Tight junctions of the proximal tubule and their channel proteins

Protection of glutamine: The NF-κB/MLCK/MLC2 signaling pathway mediated by tight junction affects oxidative stress, inflammation and apoptosis in snakehead (Channa argus)

Lipopolysaccharide (LPS) destroys intestinal mechanical barrier and causes apoptosis by triggering oxidative stress and inflammatory responses. Glutamine (Gln) can maintain normal intestinal function under various stressed or pathological conditions. Thereby, this study aims to evaluate the protection of glutamine on intestinal health of snakehead (Channa argus), specifically regarding the NF-κB/MLCK/MLC2 signaling pathway mediated by tight junction affecting oxidative stress, inflammation and apoptosis. In this work, a model of intestinal tight junction injury in intestine of snakehead was constructed by injecting 4 mg/mL LPS into anus for 96 h. Before constructing the model, fish were treated with different levels of alanyl-glutamine (Ala-Gln) (0 %, 0.3 %, 0.6 %, 0.9 %, 1.2 % and 1.5 %) for 56 days. Microstructure and ultra microstructure showed that LPS-induced obvious intestinal damage and tight connection destruction, while Gln effectively alleviated these phenomena. In addition, results also showed that Gln can effectively inhibit LPS-induced damage to intestinal tight junction (zo-1, occludin, claudin5, claudin1, nf-κb p65, mlck and mlc2), alleviate oxidative stress (nrf2, sod, gsh, gpx and cat), ameliorate intestinal inflammation (tnf-α, il-1β, il-8, tlr5 and tlr2), thereby reduce apoptosis (p38mapk, caspase9, caspase8, caspase3 and bax). Crucially, the above results were related to NF-κB/MLCK/MLC2 signaling pathway mediated by tight junction. In conclusion, Gln has a good protective effect on LPS-induced intestinal injury in northern snakehead, providing a new perspective for regulating fish intestinal health.

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.

Biophysical Analysis of a Minimalistic Kidney Model Expressing SGLT1 Reveals Crosstalk between Luminal and Lateral Membranes and a Plausible Mechanism of Isosmotic Transport

We extended our model of the S1 tubular segment to address the mechanisms by which SGLT1 interacts with lateral Na/K pumps and tight junctional complexes to generate isosmotic fluid reabsorption via tubular segment S3. The strategy applied allowed for simulation of laboratory experiments. Reproducing known experimental results constrained the range of acceptable model outputs and contributed to minimizing the free parameter space. (1) In experimental conditions, published Na and K concentrations of proximal kidney cells were found to deviate substantially from their normal physiological levels. Analysis of the mechanisms involved suggested insufficient oxygen supply as the cause and, indirectly, that a main function of the Na/H exchanger (NHE3) is to extrude protons stemming from mitochondrial energy metabolism. (2) The water path from the lumen to the peritubular space passed through aquaporins on the cell membrane and claudin-2 at paracellular tight junctions, with an additional contribution to water transport by the coupling of 1 glucose:2 Na:400 H2O in SGLT1. (3) A Na-uptake component passed through paracellular junctions via solvent drag in Na- and water-permeable claudin-2, thus bypassing the Na/K pump, in agreement with the findings of early studies. (4) Electrical crosstalk between apical rheogenic SGLT1 and lateral rheogenic Na/K pumps resulted in tight coupling of luminal glucose uptake and transepithelial water flow. (5) Isosmotic transport was achieved by Na-mediated ion recirculation at the peritubular membrane.

Proximal tubule hypertrophy and hyperfunction: a novel pathophysiological feature in disease states

The role of proximal tubules (PTs), a major component of the renal tubular structure in the renal cortex, has been examined extensively. Along with its physiological role in the reabsorption of various molecules, including electrolytes, amino acids and monosaccharides, transcellular transport of different hormones and regulation of homeostasis, pathological events affecting PTs may underlie multiple disease states. PT hypertrophy or a hyperfunctioning state, despite being a compensatory mechanism at first in response to various stimuli or alterations at tubular transport proteins, have been shown to be critical pathophysiological events leading to multiple disorders, including diabetes mellitus, obesity, metabolic syndrome and congestive heart failure. Moreover, pharmacotherapeutic agents have primarily targeted PTs, including sodium-glucose cotransporter 2, urate transporters and carbonic anhydrase enzymes. In this narrative review, we focus on the physiological role of PTs in healthy states and the current understanding of the PT pathologies leading to disease states and potential therapeutic targets.

Online monitoring of epithelial barrier kinetics and cell detachment during cisplatin-induced toxicity of renal proximal tubule cells

Real-time and non-invasive assessment of tissue health is crucial for maximizing the potential of microphysiological systems (MPS) for drug-induced nephrotoxicity screening. Although impedance has been widely considered as a measure of the barrier function, it has not been incorporated to detect cell detachment in MPS with top and bottom microfluidic channels separated by a porous membrane. During cell delamination from the porous membrane, the resistance between both channels decreases, while capacitance increases, allowing the detection of such detachment. Previously reported concepts have solely attributed the decrease in the resistance to the distortion of the barrier function, ignoring the resistance and capacitance changes due to cell detachment. Here, we report a two-channel MPS with integrated indium tin oxide (ITO) electrodes capable of measuring impedance in real time. The trans-epithelial electrical resistance (TEER) and tissue reactance (capacitance) were extracted from the impedance profiles. We attributed the anomalous initial increase observed in TEER, upon cisplatin administration, to the distortion of tight junctions. Cell detachment was captured by sudden jumps in capacitance. TEER profiles illuminated the effects of cisplatin and cimetidine treatments in a dose-dependent and polarity-dependent manner. The correspondence between TEER and barrier function was validated for a continuous tissue using the capacitance profiles. These results demonstrate that capacitance can be used as a real-time and non-invasive indicator of confluence and will support the accuracy of the drug-induced cytotoxicity assessed by TEER profiles in the two-channel MPS for the barrier function of a cell monolayer.

Knock-out of dipeptidase CN2 in human proximal tubular cells disrupts dipeptide and amino acid homeostasis and para- and transcellular solute transport

AIM

Although of potential biomedical relevance, dipeptide metabolism has hardly been studied. We found the dipeptidase carnosinase-2 (CN2) to be abundant in human proximal tubules, which regulate water and solute homeostasis. We therefore hypothesized, that CN2 has a key metabolic role, impacting proximal tubular transport function.

METHODS

A knockout of the CN2 gene (CNDP2-KO) was generated in human proximal tubule cells and characterized by metabolomics, RNA-seq analysis, paracellular permeability analysis and ion transport.

RESULTS

CNDP2-KO in human proximal tubule cells resulted in the accumulation of cellular dipeptides, reduction of amino acids and imbalance of related metabolic pathways, and of energy supply. RNA-seq analyses indicated altered protein metabolism and ion transport. Detailed functional studies demonstrated lower CNDP2-KO cell viability and proliferation, and altered ion and macromolecule transport via trans- and paracellular pathways. Regulatory and transport protein abundance was disturbed, either as a consequence of the metabolic imbalance or the resulting functional disequilibrium.

CONCLUSION

CN2 function has a major impact on intracellular amino acid and dipeptide metabolism and is essential for key metabolic and regulatory functions of proximal tubular cells. These findings deserve in vivo analysis of the relevance of CN2 for nephron function and regulation of body homeostasis.

Canonical and Non-Canonical Localization of Tight Junction Proteins during Early Murine Cranial Development

This study investigates the intricate composition and spatial distribution of tight junction complex proteins during early mouse neurulation. The analyses focused on the cranial neural tube, which gives rise to all head structures. Neurulation brings about significant changes in the neuronal and non-neuronal ectoderm at a cellular and tissue level. During this process, precise coordination of both epithelial integrity and epithelial dynamics is essential for accurate tissue morphogenesis. Tight junctions are pivotal for epithelial integrity, yet their complex composition in this context remains poorly understood. Our examination of various tight junction proteins in the forebrain region of mouse embryos revealed distinct patterns in the neuronal and non-neuronal ectoderm, as well as mesoderm-derived mesenchymal cells. While claudin-4 exhibited exclusive expression in the non-neuronal ectoderm, we demonstrated a neuronal ectoderm specific localization for claudin-12 in the developing cranial neural tube. Claudin-5 was uniquely present in mesenchymal cells. Regarding the subcellular localization, canonical tight junction localization in the apical junctions was predominant for most tight junction complex proteins. ZO-1 (zona occludens protein-1), claudin-1, claudin-4, claudin-12, and occludin were detected at the apical junction. However, claudin-1 and occludin also appeared in basolateral domains. Intriguingly, claudin-3 displayed a non-canonical localization, overlapping with a nuclear lamina marker. These findings highlight the diverse tissue and subcellular distribution of tight junction proteins and emphasize the need for their precise regulation during the dynamic processes of forebrain development. The study can thereby contribute to a better understanding of the role of tight junction complex proteins in forebrain development.

An emerging role of arecoline on growth performance, intestinal digestion and absorption capacities and intestinal structural integrity of adult grass carp (Ctenopharyngodon idella)

Arecoline is an alkaloid with important pharmacological effects in the plant areca nut, which has been demonstrated to be an agonist of muscarinic receptors (M receptor). This study explored the influences of dietary arecoline on growth performance, intestinal digestion and absorption abilities, antioxidant capacity, and the apical junction complex (AJC) of adult grass carp (Ctenopharyngodon idella). Adult grass carp (608 to 1512 g) were fed at 6 graded levels of dietary arecoline (0, 0.5, 1.0, 1.5, 2.0, and 2.5 mg/kg diet) for 9 weeks. The results suggested that appropriate dietary supplementation of arecoline (1.0 mg/kg) increased growth parameters and intestinal growth in adult grass carp (P < 0.05), enhanced digestion and absorption capacities (P < 0.05), up-regulated muscarinic receptor 3 (M3) mRNA level (P < 0.05), increased the content of neuropeptide fish substance P (P < 0.05), improved antioxidant capacity by activating the Keap1a/Nrf2 signaling pathway (P < 0.05), reduced intestinal mucosal permeability (P < 0.05), and increased mRNA levels of tight junction (TJ) and adherent junction AJ-related proteins in fish by inhibiting the RhoA/ROCK signaling pathway (RhoA/ROCK/MLCK/NMII) (P < 0.05). In addition, the appropriate arecoline supplementation for adult grass carp was determined to be 1.20, 1.21, 1.07, and 1.19 mg/kg based on percentage weight gain, lipase activity, serum diamine oxidase, and protein carbonyl, respectively. Overall, to the best of our knowledge, we investigated for the first time the effects and possible mechanisms of dietary arecoline on intestinal digestive and absorptive capacities and structural integrity in fish and evaluated the appropriate level of supplementation.

Moringa oleifera Leaves Protein Enhances Intestinal Permeability by Activating TLR4 Upstream Signaling and Disrupting Tight Junctions

Changes in intestinal mucosal barrier permeability lead to antigen sensitization and mast cell-mediated allergic reactions, which are considered to play important roles in the occurrence and development of food allergies. It has been suggested that protein causes increased intestinal permeability via mast cell degranulation, and we investigated the effect of camellia Moringa oleifera leaves protein on intestinal permeability and explored its role in the development of food allergies. The current study investigated the effect of M. oleifera leaves protein on intestinal permeability through assessments of transepithelial electrical resistance (TEER) and transmembrane transport of FITC-dextran by Caco-2 cells. The expression levels of Toll-like receptor 4 (TLR4), IL-8, Occludin, Claudin-1, and perimembrane protein family (ZO-1) were detected by real-time PCR and Western blotting. The effect of M. oleifera leaves protein on intestinal permeability was verified in mice in vivo. The serum fluorescence intensity was measured using the FITC-dextran tracer method, and the expression of tight junction proteins was detected using Western blotting. The results showed that M. oleifera leaves protein widened the gaps between Caco-2 cells, reduced transmembrane resistance, and increased permeability. This protein also reduced the mRNA and protein levels of Occludin, Claudin-1, and ZO-1. Animal experiments showed that intestinal permeability was increased, and that the expression of the tight junction proteins Occludin and Claudin-1 were downregulated in mice. This study shows that M. oleifera leaves protein has components that increase intestinal permeability, decrease tight junction protein expression, promote transmembrane transport in Caco-2 cells, and increase intestinal permeability in experimental animals. The finding that M. oleifera leaves active protein increases intestinal permeability suggests that this protein may be valuable for the prevention, diagnosis, and treatment of M. oleifera leaves allergy.

Myelin barrier breakdown, mechanical hypersensitivity, and painfulness in polyneuropathy with claudin-12 deficiency

BACKGROUND

The blood-nerve and myelin barrier shield peripheral neurons and their axons. These barriers are sealed by tight junction proteins, which control the passage of potentially noxious molecules including proinflammatory cytokines via paracellular pathways. Peripheral nerve barrier breakdown occurs in various neuropathies, such as chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) and traumatic neuropathy. Here, we studied the functional role of the tight junction protein claudin-12 in regulating peripheral nerve barrier integrity and CIDP pathogenesis.

METHODS

Sections from sural nerve biopsies from 23 patients with CIDP and non-inflammatory idiopathic polyneuropathy (PNP) were analyzed for claudin-12 and -19 immunoreactivity. Cldn12-KO mice were generated and subjected to the chronic constriction injury (CCI) model of neuropathy. These mice were then characterized using a battery of barrier and behavioral tests, histology, immunohistochemistry, and mRNA/protein expression. In phenotype rescue experiments, the proinflammatory cytokine TNFα was neutralized with the anti-TNFα antibody etanercept; the peripheral nerve barrier was stabilized with the sonic hedgehog agonist smoothened (SAG).

RESULTS

Compared to those without pain, patients with painful neuropathy exhibited reduced claudin-12 expression independently of fiber loss. Accordingly, global Cldn12-KO in male mice, but not fertile female mice, selectively caused mechanical allodynia associated with a leaky myelin barrier, increased TNFα, decreased sonic hedgehog (SHH), and loss of small axons accompanied by reduced peripheral myelin protein 22 (Pmp22). Other barriers and neurological functions remained intact. The Cldn12-KO phenotype could be rescued either by neutralizing TNFα with etanercept or stabilizing the barrier with SAG, which both also upregulated the Schwann cell barrier proteins Cldn19 and Pmp22.

CONCLUSION

These results point to a critical role for claudin-12 in maintaining the myelin barrier presumably via Pmp22 and highlight restoration of the hedgehog pathway as a potential treatment strategy for painful inflammatory neuropathy.

The role of claudin-2 in kidney function and dysfunction

Claudin-2 is a tight junction protein expressed in leaky epithelia where it forms paracellular pores permeable to cations and water. The paracellular pore formed by claudin-2 is important in energy-efficient cation and water transport in the proximal tubules of the kidneys. Mounting evidence now suggests that claudin-2 may modulate cellular processes often altered in disease, including cellular proliferation. Also, dysregulation of claudin-2 expression has been linked to various diseases, including kidney stone disease and renal cell carcinoma. However, the mechanisms linking altered claudin-2 expression and function to disease are poorly understood and require further investigation. The aim of this review is to discuss the current understanding of the role of claudin-2 in kidney function and dysfunction. We provide a general overview of the claudins and their organization in the tight junction, the expression, and function of claudin-2 in the kidney, and the evolving evidence for its role in kidney disease.

Solasodine suppresses the metastasis of gastric cancer through claudin-2 via the AMPK/STAT3/NF-κB pathway

Gastric cancer (GC) is one of the most common malignancies, and it has become the third most common malignant tumour in the world. Targeting metastasis has also become a key and difficult point in the treatment of GC. Solasodine is an active ingredient isolated from Solanum nigrumL. for the treatment of various cancers, such as breast cancer, pancreatic cancer and lung cancer. In the present study, we investigated the role and mechanism of solasodine in inhibiting GC. In vitro, we found that solasodine not only promoted cell death but also inhibited the migration and invasion of HGC27 and AGS cells. Solasodine regulated epithelial-mesenchymal transition (EMT) and reduced the expression of claudin-2 (CLDN2). Moreover, overexpression of CLDN2 inhibited the prometastatic phenotype and EMT of GC, and solasodine recovered this phenotype. Furthermore, the knockdown of CLDN2 had the opposite effect. We also found that the AMPK activators metformin and AICAR activated phosphorylation of AMPK and downregulated the expression of RhoA and CLDN2, indicating that AMPK was the upstream regulator of CLDN2. Solasodine could also activate AMP-activated protein kinase (AMPK) and inhibit the phosphorylation of STAT3 and the nuclear translocation of NF-κB. Therefore, solasodine may have prevented EMT by modulating the AMPK/STAT3/NF-κB/CLDN2 signalling pathway. In vivo, we established a xenograft model to investigate the phosphorylation of AMPK and the expression of CLDN2 from tumour tissues, and we found that solasodine inhibited tumour growth through AMPK-CLDN2 pathway. To sum up, solasodine prevented EMT by modulating the AMPK/STAT3/NF-κB/CLDN2 signalling pathway, becoming a new solution for inhibiting GC metastasis.

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.

Structure Composition and Intracellular Transport of Clathrin-Mediated Intestinal Transmembrane Tight Junction Protein

Tight junctions (TJs) are located in the apical region of the junctions between epithelial cells and are widely found in organs such as the brain, retina, intestinal epithelium, and endothelial system. As a mechanical barrier of the intestinal mucosa, TJs can not only maintain the integrity of intestinal epithelial cells but also maintain intestinal mucosal permeability by regulating the entry of ions and molecules into paracellular channels. Therefore, the formation disorder or integrity destruction of TJs can induce damage to the intestinal epithelial barrier, ultimately leading to the occurrence of various gastrointestinal diseases, such as inflammatory bowel disease (IBD), gastroesophageal reflux disease (GERD), and irritable bowel syndrome (IBS). However, a large number of studies have shown that TJs protein transport disorder from the endoplasmic reticulum to the apical membrane can lead to TJs formation disorder, in addition to disruption of TJs integrity caused by external pathological factors and reduction of TJs protein synthesis. In this review, we focus on the structural composition of TJs, the formation of clathrin-coated vesicles containing transmembrane TJs from the Golgi apparatus, and the transport process from the Golgi apparatus to the plasma membrane via microtubules and finally fusion with the plasma membrane. At present, the mechanism of the intracellular transport of TJ proteins remains unclear. More studies are needed in the future to focus on the sorting of TJs protein vesicles, regulation of transport processes, and recycling of TJ proteins, etc.

Apical Medium Flow Influences the Morphology and Physiology of Human Proximal Tubular Cells in a Microphysiological System

There is a lack of physiologically relevant in vitro human kidney models for disease modelling and detecting drug-induced effects given the limited choice of cells and difficulty implementing quasi-physiological culture conditions. We investigated the influence of fluid shear stress on primary human renal proximal tubule epithelial cells (RPTECs) cultured in the micro-physiological Vitrofluid device. This system houses cells seeded on semipermeable membranes and can be connected to a regulable pump that enables controlled, unidirectional flow. After 7 days in culture, RPTECs maintained physiological characteristics such as barrier integrity, protein uptake ability, and expression of specific transporters (e.g., aquaporin-1). Exposure to constant apical side flow did not cause cytotoxicity, cell detachment, or intracellular reactive oxygen species accumulation. However, unidirectional flow profoundly affected cell morphology and led to primary cilia lengthening and alignment in the flow direction. The dynamic conditions also reduced cell proliferation, altered plasma membrane leakiness, increased cytokine secretion, and repressed histone deacetylase 6 and kidney injury molecule 1 expression. Cells under flow also remained susceptible to colistin-induced toxicity. Collectively, the results suggest that dynamic culture conditions in the Vitrofluid system promote a more differentiated phenotype in primary human RPTECs and represent an improved in vitro kidney model.

Defective claudin-10 causes a novel variation of HELIX syndrome through compromised tight junction strand assembly

Formation of claudin-10 based tight junctions (TJs) is paramount to paracellular Na⁺ transport in multiple epithelia. Sequence variants in CLDN10 have been linked to HELIX syndrome, a salt-losing tubulopathy with altered handling of divalent cations accompanied by dysfunctional salivary, sweat, and lacrimal glands. Here, we investigate molecular basis and phenotypic consequences of a newly identified homozygous CLDN10 variant that translates into a single amino acid substitution within the fourth transmembrane helix of claudin-10. In addition to hypohidrosis (H), electrolyte (E) imbalance with impaired urine concentrating ability, and hypolacrimia (L), phenotypic findings include altered salivary electrolyte composition and amelogenesis imperfecta but neither ichthyosis (I) nor xerostomia (X). Employing cellular TJ reconstitution assays, we demonstrate perturbation of cis- and trans-interactions between mutant claudin-10 proteins. Ultrastructures of reconstituted TJ strands show disturbed continuity and reduced abundance in the mutant case. Throughout, both major isoforms, claudin-10a and claudin-10b, are differentially affected with claudin-10b showing more severe molecular alterations. However, expression of the mutant in renal epithelial cells with endogenous TJs results in wild-type-like ion selectivity and conductivity, indicating that aberrant claudin-10 is generally capable of forming functional paracellular channels. Thus, mutant proteins prove pathogenic by compromising claudin-10 TJ strand assembly. Additional ex vivo investigations indicate their insertion into TJs to occur in a tissue-specific manner.

Human duodenal organoid-derived monolayers serve as a suitable barrier model for duodenal tissue

Usually, duodenal barriers are investigated using intestinal cell lines like Caco-2, which in contrast to native tissue are limited in cell-type representation. Organoids can consist of all intestinal cell types and are supposed to better reflect the in vivo situation. Growing three-dimensionally, with the apical side facing the lumen, application of typical physiological techniques to analyze the barrier is difficult. Organoid-derived monolayers (ODMs) were developed to overcome this. After optimizing culturing conditions, ODMs were characterized and compared to Caco-2 and duodenal tissue. Tight junction composition and appearance were analyzed, and electrophysiological barrier properties, like paracellular and transcellular barrier function and macromolecule permeability, were evaluated. Furthermore, transcriptomic data were analyzed. ODMs had tight junction protein expression and paracellular barrier properties much more resembling the originating tissue than Caco-2. Transcellular barrier was similar between ODMs and native tissue but was increased in Caco-2. Transcriptomic data showed that Caco-2 expressed fewer solute carriers than ODMs and native tissue. In conclusion, while Caco-2 cells differ mostly in transcellular properties, ODMs reflect trans- and paracellular properties of the originating tissue. If cultured under optimized conditions, ODMs possess reproducible functionality, and the variety of different cell types makes them a suitable model for human tissue-specific investigations.

Claudins in kidney health and disease

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

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 physiological 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. Upon 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 consequence, there was tubular retention of calcium and magnesium, higher urine pH, and mild hypermagnesemia. A comparison of other urine and serum parameters under control conditions and sequential pharmacologic transport inhibition, as well as 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 hyperreabsorption by claudin-2 redistribution. Transcellular transport in proximal and distal segments and proximal tubular metabolic adaptation compensate for loss of paracellular anion permeability.

Localization of claudin-2 and claudin-3 in eutopic and ectopic endometrium is highly similar

PURPOSE

Claudins as the major components of tight junctions are important in maintaining cell-cell integrity and thus function as a barrier. Dysregulation of the claudins is often associated with loss of the epithelial phenotype, a process called epithelial-mesenchymal transition (EMT), which most often results in gain of migrative and invasive properties. However, the role of claudins in the endometrium or endometriosis has only rarely been examined.

METHODS

In this study, we investigated localization of claudin-2 and claudin-3 in the eutopic and ectopic endometrium with immunohistochemistry. A detailed quantification with HSCORE was performed for claudin-2 and claudin-3 in endometrium without endometriosis and in cases with endometriosis compared to the three endometriotic entities: peritoneal, ovarian, and deep-infiltrating endometriosis.

RESULTS

We found a preferential localization of both claudins in the glandular and the luminal epithelial cells in the endometrium with and without endometriosis. Quantification of localization of both claudins showed no differences in eutopic endometrium of control cases compared to cases with endometriosis. Furthermore, both claudins are localized highly similar in the ectopic compared to the eutopic endometrium, which is in clear contrast to previously published data for claudin-3.

CONCLUSION

From our results, we conclude that localization of claudin-2 and claudin-3 is highly stable in eutopic and ectopic endometrium without any loss of the epithelial phenotype and thus do not contribute to the pathogenesis of endometriosis.

Disruption of tight junction structure contributes to secretory dysfunction in IgG4-related sialadenitis

IgG4-related sialadenitis (IgG4-RS) is a chronic fibro-inflammatory disease characterized by swelling of salivary glands and varying degrees of xerostomia. Tight junctions (TJs) play an essential role in maintaining secretory function by regulating the paracellular flow of ions and water. However, whether TJs are altered and contribute to the hyposecretion in IgG4-RS is not fully understood. Here, a total of 399 differentially expressed proteins were identified in IgG4-RS submandibular glands (SMGs) and enriched in the regulation of actin cytoskeleton and the salivary secretion. Real-time PCR results showed that the mRNA levels of claudin-3, -4, -6, -7, -8, -10, -12, occludin, and ZO-1 were significantly lower, whereas claudin-1 and -5 were higher in IgG4-RS SMGs. Immunohistochemical and immunofluorescence staining revealed that claudin-1, -3, -4, occludin, and ZO-1 were mainly distributed at apicolateral membranes in acini and ducts of SMGs from controls, whereas claudin-1 protein intensity at apicolateral membrane was elevated, while the staining of claudin-3, -4, and ZO-1 were reduced in IgG4-RS SMGs. Occludin was dispersed into cytoplasm of acini and ducts in SMGs of patients. Among them, claudin-3 and ZO-1 protein levels were positively correlated with saliva flow rate. Furthermore, the decreased fluorescence intensity of F-actin at peri-apicolateral membranes and the loss of ZO-1 staining at the same location were observed in acinar and ductal cells of IgG4-RS SMGs, which might be responsible for disorganization of TJ complex. Taken together, these findings indicate that the integrity of TJ complex of SMGs is impaired and might contribute to hyposalivation of IgG4-RS patients.

Claudin-2: Roles beyond Permeability Functions

Claudin-2 is expressed in the tight junctions of leaky epithelia, where it forms cation-selective and water permeable paracellular channels. Its abundance is under fine control by a complex signaling network that affects both its synthesis and turnover in response to various environmental inputs. Claudin-2 expression is dysregulated in many pathologies including cancer, inflammation, and fibrosis. Claudin-2 has a key role in energy-efficient ion and water transport in the proximal tubules of the kidneys and in the gut. Importantly, strong evidence now also supports a role for this protein as a modulator of vital cellular events relevant to diseases. Signaling pathways that are overactivated in diseases can alter claudin-2 expression, and a good correlation exists between disease stage and claudin-2 abundance. Further, loss- and gain-of-function studies showed that primary changes in claudin-2 expression impact vital cellular processes such as proliferation, migration, and cell fate determination. These effects appear to be mediated by alterations in key signaling pathways. The specific mechanisms linking claudin-2 to these changes remain poorly understood, but adapters binding to the intracellular portion of claudin-2 may play a key role. Thus, dysregulation of claudin-2 may contribute to the generation, maintenance, and/or progression of diseases through both permeability-dependent and -independent mechanisms. The aim of this review is to provide an overview of the properties, regulation, and functions of claudin-2, with a special emphasis on its signal-modulating effects and possible role in diseases.

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

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

Claudin-2 suppresses GEF-H1, RHOA, and MRTF, thereby impacting proliferation and profibrotic phenotype of tubular cells

The tight junctional pore-forming protein claudin-2 (CLDN-2) mediates paracellular Na⁺ and water transport in leaky epithelia and alters cancer cell proliferation. Previously, we reported that tumor necrosis factor-α time-dependently alters CLDN-2 expression in tubular epithelial cells. Here, we found a similar expression pattern in a mouse kidney injury model (unilateral ureteral obstruction), consisting of an initial increase followed by a drop in CLDN-2 protein expression. CLDN-2 silencing in LLC-PK¹ tubular cells induced activation and phosphorylation of guanine nucleotide exchange factor H1 (GEF-H1), leading to Ras homolog family member A (RHOA) activation. Silencing of other claudins had no such effects, and re-expression of an siRNA-resistant CLDN-2 prevented RHOA activation, indicating specific effects of CLDN-2 on RHOA. Moreover, kidneys from CLDN-2 knockout mice had elevated levels of active RHOA. Of note, CLDN-2 silencing reduced LLC-PK1 cell proliferation and elevated expression of cyclin-dependent kinase inhibitor P27 (P27KIP1) in a GEF-H1/RHOA-dependent manner. P27KIP1 silencing abrogated the effects of CLDN-2 depletion on proliferation. CLDN-2 loss also activated myocardin-related transcription factor (MRTF), a fibrogenic RHOA effector, and elevated expression of connective tissue growth factor and smooth muscle actin. Finally, CLDN-2 down-regulation contributed to RHOA activation and smooth muscle actin expression induced by prolonged tumor necrosis factor-α treatment, because they were mitigated by re-expression of CLDN-2. Our results indicate that CLDN-2 suppresses GEF-H1/RHOA. CLDN-2 down-regulation, for example, by inflammation, can reduce proliferation and promote MRTF activation through RHOA. These findings suggest that the initial CLDN-2 elevation might aid epithelial regeneration, and CLDN-2 loss could contribute to fibrotic reprogramming.

Shenmai injection maintains blood-brain barrier integrity following focal cerebral ischemia via modulating the expression and trafficking of occludin in lipid rafts

ETHNOPHARMACOLOGICAL RELEVANCE

Shenmai injection (SMI), a traditional Chinese herbal medicine is widely used for the clinical treatment of cerebral infarction in China.

AIM OF THE STUDY

Tight junctions (TJs) are major components of the blood-brain barrier (BBB) that physically restrict the paracellular diffusion of blood-borne substances between endothelial cells into the CNS. TJ proteins are associated with cholesterol-enriched regions of plasma membrane known as lipid rafts, which are critical for the trafficking, positioning and function of TJ proteins. In this study, we investigated the effect of SMI on the expression and trafficking of the key TJ-associated protein, occludin, in lipid rafts.

MATERIALS AND METHODS

Using a neutral pH, rat cerebral microvessels were subjected to detergent-free density-gradient fractionation to isolate lipid rafts containing occludin. Transmission electron microscopy (TEM) was performed to study the effects of drug administration on ultrastructural changes to TJs. Western blotting (WB), immunofluorescence (IF), and co-immunoprecipitation (COIP) were used to observe the localization and function of TJ-associated proteins.

RESULTS

We successfully isolated cerebral microvessels and separated lipid rafts from plasma membranes. With SMI treatment, extravasation of FITC-albumin decreased around the cerebral vessels by IF, the tight junctions were found to still be intact and the basement membrane appeared to be of uniform thickness in TEM. Compared with the untreated group, the co-expression of flotillin-1 and occludin in microvascular endothelial cells was increased and distributed continuously in SMI treatment as shown in double label IF. SMI significantly increased the translocation of occludin to lipid raft fractions by WB and COIP.

CONCLUSIONS

SMI helps maintain the proper assembly of the TJ multiprotein complex in lipid rafts, thereby helping to preserve BBB functional integrity during focal cerebral ischemic insult. Our findings enhance our understanding of the mechanisms underlying the neuroprotective effect of SMI in cerebral ischemia.

Stationary and Nonstationary Ion and Water Flux Interactions in Kidney Proximal Tubule: Mathematical Analysis of Isosmotic Transport by a Minimalistic Model

Our mathematical model of epithelial transport (Larsen et al. Acta Physiol. 195:171–186, 2009) is extended by equations for currents and conductance of apical SGLT2. With independent variables of the physiological parameter space, the model reproduces intracellular solute concentrations, ion and water fluxes, and electrophysiology of proximal convoluted tubule. The following were shown:

Hypokalemia Associated With a Claudin 10 Mutation: A Case Report

Hypokalemia of renal origin can arise from genetic abnormalities in a variety of transporters or channel proteins that mediate tubular handling of potassium. Recently, mutations in claudin 10 have been documented in patients with hypokalemia in association with a range of other electrolyte abnormalities and skin and sweat gland manifestations. We report a 12-year-old Hispanic boy who presented with anhydrosis, aptyalism, alacrima, hypokalemia, and hypocalciuria, in whom we detected a homozygous mutation in the claudin 10 gene. During the 4-year follow-up period, he developed hypermagnesemia and a decline in estimated glomerular filtration rate to 59mL/min/1.73m². His unaffected parents and siblings were heterozygous for the mutation. We summarize the clinical phenotype encountered in patients with claudin 10 mutations. It is characterized by significant heterogeneity in electrolyte and extrarenal abnormalities and is associated with a risk for progressive loss of kidney function in up to 33% of cases. Awareness of this association between claudin 10 mutations and electrolyte abnormalities, namely hypokalemia and hypermagnesemia, sheds new light on the physiology of potassium and magnesium handling along the nephron and increases the likelihood of identifying the underlying tubular mechanism in patients with newly diagnosed hypokalemia with or without concomitant hypermagnesemia.

Advances in predictive in vitro models of drug-induced nephrotoxicity

In vitro screens for nephrotoxicity are currently poorly predictive of toxicity in humans. Although the functional proteins that are expressed by nephron tubules and mediate drug susceptibility are well known, current in vitro cellular models poorly replicate both the morphology and the function of kidney tubules and therefore fail to demonstrate injury responses to drugs that would be nephrotoxic in vivo. Advances in protocols to enable the directed differentiation of pluripotent stem cells into multiple renal cell types and the development of microfluidic and 3D culture systems have opened a range of potential new platforms for evaluating drug nephrotoxicity. Many of the new in vitro culture systems have been characterized by the expression and function of transporters, enzymes, and other functional proteins that are expressed by the kidney and have been implicated in drug-induced renal injury. In vitro platforms that express these proteins and exhibit molecular biomarkers that have been used as readouts of injury demonstrate improved functional maturity compared with static 2D cultures and represent an opportunity to model injury to renal cell types that have hitherto received little attention. As nephrotoxicity screening platforms become more physiologically relevant, they will facilitate the development of safer drugs and improved clinical management of nephrotoxicants.

A model of calcium transport and regulation in the proximal tubule

The objective of this study was to examine theoretically how Ca²⁺ reabsorption in the proximal tubule (PT) is modulated by Na⁺ and water fluxes, parathyroid hormone (PTH), Na⁺-glucose cotransporter (SGLT2) inhibitors, and acetazolamide. We expanded a previously published mathematical model of water and solute transport in the rat PT (Layton AT, Vallon V, Edwards A. Am J Physiol Renal Physiol 308: F1343–F1357, 2015) that did not include Ca²⁺. Our results indicate that Ca²⁺ reabsorption in the PT is primarily driven by the transepithelial Ca²⁺ concentration gradient that stems from water reabsorption, which is itself coupled to Na⁺ reabsorption. Simulated variations in permeability or transporter activity elicit opposite changes in paracellular and transcellular Ca²⁺ fluxes, whereas a simulated decrease in filtration rate lowers both fluxes. The model predicts that PTH-mediated inhibition of the apical Na⁺/H⁺ exchanger NHE3 reduces Na⁺ and Ca²⁺ transport to a similar extent. It also suggests that acetazolamide- and SGLT2 inhibitor-induced calciuria at least partly stems from reduced Ca²⁺ reabsorption in the PT. In addition, backleak of phosphate (PO₄) across tight junctions is predicted to reduce net PO₄ reabsorption by ~20% under normal conditions. When transcellular PO₄ transport is substantially reduced by PTH, paracellular PO₄ flux is reversed and contributes significantly to PO₄ reabsorption. Furthermore, PTH is predicted to exert an indirect impact on PO₄ reabsorption via its inhibitory action on NHE3. This model thus provides greater insight into the mechanisms that modulate Ca²⁺ and PO₄ reabsorption in the PT.

Cell confluence regulates claudin-2 expression: possible role for ZO-1 and Rac

Claudin-2 (Cldn-2) is a channel-forming tight junction (TJ) protein in the proximal tubules that mediates paracellular Na+ transport and has also emerged as a regulator of proliferation and migration. Expression of Cldn-2 is altered by numerous stimuli, but the underlying mechanisms remain incompletely understood. Here we show that Cldn-2 protein and mRNA expression were low in subconfluent tubular cells and increased during junction maturation. Cldn-1 or occludin did not exhibit similar confluence-dependence. Conversely, disruption of TJs by Ca2+ removal or silencing of zonula occludens-1 (ZO-1) or ZO-2 induced a large drop in Cldn-2 abundance. Immunofluorescent staining revealed a more uneven Cldn-2 staining in nascent, Cldn-1-positive TJs. Subconfluence and ZO-1 silencing augmented Cldn-2 degradation and reduced Cldn-2 promoter activity, suggesting that insertion into the TJs slows Cldn-2 turnover. Indeed, blocking endocytosis or lysosomal degradation increased Cldn-2 abundance. Cell confluence increased expression of the junctional adapters ZO-1 and -2, and the small GTPase Rac, and elevated Rac activity and p21-activated kinase (Pak) phosphorylation, suggesting that they might mediate confluence-dependent Cldn-2 regulation. Indeed, Rac silencing or Pak inhibition strongly reduced Cldn-2 protein abundance, which was likely the combined effect on turnover, as these interventions reduced Cldn-2 promoter activity and augmented Cldn-2 degradation. Taken together, our data suggest that TJ integrity and maturity, ZO-1 expression/TJ localization, and Rac/Pak control Cldn-2 degradation and synthesis. A feedback mechanism connecting Cldn-2 expression with junction remodeling, e.g., during wound healing, epithelial-mesenchymal transition, or tumor metastasis formation, may have important downstream effects on permeability, proliferation, and migration.

A look at the smelly side of physiology: transport of short chain fatty acids

Fermentative organs such as the caecum, the colon, and the rumen have evolved to produce and absorb energy rich short chain fatty acids (SCFA) from otherwise indigestible substrates. Classical models postulate diffusional uptake of the undissociated acid (HSCFA). However, in net terms, a major part of SCFA absorption occurs with uptake of Na⁺ and resembles classical, coupled electroneutral NaCl transport. Considerable evidence suggests that the anion transporting proteins expressed by epithelia of fermentative organs are poorly selective and that their main function may be to transport acetate^-, propionate^-, butyrate^- and HCO₃^- as the physiologically relevant anions. Apical uptake of SCFA thus involves non-saturable diffusion of the undissociated acid (HSCFA), SCFA^-/HCO₃^- exchange via DRA (SLC26A3) and/or SCFA^--H⁺ symport (MCT1, SLC16A1). All mechanisms lead to cytosolic acidification with stimulation of Na⁺/H⁺ exchange via NHE (SLC9A2/3). Basolaterally, Na⁺ leaves via the Na⁺/K⁺-ATPase with recirculation of K⁺. Na⁺ efflux drives the transport of SCFA^- anions through volume-regulated anion channels, such as maxi-anion channels (possibly SLCO2A1), LRRC8, anoctamins, or uncoupled exchangers. When luminal buffering is inadequate, basolateral efflux will increasingly involve SCFA^-/ HCO₃^- exchange (AE1/2, SCL4A1/2), or efflux of SCFA^- with H⁺ (MCT1/4, SLC16A1/3). Furthermore, protons can be basolaterally removed by NHE1 (SCL9A1) or NBCe1 (SLC4A4). The purpose of these transport proteins is to maximize the amount of SCFA transported from the tightly buffered ingesta while minimizing acid transport through the epithelium. As known from the rumen for many decades, a disturbance of these processes is likely to cause severe colonic disease.