Distal convoluted tubule sexual dimorphism revealed by advanced 3D imaging

Enriched Single-Nucleus RNA-Sequencing Reveals Unique Attributes of Distal Convoluted Tubule Cells

SIGNIFICANCE STATEMENT

High-resolution single-nucleus RNA-sequencing data indicate a clear separation between primary sites of calcium and magnesium handling within distal convoluted tubule (DCT). Both DCT1 and DCT2 express Slc12a3, but these subsegments serve distinctive functions, with more abundant magnesium-handling genes along DCT1 and more calcium-handling genes along DCT2. The data also provide insight into the plasticity of the distal nephron-collecting duct junction, formed from cells of separate embryonic origins. By focusing/changing gradients of gene expression, the DCT can morph into different physiological cell states on demand.

BACKGROUND

The distal convoluted tubule (DCT) comprises two subsegments, DCT1 and DCT2, with different functional and molecular characteristics. The functional and molecular distinction between these segments, however, has been controversial.

METHODS

To understand the heterogeneity within the DCT population with better clarity, we enriched for DCT nuclei by using a mouse line combining "Isolation of Nuclei Tagged in specific Cell Types" and sodium chloride cotransporter-driven inducible Cre recombinase. We sorted the fluorescently labeled DCT nuclei using Fluorescence-Activated Nucleus Sorting and performed single-nucleus transcriptomics.

RESULTS

Among 25,183 DCT cells, 75% were from DCT1 and 25% were from DCT2. In addition, there was a small population (<1%) enriched in proliferation-related genes, such as Top2a , Cenpp , and Mki67 . Although both DCT1 and DCT2 expressed sodium chloride cotransporter, magnesium transport genes were predominantly expressed along DCT1, whereas calcium, electrogenic sodium, and potassium transport genes were more abundant along DCT2. The transition between these two segments was gradual, with a transitional zone in which DCT1 and DCT2 cells were interspersed. The expression of the homeobox genes by DCT cells suggests that they develop along different trajectories.

CONCLUSIONS

Transcriptomic analysis of an enriched rare cell population using a genetically targeted approach clarifies the function and classification of distal cells. The DCT segment is short, can be separated into two subsegments that serve distinct functions, and is speculated to derive from different origins during development.

Precise reconstruction of the entire mouse kidney at cellular resolution

The kidney is an important organ for excreting metabolic waste and maintaining the stability of the body's internal environment. The renal function involves multiple complex and fine structures in the whole kidney, and any change in these structures may cause impaired nephric function. Consequently, achieving three-dimensional (3D) reconstruction of the entire kidney at a single-cell resolution is of significant importance for understanding the kidney's structural characteristics and exploring the pathogenesis of kidney diseases. In this paper, we propose a pipeline from sample preparation to optical microscopic imaging of the entire kidney, followed by data processing for 3D reconstruction of the whole mouse kidney. We employed transgenic fluorescent labeling and propidium iodide (PI) labeling to obtain detailed information about the vascular structure and cytoarchitecture of the kidney. Subsequently, the entire mouse kidney was imaged at submicron-resolution using high-definition fluorescent micro-optical sectioning tomography (HD-fMOST). Finally, we reconstructed the structures of interest through various data processing methods on the original images. This included detecting glomeruli throughout the entire kidney, as well as the segmentation and visualization of the renal arteries, veins, and three different types of nephrons. Our method provides a powerful tool for studying the renal microstructure and its spatial relationships throughout the entire kidney.

Dysregulation of the WNK4-SPAK/OSR1 pathway has a minor effect on baseline NKCC2 phosphorylation

The with-no-lysine kinase 4 (WNK4)-sterile 20/SPS-1-related proline/alanine-rich kinase (SPAK)/oxidative stress-responsive kinase 1 (OSR1) pathway mediates activating phosphorylation of the furosemide-sensitive Na+-K+-2Cl- cotransporter (NKCC2) and the thiazide-sensitive NaCl cotransporter (NCC). The commonly used pT96/pT101-pNKCC2 antibody cross-reacts with pT53-NCC in mice on the C57BL/6 background due to a five amino acid deletion. We generated a new C57BL/6-specific pNKCC2 antibody (anti-pT96-NKCC2) and tested the hypothesis that the WNK4-SPAK/OSR1 pathway strongly regulates the phosphorylation of NCC but not NKCC2. In C57BL/6 mice, anti-pT96-NKCC2 detected pNKCC2 and did not cross-react with NCC. Abundances of pT96-NKCC2 and pT53-NCC were evaluated in Wnk4-/-, Osr1-/-, Spak-/-, and Osr1-/-/Spak-/- mice and in several models of the disease familial hyperkalemic hypertension (FHHt) in which the CUL3-KLHL3 ubiquitin ligase complex that promotes WNK4 degradation is dysregulated (Cul3+/-/Δ9, Klhl3-/-, and Klhl3R528H/R528H). All mice were on the C57BL/6 background. In Wnk4-/- mice, pT53-NCC was almost absent but pT96-NKCC2 was only slightly lower. pT53-NCC was almost absent in Spak-/- and Osr1-/-/Spak-/- mice, but pT96-NKCC2 abundance did not differ from controls. pT96-NKCC2/total NKCC2 was slightly lower in Osr1-/- and Osr1-/-/Spak-/- mice. WNK4 expression colocalized not only with NCC but also with NKCC2 in Klhl3-/- mice, but pT96-NKCC2 abundance was unchanged. Consistent with this, furosemide-induced urinary Na+ excretion following thiazide treatment was similar between Klhl3-/- and controls. pT96-NKCC2 abundance was also unchanged in the other FHHt mouse models. Our data show that disruption of the WNK4-SPAK/OSR1 pathway only mildly affects NKCC2 phosphorylation, suggesting a role for other kinases in NKCC2 activation. In FHHt models NKCC2 phosphorylation is unchanged despite higher WNK4 abundance, explaining the thiazide sensitivity of FHHt.NEW & NOTEWORTHY The renal cation cotransporters NCC and NKCC2 are activated following phosphorylation mediated by the WNK4-SPAK/OSR1 pathway. While disruption of this pathway strongly affects NCC activity, effects on NKCC2 activity are unclear since the commonly used phospho-NKCC2 antibody was recently reported to cross-react with phospho-NCC in mice on the C57BL/6 background. Using a new phospho-NKCC2 antibody specific for C57BL/6, we show that inhibition or activation of the WNK4-SPAK/OSR1 pathway in mice only mildly affects NKCC2 phosphorylation.

Sex differences in renal transporters: assessment and functional consequences

Mammalian kidneys are specialized to maintain fluid and electrolyte homeostasis. The epithelial transport processes along the renal tubule that match output to input have long been the subject of experimental and theoretical study. However, emerging data have identified a new dimension of investigation: sex. Like most tissues, the structure and function of the kidney is regulated by sex hormones and chromosomes. Available data demonstrate sex differences in the abundance of kidney solute and electrolyte transporters, establishing that renal tubular organization and operation are distinctly different in females and males. Newer studies have provided insights into the physiological consequences of these sex differences. Computational simulations predict that sex differences in transporter abundance are likely driven to optimize reproduction, enabling adaptive responses to the nutritional requirements of serial pregnancies and lactation - normal life-cycle changes that challenge the ability of renal transporters to maintain fluid and electrolyte homeostasis. Later in life, females may also undergo menopause, which is associated with changes in disease risk. Although numerous knowledge gaps remain, ongoing studies will provide further insights into the sex-specific mechanisms of sodium, potassium, acid-base and volume physiology throughout the life cycle, which may lead to therapeutic opportunities.

Direct androgen receptor control of sexually dimorphic gene expression in the mammalian kidney

Mammalian organs exhibit distinct physiology, disease susceptibility, and injury responses between the sexes. In the mouse kidney, sexually dimorphic gene activity maps predominantly to proximal tubule (PT) segments. Bulk RNA sequencing (RNA-seq) data demonstrated that sex differences were established from 4 and 8 weeks after birth under gonadal control. Hormone injection studies and genetic removal of androgen and estrogen receptors demonstrated androgen receptor (AR)-mediated regulation of gene activity in PT cells as the regulatory mechanism. Interestingly, caloric restriction feminizes the male kidney. Single-nuclear multiomic analysis identified putative cis-regulatory regions and cooperating factors mediating PT responses to AR activity in the mouse kidney. In the human kidney, a limited set of genes showed conserved sex-linked regulation, whereas analysis of the mouse liver underscored organ-specific differences in the regulation of sexually dimorphic gene expression. These findings raise interesting questions on the evolution, physiological significance, disease, and metabolic linkage of sexually dimorphic gene activity.

Sex differences in renal electrolyte transport

PURPOSE OF REVIEW

Women experience unique life events, for example, pregnancy and lactation, that challenge renal regulation of electrolyte homeostasis. Recent analyses of nephron organization in female vs. male rodent kidneys, revealed distinct sexual dimorphisms in electrolyte transporter expression, abundance, and activity. This review aims to provide an overview of electrolyte transporters' organization and operation in female compared with the commonly studied male kidney, and the (patho)physiologic consequences of the differences.

RECENT FINDINGS

When electrolyte transporters are assessed in kidney protein homogenates from both sexes, relative transporter abundance ratios in females/males are less than one along proximal tubule and greater than one post macula densa, which is indicative of a 'downstream shift' in fractional reabsorption of electrolytes in females. This arrangement improves the excretion of a sodium load, challenges potassium homeostasis, and is consistent with the lower blood pressure and greater pressure natriuresis observed in premenopausal women.

SUMMARY

We summarize recently reported new knowledge about sex differences in renal transporters: abundance and expression along nephron, implications for regulation by Na + , K + and angiotensin II, and mathematical models of female nephron function.

Potassium Activates mTORC2-dependent SGK1 Phosphorylation to Stimulate Epithelial Sodium Channel: Role in Rapid Renal Responses to Dietary Potassium

SIGNIFICANCE STATEMENT

Rapid renal responses to ingested potassium are essential to prevent hyperkalemia and also play a central role in blood pressure regulation. Although local extracellular K + concentration in kidney tissue is increasingly recognized as an important regulator of K + secretion, the underlying mechanisms that are relevant in vivo remain controversial. To assess the role of the signaling kinase mTOR complex-2 (mTORC2), the authors compared the effects of K + administered by gavage in wild-type mice and knockout mice with kidney tubule-specific inactivation of mTORC2. They found that mTORC2 is rapidly activated to trigger K + secretion and maintain electrolyte homeostasis. Downstream targets of mTORC2 implicated in epithelial sodium channel regulation (SGK1 and Nedd4-2) were concomitantly phosphorylated in wild-type, but not knockout, mice. These findings offer insight into electrolyte physiologic and regulatory mechanisms.

BACKGROUND

Increasing evidence implicates the signaling kinase mTOR complex-2 (mTORC2) in rapid renal responses to changes in plasma potassium concentration [K + ]. However, the underlying cellular and molecular mechanisms that are relevant in vivo for these responses remain controversial.

METHODS

We used Cre-Lox-mediated knockout of rapamycin-insensitive companion of TOR (Rictor) to inactivate mTORC2 in kidney tubule cells of mice. In a series of time-course experiments in wild-type and knockout mice, we assessed urinary and blood parameters and renal expression and activity of signaling molecules and transport proteins after a K + load by gavage.

RESULTS

A K + load rapidly stimulated epithelial sodium channel (ENaC) processing, plasma membrane localization, and activity in wild-type, but not in knockout, mice. Downstream targets of mTORC2 implicated in ENaC regulation (SGK1 and Nedd4-2) were concomitantly phosphorylated in wild-type, but not knockout, mice. We observed differences in urine electrolytes within 60 minutes, and plasma [K + ] was greater in knockout mice within 3 hours of gavage. Renal outer medullary potassium (ROMK) channels were not acutely stimulated in wild-type or knockout mice, nor were phosphorylation of other mTORC2 substrates (PKC and Akt).

CONCLUSIONS

The mTORC2-SGK1-Nedd4-2-ENaC signaling axis is a key mediator of rapid tubule cell responses to increased plasma [K + ] in vivo . The effects of K + on this signaling module are specific, in that other downstream mTORC2 targets, such as PKC and Akt, are not acutely affected, and ROMK and Large-conductance K + (BK) channels are not activated. These findings provide new insight into the signaling network and ion transport systems that underlie renal responses to K +in vivo .

Direct androgen receptor regulation of sexually dimorphic gene expression in the mammalian kidney

Mammalian organs exhibit distinct physiology, disease susceptibility and injury responses between the sexes. In the mouse kidney, sexually dimorphic gene activity maps predominantly to proximal tubule (PT) segments. Bulk RNA-seq data demonstrated sex differences were established from 4 and 8 weeks after birth under gonadal control. Hormone injection studies and genetic removal of androgen and estrogen receptors demonstrated androgen receptor (AR) mediated regulation of gene activity in PT cells as the regulatory mechanism. Interestingly, caloric restriction feminizes the male kidney. Single-nuclear multiomic analysis identified putative cis-regulatory regions and cooperating factors mediating PT responses to AR activity in the mouse kidney. In the human kidney, a limited set of genes showed conserved sex-linked regulation while analysis of the mouse liver underscored organ-specific differences in the regulation of sexually dimorphic gene expression. These findings raise interesting questions on the evolution, physiological significance, and disease and metabolic linkage, of sexually dimorphic gene activity.

Multiomics Analyses Reveal Sex Differences in Mouse Renal Proximal Subsegments

SIGNIFICANCE STATEMENT

Sex-dependent differences in kidney function are recognized but the underlying molecular mechanisms are largely unexplored. Advances in genomics and proteomic technologies now allow extensive characterization of differences between the same cell types of males and females. Multiomics integrating RNA-seq, ATAC-seq, and proteomics data to investigate differences in gene expression, chromatin accessibility, and protein expression in proximal tubules of male and female mice identified many sex-biased genes and proteins associated with kidney functions, including metabolic and transport processes. Sex differences may also arise from variations of the interaction between transcription factors and accessible chromatin regions. A comprehensive web resource is provided to advance understanding of sex differences in cells of the proximal tubule.

BACKGROUND

Sex differences have been increasingly recognized as important in kidney physiology and pathophysiology, but limited resources are available for comprehensive interrogation of sex differences.

METHODS

RNA-seq and ATAC-seq of microdissected mouse proximal tubules and protein mass spectrometry of homogenized perfused mouse kidneys reveal differences in proximal tubule cells of males and females.

RESULTS

The transcriptomic data indicated that the major differences in the proximal tubules between the sexes were in the S2/S3 segments, and most of the sex-biased transcripts mapped to autosomes rather than to the sex chromosomes. Many of the transcripts exhibiting sex-biased expression are involved in monocarboxylic acid metabolic processes, organic anion transport, and organic acid transport. The ATAC-seq method on microdissected tubules captured chromatin accessibility. Many of the more than 7000 differentially accessible DNA regions identified were in distal regions. Motif analyses revealed a lack of direct involvement of estrogen receptors or the androgen receptor (absence of canonical hormone response elements), suggesting an indirect regulatory role of sex hormones. Instead, analyses identified several transcription factors (TFs) ( Tead1 , Nfia/b , and Pou3f3 ) whose interplay with proximal tubule-specific TFs ( e.g. , Hnf1b , Hnf4a ) may contribute to sex differences. Finally, the whole-kidney proteome was correlated with the transcriptome, and many sex-biased proteins ( e.g. , Cyp2e1, Acsm2/3) were identified.

CONCLUSIONS

Sex-dependent cis-regulatory elements interact with TFs in ways that lead to sex-biased gene expression in proximal tubule cells. These data are provided as a user-friendly web page at https://esbl.nhlbi.nih.gov/MRECA/PT/ .

Women on diuretics have a higher risk of hospital admission because of hyponatremia than men

PURPOSE

Recent studies suggest that women are more susceptible to diuretic-induced hyponatremia resulting in hospital admission than men. The aim of this study was to confirm whether these sex differences in hyponatremia-related hospital admissions in diuretic users remain after adjusting for several confounding variables such as age, dose, and concurrent medication.

METHODS

In a case-control design nested in diuretic users, cases of hyponatremia associated hospital admissions between 2005 and 2017 were identified from the PHARMO Data Network. Cases were 1:10 matched to diuretic users as controls. Odds ratios (OR) with 95%CIs were calculated for women versus men and adjusted for potential confounders (age, number of diuretics, other hyponatremia-inducing drugs, chronic disease score) using unconditional logistic regression analysis. A subgroup analysis was performed for specific diuretic groups (thiazides, loop diuretics and aldosterone antagonists).

RESULTS

Women had a statistically significantly higher risk of a hospital admission associated with hyponatremia than men while using diuretics (OR 1.86, 95%CI 1.64-2.11). Adjusting for the potential confounders resulted in an increased risk for women compared to men (ORadj 2.65, 95% CI 2.31-3.04). This higher risk in women was also seen in the three subgroup analyses after adjustment.

CONCLUSION

Our findings show a higher risk of hyponatremia-related hospital admission in women than men while using diuretics. Further research is needed to understand the underlying mechanism of this sex difference to be able to provide sex-specific recommendations.

Sex and species differences in epithelial transport in rat and mouse kidneys: Modeling and analysis

The goal of this study was to investigate the functional implications of sex and species differences in the pattern of transporters along nephrons in the rat and mouse kidney, as reported by Veiras et al. (J Am Soc Nephrol 28: 3504–3517, 2017). To do so, we developed the first sex-specific computational models of epithelial water and solute transport along the nephrons from male and female mouse kidneys, and conducted simulations along with our published rat models. These models account for the sex differences in the abundance of apical and basolateral transporters, glomerular filtration rate, and tubular dimensions. Model simulations predict that 73% and 57% of filtered Na+ is reabsorbed by the proximal tubules of male and female rat kidneys, respectively. Due to their smaller transport area and lower NHE3 activity, the proximal tubules in the mouse kidney reabsorb a significantly smaller fraction of the filtered Na+, at 53% in male and only 34% in female. The lower proximal fractional Na+ reabsorption in female kidneys of both rat and mouse is due primarily to their smaller transport area, lower Na+/H+ exchanger activity, and lower claudin-2 abundance, culminating in significantly larger fractional delivery of water and Na+ to the downstream nephron segments in female kidneys. Conversely, the female distal nephron exhibits a higher abundance of key Na+ transporters, including Na+-Cl− cotransporters in both species, epithelial Na+ channels for the female rat, and Na+-K+-Cl−cotransporters for the female mouse. The higher abundance of transporters accounts for the enhanced water and Na+ transport along the female rat and mouse distal nephrons, relative to the respective male, resulting in similar urine excretion between the sexes. Model simulations indicate that the sex and species differences in renal transporter patterns may partially explain the experimental observation that, in response to a saline load, the diuretic and natriuretic responses were more rapid in female rats than males, but no significant sex difference was found in mice. These computational models can serve as a valuable tool for analyzing findings from experimental studies conducted in rats and mice, especially those involving genetic modifications.

Acclimation to a High-Salt Diet Is Sex Dependent

Background Premenopausal women are less likely to develop hypertension and salt-related complications than are men, yet the impact of sex on mechanisms regulating Na+ homeostasis during dietary salt challenges is poorly defined. Here, we determined whether female rats have a more efficient capacity to acclimate to increased dietary salt intake challenge. Methods and Results Age-matched male and female Sprague Dawley rats maintained on a normal-salt (NS) diet (0.49% NaCl) were challenged with a 5-day high-salt diet (4.0% NaCl). We assessed serum, urinary, skin, and muscle electrolytes; total body water; and kidney Na+ transporters during the NS and high-salt diet phases. During the 5-day high-salt challenge, natriuresis increased more rapidly in females, whereas serum Na+ and body water concentration increased only in males. To determine if females are primed to handle changes in dietary salt, we asked the question whether the renal endothelin-1 natriuretic system is more active in female rats, compared with males. During the NS diet, female rats had a higher urinary endothelin-1 excretion rate than males. Moreover, Ingenuity Pathway Analysis of RNA sequencing data identified the enrichment of endothelin signaling pathway transcripts in the inner medulla of kidneys from NS-fed female rats compared with male counterparts. Notably, in human subjects who consumed an Na+-controlled diet (3314-3668 mg/day) for 3 days, women had a higher urinary endothelin-1 excretion rate than men, consistent with our findings in NS-fed rats. Conclusions These results suggest that female sex confers a greater ability to maintain Na+ homeostasis during acclimation to dietary Na+ challenges and indicate that the intrarenal endothelin-1 natriuretic pathway is enhanced in women.

Potassium and the kidney: a reciprocal relationship with clinical relevance

By controlling urinary potassium excretion, the kidneys play a key role in maintaining whole-body potassium homeostasis. Conversely, low urinary potassium excretion (as a proxy for insufficient dietary intake) is increasingly recognized as a risk factor for the progression of kidney disease. Thus, there is a reciprocal relationship between potassium and the kidney: the kidney regulates potassium balance but potassium also affects kidney function. This review explores this relationship by discussing new insights into kidney potassium handling derived from recently characterized tubulopathies and studies on sexual dimorphism. These insights reveal a central but non-exclusive role for the distal convoluted tubule in sensing potassium and subsequently modifying the activity of the sodium-chloride cotransporter. This is another example of reciprocity: activation of the sodium-chloride cotransporter not only reduces distal sodium delivery and therefore potassium secretion but also increases salt sensitivity. This mechanism helps explain the well-known relationship between dietary potassium and blood pressure. Remarkably, in children, blood pressure is related to dietary potassium but not sodium intake. To explore how potassium deficiency can cause kidney injury, we review the mechanisms of hypokalemic nephropathy and discuss if these mechanisms may explain the association between low dietary potassium intake and adverse kidney outcomes. We discuss if potassium should be repleted in patients with kidney disease and what role dietary potassium plays in the risk of hyperkalemia. Supported by data and physiology, we reach the conclusion that we should view potassium not only as a potentially dangerous cation but also as a companion in the battle against kidney disease.

Sex-specific adaptations to high-salt diet preserve electrolyte homeostasis with distinct sodium transporter profiles

Kidneys continuously filter an enormous amount of sodium and adapt kidney Na+ reabsorption to match Na+ intake to maintain circulatory volume and electrolyte homeostasis. Males (M) respond to high-salt (HS) diet by translocating proximal tubule Na+/H+ exchanger isoform 3 (NHE3) to the base of the microvilli, reducing activated forms of the distal NaCl cotransporter (NCC) and epithelial Na+ channel (ENaC). Males (M) and females (F) on normal-salt (NS) diet present sex-specific profiles of "transporters" (cotransporters, channels, pumps, and claudins) along the nephron, e.g., F exhibit 40% lower NHE3 and 200% higher NCC abundance than M. We tested the hypothesis that adaptations to HS diet along the nephron will, likewise, exhibit sexual dimorphisms. C57BL/6J mice were fed for 15 days with 4% NaCl diet (HS) versus 0.26% NaCl diet (NS). On HS, M and F exhibited normal plasma [Na+] and [K+], similar urine volume, Na+, K+, and osmolal excretion rates normalized to body weight. In F, like M, HS lowered abundance of distal NCC, phosphorylated NCC, and cleaved (activated) forms of ENaC. The adaptations associated with achieving electrolyte homeostasis exhibit sex-dependent and independent mechanisms. Sex differences in baseline "transporters" abundance persist during HS diet, yet the fold changes during HS diet (normalized to NS) are similar along the distal nephron and collecting duct. Sex-dependent differences observed along the proximal tubule during HS show that female kidneys adapt differently from patterns reported in males, yet achieve and maintain fluid and electrolyte homeostasis.

Sex differences in solute and water handling in the human kidney: Modeling and functional implications

The kidneys maintain homeostasis by controlling the amount of water and electrolytes in the blood. That function is accomplished by the nephrons, which transform glomerular filtrate into urine by a transport process mediated by membrane transporters. We postulate that the distribution of renal transporters along the nephron is markedly different between men and women, as recently shown in rodents. We hypothesize that the larger abundance of a renal Na+ transport in the proximal tubules in females may also better prepare them for the fluid retention adaptations required during pregnancy and lactation. Also, kidneys play a key role in blood pressure regulation, and a popular class of anti-hypertensive medications and angiotensin converting enzymes (ACE) inhibitors have been reported to be less effective in women. Model simulations suggest that the blunted natriuretic and diuretic effects of ACE inhibition in women can be attributed, in part, to their higher distal baseline transport capacity.

Clearly imaging and quantifying the kidney in 3D

For decades, measurements of kidney microanatomy using 2-dimensional sections has provided us with a detailed knowledge of kidney morphology under physiological and pathological conditions. However, the rapid development of tissue clearing methods in recent years, in combination with the development of novel 3-dimensional imaging modalities have provided new insights into kidney structure and function. This review article describes a range of novel insights into kidney development and disease obtained recently using these new methodological approaches. For example, in the developing kidney these approaches have provided new understandings of ureteric branching morphogenesis, nephron progenitor cell proliferation and commitment, interactions between ureteric tip cells and nephron progenitor cells, and the establishment of nephron segmentation. In whole adult mouse kidneys, tissue clearing combined with light sheet microscopy can image and quantify the total number of glomeruli, a major breakthrough in the field. Similar approaches have provided new insights into the structure of the renal vasculature and innervation, tubulointerstitial remodeling, podocyte loss and hypertrophy, cyst formation, the evolution of cellular crescents, and the structure of the glomerular filtration barrier. Many more advances in the understanding of kidney biology and pathology can be expected as additional clearing and imaging techniques are developed and adopted by more investigators.

Deletion of renal Nedd4-2 abolishes the effect of high K+ intake on Kir4.1/Kir5.1 and NCC activity in the distal convoluted tubule

High-dietary K⁺ (HK) intake inhibits basolateral Kir4.1/Kir5.1 activity in the distal convoluted tubule (DCT), and HK-induced inhibition of Kir4.1/Kir5.1 is essential for HK-induced inhibition of NaCl cotransporter (NCC). Here, we examined whether neural precursor cell expressed developmentally downregulated 4-2 (Nedd4-2) deletion compromises the effect of HK on basolateral Kir4.1/Kir5.1 and NCC in the DCT. Single-channel recording and whole cell recording showed that neither HK decreased nor low-dietary K⁺ (LK) increased basolateral Kir4.1/Kir5.1 activity of the DCT in kidney tubule-specific Nedd4-2 knockout (Ks-Nedd4-2 KO) mice. In contrast, HK inhibited and LK increased Kir4.1/Kir5.1 activity in control mice [neural precursor cell expressed developmentally downregulated 4-like (Nedd4l)^flox/flox]. Also, HK intake decreased the negativity of K⁺ current reversal potential in the DCT (depolarization) only in control mice but not in Ks-Nedd4-2 KO mice. Renal clearance experiments showed that HK intake decreased, whereas LK intake increased, hydrochlorothiazide-induced renal Na⁺ excretion only in control mice, but this effect was absent in Ks-Nedd4-2 KO mice. Western blot analysis also demonstrated that HK-induced inhibition of phosphorylated NCC (Thr⁵³) and total NCC was observed only in control mice but not in Ks-Nedd4-2 KO mice. Furthermore, expression of all three subunits of the epithelial Na⁺ channel in Ks-Nedd4-2 KO mice on HK was higher than in control mice. Thus, plasma K⁺ concentrations were similar between Nedd4l^flox/flox and Ks-Nedd4-2 KO mice on HK for 7 days despite high NCC expression. We conclude that Nedd4-2 plays a role in regulating HK-induced inhibition of Kir4.1/Kir5.1 and NCC in the DCT.NEW & NOTEWORTHY Basolateral Kir4.1/Kir5.1 in the distal convoluted tubule plays an important role as a "K⁺ sensor" in the regulation of renal K⁺ excretion after high K⁺ intake. We found that neural precursor cell expressed developmentally downregulated 4-2 (Nedd4-2) a role in mediating the effect of K⁺ diet on Kir4.1/Kir5.1 and NaCl cotransporter because high K⁺ intake failed to inhibit basolateral Kir4.1/Kir5.1 and NaCl cotransporter in kidney tubule-specific Nedd4-2 knockout mice.

Rapid development of vasopressin resistance in dietary K+ deficiency

The association between diabetes insipidus (DI) and chronic dietary K+ deprivation is well known, but it remains uncertain how the disorder develops and whether it is influenced by the sexual dimorphism in K+ handling. Here, we determined the plasma K+ (PK) threshold for DI in male and female mice and ascertained if DI is initiated by polydipsia or by a central or nephrogenic defect. C57BL6J mice were randomized to a control diet or to graded reductions in dietary K+ for 8 days, and kidney function and transporters involved in water balance were characterized. We found that male and female mice develop polyuria and secondary polydipsia. Altered water balance coincided with a decrease in aquaporin-2 (AQP2) phosphorylation and apical localization despite increased levels of the vasopressin surrogate marker copeptin. No change in the protein abundance of urea transporter-A1 was observed. The Na+-K+-2Cl- cotransporter decreased only in males. Desmopressin treatment failed to reverse water diuresis in K+-restricted mice. These findings indicate that even a small fall in PK is associated with nephrogenic DI (NDI), coincident with the development of altered AQP2 regulation, implicating low PK as a causal trigger of NDI. We found that PK decreased more in females, and, consequently, females were more prone to develop NDI. Together, these data indicate that AQP2 regulation is disrupted by a small decrease in PK and that the response is influenced by sexual dimorphism in K+ handling. These findings provide new insights into the mechanisms linking water and K+ balances and support defining the disorder as "potassium-dependent NDI."NEW & NOTEWORTHY This study shows that aquaporin-2 regulation is disrupted by a small fall in plasma potassium levels and the response is influenced by sexual dimorphism in renal potassium handling. The findings provided new insights into the mechanisms by which water balance is altered in dietary potassium deficiency and support defining the disorder as "potassium-dependent nephrogenic diabetes insipidus."

A Comprehensive Map of mRNAs and Their Isoforms across All 14 Renal Tubule Segments of Mouse

BACKGROUND

The repertoire of protein expression along the renal tubule depends both on regulation of transcription and regulation of alternative splicing that can generate multiple proteins from a single gene.

METHODS

A full-length, small-sample RNA-seq protocol profiled transcriptomes for all 14 renal tubule segments microdissected from mouse kidneys.

RESULTS

This study identified >34,000 transcripts, including 3709 that were expressed in a segment-specific manner. All data are provided as an online resource (https://esbl.nhlbi.nih.gov/MRECA/Nephron/). Many of the genes expressed in unique patterns along the renal tubule were solute carriers, transcription factors, or G protein-coupled receptors that account for segment-specific function. Mapping the distribution of transcripts associated with Wnk-SPAK-PKA signaling, renin-angiotensin-aldosterone signaling, and cystic diseases of the kidney illustrated the applications of the online resource. The method allowed full-length mapping of RNA-seq reads, which facilitated comprehensive, unbiased characterization of alternative exon usage along the renal tubule, including known isoforms of Cldn10, Kcnj1 (ROMK), Slc12a1 (NKCC2), Wnk1, Stk39 (SPAK), and Slc14a2 (UT-A urea transporter). It also identified many novel isoforms with segment-specific distribution. These included variants associated with altered protein structure (Slc9a8, Khk, Tsc22d1, and Scoc), and variants that may affect untranslated, regulatory regions of transcripts (Pth1r, Pkar1a, and Dab2).

CONCLUSIONS

Full-length, unbiased sequencing of transcripts identified gene-expression patterns along the mouse renal tubule. The data, provided as an online resource, include both quantitative and qualitative differences in transcripts. Identification of alternative splicing along the renal tubule may prove critical to understanding renal physiology and pathophysiology.

An in vivo protein landscape of the mouse DCT during high dietary K+ or low dietary Na+ intake

The hormone aldosterone is essential for maintaining K⁺ and Na⁺ balance and controlling blood pressure. Aldosterone has different effects if it is secreted due to hypovolemia or hyperkalemia. The kidney distal convoluted tubule (DCT) is believed to play a central role in mediating the differential responses to aldosterone. To determine the alterations in the DCT that may be responsible for these effects, male mice with green fluorescent protein expression specifically in the DCT were maintained on diets containing low NaCl (hypovolemic state) or high potassium citrate (hyperkalemic state) for 4 days, and DCT cells were isolated using fluorescence-activated cell sorting. This pure population of DCT cells was subjected to analysis by liquid chromatography-coupled tandem mass spectrometry. Over 3,000 proteins were identified in the DCT, creating the first proteome of the mouse DCT. Of the identified proteins, 210 proteins were altered in abundance following a low-NaCl diet and 625 proteins following the high-K⁺ diet. Many of these changes were not detectable by analyzing whole kidney samples from the same animals. When comparing responses to high-K⁺ versus low-Na⁺ diets, protein translation, chaperone-mediated protein folding, and protein ubiquitylation were likely to be significantly altered in the DCT subsequent to a high-K⁺ diet. In conclusion, this study defines an in vivo protein landscape of the DCT in male mice following either a low-NaCl or a high-K⁺ diet and acts as an essential resource for the kidney research community.NEW & NOTEWORTHY The mineralocorticoid aldosterone, essential for maintaining body K⁺ and Na⁺ balance, has different effects if secreted due to hypovolemia or hyperkalemia. Here, we used proteomics to profile kidney distal convoluted tubule (DCT) cells isolated by a novel FACS approach from mice fed a low-Na⁺ diet (mimicking hypovolemia) or a high-K⁺ diet (mimicking hyperkalemia). The study provides the first in-depth proteome of the mouse DCT and insights into how it is physiologically regulated.

Dietary sodium intake does not alter renal potassium handling and blood pressure in healthy young males

Background

The effects of sodium (Na⁺) intakes on renal handling of potassium (K⁺) are insufficiently studied.

Methods

We assessed the effect of Na⁺ on renal K⁺ handling in 16 healthy males assigned to three 7-day periods on low salt diet [LSD, 3 g sodium chloride (NaCl)/day], normal salt diet (NSD, 6 g NaCl/day) and high salt diet (HSD, 15 g NaCl/day), with constant K⁺ intake. Contributions of distal NaCl co-transporter and epithelial Na⁺ channel in the collecting system on K⁺ and Na⁺ handling were assessed at steady state by acute response to 100 mg oral hydrochlorothiazide and with addition of 10 mg of amiloride to hydrochlorothiazide, respectively.

Results

Diurnal blood pressure slightly increased from 119.30 ± 7.95 mmHg under LSD to 123.00 ± 7.50 mmHg (P = 0.02) under HSD, while estimated glomerular filtration rate increased from 133.20 ± 34.68 mL/min under LSD to 187.00 ± 49.10 under HSD (P = 0.005). The 24-h K⁺ excretion remained stable on all Na⁺ intakes (66.28 ± 19.12 mmol/24 h under LSD; 55.91 ± 21.17 mmol/24 h under NSD; and 66.81 ± 20.72 under HSD, P = 0.9). The hydrochlorothiazide-induced natriuresis was the highest under HSD (30.22 ± 12.53 mmol/h) and the lowest under LSD (15.38 ± 8.94 mmol/h, P = 0.02). Hydrochlorothiazide increased kaliuresis and amiloride decreased kaliuresis similarly on all three diets.

Conclusions

Neither spontaneous nor diuretic-induced K⁺ excretion was influenced by Na⁺ intake in healthy male subjects. However, the respective contribution of the distal convoluted tubule and the collecting duct to renal Na⁺ handling was dependent on dietary Na⁺ intake.