) in which the kidney responds inappropriately to AVP.

New Tacrine Hybrids with Natural-Based Cysteine Derivatives as Multitargeted Drugs for Potential Treatment of Alzheimer's Disease.

Alzheimer's disease (AD) is a devastating age-dependent neurodegenerative disorder. The main hallmarks are impairment of cholinergic system and accumulation in brain of beta-amyloid (Aß) aggregates, which have been associated with oxidative damage and dyshomeostasis of redox-active biometals. The absence of an efficient treatment that could delay or cure AD has been attributed to the complexity and multifactorial nature of this disease. With this in mind and the recent interest on natural-based drugs, we have explored a set of natural-based hybrid compounds by conjugation of a tacrine moiety with an S-allylcysteine (garlic constituent) or S-propargylcysteine moiety aimed at improving the cholinergic system and neuroprotective capacity. The docking modeling studies allowed the selection of linkers to optimize the bimodal drug interaction with acetylcholinesterase enzyme (AChE) active site. The compounds were evaluated for some representative biological properties, including AChE activity and Aß aggregation inhibition, as well as for their neuroprotective activity to Aß- and ROS-induced cellular toxicity. The most promising results were achieved by compounds 9d for the AChE inhibition and 9l for the remarkable prevention of superoxide production and Aß-induced cellular toxicity.

Copper(II) complexation of tacrine hybrids with potential anti-neurodegenerative roles.

The complexity and multifactorial nature of neurodegenerative diseases turn quite difficult the development of adequate drugs for their treatment. Multi-target analogues, in conjugation with natural moieties, have been developed in order to combine acetylcholinesterase (AChE) inhibition with antioxidant properties, metal-binding capacity and inhibition of amyloid-ß (Aß) aggregation. Due to the recent interest on natural-based drugs and also the importance of studying the role of transition metal ions in the disease process, we herein evaluate the copper chelating capacity and inhibitory ability for self- and Cu-induced Aß1-42 aggregation of two nature-base hybrid model compounds obtained from conjugation of a tacrine moiety with a S-allylcystein (1) or S-propargylcystein (2) moiety. Both compounds show a moderate chelating power towards Cu(II) (pCu 7.13-7.51, CL/CCu=10, CCu=10(-6)M, pH7.4), with predominant formation of 1:1 complex species (CuL, CuH-1L) for which the coordination sphere involves the N-amide and the NH2 amine of the cysteine derivative as well as the NH of tacrine. The compounds are able to improve the inhibition of Aß aggregation in the presence of Cu(II) and this is slightly more relevant for the allyl derivative (1), a stronger copper chelator, than for the propargyl (2). Moreover, the presence of a chloro atom in the tacrine moiety and the size of the chain length between the two NH groups appeared also to improve the inhibition capacity for Aß aggregation.

Design, synthesis and neuroprotective evaluation of novel tacrine-benzothiazole hybrids as multi-targeted compounds against Alzheimer's disease.

Alzheimer's disease (AD) is a multifactorial disorder with several target proteins contributing to its etiology. In search for multifunctional anti-AD drug candidates, taking into account that the acetylcholinesterase (AChE) and beta-amyloid (Aß) aggregation are particularly important targets for inhibition, the tacrine and benzothiazole (BTA) moieties were conjugated with suitable linkers in a novel series of hybrids. The designed compounds (7a-7e) were synthesized and in vitro as well as in ex vivo evaluated for their capacity for the inhibition of acetylcholinesterase (AChE) and Aß self-induced aggregation, and also for the protection of neuronal cells death (SHSY-5Y cells, AD and MCI cybrids). All the tacrine-BTA hybrids displayed high in vitro activities, namely with IC50 values in the low micromolar to sub-micromolar concentration range towards the inhibition of AChE, and high percentages of inhibition of the self-induced Aß aggregation. Among them, compound 7a, with the shortest linker, presented the best inhibitory activity of AChE (IC50=0.34 µM), while the highest activity as anti-Aß42 self-aggregation, was evidenced for compound 7b (61.3%, at 50µM. The docking studies demonstrated that all compounds are able to interact with both catalytic active site (CAS) and peripheral anionic site (PAS) of AChE. Our results show that compounds 7d and 7e improved cell viability in cells treated with Aß42 peptide. Overall, these multi-targeted hybrid compounds appear as promising lead compounds for the treatment of Alzheimer's disease.

Multifunctional iron-chelators with protective roles against neurodegenerative diseases.

The multifactorial nature of Alzheimer's disease (AD), and the absence of a disease modifying drug, makes the development of new multifunctional drugs an attractive therapeutic strategy. Taking into account the hallmarks of AD patient brains, such as low levels of acetylcholine, misfolding of proteins and associated beta-amyloid (Aß) aggregation, oxidative stress and metal dyshomeostasis, we have developed a series of compounds that merge three different approaches: metal attenuation, anti-Aß aggregation and anti-acetylcholinesterase activity. Therefore, 3-hydroxy-4-pyridinone (3,4-HP) and benzothiazole molecular moieties were selected as starting frameworks due to their well known affinity for iron and Aß peptides, respectively. The linkers between these two main functional groups were selected on the basis of virtual screening, so that the final molecule could further inhibit the acetylcholinesterase, responsible for the cholinergic losses. We describe herein the design and synthesis of the new hybrid compounds, followed by the assessment of solution properties, namely iron chelation and anti-oxidant capacity. The compounds were bioassayed for their capacity to inhibit AChE, as well as self- and Zn mediated-Aß(1-42) aggregation. Finally, we assessed their effects on the viability of neuronal cells stressed with Aß(42).