Physiology and pathophysiology of the renal Na-K-2Cl cotransporter (NKCC2)

New hemisynthetic derivatives of sphaeropsidin phytotoxins triggering severe endoplasmic reticulum swelling in cancer cells

Sphaeropsidins are iso-pimarane diterpenes produced by phytopathogenic fungi that display promising anticancer activities. Sphaeropsidin A, in particular, has been shown to counteract regulatory volume increase, a process used by cancer cells to avoid apoptosis. This study reports the hemi-synthesis of new lipophilic derivatives obtained by modifications of the C15,C16-alkene moiety. Several of these compounds triggered severe ER swelling associated with strong proteasomal inhibition and consequently cell death, a feature that was not observed with respect to mode of action of the natural product. Significantly, an analysis from the National Cancer Institute sixty cell line testing did not reveal any correlations between the most potent derivative and any other compound in the database, except at high concentrations (LC50). This study led to the discovery of a new set of sphaeropsidin derivatives that may be exploited as potential anti-cancer agents, notably due to their maintained activity towards multidrug resistant models.

Protein Quality Control of NKCC2 in Bartter Syndrome and Blood Pressure Regulation

Mutations in NKCC2 generate antenatal Bartter syndrome type 1 (type 1 BS), a life-threatening salt-losing nephropathy characterized by arterial hypotension, as well as electrolyte abnormalities. In contrast to the genetic inactivation of NKCC2, inappropriate increased NKCC2 activity has been associated with salt-sensitive hypertension. Given the importance of NKCC2 in salt-sensitive hypertension and the pathophysiology of prenatal BS, studying the molecular regulation of this Na-K-2Cl cotransporter has attracted great interest. Therefore, several studies have addressed various aspects of NKCC2 regulation, such as phosphorylation and post-Golgi trafficking. However, the regulation of this cotransporter at the pre-Golgi level remained unknown for years. Similar to several transmembrane proteins, export from the ER appears to be the rate-limiting step in the cotransporter's maturation and trafficking to the plasma membrane. The most compelling evidence comes from patients with type 5 BS, the most severe form of prenatal BS, in whom NKCC2 is not detectable in the apical membrane of thick ascending limb (TAL) cells due to ER retention and ER-associated degradation (ERAD) mechanisms. In addition, type 1 BS is one of the diseases linked to ERAD pathways. In recent years, several molecular determinants of NKCC2 export from the ER and protein quality control have been identified. The aim of this review is therefore to summarize recent data regarding the protein quality control of NKCC2 and to discuss their potential implications in BS and blood pressure regulation.

Contemporary Decongestive Strategies in Acute Heart Failure

Congestion is the primary driver of hospital admissions in patients with heart failure and the key determinant of their outcome. Although intravenous loop diuretics remain the predominant agents used in the setting of acute heart failure, the therapeutic response is known to be variable, with a significant subset of patients discharged from the hospital with residual hypervolemia. In this context, urinary sodium excretion has gained attention both as a marker of response to loop diuretics and as a marker of prognosis that may be a useful clinical tool to guide therapy. Several decongestive strategies have been explored to improve diuretic responsiveness and removal of excess fluid. Sequential nephron blockade through combination diuretic therapy is one of the most used methods to enhance natriuresis and counter diuretic resistance. In this article, I provide an overview of the contemporary decongestive approaches and discuss the clinical data on the use of add-on diuretic therapy. I also discuss mechanical removal of excess fluid through extracorporeal ultrafiltration with a brief review of the results of landmark studies. Finally, I provide a short overview of the strategies that are currently under investigation and may prove helpful in this setting.

Effects of furosemide, acetazolamide and amiloride on renal cortical and medullary tissue oxygenation in non-anaesthetised healthy sheep

It has been proposed that diuretics can improve renal tissue oxygenation through inhibition of tubular sodium reabsorption and reduced metabolic demand. However, the impact of clinically used diuretic drugs on the renal cortical and medullary microcirculation is unclear. Therefore, we examined the effects of three commonly used diuretics, at clinically relevant doses, on renal cortical and medullary perfusion and oxygenation in non-anaesthetised healthy sheep. Merino ewes received acetazolamide (250 mg; n = 9), furosemide (20 mg; n = 10) or amiloride (10 mg; n = 7) intravenously. Systemic and renal haemodynamics, renal cortical and medullary tissue perfusion andP O 2 ${P_{{{\mathrm{O}}_{\mathrm{2}}}}}$ , and renal function were then monitored for up to 8 h post-treatment. The peak diuretic response occurred 2 h (99.4 ± 14.8 mL/h) after acetazolamide, at which stage cortical and medullary tissue perfusion andP O 2 ${P_{{{\mathrm{O}}_{\mathrm{2}}}}}$ were not significantly different from their baseline levels. The peak diuretic response to furosemide occurred at 1 h (196.5 ± 12.3 mL/h) post-treatment but there were no significant changes in cortical and medullary tissue oxygenation during this period. However, cortical tissueP O 2 ${P_{{{\mathrm{O}}_{\mathrm{2}}}}}$ fell from 40.1 ± 3.8 mmHg at baseline to 17.2 ± 4.4 mmHg at 3 h and to 20.5 ± 5.3 mmHg at 6 h after furosemide administration. Amiloride did not produce a diuretic response and was not associated with significant changes in cortical or medullary tissue oxygenation. In conclusion, clinically relevant doses of diuretic agents did not improve regional renal tissue oxygenation in healthy animals during the 8 h experimentation period. On the contrary, rebound renal cortical hypoxia may develop after dissipation of furosemide-induced diuresis.

Juxtaglomerular apparatus-mediated homeostatic mechanisms: therapeutic implication for chronic kidney disease

INTRODUCTION

Juxtaglomerular apparatus (JGA)-mediated homeostatic mechanism links to how sodium-glucose cotransporter 2 inhibitors (SGLT2is) slow progression of chronic kidney disease (CKD) and may link to how tolvaptan slows renal function decline in autosomal dominant polycystic kidney disease (ADPKD).

AREA COVERED

JGA-mediated homeostatic mechanism has been hypothesized based on investigations of tubuloglomerular feedback and renin-angiotensin system. We reviewed clinical trials of SGLT2is and tolvaptan to assess the relationship between this mechanism and these drugs.

EXPERT OPINION

When sodium load to macula densa (MD) increases, MD increases adenosine production, constricting afferent arteriole (Af-art) and protecting glomeruli. Concurrently, MD signaling suppresses renin secretion, increases urinary sodium excretion, and counterbalances reduced sodium filtration. However, when there is marked increase in sodium load per-nephron, as in advanced CKD, MD adenosine production increases, relaxing Af-art and maintaining sodium homeostasis at the expense of glomeruli. The beneficial effects of tolvaptan on renal function in ADPKD may also depend on the JGA-mediated homeostatic mechanisms since tolvaptan inhibits sodium reabsorption in the thick ascending limb.The JGA-mediated homeostatic mechanism regulates Af-arts, constricting to relaxing according to homeostatic needs. Understanding this mechanism may contribute to the development of pharmacotherapeutic compounds and better care for patients with CKD.

Sex differences in blood pressure regulation and hypertension: renal, hemodynamic, and hormonal mechanisms

The teleology of sex differences has been argued since at least as early as Aristotle's controversial Generation of Animals more than 300 years BC, which reflects the sex bias of the time to contemporary readers. Although the question "why are the sexes different" remains a topic of debate in the present day in metaphysics, the recent emphasis on sex comparison in research studies has led to the question "how are the sexes different" being addressed in health science through numerous observational studies in both health and disease susceptibility, including blood pressure regulation and hypertension. These efforts have resulted in better understanding of differences in males and females at the molecular level that partially explain their differences in vascular function and renal sodium handling and hence blood pressure and the consequential cardiovascular and kidney disease risks in hypertension. This review focuses on clinical studies comparing differences between men and women in blood pressure over the life span and response to dietary sodium and highlights experimental models investigating sexual dimorphism in the renin-angiotensin-aldosterone, vascular, sympathetic nervous, and immune systems, endothelin, the major renal sodium transporters/exchangers/channels, and the impact of sex hormones on these systems in blood pressure homeostasis. Understanding the mechanisms governing sex differences in blood pressure regulation could guide novel therapeutic approaches in a sex-specific manner to lower cardiovascular risks in hypertension and advance personalized medicine.

Effects of phenylbutazone, firocoxib, and dipyrone on the diuretic response to furosemide in horses

BACKGROUND

Treatment with phenylbutazone (nonselective COX inhibitor) decreases the diuretic and natriuretic effects of furosemide by nearly 30% but the effects of COX-2 specific inhibitors (firocoxib) and atypical NSAIDs (dipyrone) are unknown.

HYPOTHESIS

Furosemide-induced diuresis after pretreatment with firocoxib or dipyrone is diminished to a lesser extent than after pretreatment with phenylbutazone.

ANIMALS

Eight healthy mares.

METHODS

Each mare received 4 treatments in a prospective experimental crossover study using a replicated 4 × 4 Latin Square design: furosemide alone (FU), furosemide and phenylbutazone (PB), furosemide and firocoxib (FX), and furosemide and dipyrone (DP). After 24 hours of NSAID treatment at recommended dosages, ureteral catheters were placed for continual urine collection. After a 30-minute baseline collection period, furosemide (1.0 mg/kg, IV) was administered, and urine and blood samples were collected for 4 hours. Data were assessed by repeated measures ANOVA.

RESULTS

Four-hour urine volume was (mean ± SD) ~25% less (P < .001) after pretreatment with all NSAIDs (PB 19.1 ± 2.1 mL/kg, FX 17.7 ± 3.5 mL/kg, DP 19.1 ± 3.9 mL/kg), as compared to FU (23.4 ± 5.1 mL/kg) (P < .001), but there were no differences between PB, FX, or DP. Interindividual variability in furosemide diuresis after pretreatment with different NSAIDs was observed.

CONCLUSIONS AND CLINICAL IMPORTANCE

Though COX-2 selective NSAIDs and dipyrone might have less severe or fever gastrointestinal adverse effects in horses, our data suggest minimal differences in effects on furosemide-induced diuresis, and possibly, risk of nephrotoxicosis.

A better explanation of countercurrent multiplication in the formation of the corticopapillary osmotic gradient in the outer medulla

Countercurrent multiplication (CCM) is widely accepted as the mechanism for the generation of the corticopapillary osmotic gradient in the outer medulla of mammalian kidneys. However, several issues in the literature cause the current explanations of CCM to be inefficient and incomplete. As a result, it is challenging to clearly explain CCM in physiology education. The goal of this article is to share a modified version of CCM with more understandable explanation in the hopes of motivating peer discussion, further improvement, and future research. To reach this goal, the logical processes leading to CCM are first analyzed, which results in a set of formulas that serve as the principles governing CCM. Next, the cessation of CCM is addressed to provide a complete picture of the modified version of CCM. Throughout these two steps, the issues mentioned above are identified and addressed so that how the modified version of CCM eliminates these issues becomes clear. The formulas mentioned above are provided in the Tables S1, S2, and S3 (all Supplemental material is available in the Supplemental Excel File at https://doi.org/10.6084/m9.figshare.23515614) to explain how the interstitial and intrathick ascending limb osmotic concentration (OC) values used in the figures in this article are simulated and how alternative OC values can be generated from Tables S1 and S2 to illustrate CCM.NEW & NOTEWORTHY Countercurrent multiplication is widely accepted as the mechanism for the generation of the corticopapillary osmotic gradient in the outer medulla of mammalian kidneys, but the current explanations of it in textbooks and the literature are inefficient and incomplete, leading to confusion for students. This article shares a modified version of countercurrent multiplication with more understandable explanation as a way of motivating peer discussion, further improvement, and future research.

Potassium homeostasis - Physiology and pharmacology in a clinical context

Membrane voltage controls the function of excitable cells and is mainly a consequence of the ratio between the extra- and intracellular potassium concentration. Potassium homeostasis is safeguarded by balancing the extra-/intracellular distribution and systemic elimination of potassium to the dietary potassium intake. These processes adjust the plasma potassium concentration between 3.5 and 4.5 mmol/L. Several genetic and acquired diseases but also pharmacological interventions cause dyskalemias that are associated with increased morbidity and mortality. The thresholds at which serum K+ not only associates but also causes increased mortality are hotly debated. We discuss physiologic, pathophysiologic, and pharmacologic aspects of potassium regulation and provide informative case vignettes. Our aim is to help clinicians, epidemiologists, and pharmacologists to understand the complexity of the potassium homeostasis in health and disease and to initiate appropriate treatment strategies in dyskalemic patients.

Sex and race differences in urinary Tumor Necrosis Factor-α (TNF-α) levels: Secondary analysis of the DASH-sodium trial

Previous work in mouse models shows that urinary TNF-α levels become elevated when dietary salt (NaCl) intake increases. To examine if this relationship exists in humans, we conducted a secondary analysis of the Dietary Approaches to Stop Hypertension (DASH)-Sodium trial to determine levels of urinary TNF-α in 367 subjects categorized by race, sex, and blood pressure. The DASH-Sodium trial is a multicenter feeding trial in which subjects were randomly assigned to either the DASH or control diet, and high, medium, and low sodium in random order. Multivariable linear regression was used to model baseline TNF-α and a mixed model was used to model TNF-α as a function of dietary intervention. At baseline, with all subjects on a "typical American diet", urinary TNF-α levels were lowest in Black, p = 0.002 and male subjects, p < 0.001. After randomization to either the DASH or control diet, with increasing levels of sodium, urinary TNF-α levels increased only in subjects on the control diet, p < 0.05. As in the baseline analysis, TNF-α levels were highest in White females, then White males, Black females and lowest in Black males. The results indicate that urinary TNF-α levels in DASH-Sodium subjects are regulated by NaCl intake, modulated by the DASH diet, and influenced by both race and sex. The inherent differences between subgroups support studies in mice showing that increases in renal TNF-α minimize the extent salt-dependent activation of NKCC2.

A profile of SGLT-2 inhibitors in hyponatremia: The evidence to date

Hyponatremia is the most common electrolyte disorder in clinical practice, which may lead to life-threatening complications. Several lines of evidence suggest that hyponatremia is associated not only with significant increases in length of stay, cost, and financial burden, but also with increased morbidity and mortality. Hyponatremia is also considered to be a negative prognostic factor in patients with heart failure and cancer. Although multiple therapeutic methods are available for treating hyponatremia, most have some limitations, such as poor compliance, rapid correction of serum Na⁺, other negative side effects and high cost. Given these limitations, identifying novel therapies for hyponatremia is essential. Recent clinical studies have shown that SGLT-2 inhibitors (SGLT 2i) significantly increased serum Na⁺ levels and were well tolerated by patients who underwent this treatment. Therefore, oral administration of SGLT 2i appears to be an effective treatment for hyponatremia. This article will briefly review the etiology of hyponatremia and integrated control of sodium within the kidney, current therapies for hyponatremia, potential mechanisms and efficacy of SGLT 2i for hyponatremia, and the benefits in cardiovascular, cancer, and kidney disease by regulating sodium and water balance.

Na+/Cl- cotransporter 2 is not fish-specific and is widely found in amphibians, non-avian reptiles, and select mammals

Solute carrier 12 (Slc12) is a family of electroneutral cation-coupled chloride (Cl-) cotransporters. Na+/K+/2Cl- (Nkcc) and Na+/Cl- cotransporters (Ncc) belong to the Nkcc/Ncc subfamily. Human and mouse possess one gene for the Na+/Cl- cotransporter (ncc gene: slc12a3), whereas teleost fishes possess multiple ncc genes, slc12a3 (ncc1) and slc12a10 (ncc2), in addition to their species-specific paralogs. Amphibians and squamates have two ncc genes: slc12a3 (ncc1) and ncc3. However, the evolutionary relationship between slc12a10 and ncc3 remains unresolved, and the presence of slc12a10 (ncc2) in mammals has not been clarified. Synteny and phylogenetic analyses of vertebrate genome databases showed that ncc3 is the ortholog of slc12a10, and slc12a10 is present in most ray-finned fishes, coelacanths, amphibians, reptiles, and a few mammals (e.g., platypus and horse) but pseudogenized or deleted in birds, most mammals, and some ray-finned fishes (pufferfishes). This shows that slc12a10 is widely present among bony vertebrates and pseudogenized or deleted independently in multiple lineages. Notably, as compared with some fish that show varied slc12a10 tissue expression profile, spotted gar, African clawed frog, red-eared slider turtle, and horse express slc12a10 in the ovaries or premature gonads. In horse tissues, an unexpectedly large number of splicing variants for Slc12a10 have been cloned, many of which encode truncated forms of Slc12a10, suggesting that the functional constraints of horse slc12a10 are weakened, which may be in the process of becoming a pseudogene. Our results elaborate on the evolution of Nkcc/Ncc subfamily of Slc12 in vertebrates.NEW & NOTEWORTHY slc12a10 is not a fish-specific gene and is present in a few mammals (e.g., platypus and horse), non-avian reptiles, amphibians, but was pseudogenized or deleted in most mammals (e.g., human, mouse, cat, cow, and rhinoceros), birds, and some ray-finned fishes (pufferfishes).

MicroRNA-195a-5p Regulates Blood Pressure by Inhibiting NKCC2A

BACKGROUND

Previous studies showed that miR-195a-5p was among the most abundant microRNAs (miRNAs) expressed in the kidney.

METHODS

Lentivirus silencing of tumor necrosis factor-α (TNF) was performed in vivo and in vitro. Luciferase reporter assays confirmed that bumetanide-sensitive Na+-K+-2Cl- cotransporter isoform A (NKCC2A) mRNA is targeted and repressed by miR-195a-5p. Radiotelemetry was used to measure mean arterial pressure.

RESULTS

TNF upregulates mmu-miR-195a-5p, and -203 and downregulates mmu-miR-30c and -100 in the medullary thick ascending limb of male mice. miR-195a-5p was >3-fold higher in the renal outer medulla of mice given an intrarenal injection of murine recombinant TNF, whereas silencing TNF inhibited miR-195a-5p expression by ≈51%. Transient transfection of a miR-195a-5p mimic into medullary thick ascending limb cells suppressed NKCC2A mRNA by ≈83%, whereas transfection with Anti-miR-195a-5p increased NKCC2A mRNA. Silencing TNF in medullary thick ascending limb cells prevented increases in miR-195 induced by 400 mosmol/kg H2O medium, an effect reversed by transfection with a miR-195a-5p mimic. Expression of phosphorylated NKCC2 increased 1.5-fold in medullary thick ascending limb cells transfected with Anti-miR-195a-5p and a miR-195a-5p mimic prevented the increase, which was induced by silencing TNF in cells exposed to 400 mosmol/kg H2O medium after osmolality was increased by adding NaCl. Intrarenal injection of TNF suppressed NKCC2A mRNA, whereas injection of miR-195a-5p prevented the increase of NKCC2A mRNA abundance and phosphorylated NKCC2 expression when TNF was silenced. Intrarenal injection with miR-195a-5p markedly attenuated MAP after renal silencing of TNF in mice given 1% NaCl.

CONCLUSIONS

The study identifies miR-195a-5p as a salt-sensitive and TNF-inducible miRNA that attenuates NaCl-mediated increases in blood pressure by inhibiting NKCC2A.

Controls of Central and Peripheral Blood Pressure and Hemorrhagic/Hypovolemic Shock

The pressure exerted on the heart and blood vessels because of blood flow is considered an essential parameter for cardiovascular function. It determines sufficient blood perfusion, and transportation of nutrition, oxygen, and other essential factors to every organ. Pressure in the primary arteries near the heart and the brain is known as central blood pressure (CBP), while that in the peripheral arteries is known as peripheral blood pressure (PBP). Usually, CBP and PBP are correlated; however, various types of shocks and cardiovascular disorders interfere with their regulation and differently affect the blood flow in vital and accessory organs. Therefore, understanding blood pressure in normal and disease conditions is essential for managing shock-related cardiovascular implications and improving treatment outcomes. In this review, we have described the control systems (neural, hormonal, osmotic, and cellular) of blood pressure and their regulation in hemorrhagic/hypovolemic shock using centhaquine (Lyfaquin^®) as a resuscitative agent.

Apelin is expressed throughout the human kidney, is elevated in chronic kidney disease & associates independently with decline in kidney function

AIMS

Chronic kidney disease (CKD) is common and cardiovascular disease (CVD) is its commonest complication. The apelin system is a potential therapeutic target for CVD but data relating to apelin in CKD are limited. We examined expression of the apelin system in human kidney, and investigated apelin and Elabela/Toddler (ELA), the endogenous ligands for the apelin receptor, in patients with CKD.

METHODS

Using autoradiography, immunohistochemistry and enzyme-linked immunosorbent assay, we assessed expression of apelin, ELA and the apelin receptor in healthy human kidney, and measured plasma apelin and ELA in 155 subjects (128 patients with CKD, 27 matched controls) followed up for 5 years. Cardiovascular assessments included blood pressure, arterial stiffness (pulse wave velocity) and brachial artery flow-mediated dilation. Surrogate markers of endothelial function (plasma asymmetric dimethylarginine and endothelin-1) and inflammation (C-reactive protein and interleukin-6) were measured.

RESULTS

The apelin system was expressed in healthy human kidney, throughout the nephron. Plasma apelin concentrations were 60% higher in women than men (6.48 [3.62-9.89] vs. 3.95 [2.02-5.85] pg/mL; P < .0001), and increased as glomerular filtration rate declined (R = -0.41, P < .0001), and albuminuria rose (R = 0.52, P < .0001). Plasma apelin and ELA were associated with vascular dysfunction. Plasma apelin associated independently with a 50% decline in glomerular filtration rate at 5 years.

CONCLUSION

We show for the first time that the apelin system is expressed in healthy human kidney. Plasma apelin is elevated in CKD and may be a potential biomarker of risk of decline in kidney function. Clinical studies exploring the therapeutic potential of apelin agonism in CKD are warranted.

Structural Pharmacology of Cation-Chloride Cotransporters

Loop and thiazide diuretics have been cornerstones of clinical management of hypertension and fluid overload conditions for more than five decades. The hunt for their molecular targets led to the discovery of cation-chloride cotransporters (CCCs) that catalyze electroneutral movement of Cl^- together with Na⁺ and/or K⁺. CCCs consist of two 1 Na⁺-1 K⁺-2 Cl^- (NKCC1-2), one 1 Na⁺-1 Cl^- (NCC), and four 1 K⁺-1 Cl^- (KCC1-4) transporters in human. CCCs are fundamental in trans-epithelia ion secretion and absorption, homeostasis of intracellular Cl^- concentration and cell volume, and regulation of neuronal excitability. Malfunction of NKCC2 and NCC leads to abnormal salt and water retention in the kidney and, consequently, imbalance in electrolytes and blood pressure. Mutations in KCC2 and KCC3 are associated with brain disorders due to impairments in regulation of excitability and possibly cell volume of neurons. A recent surge of structures of CCCs have defined their dimeric architecture, their ion binding sites, their conformational changes associated with ion translocation, and the mechanisms of action of loop diuretics and small molecule inhibitors. These breakthroughs now set the stage to expand CCC pharmacology beyond loop and thiazide diuretics, developing the next generation of diuretics with improved potency and specificity. Beyond drugging renal-specific CCCs, brain-penetrable therapeutics are sorely needed to target CCCs in the nervous system for the treatment of neurological disorders and psychiatric conditions.

Alleviation of liver cirrhosis and associated portal-hypertension by Astragalus species in relation to their UPLC-MS/MS metabolic profiles: a mechanistic study

Liver cirrhosis is a late-stage liver disease characterized by excessive fibrous deposition triggering portal-hypertension (PH); the prime restrainer for cirrhosis-related complications. Remedies that can dually oppose hepatic fibrosis and lower PH, may prevent progression into decompensated-cirrhosis. Different Astragalus-species members have shown antifibrotic and diuretic actions with possible subsequent PH reduction. However, A.spinosus and A.trigonus were poorly tested for eliciting these actions. Herein, A.spinosus and A.trigonus roots and aerial parts extracts were subjected to comprehensive metabolic-fingerprinting using UHPLC-MS/MS resulting in 56 identified phytoconstituents, followed by chemometric untargeted analysis that revealed variable metabolic profiles exemplified by different species and organ types. Consequently, tested extracts were in-vivo evaluated for potential antifibrotic/anticirrhotic activity by assessing specific markers. The mechanistic prospective to induce diuresis was investigated by analyzing plasma aldosterone and renal-transporters gene-expression. Serum apelin and dimethylarginine-dimethylaminohydrolase-1 were measured to indicate the overall effect on PH. All extracts amended cirrhosis and PH to varying extents and induced diuresis via different mechanisms. Further, An OPLS model was built to generate a comprehensive metabolic-profiling of A.spinosus and A.trigonus secondary-metabolites providing a chemical-based evidence for their efficacious consistency. In conclusion, A.spinosus and A.trigonus organs comprised myriad pharmacologically-active constituents that act synergistically to ameliorate cirrhosis and associated PH.

Effects of Sodium-Glucose Cotransporter 2 Inhibitors on Water and Sodium Metabolism

Sodium-glucose cotransporter 2 (SGLT2) inhibitors exert hypoglycemic and diuretic effects by inhibiting the absorption of sodium and glucose from the proximal tubule. Currently available data indicate that SGLT2 inhibitors transiently enhance urinary sodium excretion and urinary volume. When combined with loop diuretics, SGLT2 inhibitors exert a synergistic natriuretic effect. The favorable diuretic profile of SGLT2 inhibitors may confer benefits to volume management in patients with heart failure but this natriuretic effect may not be the dominant mechanism for the superior long-term outcomes observed with these agents in patients with heart failure. The first part of this review explores the causes of transient natriuresis and the diuretic mechanisms of SGLT2 inhibitors. The second part provides an overview of the synergistic effects of combining SGLT2 inhibitors with loop diuretics, and the third part summarizes the mechanisms of cardiovascular protection associated with the diuretic effects of SGLT2 inhibitors.

Genetic Heterogeneity in Bartter Syndrome: Clinical and Practical Importance

Bartter syndrome (BS) is a rare tubulopathy that causes polyuria, hypokalemia, hypochloremic metabolic alkalosis, and normotensive hyperreninemic hyperaldosteronism. It is characterized by locus, clinical, and allelic heterogeneity. Types 1-4 of BS are inherited according to an autosomal recessive pattern, while type 5, which is transient, is X linked. There are specific correlations between the clinical expression and the molecular defect, but since it is a rare disease, such studies are rare. Therapeutic interventions are different, being correlated with types of BS.

Golgi Alpha1,2-Mannosidase IA Promotes Efficient Endoplasmic Reticulum-Associated Degradation of NKCC2

Mutations in the apically located kidney Na-K-2Cl cotransporter NKCC2 cause type I Bartter syndrome, a life-threatening kidney disorder. We previously showed that transport from the ER represents the limiting phase in NKCC2 journey to the cell surface. Yet very little is known about the ER quality control components specific to NKCC2 and its disease-causing mutants. Here, we report the identification of Golgi alpha1, 2-mannosidase IA (ManIA) as a novel binding partner of the immature form of NKCC2. ManIA interaction with NKCC2 takes place mainly at the cis-Golgi network. ManIA coexpression decreased total NKCC2 protein abundance whereas ManIA knock-down produced the opposite effect. Importantly, ManIA coexpression had a more profound effect on NKCC2 folding mutants. Cycloheximide chase assay showed that in cells overexpressing ManIA, NKCC2 stability and maturation are heavily hampered. Deleting the cytoplasmic region of ManIA attenuated its interaction with NKCC2 and inhibited its effect on the maturation of the cotransporter. ManIA-induced reductions in NKCC2 expression were offset by the proteasome inhibitor MG132. Likewise, kifunensine treatment greatly reduced ManIA effect, strongly suggesting that mannose trimming is involved in the enhanced ERAD of the cotransporter. Moreover, depriving ManIA of its catalytic domain fully abolished its effect on NKCC2. In summary, our data demonstrate the presence of a ManIA-mediated ERAD pathway in renal cells promoting retention and degradation of misfolded NKCC2 proteins. They suggest a model whereby Golgi ManIA contributes to ERAD of NKCC2, by promoting the retention, recycling, and ERAD of misfolded proteins that initially escape protein quality control surveillance within the ER.

Anatomophysiology of the Henle's Loop: Emphasis on the Thick Ascending Limb

The loop of Henle plays a variety of important physiological roles through the concerted actions of ion transport systems in both its apical and basolateral membranes. It is involved most notably in extracellular fluid volume and blood pressure regulation as well as Ca²⁺ , Mg²⁺ , and acid-base homeostasis because of its ability to reclaim a large fraction of the ultrafiltered solute load. This nephron segment is also involved in urinary concentration by energizing several of the steps that are required to generate a gradient of increasing osmolality from cortex to medulla. Another important role of the loop of Henle is to sustain a process known as tubuloglomerular feedback through the presence of specialized renal tubular cells that lie next to the juxtaglomerular arterioles. This article aims at describing these physiological roles and at discussing a number of the molecular mechanisms involved. It will also report on novel findings and uncertainties regarding the realization of certain processes and on the pathophysiological consequences of perturbed salt handling by the thick ascending limb of the loop of Henle. Since its discovery 150 years ago, the loop of Henle has remained in the spotlight and is now generating further interest because of its role in the renal-sparing effect of SGLT2 inhibitors. © 2022 American Physiological Society. Compr Physiol 12:1-21, 2022.

Renal expression and urinary excretion of aquaporin-2 in postobstructive uropathy in rats

This work assessed the time course of water renal management together with aquaporin-2 (AQP2) kidney expression and urinary AQP2 levels (AQP2u) in obstructive nephropathy. Adult male Wistar rats were monitored after 1, 2, and 7 days of bilateral ureteral release (bilateral ureteral obstruction (BUO); BUO-1, BUO-2 and BUO-7). Renal water handling was evaluated using conventional clearance techniques. AQP2 levels were assessed by immunoblotting and immunohistochemical techniques. AQP2 expression in apical membranes was downregulated in BUO-1 rats and upregulated both in BUO-2 and BUO-7 animals. AQP2 protein expression in whole cell lysate fraction from kidney cortex and medulla were significantly decreased in all the experimental groups. Concomitantly, mRNA levels of AQP2 decreased in renal medulla of all groups and in renal cortex from BUO-1; however, in renal cortex from BUO-2 and BUO-7 a recovery and an increase in the level of AQP2 mRNA were, respectively, observed. BUO-7 group showed a significant increase in AQP2u. The alterations observed in apical membranes AQP2 expression could explain, at least in part, the evolution time of water kidney management in the postobstructive phase of BUO. Additionally, the AQP2u increase after 7 days of ureteral release may be postulated as a biomarker of improvement in the kidney function.

The role of GABAergic signalling in neurodevelopmental disorders

GABAergic inhibition shapes the connectivity, activity and plasticity of the brain. A series of exciting new discoveries provides compelling evidence that disruptions in a number of key facets of GABAergic inhibition have critical roles in the aetiology of neurodevelopmental disorders (NDDs). These facets include the generation, migration and survival of GABAergic neurons, the formation of GABAergic synapses and circuit connectivity, and the dynamic regulation of the efficacy of GABAergic signalling through neuronal chloride transporters. In this Review, we discuss recent work that elucidates the functions and dysfunctions of GABAergic signalling in health and disease, that uncovers the contribution of GABAergic neural circuit dysfunction to NDD aetiology and that leverages such mechanistic insights to advance precision medicine for the treatment of NDDs.

Regulatory control of the Na-Cl co-transporter NCC and its therapeutic potential for hypertension

Hypertension is the largest risk factor for cardiovascular disease, the leading cause of mortality worldwide. As blood pressure regulation is influenced by multiple physiological systems, hypertension cannot be attributed to a single identifiable etiology. Three decades of research into Mendelian forms of hypertension implicated alterations in the renal tubular sodium handling, particularly the distal convoluted tubule (DCT)-native, thiazide-sensitive Na-Cl cotransporter (NCC). Altered functions of the NCC have shown to have profound effects on blood pressure regulation as illustrated by the over activation and inactivation of the NCC in Gordon's and Gitelman syndromes respectively. Substantial progress has uncovered multiple factors that affect the expression and activity of the NCC. In particular, NCC activity is controlled by phosphorylation/dephosphorylation, and NCC expression is facilitated by glycosylation and negatively regulated by ubiquitination. Studies have even found parvalbumin to be an unexpected regulator of the NCC. In recent years, there have been considerable advances in our understanding of NCC control mechanisms, particularly via the pathway containing the with-no-lysine [K] (WNK) and its downstream target kinases, SPS/Ste20-related proline-alanine-rich kinase (SPAK) and oxidative stress responsive 1 (OSR1), which has led to the discovery of novel inhibitory molecules. This review summarizes the currently reported regulatory mechanisms of the NCC and discusses their potential as therapeutic targets for treating hypertension.

Renal and non-renal response of ABC and SLC transporters in chronic kidney disease

INTRODUCTION

The solute carrier (SLC) and the ATP-binding cassette (ABC) transporter superfamilies play essential roles in the disposition of small molecules (endogenous metabolites, uremic toxins, drugs) in the blood, kidney, liver, intestine, and other organs. In chronic kidney disease (CKD), the loss of renal function is associated with altered function of remote organs. As renal function declines, many molecules accumulate in the plasma. Many studies now support the view that ABC and SLC transporters as well as drug metabolizing enzymes (DMEs) in renal and non-renal tissues are directly or indirectly affected by the presence of various types of uremic toxins, including those derived from the gut microbiome; this can lead to aberrant inter-organ communication.

AREAS COVERED

Here, the expression, localization and/or function of various SLC and ABC transporters as well as DMEs in the kidney and other organs are discussed in the context of CKD and systemic pathophysiology.

EXPERT OPINION

According to the Remote Sensing and Signaling Theory (RSST), a transporter and DME-centric network that optimizes local and systemic metabolism maintains homeostasis in the steady state and resets homeostasis following perturbations due to renal dysfunction. The implications of this view for pharmacotherapy of CKD are also discussed.

Single-cell mRNA profiling reveals changes in solute carrier expression and suggests a metabolic switch during zebrafish pronephros development

Developing organisms need to adapt to environmental variations as well as to rapid changes in substrate availability and energy demands imposed by fast-growing tissues and organs. Little is known about the adjustments that kidneys undergo in response to these challenges. We performed single-cell RNA sequencing of zebrafish pronephric duct cells to understand how the developing kidney responds to changes in filtered substrates and intrinsic energy requirements. We found high levels of glucose transporters early in development and increased expression of monocarboxylate transporters at later times. This indicates that the zebrafish embryonic kidney displays a high glucose transporting capacity during early development, which is replaced by the ability to absorb monocarboxylates and amino acids at later stages. This change in transport capacity was accompanied by the upregulation of mitochondrial carriers, indicating a switch to increased oxidative phosphorylation to meet the increasing energy demand of a developing kidney.NEW & NOTEWORTHY The zebrafish embryonic kidney has high levels of glucose transporters during early development, which are replaced by monocarboxylate and amino acid transporters later on. Inhibition of Na⁺-glucose cotransporter-dependent glucose transport by sotagliflozin also increased slc2a1a expression, supporting the idea that the glucose transport capacity is dynamically adjusted during zebrafish pronephros development. Concurrent upregulation of mitochondrial SCL25 transporters at later stages supports the idea that the pronephros adjusts to changing substrate supplies and/or energy demands during embryonic development.

Differential Effects of STCH and Stress-Inducible Hsp70 on the Stability and Maturation of NKCC2

Mutations in the Na-K-2Cl co-transporter NKCC2 lead to type I Bartter syndrome, a life-threatening kidney disease. We previously showed that export from the ER constitutes the limiting step in NKCC2 maturation and cell surface expression. Yet, the molecular mechanisms involved in this process remain obscure. Here, we report the identification of chaperone stress 70 protein (STCH) and the stress-inducible heat shock protein 70 (Hsp70), as two novel binding partners of the ER-resident form of NKCC2. STCH knock-down increased total NKCC2 expression whereas Hsp70 knock-down or its inhibition by YM-01 had the opposite effect. Accordingly, overexpressing of STCH and Hsp70 exerted opposite actions on total protein abundance of NKCC2 and its folding mutants. Cycloheximide chase assay showed that in cells over-expressing STCH, NKCC2 stability and maturation are heavily impaired. In contrast to STCH, Hsp70 co-expression increased NKCC2 maturation. Interestingly, treatment by protein degradation inhibitors revealed that in addition to the proteasome, the ER associated degradation (ERAD) of NKCC2 mediated by STCH, involves also the ER-to-lysosome-associated degradation pathway. In summary, our data are consistent with STCH and Hsp70 having differential and antagonistic effects with regard to NKCC2 biogenesis. These findings may have an impact on our understanding and potential treatment of diseases related to aberrant NKCC2 trafficking and expression.

Differential Effects of STCH and Stress-Inducible Hsp70 on the Stability and Maturation of NKCC2

Mutations in the Na-K-2Cl co-transporter NKCC2 lead to type I Bartter syndrome, a life-threatening kidney disease. We previously showed that export from the ER constitutes the limiting step in NKCC2 maturation and cell surface expression. Yet, the molecular mechanisms involved in this process remain obscure. Here, we report the identification of chaperone stress 70 protein (STCH) and the stress-inducible heat shock protein 70 (Hsp70), as two novel binding partners of the ER-resident form of NKCC2. STCH knock-down increased total NKCC2 expression whereas Hsp70 knock-down or its inhibition by YM-01 had the opposite effect. Accordingly, overexpressing of STCH and Hsp70 exerted opposite actions on total protein abundance of NKCC2 and its folding mutants. Cycloheximide chase assay showed that in cells over-expressing STCH, NKCC2 stability and maturation are heavily impaired. In contrast to STCH, Hsp70 co-expression increased NKCC2 maturation. Interestingly, treatment by protein degradation inhibitors revealed that in addition to the proteasome, the ER associated degradation (ERAD) of NKCC2 mediated by STCH, involves also the ER-to-lysosome-associated degradation pathway. In summary, our data are consistent with STCH and Hsp70 having differential and antagonistic effects with regard to NKCC2 biogenesis. These findings may have an impact on our understanding and potential treatment of diseases related to aberrant NKCC2 trafficking and expression.

Mutation affecting the conserved acidic WNK1 motif causes inherited hyperkalemic hyperchloremic acidosis

Gain-of-function mutations in with no lysine (K) 1 (WNK1) and WNK4 genes are responsible for familial hyperkalemic hypertension (FHHt), a rare, inherited disorder characterized by arterial hypertension and hyperkalemia with metabolic acidosis. More recently, FHHt-causing mutations in the Kelch-like 3-Cullin 3 (KLHL3-CUL3) E3 ubiquitin ligase complex have shed light on the importance of WNK's cellular degradation on renal ion transport. Using full exome sequencing for a 4-generation family and then targeted sequencing in other suspected cases, we have identified new missense variants in the WNK1 gene clustering in the short conserved acidic motif known to interact with the KLHL3-CUL3 ubiquitin complex. Affected subjects had an early onset of a hyperkalemic hyperchloremic phenotype, but normal blood pressure values"Functional experiments in Xenopus laevis oocytes and HEK293T cells demonstrated that these mutations strongly decrease the ubiquitination of the kidney-specific isoform KS-WNK1 by the KLHL3-CUL3 complex rather than the long ubiquitous catalytically active L-WNK1 isoform. A corresponding CRISPR/Cas9 engineered mouse model recapitulated both the clinical and biological phenotypes. Renal investigations showed increased activation of the Ste20 proline alanine-rich kinase-Na+-Cl- cotransporter (SPAK-NCC) phosphorylation cascade, associated with impaired ROMK apical expression in the distal part of the renal tubule. Together, these new WNK1 genetic variants highlight the importance of the KS-WNK1 isoform abundance on potassium homeostasis.

High glucose-induced effects on Na+-K+-2Cl- cotransport and Na+/H+ exchange of blood-brain barrier endothelial cells: involvement of SGK1, PKCβII, and SPAK/OSR1

Hyperglycemia exacerbates edema formation and worsens neurological outcome in ischemic stroke. Edema formation in the early hours of stroke involves transport of ions and water across an intact blood-brain barrier (BBB), and swelling of astrocytes. We showed previously that high glucose (HG) exposures of 24 hours to 7 days increase abundance and activity of BBB Na⁺-K⁺-2Cl^- cotransport (NKCC) and Na⁺/H⁺ exchange 1 (NHE1). Further, bumetanide and HOE-642 inhibition of these transporters significantly reduces edema and infarct following middle cerebral artery occlusion in hyperglycemic rats, suggesting that NKCC and NHE1 are effective therapeutic targets for reducing edema in hyperglycemic stroke. The mechanisms underlying hyperglycemia effects on BBB NKCC and NHE1 are not known. In the present study we investigated whether serum-glucocorticoid regulated kinase 1 (SGK1) and protein kinase C beta II (PKCβII) are involved in HG effects on BBB NKCC and NHE1. We found transient increases in phosphorylated SGK1 and PKCβII within the first hour of HG exposure, after 5-60 min for SGK1 and 5 min for PKCβII. However, no changes were observed in cerebral microvascular endothelial cell SGK1 or PKCβII abundance or phosphorylation (activity) after 24 or 48 h HG exposures. Further, we found that HG-induced increases in NKCC and NHE1 abundance were abolished by inhibition of SGK1 but not PKCβII, whereas the increases in NKCC and NHE activity were abolished by inhibition of either kinase. Finally, we found evidence that STE20/SPS1-related proline/alanine-rich kinase and oxidative stress-responsive kinase-1 (SPAK/OSR1) participate in the HG-induced effects on BBB NKCC.

ENaC expression correlates with the acute furosemide-induced K+ excretion

BACKGROUND

In the aldosterone-sensitive distal nephron (ASDN), epithelial sodium channel (ENaC)-mediated Na+ absorption drives K+ excretion. K+ excretion depends on the delivery of Na+ to the ASDN and molecularly activated ENaC. Furosemide is known as a K+ wasting diuretic as it greatly enhances Na+ delivery to the ASDN. Here, we studied the magnitude of acute furosemide-induced kaliuresis under various states of basal molecular ENaC activity.

METHODS

C57/Bl6J mice were subjected to different dietary regimens that regulate molecular ENaC expression and activity levels. The animals were anesthetized and bladder-catheterized. Diuresis was continuously measured before and after administration of furosemide (2 µg/g BW) or benzamil (0.2 µg/g BW). Flame photometry was used to measure urinary [Na+ ] and [K+ ]. The kidneys were harvested and, subsequently, ENaC expression and cleavage activation were determined by semiquantitative western blotting.

RESULTS

A low K+ and a high Na+ diet markedly suppressed ENaC protein expression, cleavage activation, and furosemide-induced kaliuresis. In contrast, furosemide-induced kaliuresis was greatly enhanced in animals fed a high K+ or low Na+ diet, conditions with increased ENaC expression. The furosemide-induced diuresis was similar in all dietary groups.

CONCLUSION

Acute furosemide-induced kaliuresis differs greatly and depends on the a priori molecular expression level of ENaC. Remarkably, it can be even absent in animals fed a high Na+ diet, despite a marked increase of tubular flow and urinary Na+ excretion. This study provides auxiliary evidence that acute ENaC-dependent K+ excretion requires both Na+ as substrate and molecular activation of ENaC.

A long-term study of the effects of SLC12A1 homozygous mutation (g.62382825G>A, p.Pro372Leu) in Japanese Black cattle

Recessive missense mutation in the solute carrier family 12, member 1 (SLC12A1) gene (g.62382825G>A) is associated with hydrallantois, which is the accumulation of fluid in the allantoic cavity of a pregnant animal, and usually causes fetal death in Japanese Black cattle. However, the symptoms of a homozygote with this mutation that do not result in fetal death have not previously been tracked and evaluated. In the present study, we observed a homozygote with the SLC12A1 risk allele over a long-term period. The calf did not show any obvious clinical symptoms, although it did exhibit a slight growth retardation that accompanied mild calciuria. At 28 months of age, the homozygote showed renal dysfunction, which in turn resulted in hydronephrosis. The time course of the symptoms was consistent with the phenotype of Bartter syndrome in humans. Additionally, the risk heterozygous genotype did not any effects on carcass traits, which indicates that eliminating the risk allele would not have any unfavorable effects. Therefore, we emphasize that both the fetal- and late-stage symptoms associated with the SLC12A1 risk allele compromise animal welfare, and consequently may result in severe economic losses for individual farmers if the SLC12A1 risk allele is not eliminated from the population.

Conditional Myh9 and Myh10 inactivation in adult mouse renal epithelium results in progressive kidney disease

Actin-associated nonmuscle myosin II (NM2) motor proteins play critical roles in a myriad of cellular functions, including endocytosis and organelle transport pathways. Cell type–specific expression and unique subcellular localization of the NM2 proteins, encoded by the Myh9 and Myh10 genes, in the mouse kidney tubules led us to hypothesize that these proteins have specialized functional roles within the renal epithelium. Inducible conditional knockout (cKO) of Myh9 and Myh10 in the renal tubules of adult mice resulted in progressive kidney disease. Prior to overt renal tubular injury, we observed intracellular accumulation of the glycosylphosphatidylinositol-anchored protein uromodulin (UMOD) and gradual loss of Na⁺ K⁺ 2Cl^– cotransporter from the apical membrane of the thick ascending limb epithelia. The UMOD accumulation coincided with expansion of endoplasmic reticulum (ER) tubules and activation of ER stress and unfolded protein response pathways in Myh9&10-cKO kidneys. We conclude that NM2 proteins are required for localization and transport of UMOD and loss of function results in accumulation of UMOD and ER stress–mediated progressive renal tubulointerstitial disease. These observations establish cell type–specific role(s) for NM2 proteins in regulation of specialized renal epithelial transport pathways and reveal the possibility that human kidney disease associated with MYH9 mutations could be of renal epithelial origin.

Adult mouse renal epithelium specific knockout of Myh9 and Myh10 genes result in kidney disease.

Multiple Facets and Roles of Na+-K+-Cl−Cotransport: Mechanisms and Therapeutic Implications

The Na+-K+-Cl−cotransporters play key physiological and pathophysiological roles by regulating the membrane potential of many cell types and the movement of fluid across a variety of epithelial or endothelial structures. As such, they should soon become invaluable targets for the treatment of various disorders including pain, epilepsy, brain edema, and hypertension. This review highlights the nature of these roles, the mechanisms at play, and the unresolved issues in the field.

Differential diagnosis of perinatal Bartter, Bartter and Gitelman syndromes

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

OEsophageal Ion Transport Mechanisms and Significance Under Pathological Conditions

Ion transporters play an important role in several physiological functions, such as cell volume regulation, pH homeostasis and secretion. In the oesophagus, ion transport proteins are part of the epithelial resistance, a mechanism which protects the oesophagus against reflux-induced damage. A change in the function or expression of ion transporters has significance in the development or neoplastic progression of Barrett’s oesophagus (BO). In this review, we discuss the physiological and pathophysiological roles of ion transporters in the oesophagus, highlighting transport proteins which serve as therapeutic targets or prognostic markers in eosinophilic oesophagitis, BO and esophageal cancer. We believe that this review highlights important relationships which might contribute to a better understanding of the pathomechanisms of esophageal diseases.

Molecular regulation of NKCC2 in blood pressure control and hypertension

PURPOSE OF REVIEW

The apical Na/K/2Cl cotransporter (NKCC2) mediates NaCl reabsorption by the thick ascending limb, contributing to maintenance of blood pressure (BP). Despite effective NKCC2 inhibition by loop diuretics, these agents are not viable for long-term management of BP due to side effects. Novel molecular mechanisms that control NKCC2 activity reveal an increasingly complex picture with interacting layers of NKCC2 regulation. Here, we review the latest developments that shine new light on NKCC2-mediated control of BP and potential new long-term therapies to treat hypertension.

RECENT FINDINGS

Emerging molecular NKCC2 regulators, often binding partners, reveal a complex overlay of interacting mechanisms aimed at fine tuning NKCC2 activity. Different factors achieve this by shifting the balance between trafficking steps like exocytosis, endocytosis, recycling and protein turnover, or by balancing phosphorylation vs. dephosphorylation. Further molecular details are also emerging on previously known pathways of NKCC2 regulation, and recent in-vivo data continues to place NKCC2 regulation at the center of BP control.

SUMMARY

Several layers of emerging molecular mechanisms that control NKCC2 activity may operate simultaneously, but they can also be controlled independently. This provides an opportunity to identify new pharmacological targets to fine-tune NKCC2 activity for BP management.

Animal Models and Renal Biomarkers of Diabetic Nephropathy

Diabetes mellitus (DM) is the first cause of end stage chronic kidney disease (CKD). Animal models of the disease can shed light on the pathogenesis of the diabetic nephropathy (DN) and novel and earlier biomarkers of the condition may help to improve diagnosis and prognosis. This review summarizes the most important features of animal models used in the study of DN and updates the most recent progress in biomarker research.

Vasopressin receptor subtypes and renal sodium transport

In mammals, three subtypes of V-receptors have been identified in the kidney. The effects of vasopressin, a hormone synthesized in the hypothalamus, are triggered by three distinct receptor isoforms: V2, V1a, and V1b. Stimulation of V2-receptors regulates urine osmotic concentration by increasing sodium reabsorption in the thick ascending limb of the loop of Henle and enhancing osmotic permeability of the epithelium cells in the collecting duct. Stimulation of V1a-receptors inhibits renal sodium reabsorption and induces natriuresis, comparable to the effect of the diuretic furosemide, in the thick ascending limb of the loop of Henle. Stimulation of V1b-receptors induces potassium secretion in the final parts of the distal segments and initial parts of the collecting ducts. In this review, we discuss the role of vasopressin and its interaction with V-receptor subtypes in natriuresis and for stabilizing the physicochemical parameters of the internal environment and water-salt homeostasis in humans. A better understanding of these systems and their regulation is necessary to facilitate identification of additional system components and mechanisms, clarify their contribution during various normal and pathological functional states, and suggest novel strategies for the development of therapeutic interventions.

Impaired glucose tolerance, glucagon, and insulin responses in mice lacking the loop diuretic-sensitive Nkcc2a transporter

The Na⁺K⁺2Cl^- cotransporter-2 (Nkcc2, Slc12a1) is abundantly expressed in the kidney and its inhibition with the loop-diuretics bumetanide and furosemide has been linked to transient or permanent hyperglycemia in mice and humans. Notably, Slc12a1 is expressed at low levels in hypothalamic neurons and in insulin-secreting β-cells of the endocrine pancreas. The present study was designed to determine if global elimination of one of the Slc12a1 products, i.e., Nkcc2 variant a (Nkcc2a), the main splice version of Nkcc2 found in insulin-secreting β-cells, has an impact on the insulin and glucagon secretory responses and fuel homeostasis in vivo. We have used dynamic tests of glucose homeostasis in wild-type mice and mice lacking both alleles of Nkcc2a (Nkcc2a^KO) and assessed their islet secretory responses in vitro. Under basal conditions, Nkcc2a^KO mice have impaired glucose homeostasis characterized by increased blood glucose, intolerance to the sugar, delayed/blunted in vivo insulin and glucagon responses to glucose, and increased glycemic responses to the gluconeogenic substrate alanine. Further, we provide evidence of conserved quantitative secretory responses of Nkcc2a^KO islets within a context of increased islet size related to hyperplastic/hypertrophic glucagon- and insulin-positive cells (α-cells and β-cells, respectively), normal total islet Cl^- content, and reduced β-cell expression of the Cl^- extruder Kcc2.

Tyrosine phosphorylation modulates cell surface expression of chloride cotransporters NKCC2 and KCC3

Cellular chloride transport has a fundamental role in cell volume regulation and renal salt handling. Cellular chloride entry or exit are mediated at the plasma membrane by cotransporter proteins of the solute carrier 12 family. For example, NKCC2 resorbs chloride with sodium and potassium ions at the apical membrane of epithelial cells in the kidney, whereas KCC3 releases chloride with potassium ions at the basolateral membrane. Their ion transport activity is regulated by protein phosphorylation in response to signaling pathways. An additional regulatory mechanism concerns the amount of cotransporter molecules inserted into the plasma membrane. Here we describe that tyrosine phosphorylation of NKCC2 and KCC3 regulates their plasma membrane expression levels. We identified that spleen tyrosine kinase (SYK) phosphorylates a specific N-terminal tyrosine residue in each cotransporter. Experimental depletion of endogenous SYK or pharmacological inhibition of its kinase activity increased the abundance of NKCC2 at the plasma membrane of human embryonic kidney cells. In contrast, overexpression of a constitutively active SYK mutant decreased NKCC2 membrane abundance. Intriguingly, the same experimental approaches revealed the opposite effect on KCC3 abundance at the plasma membrane, compatible with the known antagonistic roles of NKCC and KCC cotransporters in cell volume regulation. Thus, we identified a novel pathway modulating the cell surface expression of NKCC2 and KCC3 and show that this same pathway has opposite functional outcomes for these two cotransporters. The findings have several biomedical implications considering the role of these cotransporters in regulating blood pressure and cell volume.

Nephrolithiasis secondary to inherited defects in the thick ascending loop of henle and connecting tubules

Twin and genealogy studies suggest a strong genetic component of nephrolithiasis. Likewise, urinary traits associated with renal stone formation were found to be highly heritable, even after adjustment for demographic, anthropometric and dietary covariates. Recent high-throughput sequencing projects of phenotypically well-defined cohorts of stone formers and large genome-wide association studies led to the discovery of many new genes associated with kidney stones. The spectrum ranges from infrequent but highly penetrant variants (mutations) causing mendelian forms of nephrolithiasis (monogenic traits) to common but phenotypically mild variants associated with nephrolithiasis (polygenic traits). About two-thirds of the genes currently known to be associated with nephrolithiasis code for membrane proteins or enzymes involved in renal tubular transport. The thick ascending limb of Henle and connecting tubules are of paramount importance for renal water and electrolyte handling, urinary concentration and maintenance of acid-base homeostasis. In most instances, pathogenic variants in genes involved in thick ascending limb of Henle and connecting tubule function result in phenotypically severe disease, frequently accompanied by nephrocalcinosis with progressive CKD and to a variable degree by nephrolithiasis. The aim of this article is to review the current knowledge on kidney stone disease associated with inherited defects in the thick ascending loop of Henle and the connecting tubules. We also highlight recent advances in the field of kidney stone genetics that have implications beyond rare disease, offering new insights into the most common type of kidney stone disease, i.e., idiopathic calcium stone disease.

Mammalian urine concentration: a review of renal medullary architecture and membrane transporters

Mammalian kidneys play an essential role in balancing internal water and salt concentrations. When water needs to be conserved, the renal medulla produces concentrated urine. Central to this process of urine concentration is an osmotic gradient that increases from the corticomedullary boundary to the inner medullary tip. How this gradient is generated and maintained has been the subject of study since the 1940s. While it is generally accepted that the outer medulla contributes to the gradient by means of an active process involving countercurrent multiplication, the source of the gradient in the inner medulla is unclear. The last two decades have witnessed advances in our understanding of the urine-concentrating mechanism. Details of medullary architecture and permeability properties of the tubules and vessels suggest that the functional and anatomic relationships of these structures may contribute to the osmotic gradient necessary to concentrate urine. Additionally, we are learning more about the membrane transporters involved and their regulatory mechanisms. The role of medullary architecture and membrane transporters in the mammalian urine-concentrating mechanism are the focus of this review.

Water Homeostasis and Cell Volume Maintenance and Regulation

From early unicellular organisms that formed in salty water environments to complex organisms that live on land away from water, cells have had to protect a homeostatic internal environment favorable to the biochemical reactions necessary for life. In this chapter, we will outline what steps were necessary to conserve the water within our cells and how mechanisms have evolved to maintain and regulate our cellular and organismal volume. We will first examine whole body water homeostasis and the relationship between kidney function, regulation of blood pressure, and blood filtration in the process of producing urine. We will then discuss how the composition of the lipid-rich bilayer affects its permeability to water and salts, and how the cell uses this differential to drive physiological and biochemical cellular functions. The capacity to maintain cell volume is vital to epithelial transport, neurotransmission, cell cycle, apoptosis, and cell migration. Finally, we will wrap up the chapter by discussing in some detail specific channels, cotransporters, and exchangers that have evolved to facilitate the movement of cations and anions otherwise unable to cross the lipid-rich bilayer and that are involved in maintaining or regulating cell volume.

Hydrogen sulfide upregulates renal AQP-2 protein expression and promotes urine concentration

Increasing evidence supports the important role of H₂S in renal physiology and the pathogenesis of kidney injury. Whether H₂S regulates water metabolism in the kidney and the potential mechanism are still unknown. The present study was conducted to determine the role of H₂S in urine concentration. Inhibition of both cystathionine-γ-lyase (CSE) and cystathionine-β-synthase (CBS), 2 major enzymes for endogenous H₂S production, with propargylglycine (PPG) and amino-oxyacetate (AOAA), respectively, caused increased urine output and reduced urine osmolality in mice that was associated with decreased expression of aquaporin (AQP)-2 in the renal inner medulla. Mice treated with both PPG and AOAA developed a urine concentration defect in response to dehydration that was accompanied by reduced AQP-2 protein expression. Inhibition of CSE alone was associated with a mild decrease in AQP-2 protein level in the renal medulla of heterozygous CBS mice. GYY4137, a slow H₂S donor, markedly improved urine concentration and prevented the down-regulation of renal AQP-2 protein expression in mice with lithium-induced nephrogenic diabetes insipidus (NDI). GYY4137 significantly increased cAMP levels in cell lysates prepared from inner medullary collecting duct (IMCD) suspensions. AQP-2 protein expression was also upregulated, but was significantly inhibited by the adenyl cyclase inhibitor MDL12330A or the PKA inhibitor H89, but not the vasopressin 2 receptor (V₂R) antagonist tolvaptan. Inhibition of endogenous H₂S production impaired urine concentration in mice, whereas an exogenous H₂S donor improved urine concentration in lithium-induced NDI by increasing AQP-2 expression in the collecting duct principal cells. H₂S upregulated AQP-2 protein expression, probably via the cAMP-PKA pathway.-Luo, R., Hu, S., Liu, Q., Han, M., Wang, F., Qiu, M., Li, S., Li, X., Yang, T., Fu, X., Wang, W., Li, C. Hydrogen sulfide upregulates renal AQP-2 protein expression and promotes urine concentration.

Azosemide is more potent than bumetanide and various other loop diuretics to inhibit the sodium-potassium-chloride-cotransporter human variants hNKCC1A and hNKCC1B

The Na⁺–K⁺–2Cl⁻ cotransporter NKCC1 plays a role in neuronal Cl⁻ homeostasis secretion and represents a target for brain pathologies with altered NKCC1 function. Two main variants of NKCC1 have been identified: a full-length NKCC1 transcript (NKCC1A) and a shorter splice variant (NKCC1B) that is particularly enriched in the brain. The loop diuretic bumetanide is often used to inhibit NKCC1 in brain disorders, but only poorly crosses the blood-brain barrier. We determined the sensitivity of the two human NKCC1 splice variants to bumetanide and various other chemically diverse loop diuretics, using the Xenopus oocyte heterologous expression system. Azosemide was the most potent NKCC1 inhibitor (IC₅₀s 0.246 µM for hNKCC1A and 0.197 µM for NKCC1B), being about 4-times more potent than bumetanide. Structurally, a carboxylic group as in bumetanide was not a prerequisite for potent NKCC1 inhibition, whereas loop diuretics without a sulfonamide group were less potent. None of the drugs tested were selective for hNKCC1B vs. hNKCC1A, indicating that loop diuretics are not a useful starting point to design NKCC1B-specific compounds. Azosemide was found to exert an unexpectedly potent inhibitory effect and as a non-acidic compound, it is more likely to cross the blood-brain barrier than bumetanide.

Axial and cellular heterogeneity in electrolyte transport pathways along the thick ascending limb

The thick ascending limb (TAL) extends from the border of the inner medulla to the renal cortex, thus ascending through regions with wide differences in tissue solute and electrolyte concentrations. Structural and functional differences between TAL cells in the medulla (mTAL) and the cortex (cTAL) would therefore be useful to adapt TAL transport function to a changing external fluid composition. While mechanisms common to all TAL cells play a central role in the reclamation of about 25% of the NaCl filtered by the kidney, morphological features, Na⁺ / K⁺ -ATPase activity, NKCC2 splicing and phosphorylation do vary between segments and cells. The TAL contributes to K⁺ homeostasis and TAL cells with high or low basolateral K⁺ conductances have been identified which may be involved in K⁺ reabsorption and secretion respectively. Although transport rates for HCO3- do not differ between mTAL and cTAL, divergent axial and cellular expression of H⁺ transport proteins in TAL have been documented. The reabsorption of the divalent cations Ca²⁺ and Mg²⁺ is highest in cTAL and paralleled by differences in divalent cation permeability and the expression of select claudins. Morphologically, two cell types with different cell surface phenotypes have been described that still need to be linked to specific functional characteristics. The unique external environment and its change along the longitudinal axis require an axial functional heterogeneity for the TAL to optimally participate in conserving electrolyte homeostasis. Despite substantial progress in understanding TAL function, there are still considerable knowledge gaps that are just beginning to become bridged.