L-WNK1 is required for BK channel activation in intercalated cells

Aldosterone-independent regulation of K + secretion in the distal nephron

PURPOSE OF REVIEW

Maintenance of plasma K + concentration within a narrow range is critical to all cellular functions. The kidneys are the central organ for K + excretion, and robust renal excretory responses to dietary K + loads are essential for survival. Recent advances in the field have challenged the view that aldosterone is at the center of K + regulation. This review will examine recent findings and propose a new mechanism for regulating K + secretion.

RECENT FINDINGS

Local aldosterone-independent response systems in the distal nephron are increasingly recognized as key components of the rapid response to an acute K + load, as well as playing an essential role in sustained responses to increased dietary K + . The master kinase mTOR, best known for its role in mediating the effects of growth factors and insulin on growth and cellular metabolism, is central to these aldosterone-independent responses. Recent studies have shown that mTOR, particularly in the context of the "type 2" complex (mTORC2), is regulated by K + in a cell-autonomous fashion.

SUMMARY

New concepts related to cell-autonomous K + signaling and how it interfaces with aldosterone-dependent regulation are emerging. The underlying signaling pathways and effectors of regulated K + secretion, as well as implications for the aldosterone paradox and disease pathogenesis are discussed.

Cannabinoid receptor type 1 activation causes a water diuresis by inducing an acute central diabetes insipidus in mice

Cannabis and synthetic cannabinoid consumption are increasing worldwide. Cannabis contains numerous phytocannabinoids that act on the G protein-coupled cannabinoid receptor type 1 (CB1R) and cannabinoid receptor type 2 expressed throughout the body, including the kidney. Essentially every organ, including the kidney, produces endocannabinoids, which are endogenous ligands to these receptors. Cannabinoids acutely increase urine output in rodents and humans, thus potentially influencing total body water and electrolyte homeostasis. As the kidney collecting duct (CD) regulates total body water, acid/base, and electrolyte balance through specific functions of principal cells (PCs) and intercalated cells (ICs), we examined the cell-specific immunolocalization of CB1R in the mouse CD. Antibodies against either the C-terminus or N-terminus of CB1R consistently labeled aquaporin 2 (AQP2)-negative cells in the cortical and medullary CD and thus presumably ICs. Given the well-established role of ICs in urinary acidification, we used a clearance approach in mice that were acid loaded with 280 mM NH4Cl for 7 days and nonacid-loaded mice treated with the cannabinoid receptor agonist WIN55,212-2 (WIN) or a vehicle control. Although WIN had no effect on urinary acidification, these WIN-treated mice had less apical + subapical AQP2 expression in PCs compared with controls and developed acute diabetes insipidus associated with the excretion of large volumes of dilute urine. Mice maximally concentrated their urine when WIN and 1-desamino-8-d-arginine vasopressin [desmopressin (DDAVP)] were coadministered, consistent with central rather than nephrogenic diabetes insipidus. Although ICs express CB1R, the physiological role of CB1R in this cell type remains to be determined.NEW & NOTEWORTHY The CB1R agonist WIN55,212-2 induces central diabetes insipidus in mice. This research integrates existing knowledge regarding the diuretic effects of cannabinoids and the influence of CB1R on vasopressin secretion while adding new mechanistic insights about total body water homeostasis. Our findings provide a deeper understanding about the potential clinical impact of cannabinoids on human physiology and may help identify targets for novel therapeutics to treat water and electrolyte disorders such as hyponatremia and volume overload.

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.

Epithelial Na + Channels Function as Extracellular Sensors

The epithelial Na + channel (ENaC) resides on the apical surfaces of specific epithelia in vertebrates and plays a critical role in extracellular fluid homeostasis. Evidence that ENaC senses the external environment emerged well before the molecular identity of the channel was reported three decades ago. This article discusses progress toward elucidating the mechanisms through which specific external factors regulate ENaC function, highlighting insights gained from structural studies of ENaC and related family members. It also reviews our understanding of the role of ENaC regulation by the extracellular environment in physiology and disease. After familiarizing the reader with the channel's physiological roles and structure, we describe the central role protein allostery plays in ENaC's sensitivity to the external environment. We then discuss each of the extracellular factors that directly regulate the channel: proteases, cations and anions, shear stress, and other regulators specific to particular extracellular compartments. For each regulator, we discuss the initial observations that led to discovery, studies investigating molecular mechanism, and the physiological and pathophysiological implications of regulation. © 2024 American Physiological Society. Compr Physiol 14:5407-5447, 2024.

PIEZO1 is a distal nephron mechanosensor and is required for flow-induced K+ secretion

Ca2+-activated BK channels in renal intercalated cells (ICs) mediate luminal flow-induced K+ secretion (FIKS), but how ICs sense increased flow remains uncertain. We examined whether PIEZO1, a mechanosensitive Ca2+-permeable channel expressed in the basolateral membranes of ICs, is required for FIKS. In isolated cortical collecting ducts (CCDs), the mechanosensitive cation-selective channel inhibitor GsMTx4 dampened flow-induced increases in intracellular Ca2+ concentration ([Ca2+]i), whereas the PIEZO1 activator Yoda1 increased [Ca2+]i and BK channel activity. CCDs from mice fed a high-K+ (HK) diet exhibited a greater Yoda1-dependent increase in [Ca2+]i than CCDs from mice fed a control K+ diet. ICs in CCDs isolated from mice with a targeted gene deletion of Piezo1 in ICs (IC-Piezo1-KO) exhibited a blunted [Ca2+]i response to Yoda1 or increased flow, with an associated loss of FIKS in CCDs. Male IC-Piezo1-KO mice selectively exhibited an increased blood [K+] in response to an oral K+ bolus and blunted urinary K+ excretion following a volume challenge. Whole-cell expression of BKα subunit was reduced in ICs of IC-Piezo1-KO mice fed an HK diet. We conclude that PIEZO1 mediates flow-induced basolateral Ca2+ entry into ICs, is upregulated in the CCD in response to an HK diet, and is necessary for FIKS.

Tubular deficiency of ABCA1 augments cholesterol- and Na+-dependent effects on systemic blood pressure in male mice

Dyslipidemia, with changes in plasma membrane (PM) composition, is associated with hypertension, while rising PM cholesterol induces Na+ channel activity. We hypothesize that ablation of renal tubular ABCA1, a cholesterol efflux protein, leads to cholesterol- and Na+-dependent changes in blood pressure (BP). Transgenic mice (TgPAX8rtTA;tetO-Cre/+) expressing a doxycycline (dox)-inducible CRE recombinase were bred with mice expressing floxed ABCA1 to generate renal tubules deficient in ABCA1 (ABCA1FF). Tail-cuff systolic BP (SBP) was measured in mice on specific diets. Immunoblotting was performed on whole and PM protein lysates of kidney from mice completing experimental diets. Cortical PM of ABCA1FF showed reduced ABCA1 (60 ± 28%; n = 10, P < 0.05) compared with wild-type littermates (WT; n = 9). Tail-cuff SBP of ABCA1FF (n = 11) was not only greater post dox, but also during cholesterol or high Na+ feeding (P < 0.05) compared with WT mice (n = 15). A Na+-deficient diet abolished the difference, while 6 wk of cholesterol diet raised SBP in ABCA1FF compared with mice before cholesterol feeding (P < 0.05). No difference in α-ENaC protein abundance was noted in kidney lysate; however, γ-ENaC increased in ABCA1FF mice versus WT mice. In kidney membranes, NKCC2 abundance was greater in ABCA1FF versus WT mice. Cortical lysates of ABCA1FF mouse kidneys expressed less renin and angiotensin I receptor than WT mouse kidneys. Furosemide injection induced a greater diuretic effect in ABCA1FF (n = 7; 45.2 ± 8.7 µL/g body wt) versus WT (n = 7; 33.1 ± 6.9 µL/g body wt; P < 0.05) but amiloride did not. Tubular ABCA1 deficiency induces cholesterol-dependent rise in SBP and modest Na+ sensitivity of SBP, which we speculate is partly related to Na+ transporters and channels.NEW & NOTEWORTHY Cholesterol has been linked to greater Na+ channel activity in kidney cells, which may predispose to systemic hypertension. We showed that when ABCA1, a protein that removes cholesterol from tissues, is ablated from mouse kidneys, systemic blood pressure is greater than normal mice. Dietary cholesterol further increases blood pressure in transgenic mice, whereas low dietary salt intake reduced blood pressure to that of normal mice. Thus, we speculate that diseases and pharmaceuticals that reduce renal ABCA1 expression, like diabetes and calcineurin inhibitors, respectively, contribute to the prominence of hypertension in their clinical presentation.

Navigating the kidney organoid: insights into assessment and enhancement of nephron function

Kidney organoids are three-dimensional structures generated from pluripotent stem cells (PSCs) that are capable of recapitulating the major structures of mammalian kidneys. As this technology is expected to be a promising tool for studying renal biology, drug discovery, and regenerative medicine, the functional capacity of kidney organoids has emerged as a critical question in the field. Kidney organoids produced using several protocols harbor key structures of native kidneys. Here, we review the current state, recent advances, and future challenges in the functional characterization of kidney organoids, strategies to accelerate and enhance kidney organoid functions, and access to PSC resources to advance organoid research. The strategies to construct physiologically relevant kidney organoids include the use of organ-on-a-chip technologies that integrate fluid circulation and improve organoid maturation. These approaches result in increased expression of the major tubular transporters and elements of mechanosensory signaling pathways suggestive of improved functionality. Nevertheless, continuous efforts remain crucial to create kidney tissue that more faithfully replicates physiological conditions for future applications in kidney regeneration medicine and their ethical use in patient care.NEW & NOTEWORTHY Kidney organoids are three-dimensional structures derived from stem cells, mimicking the major components of mammalian kidneys. Although they show great promise, their functional capacity has become a critical question. This review explores the advancements and challenges in evaluating and enhancing kidney organoid function, including the use of organ-on-chip technologies, multiomics data, and in vivo transplantation. Integrating these approaches to further enhance their physiological relevance will continue to advance disease modeling and regenerative medicine applications.

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.

Pendrin regulation is prioritized by anion in high-potassium diets

The Cl-/[Formula: see text] exchanger pendrin in the kidney maintains acid-base balance and intravascular volume. Pendrin is upregulated in models associated with high circulating aldosterone concentration, such as dietary NaCl restriction or an aldosterone infusion. However, it has not been established if pendrin is similarly regulated by aldosterone with a high-K+ diet because the effects of accompanying anions have not been considered. Here, we explored how pendrin is modulated by different dietary potassium salts. Wild-type (WT) and aldosterone synthase (AS) knockout (KO) mice were randomized to control, high-KHCO3, or high-KCl diets. Dietary KCl and KHCO3 loading increased aldosterone in WT mice to the same extent but had opposite effects on pendrin abundance. KHCO3 loading increased pendrin protein and transcript abundance. Conversely, high-KCl diet feeding caused pendrin to decrease within 8 h of switching from the high-KHCO3 diet, coincident with an increase in plasma Cl- and a decrease in [Formula: see text]. In contrast, switching the high-KCl diet to the high-KHCO3 diet caused pendrin to increase in WT mice. Experiments in AS KO mice revealed that aldosterone is necessary to optimally upregulate pendrin protein in response to the high-KHCO3 diet but not to increase pendrin mRNA. We conclude that pendrin is differentially regulated by different dietary potassium salts and that its regulation is prioritized by the dietary anion, providing a mechanism to prevent metabolic alkalosis with high-K+ base diets and safeguard against hyperchloremic acidosis with consumption of high-KCl diets.NEW & NOTEWORTHY Regulation of the Cl-/[Formula: see text] exchanger pendrin has been suggested to explain the aldosterone paradox. A high-K+ diet has been proposed to downregulate a pendrin-mediated K+-sparing NaCl reabsorption pathway to maximize urinary K+ excretion. Here, we challenged the hypothesis, revealing that the accompanying anion, not K+, drives pendrin expression. Pendrin is downregulated with a high-KCl diet, preventing acidosis, and upregulated with an alkaline-rich high-K+ diet, preventing metabolic alkalosis. Pendrin regulation is prioritized for acid-base balance.

WNK1 is a chloride-stimulated scaffold that regulates mTORC2 activity and ion transport

Mammalian (or mechanistic) target of rapamycin complex 2 (mTORC2) is a kinase complex that targets predominantly Akt family proteins, SGK1 and protein kinase C (PKC), and has well-characterized roles in mediating hormone and growth factor effects on a wide array of cellular processes. Recent evidence suggests that mTORC2 is also directly stimulated in renal tubule cells by increased extracellular K+ concentration, leading to activation of the Na+ channel, ENaC, and increasing the electrical driving force for K+ secretion. We identify here a signaling mechanism for this local effect of K+. We show that an increase in extracellular [K+] leads to a rise in intracellular chloride (Cl-), which stimulates a previously unknown scaffolding activity of the protein 'with no lysine-1' (WNK1) kinase. WNK1 interacts selectively with SGK1 and recruits it to mTORC2, resulting in enhanced SGK1 phosphorylation and SGK1-dependent activation of ENaC. This scaffolding effect of WNK1 is independent of its own kinase activity and does not cause a generalized stimulation of mTORC2 kinase activity. These findings establish a novel WNK1-dependent regulatory mechanism that harnesses mTORC2 kinase activity selectively toward SGK1 to control epithelial ion transport and electrolyte homeostasis.

WNK1 in the kidney

PURPOSE OF REVIEW

The aim of this manuscript was to review recent evidence uncovering the roles of the With No lysine (K) kinase 1 (WNK1) in the kidney.

RECENT FINDINGS

Analyses of microdissected mouse nephron segments have revealed the abundance of long-WNK1 and kidney-specific-WNK1 transcripts in different segments. The low levels of L-WNK1 transcripts in the distal convoluted tubule (DCT) stand out and support functional evidence on the lack of L-WNK1 activity in this segment. The recent description of familial hyperkalaemic hypertension (FHHt)-causative mutations affecting the acidic domain of WNK1 supports the notion that KS-WNK1 activates the Na+:Cl- cotransporter NCC. The high sensitivity of KS-WNK1 to KLHL3-targeted degradation and the low levels of L-WNK1 in the DCT, led to propose that this type of FHHt is mainly due to increased KS-WNK1 protein in the DCT. The observation that KS-WNK1 renal protein expression is induced by low K+ diet and recent reassessment of the phenotype of KS-WNK1-/- mice suggested that KS-WNK1 may be necessary to achieve maximal NCC activation under this condition. Evidences on the regulation of other renal transport proteins by WNK1 are also summarized.

SUMMARY

The diversity of WNK1 transcripts in the kidney has complicated the interpretation of experimental data. Integration of experimental data with the knowledge of isoform abundance in renal cell types is necessary in future studies about WNK1 function in the kidney.

Physiology assays in human kidney organoids

Kidney organoids derived from human pluripotent stem cells constitute a novel model of disease, development, and regenerative therapy. Organoids are human, experimentally accessible, high throughput, and enable reconstitution of tissue-scale biology in a petri dish. Although gene expression patterns in organoid cells have been analyzed extensively, less is known about the functionality of these structures. Here, we review assays of physiological function in human kidney organoids, including best practices for quality control, and future applications. Tubular structures in organoids accumulate specific molecules through active transport, including dextran and organic anions, and swell with fluid in response to cAMP stimulation. When engrafted into animal models in vivo, organoids form vascularized glomerulus-like structures capable of size-selective filtration. Organoids exhibit metabolic, endocrine, injury, and infection phenotypes, although their specificity is not yet fully clear. To properly interpret organoid physiology assays, it is important to incorporate appropriate negative and positive controls, statistical methods, data presentation, molecular mechanisms, and clinical data sets. Improvements in organoid perfusion, patterning, and maturation are needed to enable branching morphogenesis, urine production, and renal replacement. Reconstituting renal physiology with kidney organoids is a new field with potential to provide fresh insights into classical phenomena.

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.

Salt sensitivity of volume and blood pressure in a mouse with globally reduced ENaC γ-subunit expression

The epithelial Na+ channel (ENaC) promotes the absorption of Na+ in the aldosterone-sensitive distal nephron, colon, and respiratory epithelia. Deletion of genes encoding subunits of ENaC results in early postnatal mortality. Here, we present the initial characterization of a mouse with dramatically suppressed expression of the ENaC γ-subunit. We used this hypomorphic (γmt) allele to explore the importance of this subunit in homeostasis of electrolytes and body fluid volume. At baseline, γ-subunit expression in γmt/mt mice was markedly suppressed in the kidney and lung, whereas electrolytes resembled those of littermate controls. Aldosterone levels in γmt/mt mice exceeded those seen in littermate controls. Quantitative magnetic resonance measurement of body composition revealed similar baseline body water, lean tissue mass, and fat tissue mass in γmt/mt mice and controls. γmt/mt mice exhibited a more rapid decline in body water and lean tissue mass in response to a low-Na+ diet than the controls. Replacement of drinking water with 2% saline selectively and transiently increased body water and lean tissue mass in γmt/mt mice relative to the controls. Lower blood pressures were variably observed in γmt/mt mice on a high-salt diet compared with the controls. γmt/mt also exhibited reduced diurnal blood pressure variation, a "nondipping" phenotype, on a high-Na+ diet. Although ENaC in the renal tubules and colon works to prevent extracellular fluid volume depletion, our observations suggest that ENaC in other tissues may participate in regulating extracellular fluid volume and blood pressure.NEW & NOTEWORTHY A mouse with globally suppressed expression of the epithelial Na+ channel γ-subunit showed enhanced sensitivity to dietary salt, including a transient increase in total body fluid, reduced blood pressure, and reduced diurnal blood pressure variation when given a dietary NaCl challenge. These results point to a role for the epithelial Na+ channel in regulating body fluid and blood pressure beyond classical transepithelial Na+ transport mechanisms.