Intracellular Chloride and Scaffold Protein Mo25 Cooperatively Regulate Transepithelial Ion Transport through WNK Signaling in the Malpighian Tubule

A balancing act: drifting away from the reflexive use of "ab"normal saline

Maintenance intravenous fluids are the most frequently ordered medications for hospitalized children. Since the American Association of Pediatrics published national guidelines, there has been an increased reflexive use of isotonic solutions, especially 0.9% saline, as a prophylaxis against hyponatremia. In this educational review, we discuss the potential deleterious effects of using 0.9% saline, including the development of hyperchloremia, metabolic acidosis, acute kidney injury, hyperkalemia, and a proinflammatory state. Balanced solutions with anion buffers cause relatively minimal harm when used in most children. While the literature supporting one fluid choice over the other is variable, we highlight the benefits of balanced solutions over saline and the importance of prescribing fluid therapy that is individualized for each patient.

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

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

LKB1 biology: assessing the therapeutic relevancy of LKB1 inhibitors

Liver Kinase B1 (LKB1), encoded by Serine-Threonine Kinase 11 (STK11), is a master kinase that regulates cell migration, polarity, proliferation, and metabolism through downstream adenosine monophosphate-activated protein kinase (AMPK) and AMPK-related kinase signalling. Since genetic screens identified STK11 mutations in Peutz-Jeghers Syndrome, STK11 mutants have been implicated in tumourigenesis labelling it as a tumour suppressor. In support of this, several compounds reduce tumour burden through upregulating LKB1 signalling, and LKB1-AMPK agonists are cytotoxic to tumour cells. However, in certain contexts, its role in cancer is paradoxical as LKB1 promotes tumour cell survival by mediating resistance against metabolic and oxidative stressors. LKB1 deficiency has also enhanced the selectivity and cytotoxicity of several cancer therapies. Taken together, there is a need to develop LKB1-specific pharmacological compounds, but prior to developing LKB1 inhibitors, further work is needed to understand LKB1 activity and regulation. However, investigating LKB1 activity is strenuous as cell/tissue type, mutations to the LKB1 signalling pathway, STE-20-related kinase adaptor protein (STRAD) binding, Mouse protein 25-STRAD binding, splicing variants, nucleocytoplasmic shuttling, post-translational modifications, and kinase conformation impact the functional status of LKB1. For these reasons, guidelines to standardize experimental strategies to study LKB1 activity, associate proteins, spliced isoforms, post-translational modifications, and regulation are of upmost importance to the development of LKB1-specific therapies. Therefore, to assess the therapeutic relevancy of LKB1 inhibitors, this review summarizes the importance of LKB1 in cell physiology, highlights contributors to LKB1 activation, and outlines the benefits and risks associated with targeting LKB1.

Potassium-Switch Signaling Pathway Dictates Acute Blood Pressure Response to Dietary Potassium

BACKGROUND

Potassium (K+)-deficient diets, typical of modern processed foods, increase blood pressure (BP) and NaCl sensitivity. A K+-dependent signaling pathway in the kidney distal convoluted tubule, coined the K+ switch, that couples extracellular K+ sensing to activation of the thiazide-sensitive NaCl cotransporter (NCC) and NaCl retention has been implicated, but causality has not been established.

METHODS

To test the hypothesis that small, physiological changes in plasma K+ (PK+) are translated to BP through the switch pathway, a genetic approach was used to activate the downstream switch kinase, SPAK (SPS1-related proline/alanine-rich kinase), within the distal convoluted tubule. The CA-SPAK (constitutively active SPS1-related proline/alanine-rich kinase mice) were compared with control mice over a 4-day PK+ titration (3.8-5.1 mmol) induced by changes in dietary K+. Arterial BP was monitored using radiotelemetry, and renal function measurements, NCC abundance, phosphorylation, and activity were made.

RESULTS

As PK+ decreased in control mice, BP progressively increased and became sensitive to dietary NaCl and hydrochlorothiazide, coincident with increased NCC phosphorylation and urinary sodium retention. By contrast, BP in CA-SPAK mice was elevated, resistant to the PK+ titration, and sensitive to hydrochlorothiazide and salt at all PK+ levels, concomitant with sustained and elevated urinary sodium retention and NCC phosphorylation and activity. Thus, genetically locking the switch on drives NaCl sensitivity and prevents the response of BP to potassium.

CONCLUSIONS

Low K+, common in modern ultraprocessed diets, presses the K+-switch pathway to turn on NCC activity, increasing sodium retention, BP, and salt sensitivity.

Circadian Rhythm Regulation by Pacemaker Neuron Chloride Oscillation in Flies

Circadian rhythms in physiology and behavior sync organisms to external environmental cycles. Here, circadian oscillation in intracellular chloride in central pacemaker neurons of the fly, Drosophila melanogaster, is reviewed. Intracellular chloride links SLC12 cation-coupled chloride transporter function with kinase signaling and the regulation of inwardly rectifying potassium channels.

Kinase Scaffold Cab39 Is Necessary for Phospho-Activation of the Thiazide-Sensitive NCC

BACKGROUND

Potassium regulates the WNK (with no lysine kinase)-SPAK (STE20/SPS1-related proline/alanine-rich kinase) signaling axis, which in turn controls the phosphorylation and activation of the distal convoluted tubule thiazide-sensitive NCC (sodium-chloride cotransporter) for sodium-potassium balance. Although their roles in the kidney have not been investigated, it has been postulated that Cab39 (calcium-binding protein 39) or Cab39l (Cab39-like) is required for SPAK/OSR1 (oxidative stress response 1) activation. This study demonstrates how they control the WNK-SPAK/OSR1-NCC pathway.

METHODS

We created a global knockout of Cab39l and a tamoxifen-inducible, NCC-driven, Cab39 knockout. The 2 lines were crossed to generate Cab39-DKO (Cab39 double knockout) animals. Mice were studied under control and low-potassium diet, which activates WNK-SPAK/OSR1-NCC phosphorylation. Western blots were used to assess the expression and phosphorylation of proteins. Blood and urine electrolytes were measured to test for compromised NCC function. Immunofluorescence studies were conducted to localize SPAK and OSR1.

RESULTS

Both Cab39l and Cab39 are expressed in distal convoluted tubule, and only the elimination of both leads to a striking absence of NCC phosphorylation. Cab39-DKO mice exhibited a loss-of-NCC function, like in Gitelman syndrome. In contrast to the apical membrane colocalization of SPAK with NCC in wild-type mice, SPAK and OSR1 become confined to intracellular puncta in the Cab39-DKO mice.

CONCLUSIONS

In the absence of Cab39 proteins, NCC cannot be phosphorylated, resulting in a Gitelman-like phenotype. Cab39 proteins function to localize SPAK at the apical membrane with NCC, reminiscent of the Cab39 yeast homolog function, translocating kinases during cytokinesis.

Intracellular Ca2+ oscillation frequency and amplitude modulation mediate epithelial apical and basolateral membranes crosstalk

Since the early seminal studies on epithelial solute transport, it has been understood that there must be crosstalk among different members of the transport machinery to coordinate their activity and, thus, generate localized electrochemical gradients that force solute flow in the required direction that would otherwise be thermodynamically unfavorable. However, mechanisms underlying intracellular crosstalk remain unclear. We present evidence that crosstalk between apical and basolateral membrane transporters is mediated by intracellular Ca2+ signaling in insect renal epithelia. Ion flux across the basolateral membrane is encoded in the intracellular Ca2+ oscillation frequency and amplitude modulation and that information is used by the apical membrane to adjust ion flux accordingly. Moreover, imposing experimentally generated intracellular Ca2+ oscillation modulation causes cells to predictably adjust their ion transport properties. Our results suggest that intracellular Ca2+ oscillation frequency and amplitude modulation encode information on transmembrane ion flux that is required for crosstalk.

Mechanical stimulation from the surrounding tissue activates mitochondrial energy metabolism in Drosophila differentiating germ cells

In multicellular lives, the differentiation of stem cells and progenitor cells is often accompanied by a transition from glycolysis to mitochondrial oxidative phosphorylation (OXPHOS). However, the underlying mechanism of this metabolic transition remains largely unknown. In this study, we investigate the role of mechanical stress in activating OXPHOS during differentiation of the female germline cyst in Drosophila. We demonstrate that the surrounding somatic cells flatten the 16-cell differentiating cyst, resulting in an increase of the membrane tension of germ cells inside the cyst. This mechanical stress is necessary to maintain cytosolic Ca2+ concentration in germ cells through a mechanically activated channel, transmembrane channel-like. The sustained cytosolic Ca2+ triggers a CaMKI-Fray-JNK signaling relay, leading to the transcriptional activation of OXPHOS in differentiating cysts. Our findings demonstrate a molecular link between cell mechanics and mitochondrial energy metabolism, with implications for other developmentally orchestrated metabolic transitions in mammals.

Dietary potassium stimulates Ppp1Ca-Ppp1r1a dephosphorylation of kidney NaCl cotransporter and reduces blood pressure

Consumption of low dietary potassium, common with ultraprocessed foods, activates the thiazide-sensitive sodium chloride cotransporter (NCC) via the with no (K) lysine kinase/STE20/SPS1-related proline-alanine-rich protein kinase (WNK/SPAK) pathway to induce salt retention and elevate blood pressure (BP). However, it remains unclear how high-potassium "DASH-like" diets (dietary approaches to stop hypertension) inactivate the cotransporter and whether this decreases BP. A transcriptomics screen identified Ppp1Ca, encoding PP1A, as a potassium-upregulated gene, and its negative regulator Ppp1r1a, as a potassium-suppressed gene in the kidney. PP1A directly binds to and dephosphorylates NCC when extracellular potassium is elevated. Using mice genetically engineered to constitutively activate the NCC-regulatory kinase SPAK and thereby eliminate the effects of the WNK/SPAK kinase cascade, we confirmed that PP1A dephosphorylated NCC directly in a potassium-regulated manner. Prior adaptation to a high-potassium diet was required to maximally dephosphorylate NCC and lower BP in constitutively active SPAK mice, and this was associated with potassium-dependent suppression of Ppp1r1a and dephosphorylation of its cognate protein, inhibitory subunit 1 (I1). In conclusion, potassium-dependent activation of PP1A and inhibition of I1 drove NCC dephosphorylation, providing a mechanism to explain how high dietary K+ lowers BP. Shifting signaling of PP1A in favor of activation of WNK/SPAK may provide an improved therapeutic approach for treating salt-sensitive hypertension.

Unanticipated domain requirements for Drosophila Wnk kinase in vivo

WNK (With no Lysine [K]) kinases have critical roles in the maintenance of ion homeostasis and the regulation of cell volume. Their overactivation leads to pseudohypoaldosteronism type II (Gordon syndrome) characterized by hyperkalemia and high blood pressure. More recently, WNK family members have been shown to be required for the development of the nervous system in mice, zebrafish, and flies, and the cardiovascular system of mice and fish. Furthermore, human WNK2 and Drosophila Wnk modulate canonical Wnt signaling. In addition to a well-conserved kinase domain, animal WNKs have a large, poorly conserved C-terminal domain whose function has been largely mysterious. In most but not all cases, WNKs bind and activate downstream kinases OSR1/SPAK, which in turn regulate the activity of various ion transporters and channels. Here, we show that Drosophila Wnk regulates Wnt signaling and cell size during the development of the wing in a manner dependent on Fray, the fly homolog of OSR1/SPAK. We show that the only canonical RF(X)V/I motif of Wnk, thought to be essential for WNK interactions with OSR1/SPAK, is required to interact with Fray in vitro. However, this motif is unexpectedly dispensable for Fray-dependent Wnk functions in vivo during fly development and fluid secretion in the Malpighian (renal) tubules. In contrast, a structure function analysis of Wnk revealed that the less-conserved C-terminus of Wnk, that recently has been shown to promote phase transitions in cell culture, is required for viability in vivo. Our data thus provide novel insights into unexpected in vivo roles of specific WNK domains.

Hydrostatic Pressure Sensing by WNK kinases

Previous study has demonstrated that the WNK kinases 1 and 3 are direct osmosensors consistent with their established role in cell-volume control. WNK kinases may also be regulated by hydrostatic pressure. Hydrostatic pressure applied to cells in culture with N2 gas or to Drosophila Malpighian tubules by centrifugation induces phosphorylation of downstream effectors of endogenous WNKs. In vitro, the autophosphorylation and activity of the unphosphorylated kinase domain of WNK3 (uWNK3) is enhanced to a lesser extent than in cells by 190 kPa applied with N2 gas. Hydrostatic pressure measurably alters the structure of uWNK3. Data from size exclusion chromatography in line with multi-angle light scattering (SEC-MALS), SEC alone at different back pressures, analytical ultracentrifugation (AUC), NMR, and chemical crosslinking indicate a change in oligomeric structure in the presence of hydrostatic pressure from a WNK3 dimer to a monomer. The effects on the structure are related to those seen with osmolytes. Potential mechanisms of hydrostatic pressure activation of uWNK3 and the relationships of pressure activation to WNK osmosensing are discussed.

Intracellular Ion Control of WNK Signaling

The with no lysine (K) (WNK) kinases are an evolutionarily ancient group of kinases with atypical placement of the catalytic lysine and diverse physiological roles. Recent studies have shown that WNKs are directly regulated by chloride, potassium, and osmotic pressure. Here, we review the discovery of WNKs as chloride-sensitive kinases and discuss physiological contexts in which chloride regulation of WNKs has been demonstrated. These include the kidney, pancreatic duct, neurons, and inflammatory cells. We discuss the interdependent relationship of osmotic pressure and intracellular chloride in cell volume regulation. We review the recent demonstration of potassium regulation of WNKs and speculate on possible physiological roles. Finally, structural and mechanistic aspects of intracellular ion and osmotic pressure regulation of WNKs are discussed.

Regulation of Distal Nephron Transport by Intracellular Chloride and Potassium

BACKGROUND

Low potassium increases the phosphorylation and activity of the sodium chloride cotransporter (NCC) in the distal convoluted tubule of the nephron, which contributes to the hypertensive effect of the modern low potassium/high sodium diet. A central mediator of potassium regulation of NCC is the chloride-sensitive With No Lysine [K] (WNK) kinase.

SUMMARY

Chloride directly inhibits WNKs by binding to the active site. The mechanisms underlying WNK regulation by extracellular potassium are reviewed, as well as the modulatory effect of kidney-specific-WNK1. WNK1, but not WNK1 kinase activity, is also required for the aldosterone-independent regulation of the epithelial sodium channel by potassium. Whether intracellular chloride could be involved in this process is discussed. Recent studies demonstrating direct regulation of WNKs by intracellular potassium are also reviewed, and the potential physiological relevance to renal epithelial ion transport is discussed.

KEY MESSAGES

WNKs are sensors of the intracellular ionic milieu. In the nephron, changes in extracellular ion concentrations, resulting in changes in intracellular ion concentration, regulate WNK activity and downstream transporters and channels to maintain total body ion homeostasis.

Chloride oscillation in pacemaker neurons regulates circadian rhythms through a chloride-sensing WNK kinase signaling cascade

Central pacemaker neurons regulate circadian rhythms and undergo diurnal variation in electrical activity in mammals and flies.1,2 Circadian variation in the intracellular chloride concentration of mammalian pacemaker neurons has been proposed to influence the response to GABAergic neurotransmission through GABAA receptor chloride channels.3 However, results have been contradictory,4-9 and a recent study demonstrated circadian variation in pacemaker neuron chloride without an effect on GABA response.10 Therefore, whether and how intracellular chloride regulates circadian rhythms remains controversial. Here, we demonstrate a signaling role for intracellular chloride in the Drosophila small ventral lateral (sLNv) pacemaker neurons. In control flies, intracellular chloride increases in sLNvs over the course of the morning. Chloride transport through sodium-potassium-2-chloride (NKCC) and potassium-chloride (KCC) cotransporters is a major determinant of intracellular chloride concentrations.11Drosophila melanogaster with loss-of-function mutations in the NKCC encoded by Ncc69 have abnormally low intracellular chloride 6 h after lights on, loss of morning anticipation, and a prolonged circadian period. Loss of kcc, which is expected to increase intracellular chloride, suppresses the long-period phenotype of Ncc69 mutant flies. Activation of a chloride-inhibited kinase cascade, consisting of WNK (with no lysine [K]) kinase and its downstream substrate, Fray, is necessary and sufficient to prolong period length. Fray activation of an inwardly rectifying potassium channel, Irk1, is also required for the long-period phenotype. These results indicate that the NKCC-dependent rise in intracellular chloride in Drosophila sLNv pacemakers restrains WNK-Fray signaling and overactivation of an inwardly rectifying potassium channel to maintain normal circadian period length.

Updates on ion and water transport by the Malpighian tubule

The Malpighian (renal) tubule is capable of transporting fluid at remarkable rates. This review will focus on recent insights into the mechanisms by which these high rates are achieved and controlled, with particular reference to the tubules of Drosophila melanogaster, in which the combination of physiology and genetics has led to particularly rapid progress. Like many vertebrate epithelia, the Drosophila tubule has specialized cell types, with active cation transport confined to a large, metabolically active principal cell; whereas the smaller intercalated stellate cell controls chloride and water shunts to achieve net fluid secretion. Recently, the genes underlying many of these processes have been identified, functionally validated and localized within the tubule. The imminent arrival of new types of post-genomic data (notably single cell sequencing) will herald an exciting era of new discovery.

Roles of WNK4 and SPAK in K+-mediated dephosphorylation of the NaCl cotransporter

Phosphorylation of the thiazide-sensitive NaCl cotransporter (NCC) in the distal convoluted tubule (DCT) is altered rapidly in response to changes in extracellular K+ concentration ([K+]). High extracellular [K+] is believed to activate specific phosphatases to dephosphorylate NCC, thereby reducing its activity. This process is defective in the human disease familial hyperkalemic hypertension, in which extracellular [K+] fails to dephosphorylate NCC, suggesting an interplay between NCC-activating and NCC-inactivating switches. Here, we explored the role of STE20/SPS1-related proline-alanine-rich protein kinase (SPAK) and intracellular Cl- concentration in the rapid effects of extracellular K+ on NCC phosphorylation. SPAK was found to be rapidly dephosphorylated in vitro in human embryonic kidney cells and ex vivo in kidney slices by high [K+]. Acute high-K+ challenge resulted in DCT1-specific SPAK dephosphorylation in vivo and dissolution of SPAK puncta. In line with the postulate of interplay between activating and inactivating switches, we found that the "on" switch, represented by with no lysine kinase 4 (WNK4)-SPAK, must be turned off for rapid NCC dephosphorylation by high [K+]. Longer-term WNK-SPAK-mediated stimulation, however, altered the sensitivity of the system, as it attenuated rapid NCC dephosphorylation due to acute K+ loading. Although blockade of protein phosphatase (PP)1 increased NCC phosphorylation at baseline, neither PP1 nor PP3, singly or in combination, was essential for NCC dephosphorylation. Overall, our data suggest that NCC phosphorylation is regulated by a dynamic equilibrium between activating kinases and inactivating phosphatases, with kinase inactivation playing a key role in the rapid NCC dephosphorylation by high extracellular K+.NEW & NOTEWORTHY Although a great deal is known about mechanisms by which thiazide-sensitive NaCl cotransporter is phosphorylated and activated, much less is known about dephosphorylation. Here, we show that rapid dephosphorylation by high K+ depends on the Cl- sensitivity of with no lysine kinase 4 and the rapid dephosphorylation of STE20/SPS1-related proline-alanine-rich protein kinase, primarily along the early distal convoluted tubule.

Advances in the development of novel compounds targeting cation-chloride cotransporter physiology

For about half a century, the pharmacology of electroneutral cation-chloride cotransporters has been dominated by a few drugs that are widely used in clinical medicine. Because these diuretic drugs are so good at what they do, there has been little incentive in expanding their pharmacology. The increasing realization that cation-chloride cotransporters are involved in many other key physiological processes and the knowledge that different tissues express homologous proteins with matching transport functions have rekindled interest in drug discovery. This review summarizes the methods available to assess the function of these transporters and describe the multiple efforts that have made to identify new compounds. We describe multiple screens targeting KCC2 function and one screen designed to find compounds that discriminate between NKCC1 and NKCC2. Two of the KCC2 screens identified new inhibitors that are 3-4 orders of magnitude more potent than furosemide. Additional screens identified compounds that purportedly increase cell surface expression of the cotransporter, as well as several FDA-approved drugs that increase KCC2 transcription and expression. The technical details of each screen biased them toward specific processes in the life cycle of the transporter, making these efforts independent and complementary. In addition, each drug discovery effort contributes to our understanding of the biology of the cotransporters.

WNKs are potassium-sensitive kinases

With no lysine (K) (WNK) kinases regulate epithelial ion transport in the kidney to maintain homeostasis of electrolyte concentrations and blood pressure. Chloride ion directly binds WNK kinases to inhibit autophosphorylation and activation. Changes in extracellular potassium are thought to regulate WNKs through changes in intracellular chloride. Prior studies demonstrate that in some distal nephron epithelial cells, intracellular potassium changes with chronic low- or high-potassium diet. We, therefore, investigated whether potassium regulates WNK activity independent of chloride. We found decreased activity of Drosophila WNK and mammalian WNK3 and WNK4 in fly Malpighian (renal) tubules bathed in high extracellular potassium, even when intracellular chloride was kept constant at either ∼13 mM or 26 mM. High extracellular potassium also inhibited chloride-insensitive mutants of WNK3 and WNK4. High extracellular rubidium was also inhibitory and increased tubule rubidium. The Na⁺/K⁺-ATPase inhibitor, ouabain, which is expected to lower intracellular potassium, increased tubule Drosophila WNK activity. In vitro, potassium increased the melting temperature of Drosophila WNK, WNK1, and WNK3 kinase domains, indicating ion binding to the kinase. Potassium inhibited in vitro autophosphorylation of Drosophila WNK and WNK3, and also inhibited WNK3 and WNK4 phosphorylation of their substrate, Ste20-related proline/alanine-rich kinase (SPAK). The greatest sensitivity of WNK4 to potassium occurred in the range of 80-180 mM, encompassing physiological intracellular potassium concentrations. Together, these data indicate chloride-independent potassium inhibition of Drosophila and mammalian WNK kinases through direct effects of potassium ion on the kinase.

WNK4 kinase: role of chloride sensing in the distal convoluted tubule

PURPOSE OF REVIEW

This review focuses on recent efforts in identifying with-no-lysine kinase 4 (WNK4) as a physiological intracellular chloride sensor and exploring regulators of intracellular chloride concentration ([Cl-]i) in the distal convoluted tubule (DCT).

RECENT FINDINGS

The discovery of WNK1's chloride-binding site provides the mechanistic details of the chloride-sensing regulation of WNK kinases. The subsequent in-vitro studies reveal that the chloride sensitivities of WNK kinases were variable. Because of its highest chloride sensitivity and dominant expression, WNK4 emerges as the leading candidate of the chloride sensor in DCT. The presentation of hypertension and increased sodium-chloride cotransporter (NCC) activity in chloride-insensitive WNK4 mice proved that WNK4 is inhibitable by physiological [Cl-]i in DCT. The chloride-mediated WNK4 regulation is responsible for hypokalemia-induced NCC activation but unnecessary for hyperkalemia-induced NCC deactivation. This chloride-sensing mechanism requires basolateral potassium and chloride channels or cotransporters, including Kir4.1/5.1, ClC-Kb, and possibly KCCs, to modulate [Cl-]i in response to the changes of plasma potassium.

SUMMARY

WNK4 is both a master NCC stimulator and an in-vivo chloride sensor in DCT. The understanding of chloride-mediated regulation of WNK4 explains the inverse relationship between dietary potassium intake and NCC activity.

NKCC1: Newly Found as a Human Disease-Causing Ion Transporter

Among the electroneutral Na⁺-dependent chloride transporters, NKCC1 had until now evaded identification as a protein causing human diseases. The closely related SLC12A transporters, NKCC2 and NCC have been identified some 25 years ago as responsible for Bartter and Gitelman syndromes: two renal-dependent salt wasting disorders. Absence of disease was most surprising since the NKCC1 knockout mouse was shown in 1999 to be viable, albeit with a wide range of deleterious phenotypes. Here we summarize the work of the past 5 years that introduced us to clinical cases involving NKCC1. The most striking cases are of 3 children with inherited mutations, who have complete absence of NKCC1 expression. These cases establish that lack of NKCC1 causes deafness; CFTR-like secretory defects with mucus accumulation in lung and intestine; severe xerostomia, hypotonia, dysmorphic facial features, and severe neurodevelopmental disorder. Another intriguing case is of a patient with a dominant deleterious SLC12A2 allele. This de novo mutation introduced a premature stop codon leading to a truncated protein. This mutant transporter seems to exert dominant-negative effect on wild-type transporter only in epithelial cells. The patient who suffers from lung, bladder, intestine, pancreas, and multiple endocrine abnormalities has, however, normal hearing and cognition. Finally, new reports substantiate the haploinsufficiency prediction of the SLC12A2 gene. Cases with single allele mutations in SLC12A2 have been linked to hearing loss and neurodevelopmental disorders.

Physiological Processes Modulated by the Chloride-Sensitive WNK-SPAK/OSR1 Kinase Signaling Pathway and the Cation-Coupled Chloride Cotransporters

The role of Cl^– as an intracellular signaling ion has been increasingly recognized in recent years. One of the currently best described roles of Cl^– in signaling is the modulation of the With-No-Lysine (K) (WNK) – STE20-Proline Alanine rich Kinase (SPAK)/Oxidative Stress Responsive Kinase 1 (OSR1) – Cation-Coupled Cl^–Cotransporters (CCCs) cascade. Binding of a Cl^– anion to the active site of WNK kinases directly modulates their activity, promoting their inhibition. WNK activation due to Cl^– release from the binding site leads to phosphorylation and activation of SPAK/OSR1, which in turn phosphorylate the CCCs. Phosphorylation by WNKs-SPAK/OSR1 of the Na⁺-driven CCCs (mediating ions influx) promote their activation, whereas that of the K⁺-driven CCCs (mediating ions efflux) promote their inhibition. This results in net Cl^– influx and feedback inhibition of WNK kinases. A wide variety of alterations to this pathway have been recognized as the cause of several human diseases, with manifestations in different systems. The understanding of WNK kinases as Cl^– sensitive proteins has allowed us to better understand the mechanistic details of regulatory processes involved in diverse physiological phenomena that are reviewed here. These include cell volume regulation, potassium sensing and intracellular signaling in the renal distal convoluted tubule, and regulation of the neuronal response to the neurotransmitter GABA.

Dietary potassium and the kidney: lifesaving physiology

Potassium often has a negative connotation in Nephrology as patients with chronic kidney disease (CKD) are prone to develop hyperkalaemia. Approaches to the management of chronic hyperkalaemia include a low potassium diet or potassium binders. Yet, emerging data indicate that dietary potassium may be beneficial for patients with CKD. Epidemiological studies have shown that a higher urinary potassium excretion (as proxy for higher dietary potassium intake) is associated with lower blood pressure (BP) and lower cardiovascular risk, as well as better kidney outcomes. Considering that the composition of our current diet is characterized by a high sodium and low potassium content, increasing dietary potassium may be equally important as reducing sodium. Recent studies have revealed that dietary potassium modulates the activity of the thiazide-sensitive sodium-chloride cotransporter in the distal convoluted tubule (DCT). The DCT acts as a potassium sensor to control the delivery of sodium to the collecting duct, the potassium-secreting portion of the kidney. Physiologically, this allows immediate kaliuresis after a potassium load, and conservation of potassium during potassium deficiency. Clinically, it provides a novel explanation for the inverse relationship between dietary potassium and BP. Moreover, increasing dietary potassium intake can exert BP-independent effects on the kidney by relieving the deleterious effects of a low potassium diet (inflammation, oxidative stress and fibrosis). The aim of this comprehensive review is to link physiology with clinical medicine by proposing that the same mechanisms that allow us to excrete an acute potassium load also protect us from hypertension, cardiovascular disease and CKD.

Distal convoluted tubule Cl- concentration is modulated via K+ channels and transporters

Cl^--sensitive with-no-lysine kinase (WNK) plays a key role in regulating the thiazide-sensitive Na⁺-Cl^- cotransporter (NCC) in the distal convoluted tubule (DCT). Cl^- enters DCT cells through NCC and leaves the cell across the basolateral membrane via the Cl^- channel ClC-K2 or K⁺-Cl^- cotransporter (KCC). While KCC is electroneutral, Cl^- exit via ClC-K2 is electrogenic. Therefore, an alteration in DCT basolateral K⁺ channel activity is expected to influence Cl^- movement across the basolateral membrane. Although a role for intracellular Cl^- in the regulation of WNK and NCC has been established, intracellular Cl^- concentrations ([Cl^-]_i) have not been directly measured in the mammalian DCT. Therefore, to measure [Cl^-]_i in DCT cells, we generated a transgenic mouse model expressing an optogenetic kidney-specific Cl-Sensor and measured Cl^- fluorescent imaging in the isolated DCT. Basal measurements indicated that the mean [Cl^-]_i was ~7 mM. Stimulation of Cl^- exit with low-Cl^- hypotonic solutions decreased [Cl^-]_i, whereas inhibition of KCC by DIOA or inhibition of ClC-K2 by NPPB increased [Cl^-]_i, suggesting roles for both KCC and ClC-K2 in the modulation of [Cl^-]_i . Blockade of basolateral K⁺ channels (Kir4.1/5.1) with barium significantly increased [Cl^-]_i. Finally, a decrease in extracellular K⁺ concentration transiently decreased [Cl^-]_i, whereas raising extracellular K⁺ transiently increased [Cl^-]_i, further suggesting a role for Kir4.1/5.1 in the regulation of [Cl^-]_i. We conclude that the alteration in ClC-K2, KCC, and Kir4.1/5.1 activity influences [Cl^-]_i in the DCT.

The Drosophila Malpighian tubule as a model for mammalian tubule function

PURPOSE OF REVIEW

Studies of the genetic model organism, Drosophila melanogaster, have unraveled molecular pathways relevant to human physiology and disease. The Malpighian tubule, the Drosophila renal epithelium, is described here, including tools available to study transport; conserved transporters, channels, and the signaling pathways regulating them; and fly models of kidney stone disease.

RECENT FINDINGS

Tools to measure Malpighian tubule transport continue to advance, including use of a transgenic sensor to quantify intracellular pH and proton fluxes. A recent study generated an RNA-sequencing-based atlas of tissue-specific gene expression, with resulting insights into Malpighian tubule gene expression of transporters and channels. Advances have been made in understanding the molecular physiology of the With No Lysine kinase-Ste20-related proline/alanine rich kinase/oxidative stress response kinase cascade that regulates epithelial ion transport in flies and mammals. New studies in Drosophila kidney stone models have characterized zinc transporters and used Malpighian tubules to study the efficacy of a plant metabolite in decreasing stone burden.

SUMMARY

Study of the Drosophila Malpighian tubule affords opportunities to better characterize the molecular physiology of epithelial transport mechanisms relevant to mammalian renal physiology.

The septate junction protein Mesh is required for epithelial morphogenesis, ion transport, and paracellular permeability in the Drosophila Malpighian tubule

Septate junctions (SJs) are occluding cell-cell junctions that have roles in paracellular permeability and barrier function in the epithelia of invertebrates. Arthropods have two types of SJs, pleated SJs and smooth SJs (sSJs). In Drosophila melanogaster, sSJs are found in the midgut and Malpighian tubules, but the functions of sSJs and their protein components in the tubule epithelium are unknown. Here we examined the role of the previously identified integral sSJ component, Mesh, in the Malpighian tubule. We genetically manipulated mesh specifically in the principal cells of the tubule at different life stages. Tubules of flies with developmental mesh knockdown revealed defects in epithelial architecture, sSJ molecular and structural organization, and lack of urine production in basal and kinin-stimulated conditions, resulting in edema and early adult lethality. Knockdown of mesh during adulthood did not disrupt tubule epithelial and sSJ integrity but decreased the transepithelial potential, diminished transepithelial fluid and ion transport, and decreased paracellular permeability to 4-kDa dextran. Drosophila kinin decreased transepithelial potential and increased chloride permeability, and it stimulated fluid secretion in both control and adult mesh knockdown tubules but had no effect on 4-kDa dextran flux. Together, these data indicate roles for Mesh in the developmental maturation of the Drosophila Malpighian tubule and in ion and macromolecular transport in the adult tubule.

Kidney Is Essential for Blood Pressure Modulation by Dietary Potassium

Eating more potassium may reduce blood pressure and the occurrence of other cardiovascular diseases by actions on various systems, including the vasculature, the sympathetic nervous system, systemic metabolism, and body fluid volume. Among these, the kidney plays a major role in the potassium-rich diet-mediated blood pressure reduction. PURPOSE OF REVIEW: To provide an overview of recent discoveries about the mechanisms by which a potassium-rich diet leads to natriuresis. RECENT FINDINGS: Although the distal convoluted tubule (DCT) is a short part of the nephron that reabsorbs salt, via the sodium-chloride cotransporter (NCC), it is highly sensitive to changes in plasma potassium concentration. Activation or inhibition of NCC raises or lowers blood pressure. Recent work suggests that extracellular potassium concentration is sensed by the DCT via intracellular chloride concentration which regulates WNK kinases in the DCT. High-potassium diet targets NCC in the DCT, resulting in natriuresis and fluid volume reduction, which are protective from hypertension and other cardiovascular problems.

Regulation of the Renal NaCl Cotransporter and Its Role in Potassium Homeostasis

Daily dietary potassium (K+) intake may be as large as the extracellular K+ pool. To avoid acute hyperkalemia, rapid removal of K+ from the extracellular space is essential. This is achieved by translocating K+ into cells and increasing urinary K+ excretion. Emerging data now indicate that the renal thiazide-sensitive NaCl cotransporter (NCC) is critically involved in this homeostatic kaliuretic response. This suggests that the early distal convoluted tubule (DCT) is a K+ sensor that can modify sodium (Na+) delivery to downstream segments to promote or limit K+ secretion. K+ sensing is mediated by the basolateral K+ channels Kir4.1/5.1, a capacity that the DCT likely shares with other nephron segments. Thus, next to K+-induced aldosterone secretion, K+ sensing by renal epithelial cells represents a second feedback mechanism to control K+ balance. NCC’s role in K+ homeostasis has both physiological and pathophysiological implications. During hypovolemia, NCC activation by the renin-angiotensin system stimulates Na+ reabsorption while preventing K+ secretion. Conversely, NCC inactivation by high dietary K+ intake maximizes kaliuresis and limits Na+ retention, despite high aldosterone levels. NCC activation by a low-K+ diet contributes to salt-sensitive hypertension. K+-induced natriuresis through NCC offers a novel explanation for the antihypertensive effects of a high-K+ diet. A possible role for K+ in chronic kidney disease is also emerging, as epidemiological data reveal associations between higher urinary K+ excretion and improved renal outcomes. This comprehensive review will embed these novel insights on NCC regulation into existing concepts of K+ homeostasis in health and disease.

Mechanisms and regulation of chloride transport in the Malpighian tubules of the larval cabbage looper Trichoplusia ni

Malpighian tubules (MTs) and the hindgut together constitute the excretory system of insects. Larvae of lepidopterans (butterflies and moths) demonstrate the so-called cryptonephric arrangement, where the distal blind end of each MT is embedded into the rectal complex. The rest of the free tubule is modified into several distinct regions that differ greatly in the transport of cations and water. However, relatively little is known about the transport of counter-anions (e.g., Cl^- and HCO₃^-) by the MTs of lepidopteran larvae. In the current study we used ion-selective microelectrodes to characterize Cl^- transport in the distinct regions of the free MT of the larval Trichoplusia ni. Firstly, we note that Cl^- transport in the MTs is sensitive to the Cl^- concentration of the bathing saline, and several regions of the MTs are capable of either secreting or reabsorbing Cl^-. In the distal ileac plexus (DIP), a region previously characterized by cellular heterogeneity and its ability to switch between cation secretion and reabsorption, principal cells (PCs) toggled between Cl^- reabsorption (in high-Cl^- saline) and Cl^- secretion (in low-Cl^- saline). In contrast, secondary cells (SCs) in the DIP secreted Cl^- regardless of saline Cl^- concentration. Mechanistically, we have detected a number of 'leak' and ligand-gated Cl^- channels (ClC) and demonstrated that Cl^- channels are involved in Cl^- secretion. Additionally, we demonstrated that the lumen-positive transepithelial potential increased in response to glycine. Using the scanning ion-selective electrode technique we demonstrated that glycine stimulated Cl^- secretion by SCs, but not by PCs. In contrast, when MTs were deprived of glycine, a decrease in Cl^- secretion, coupled with a decrease in the TEP, was observed. In contrast to the effects of glycine, an active dose of helicokinin reduced Cl^- secretion by PCs, but not by SCs. Lastly, we detected expression of chloride-bicarbonate exchangers (CBE) in all regions of the free tubule. Scans of H⁺ transport across the tubule indicated that base equivalents are likely reabsorbed across the ileac plexus. Blocking ClC or CBE led to secretion of a more basic fluid, indicating lack of base reabsorption. We suggest that the transport of Cl^- in the MTs of larval lepidopterans (i) may be correlated with the reabsorption of base, (ii) may be sensitive to Cl^- concentration in the haemolymph, and (iii) could be regulated by helicokinin and glycine.

Intracellular chloride: a regulator of transepithelial transport in the distal nephron

PURPOSE OF REVIEW

This review focuses on the role of intracellular chloride in regulating transepithelial ion transport in the distal convoluted tubule (DCT) in response to perturbations in plasma potassium homeostasis.

RECENT FINDINGS

Low dietary potassium increases the phosphorylation and activity of the sodium chloride cotransporter (NCC) in the DCT, and vice versa, affecting sodium-dependent potassium secretion in the downstream aldosterone-sensitive distal nephron. In cells, NCC phosphorylation is increased by lowering of intracellular chloride, via activation of the chloride-sensitive with no lysine (WNK)-SPAK/OSR1 (Ste20-related proline/alanine-rich kinase/oxidative stress response) kinase cascade. In-vivo studies have demonstrated pathway activation in the kidney in response to low dietary potassium. A possible mechanism is lowering of DCT intracellular chloride in response to low potassium because of parallel basolateral potassium and chloride channels. Recent studies support a role for these channels in the response of NCC to varying potassium. Studies examining chloride-insensitive WNK mutants, in the Drosophila renal tubule and in the mouse, lend further support to a role for chloride in regulating WNK activity and transepithelial ion transport. Caveats, alternatives, and future directions are also discussed.

SUMMARY

Chloride sensing by WNK kinase provides a mechanism to allow coupling of extracellular potassium with NCC phosphorylation and activity to maintain potassium homeostasis.

Three-dimensional architecture of pericardial nephrocytes in Drosophila melanogaster revealed by FIB/SEM tomography

Nephrocytes are similar in structure to podocytes and play a role in the isolation of toxic substances from hemolymph in insects. Drosophila melanogaster nephrocytes have recently been used to study podocyte function and disease. However, the three-dimensional ultrastructure of nephrocytes is not clearly understood because their surrounding basement membrane makes it difficult to observe using conventional scanning electron microscopy. We reconstructed the three-dimensional ultrastructure of Drosophila pericardial nephrocytes using serial focused-ion beam/scanning electron microscopy (FIB/SEM) images. The basal surfaces were occupied by foot processes and slit-like spaces between them. The slit-like spaces corresponded to the podocyte filtration slits and were formed by longitudinal infolding/invagination of the basal plasma membrane. The basal surface between the slit-like spaces became the foot processes, which ran almost linearly, and had a "washboard-like" appearance. Both ends of the foot processes were usually anastomosed to neighboring foot processes and thus free ends were rarely observed. We demonstrated that FIB/SEM is a powerful tool to better understand the three-dimensional architecture of nephrocytes.

Novel exc Genes Involved in Formation of the Tubular Excretory Canals of Caenorhabditis elegans

Regulation of luminal diameter is critical to the function of small single-celled tubes, of which the seamless tubular excretory canals of Caenorhabditis elegans provide a tractable genetic model. Mutations in several sets of genes exhibit the Exc phenotype, in which canal luminal growth is visibly altered. Here, a focused reverse genomic screen of genes highly expressed in the canals found 18 genes that significantly affect luminal outgrowth or diameter. These genes encode novel proteins as well as highly conserved proteins involved in processes including gene expression, cytoskeletal regulation, and vesicular and transmembrane transport. In addition, two genes act as suppressors on a pathway of conserved genes whose products mediate vesicle movement from early to recycling endosomes. The results provide new tools for understanding the integration of cytoplasmic structure and physiology in forming and maintaining the narrow diameter of single-cell tubules.

The glial sodium-potassium-2-chloride cotransporter is required for synaptic transmission in the Drosophila visual system

The Drosophila Ncc69 gene encodes a Na⁺-K⁺-2Cl⁻-cotransporter (NKCC) that is critical for regulating intra- and extracellular ionic conditions in different tissues. Here, we show that the Ncc69 transporter is necessary for fly vision and that its expression is required non-autonomously in glia to maintain visual synaptic transmission. Flies mutant for Ncc69 exhibit normal photoreceptor depolarization in response to a light pulse but lack the ON and OFF-transients characteristic of postsynaptic responses of lamina neurons, indicating a failure in synaptic transmission. We also find that synaptic transmission requires the Ncc69 regulatory kinases WNK and Fray in glia. The ERG phenotype is associated with a defect in the recycling of the histamine neurotransmitter. Ncc69 mutants exhibit higher levels of the transport metabolite carcinine in lamina cartridges, with its accumulation most intense in the extracellular space. Our work reveals a novel role of glial NKCC transporters in synaptic transmission, possibly through regulating extracellular ionic conditions.

Ab-normal saline in abnormal kidney function: risks and alternatives

Intravenous 0.9% saline has saved countless lives since it was introduced over a century ago. It remains the most widespread crystalloid in both adult and pediatric practice. However, in recent years, evidence of deleterious effects is accruing. These include increased mortality, acute kidney injury (AKI), metabolic acidosis, and coagulopathy. The predominant cause for these sequelae appears to be the excess chloride concentration of 0.9% saline relative to plasma. This has led to development of balanced isotonic solutions such as PlasmaLyte. This review summarizes current evidence for adverse effects of chloride-rich intravenous fluid and considers whether 0.9% saline should still be used in 2018 or abandoned as a historical treatment in favor of balanced crystalloid solutions.

WNK-SPAK/OSR1 signaling: lessons learned from an insect renal epithelium

WNK [with no lysine (K)] kinases regulate renal epithelial ion transport to maintain homeostasis of electrolyte concentrations, extracellular volume, and blood pressure. The SLC12 cation-chloride cotransporters, including the sodium-potassium-2-chloride (NKCC) and sodium chloride cotransporters (NCC), are targets of WNK regulation via the intermediary kinases SPAK (Ste20-related proline/alanine-rich kinase) and OSR1 (oxidative stress response). The pathway is activated by low dietary potassium intake, resulting in increased phosphorylation and activity of NCC. Chloride regulates WNK kinases in vitro by binding to the active site and inhibiting autophosphorylation and has been proposed to modulate WNK activity in the distal convoluted tubule in response to low dietary potassium. WNK-SPAK/OSR1 regulation of NKCC-dependent ion transport is evolutionarily ancient, and it occurs in the Drosophila Malpighian (renal) tubule. Here, we review recent studies from the Drosophila tubule demonstrating cooperative roles for chloride and the scaffold protein Mo25 (mouse protein-25, also known as calcium-binding protein-39) in the regulation of WNK-SPAK/OSR1 signaling in a transporting renal epithelium. Insights gained from this genetically manipulable and physiologically accessible epithelium shed light on molecular mechanisms of regulation of the WNK-SPAK/OSR1 pathway, which is important in human health and disease.