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.

Matrix Metalloproteinase-7 in Urinary Extracellular Vesicles Identifies Rapid Disease Progression in Autosomal Dominant Polycystic Kidney Disease.

SIGNIFICANCE STATEMENT: There is an unmet need for biomarkers of disease progression in autosomal dominant polycystic kidney disease (ADPKD). This study investigated urinary extracellular vesicles (uEVs) as a source of such biomarkers. Proteomic analysis of uEVs identified matrix metalloproteinase 7 (MMP-7) as a biomarker predictive of rapid disease progression. In validation studies, MMP-7 was predictive in uEVs but not in whole urine, possibly because uEVs are primarily secreted by tubular epithelial cells. Indeed, single-nucleus RNA sequencing showed that MMP-7 was especially increased in proximal tubule and thick ascending limb cells, which were further characterized by a profibrotic phenotype. Together, these data suggest that MMP-7 is a biologically plausible and promising uEV biomarker for rapid disease progression in ADPKD. BACKGROUND: In ADPKD, there is an unmet need for early markers of rapid disease progression to facilitate counseling and selection for kidney-protective therapy. Our aim was to identify markers for rapid disease progression in uEVs. METHODS: Six paired case-control groups ( n =10-59/group) of cases with rapid disease progression and controls with stable disease were formed from two independent ADPKD cohorts, with matching by age, sex, total kidney volume, and genetic variant. Candidate uEV biomarkers were identified by mass spectrometry and further analyzed using immunoblotting and an ELISA. Single-nucleus RNA sequencing of healthy and ADPKD tissue was used to identify the cellular origin of the uEV biomarker. RESULTS: In the discovery proteomics experiments, the protein abundance of MMP-7 was significantly higher in uEVs of patients with rapid disease progression compared with stable disease. In the validation groups, a significant >2-fold increase in uEV-MMP-7 in patients with rapid disease progression was confirmed using immunoblotting. By contrast, no significant difference in MMP-7 was found in whole urine using ELISA. Compared with healthy kidney tissue, ADPKD tissue had significantly higher MMP-7 expression in proximal tubule and thick ascending limb cells with a profibrotic phenotype. CONCLUSIONS: Among patients with ADPKD, rapid disease progressors have higher uEV-associated MMP-7. Our findings also suggest that MMP-7 is a biologically plausible biomarker for more rapid disease progression.

Kidney Angiotensin in Cardiovascular Disease: Formation and Drug Targeting.

The concept of local formation of angiotensin II in the kidney has changed over the last 10-15 years. Local synthesis of angiotensinogen in the proximal tubule has been proposed, combined with prorenin synthesis in the collecting duct. Binding of prorenin via the so-called (pro)renin receptor has been introduced, as well as megalin-mediated uptake of filtered plasma-derived renin-angiotensin system (RAS) components. Moreover, angiotensin metabolites other than angiotensin II [notably angiotensin-(1-7)] exist, and angiotensins exert their effects via three different receptors, of which angiotensin II type 2 and Mas receptors are considered renoprotective, possibly in a sex-specific manner, whereas angiotensin II type 1 (AT1) receptors are believed to be deleterious. Additionally, internalized angiotensin II may stimulate intracellular receptors. Angiotensin-converting enzyme 2 (ACE2) not only generates angiotensin-(1-7) but also acts as coronavirus receptor. Multiple, if not all, cardiovascular diseases involve the kidney RAS, with renal AT1 receptors often being claimed to exert a crucial role. Urinary RAS component levels, depending on filtration, reabsorption, and local release, are believed to reflect renal RAS activity. Finally, both existing drugs (RAS inhibitors, cyclooxygenase inhibitors) and novel drugs (angiotensin receptor/neprilysin inhibitors, sodium-glucose cotransporter-2 inhibitors, soluble ACE2) affect renal angiotensin formation, thereby displaying cardiovascular efficacy. Particular in the case of the latter three, an important question is to what degree they induce renoprotection (e.g., in a renal RAS-dependent manner). This review provides a unifying view, explaining not only how kidney angiotensin formation occurs and how it is affected by drugs but also why drugs are renoprotective when altering the renal RAS. SIGNIFICANCE STATEMENT: Angiotensin formation in the kidney is widely accepted but little understood, and multiple, often contrasting concepts have been put forward over the last two decades. This paper offers a unifying view, simultaneously explaining how existing and novel drugs exert renoprotection by interfering with kidney angiotensin formation.

Inducible Deletion of the Prostaglandin EP3 Receptor in Kidney Tubules of Male and Female Mice Has No Major Effect on Water Homeostasis.

The prostaglandin E2 (PGE2) receptor EP3 has been detected in the thick ascending limb (TAL) and the collecting duct of the kidney, where its actions are proposed to inhibit water reabsorption. However, EP3 is also expressed in other cell types, including vascular endothelial cells. The aim here was to determine the contribution of EP3 in renal water handling in male and female adult mice by phenotyping a novel mouse model with doxycycline-dependent deletion of EP3 throughout the kidney tubule (EP3-/- mice). RNAscope demonstrated that EP3 was highly expressed in the cortical and medullary TAL of adult mice. Compared to controls EP3 mRNA expression was reduced by >80% in whole kidney (RT-qPCR) and non-detectable (RNAscope) in renal tubules of EP3-/- mice. Under basal conditions, there were no significant differences in control and EP3-/- mice of both genders in food and water intake, bodyweight, urinary output or clinical biochemistries. No differences were detectable between genotypes in handling of an acute water load, or in their response to the vasopressin analogue dDAVP. No differences in water handling were observed when PGE2 production was enhanced using a 1% NaCl load. Expression of proteins involved in kidney water handling were not different between genotypes. This study demonstrates that renal tubular EP3 is not essential for body fluid homeostasis in males or females, even when PGE2 levels are high. The mouse model is a novel tool for examining the role of EP3 in kidney function independently of potential developmental abnormalities or systemic effects.

Effect of the DASH diet on the sodium-chloride cotransporter and aquaporin-2 in urinary extracellular vesicles.

The dietary approach to stop hypertension (DASH) diet combines the antihypertensive effect of a low sodium and high potassium diet. In particular, the potassium component of the diet acts as a switch in the distal convoluted tubule to reduce sodium reabsorption, similar to a diuretic but without the side effects. Previous trials to understand the mechanism of the DASH diet were based on animal models and did not characterize changes in human ion channel protein abundance. More recently, protein cargo of urinary extracellular vesicles (uEVs) has been shown to mirror tissue content and physiological changes within the kidney. We designed an inpatient open label nutritional study transitioning hypertensive volunteers from an American style diet to DASH diet to examine physiological changes in adults with stage 1 hypertension otherwise untreated (Sacks FM, Svetkey LP, Vollmer WM, Appel LJ, Bray GA, Harsha D, Obarzanek E, Conlin PR, Miller ER 3rd, Simons-Morton DG, Karanja N, Lin PH; DASH-Sodium Collaborative Research Group. N Engl J Med 344: 3-10, 2001). Urine samples from this study were used for proteomic characterization of a large range of pure uEVs (small to large) to reveal kidney epithelium changes in response to the DASH diet. These samples were collected from nine volunteers at three time points, and mass spectrometry identified 1,800 proteins from all 27 samples. We demonstrated an increase in total SLC12A3 [sodium-chloride cotransporter (NCC)] abundance and a decrease in aquaporin-2 (AQP2) in uEVs with this mass spectrometry analysis, immunoblotting revealed a significant increase in the proportion of activated (phosphorylated) NCC to total NCC and a decrease in AQP2 from day 5 to day 11. This data demonstrates that the human kidney's response to nutritional interventions may be captured noninvasively by uEV protein abundance changes. Future studies need to confirm these findings in a larger cohort and focus on which factor drove the changes in NCC and AQP2, to which degree NCC and AQP2 contributed to the antihypertensive effect and address if some uEVs function also as a waste pathway for functionally inactive proteins rather than mirroring protein changes.NEW & NOTEWORTHY Numerous studies link DASH diet to lower blood pressure, but its mechanism is unclear. Urinary extracellular vesicles (uEVs) offer noninvasive insights, potentially replacing tissue sampling. Transitioning to DASH diet alters kidney transporters in our stage 1 hypertension cohort: AQP2 decreases, NCC increases in uEVs. This aligns with increased urine volume, reduced sodium reabsorption, and blood pressure decline. Our data highlight uEV protein changes as diet markers, suggesting some uEVs may function as waste pathways. We analyzed larger EVs alongside small EVs, and NCC in immunoblots across its molecular weight range.

Animal models to study cognitive impairment of chronic kidney disease.

Mild cognitive impairment (MCI) is common in people with chronic kidney disease (CKD), and its prevalence increases with progressive loss of kidney function. MCI is characterized by a decline in cognitive performance greater than expected for an individual age and education level but with minimal impairment of instrumental activities of daily living. Deterioration can affect one or several cognitive domains (attention, memory, executive functions, language, and perceptual motor or social cognition). Given the increasing prevalence of kidney disease, more and more people with CKD will also develop MCI causing an enormous disease burden for these individuals, their relatives, and society. However, the underlying pathomechanisms are poorly understood, and current therapies mostly aim at supporting patients in their daily lives. This illustrates the urgent need to elucidate the pathogenesis and potential therapeutic targets and test novel therapies in appropriate preclinical models. Here, we will outline the necessary criteria for experimental modeling of cognitive disorders in CKD. We discuss the use of mice, rats, and zebrafish as model systems and present valuable techniques through which kidney function and cognitive impairment can be assessed in this setting. Our objective is to enable researchers to overcome hurdles and accelerate preclinical research aimed at improving the therapy of people with CKD and MCI.

Discrepancies between transcutaneous and estimated glomerular filtration rates in rats with chronic kidney disease.

Accurate assessment of the glomerular filtration rate (GFR) is crucial for researching kidney disease in rats. Although validation of methods that assess GFR is crucial, large-scale comparisons between different methods are lacking. Both transcutaneous GFR (tGFR) and a newly developed estimated GFR (eGFR) equation by our group provide a low-invasive approach enabling repeated measurements. The tGFR is a single bolus method using FITC-labeled sinistrin to measure GFR based on half-life of the transcutaneous signal, whilst the eGFR is based on urinary sinistrin clearance. Here, we retrospectively compared tGFR, using both 1- and 3- compartment models (tGFR_1c and tGFR_3c, respectively) to the eGFR in a historic cohort of 43 healthy male rats and 84 male rats with various models of chronic kidney disease. The eGFR was on average considerably lower than tGFR-1c and tGFR-3c (mean differences 855 and 216 µL/min, respectively) and only 20 and 47% of measurements were within 30% of each other, respectively. The relative difference between eGFR and tGFR was highest in rats with the lowest GFR. Possible explanations for the divergence are problems inherent to tGFR, such as technical issues with signal measurement, description of the signal kinetics, and translation of half-life to tGFR, which depends on distribution volume. The unknown impact of isoflurane anesthesia used in determining mGFR remains a limiting factor. Thus, our study shows that there is a severe disagreement between GFR measured by tGFR and eGFR, stressing the need for more rigorous validation of the tGFR and possible adjustments to the underlying technique.

Calcineurin inhibitor effects on kidney electrolyte handling and blood pressure: tacrolimus versus voclosporin.

BACKGROUND AND HYPOTHESIS: Calcineurin inhibitors affect kidney electrolyte handling and blood pressure through an effect on the distal tubule. The second generation calcineurin inhibitor voclosporin causes hypomagnesemia and hypercalciuria less often than tacrolimus. This suggests different effects on the distal tubule, but this has not yet been investigated experimentally. METHODS: Rats were treated with voclosporin, tacrolimus or vehicle for 28 days. Dosing was based on a pilot experiment to achieve clinically therapeutic concentrations. Drug effects were assessed by electrolyte handling at day 18 and 28, thiazide testing at day 20, telemetric blood pressure recordings, and analysis of mRNA and protein levels of distal tubular transporters at day 28. RESULTS: Compared to vehicle, tacrolimus but not voclosporin significantly increased the fractional excretions of calcium (>4-fold), magnesium and chloride (both 1.5-fold) and caused hypomagnesemia. Tacrolimus but not voclosporin significantly reduced distal tubular transporters at mRNA and/or protein level, including the sodium-chloride cotransporter, transient receptor melastatin 6, transient receptor potential vanilloid 5, cyclin M2, sodium-calcium exchanger and calbindin-D28K. Tacrolimus but not voclosporin reduced the mRNA level and urinary excretion of epidermal growth factor. The saluretic response to hydrochlorothiazide at day 20 was similar in the voclosporin and vehicle groups, whereas it was lower in the tacrolimus group. The phosphorylated form of the sodium-chloride cotransporter was significantly higher at day 28 in rats treated with voclosporin than in those treated with tacrolimus. Tacrolimus transiently increased blood pressure, whereas voclosporin caused a gradual but persistent increase in blood pressure which was further characterized by high renin, normal aldosterone, and low endothelin-1. CONCLUSIONS: In contrast to tacrolimus, voclosporin does not cause hypercalciuria and hypomagnesemia, but similarly causes hypertension. Our data reveal differences between the distal tubular effects of tacrolimus and voclosporin and provide a pathophysiological basis for the clinically observed differences between the two calcineurin inhibitors.

Hyponatremia is associated with unfavorable outcomes after reperfusion treatment in acute ischemic stroke.

BACKGROUND AND PURPOSE: In patients with acute ischemic stroke, hyponatremia (plasma sodium < 136 mmol/L) is common and associated with unfavorable outcomes. However, data are limited for patients who underwent intravenous thrombolysis (IVT) and/or endovascular thrombectomy (EVT). Therefore, our aim was to assess the impact of hyponatremia on postreperfusion outcomes. METHODS: We analyzed data of consecutive patients who presented with acute ischemic stroke and were treated with IVT and/or EVT at Isala Hospital, the Netherlands, in 2019 and 2020. The primary outcome measure was the adjusted common odds ratio (acOR) for a worse modified Rankin Scale (mRS) score at 3-month follow-up. Secondary outcomes included symptomatic intracranial hemorrhage, in-hospital mortality, infarct core, and penumbra volumes. RESULTS: Of the 680 patients (median age = 73 years, 49% female, median National Institutes of Health Stroke Scale = 5), 430 patients (63%) were treated with IVT, 120 patients (18%) with EVT, and 130 patients (19%) with both. Ninety-two patients (14%) were hyponatremic on admission. Hyponatremia was associated with a worse mRS score at 3 months (acOR = 1.76, 95% confidence interval [CI] = 1.12-2.76) and in-hospital mortality (aOR = 2.39, 95% CI = 1.23-4.67), but not with symptomatic intracranial hemorrhage (OR = 1.17, 95% CI = 0.39-3.47). Hyponatremia was also associated with a larger core (17.2 mL, 95% CI = 2.9-31.5) and core to penumbra ratio (55.0%, 95% CI = 7.1-102.9). CONCLUSIONS: Admission hyponatremia in patients with acute ischemic stroke treated with IVT and/or EVT was independently associated with unfavorable postreperfusion outcomes, a larger infarct core, and a larger core to penumbra ratio. Future studies should address whether correction of hyponatremia improves the prognosis.

Interactions of the Immune System with Human Kidney Organoids.

Kidney organoids are an innovative tool in transplantation research. The aim of the present study was to investigate whether kidney organoids are susceptible for allo-immune attack and whether they can be used as a model to study allo-immunity in kidney transplantation. Human induced pluripotent stem cell-derived kidney organoids were co-cultured with human peripheral blood mononuclear cells (PBMC), which resulted in invasion of allogeneic T-cells around nephron structures and macrophages in the stromal cell compartment of the organoids. This process was associated with the induction of fibrosis. Subcutaneous implantation of kidney organoids in immune-deficient mice followed by adoptive transfer of human PBMC led to the invasion of diverse T-cell subsets. Single cell transcriptomic analysis revealed that stromal cells in the organoids upregulated expression of immune response genes upon immune cell invasion. Moreover, immune regulatory PD-L1 protein was elevated in epithelial cells while genes related to nephron differentiation and function were downregulated. This study characterized the interaction between immune cells and kidney organoids, which will advance the use of kidney organoids for transplantation research.

Mild-to-Moderate Kidney Dysfunction and Cardiovascular Disease: Observational and Mendelian Randomization Analyses.

BACKGROUND: End-stage renal disease is associated with a high risk of cardiovascular events. It is unknown, however, whether mild-to-moderate kidney dysfunction is causally related to coronary heart disease (CHD) and stroke. METHODS: Observational analyses were conducted using individual-level data from 4 population data sources (Emerging Risk Factors Collaboration, EPIC-CVD [European Prospective Investigation into Cancer and Nutrition-Cardiovascular Disease Study], Million Veteran Program, and UK Biobank), comprising 648 135 participants with no history of cardiovascular disease or diabetes at baseline, yielding 42 858 and 15 693 incident CHD and stroke events, respectively, during 6.8 million person-years of follow-up. Using a genetic risk score of 218 variants for estimated glomerular filtration rate (eGFR), we conducted Mendelian randomization analyses involving 413 718 participants (25 917 CHD and 8622 strokes) in EPIC-CVD, Million Veteran Program, and UK Biobank. RESULTS: There were U-shaped observational associations of creatinine-based eGFR with CHD and stroke, with higher risk in participants with eGFR values <60 or >105 mL·min-1·1.73 m-2, compared with those with eGFR between 60 and 105 mL·min-1·1.73 m-2. Mendelian randomization analyses for CHD showed an association among participants with eGFR <60 mL·min-1·1.73 m-2, with a 14% (95% CI, 3%-27%) higher CHD risk per 5 mL·min-1·1.73 m-2 lower genetically predicted eGFR, but not for those with eGFR >105 mL·min-1·1.73 m-2. Results were not materially different after adjustment for factors associated with the eGFR genetic risk score, such as lipoprotein(a), triglycerides, hemoglobin A1c, and blood pressure. Mendelian randomization results for stroke were nonsignificant but broadly similar to those for CHD. CONCLUSIONS: In people without manifest cardiovascular disease or diabetes, mild-to-moderate kidney dysfunction is causally related to risk of CHD, highlighting the potential value of preventive approaches that preserve and modulate kidney function.

Novel mechanisms of salt-sensitive hypertension.

A high dietary sodium-consumption level is considered the most important lifestyle factor that can be modified to help prevent an increase in blood pressure and the development of hypertension. Despite numerous studies over the past decades, the pathophysiology explaining why some people show a salt-sensitive blood pressure response and others do not is incompletely understood. Here, a brief overview of the latest mechanistic insights is provided, focusing on the mononuclear phagocytic system and inflammation, the gut-kidney axis, and epigenetics. The article also discusses the effects of 3 types of novel drugs on salt-sensitive hypertension-sodium-glucose cotransporter 2 inhibitors, nonsteroidal mineralocorticoid receptor antagonists, and aldosterone synthase inhibitors. The conclusion is that besides kidney-centered mechanisms, vasoconstrictor mechanisms are also relevant for both the understanding and treatment of this blood pressure phenotype.

Dietary sodium restriction prevents vascular endothelial growth factor inhibitor-induced hypertension.

BACKGROUND: Vascular endothelial growth factor inhibitors (VEGFIs) are effective anticancer agents which often induce hypertension. VEGFI-induced hypertension is sodium-sensitive in animal studies. Therefore, the efficacy of dietary sodium restriction (DSR) to prevent VEGFI-induced hypertension in cancer patients was studied. METHODS: Cancer patients with VEGFI-induced hypertension (day mean >135/85 mmHg or a rise in systolic and/or diastolic BP ≥ 20 mmHg) were treated with DSR (aiming at <4 g salt/day). The primary endpoint was the difference in daytime mean arterial blood pressure (MAP) increase between the treatment cycle with and without DSR. RESULTS: During the first VEGFI treatment cycle without DSR, mean daytime MAP increased from 95 to 110 mmHg. During the subsequent treatment cycle with DSR, mean daytime MAP increased from 94 to 102 mmHg. Therefore, DSR attenuated the increase in mean daytime MAP by 7 mmHg (95% CI 1.3-12.0, P = 0.009). DSR prevented the rise in the endothelin-1/renin ratio that normally accompanies VEGFI-induced hypertension (P = 0.020) and prevented the onset of proteinuria: 0.15 (0.10-0.25) g/24 h with DSR versus 0.19 (0.11-0.32) g/24 h without DSR; P = 0.005. DISCUSSION: DSR significantly attenuated VEGFI induced BP rise and proteinuria and thus is an effective non-pharmacological intervention.

Serum Potassium and Risk of Death or Kidney Replacement Therapy in Older People With CKD Stages 4-5: Eight-Year Follow-up.

RATIONALE & OBJECTIVE: Hypokalemia may accelerate kidney function decline. Both hypo- and hyperkalemia can cause sudden cardiac death. However, little is known about the relationship between serum potassium and death or the occurrence of kidney failure requiring replacement therapy (KRT). We investigated this relationship in older people with chronic kidney disease (CKD) stage 4-5. STUDY DESIGN: Prospective observational cohort study. SETTING & PARTICIPANTS: We followed 1,714 patients (≥65 years old) from the European Quality (EQUAL) study for 8 years from their first estimated glomerular filtration rate (eGFR)<20mL/min/1.73m2 measurement. EXPOSURE: Serum potassium was measured every 3 to 6 months and categorized as≤3.5,>3.5-≤4.0,>4.0-≤4.5,>4.5-≤5.0 (reference),>5.0-≤5.5, >5.5-≤6.0, and>6.0mmol/L. OUTCOME: The combined outcome death before KRT or start of KRT. ANALYTICAL APPROACH: The association between categorical and continuous time-varying potassium and death or KRT start was examined using Cox proportional hazards and restricted cubic spline analyses, adjusted for age, sex, diabetes, cardiovascular disease, renin-angiotensin-aldosterone system (RAAS) inhibition, eGFR, and subjective global assessment (SGA). RESULTS: At baseline, 66% of participants were men, 42% had diabetes, 47% cardiovascular disease, and 54% used RAAS inhibitors. Their mean age was 76±7 (SD) years, mean eGFR was 17±5 (SD) mL/min/1.73m2, and mean SGA was 6.0±1.0 (SD). Over 8 years, 414 (24%) died before starting KRT, and 595 (35%) started KRT. Adjusted hazard ratios for death or KRT according to the potassium categories were 1.6 (95% CI, 1.1-2.3), 1.4 (95% CI, 1.1-1.7), 1.1 (95% CI, 1.0-1.4), 1 (reference), 1.1 (95% CI, 0.9-1.4), 1.8 (95% CI, 1.4-2.3), and 2.2 (95% CI, 1.5-3.3). Hazard ratios were lowest at a potassium of about 4.9mmol/L. LIMITATIONS: Shorter intervals between potassium measurements would have allowed for more precise estimations. CONCLUSIONS: We observed a U-shaped relationship between serum potassium and death or KRT start among patients with incident CKD 4-5, with a nadir risk at a potassium level of 4.9mmol/L. These findings underscore the potential importance of preventing both high and low potassium in patients with CKD 4-5. PLAIN-LANGUAGE SUMMARY: Abnormal potassium blood levels may increase the risk of death or kidney function decline, especially in older people with chronic kidney disease (CKD). We studied 1,714 patients aged≥65 years with advanced CKD from the European Quality (EQUAL) study and followed them for 8 years. We found that both low and high levels of potassium were associated with an increased risk of death or start of kidney replacement therapy, with the lowest risk observed at a potassium level of 4.9 mmol/L. In patients with CKD, the focus is often on preventing high blood potassium. However, this relatively high optimum potassium level stresses the potential importance of also preventing low potassium levels in older patients with advanced CKD.

Kidney Organoids Are Capable of Forming Tumors, but Not Teratomas.

Induced pluripotent stem cell (iPSC)-derived kidney organoids are a potential tool for the regeneration of kidney tissue. They represent an early stage of nephrogenesis and have been shown to successfsully vascularize and mature further in vivo. However, there are concerns regarding the long-term safety and stability of iPSC derivatives. Specifically, the potential for tumorigenesis may impede the road to clinical application. To study safety and stability of kidney organoids, we analyzed their potential for malignant transformation in a teratoma assay and following long-term subcutaneous implantation in an immune-deficient mouse model. We did not detect fully functional residual iPSCs in the kidney organoids as analyzed by gene expression analysis, single-cell sequencing and immunohistochemistry. Accordingly, kidney organoids failed to form teratoma. Upon long-term subcutaneous implantation of whole organoids in immunodeficient IL2Ry-/-RAG2-/- mice, we observed tumor formation in 5 out of 103 implanted kidney organoids. These tumors were composed of WT1+CD56+ immature blastemal cells and showed histological resemblance with Wilms tumor. No genetic changes were identified that contributed to the occurrence of tumorigenic cells within the kidney organoids. However, assessment of epigenetic changes revealed a unique cluster of differentially methylated genes that were also present in undifferentiated iPSCs. We discovered that kidney organoids have the capacity to form tumors upon long-term implantation. The presence of epigenetic modifications combined with the lack of environmental cues may have caused an arrest in terminal differentiation. Our results indicate that the safe implementation of kidney organoids should exclude the presence of pro-tumorigenic methylation in kidney organoids.

Low Urinary Potassium Excretion Is Associated with Higher Risk of All-Cause Mortality in Patients with Type 2 Diabetes: Results of the Dutch Diabetes and Lifestyle Cohort Twente (DIALECT).

BACKGROUND: Low 24-h urinary potassium excretion, reflecting low potassium intake, is associated with premature mortality in the general population. OBJECTIVES: To determine whether urinary potassium excretion is associated with all-cause mortality in patients with type 2 diabetes. METHODS: We performed a prospective cohort study in 654 patients with type 2 diabetes in the Diabetes and Lifestyle Cohort Twente (DIALECT). Sex-specific tertiles of 24-h urinary potassium excretion were analyzed in a multivariable Cox regression model with all-cause mortality. The outpatient program of the hospital uses a continuous surveillance system by the municipal registry of death to ensure up-to-date information on the patient's status (alive or deceased). FFQs were used to study associations between urinary potassium excretion and food products. RESULTS: Urinary potassium excretion at baseline was 84 ± 25 mmol/d in males and 65 ± 22 mmol/d in females, corresponding to estimated potassium intakes of 4250 ± 1270 mg/d and 3300 ± 875 mg/d. During a median follow-up of 5.2 (IQR: 2.7-7.9] y, 96 participants died. In a fully adjusted model, patients in the lowest sex-specific tertile had a higher risk of all-cause mortality, compared with patients in the highest sex-specific tertile (HR: 2.09; 95% CI: 1.06, 4.10; P = 0.03). Patients in the lowest sex-specific tertile consumed fewer fruits and vegetables, dairy, coffee, and potato products compared with patients in the highest sex-specific tertile (all P < 0.05). CONCLUSIONS: Low potassium intake is associated with a higher risk of all-cause mortality in Dutch patients with type 2 diabetes. Intervention studies are needed to determine whether potassium supplementation improves longevity in patients with type 2 diabetes. This trial was registered in the Dutch Trial Register as NTR trial code 5855.

Urinary extracellular vesicles and tubular transport.

Tubular transport is a key function of the kidney to maintain electrolyte and acid-base homeostasis. Urinary extracellular vesicles (uEVs) harbor water, electrolyte, and acid-base transporters expressed at the apical plasma membrane of tubular epithelial cells. Within the uEV proteome, the correlations between kidney and uEV protein abundances are strongest for tubular transporters. Therefore, uEVs offer a noninvasive approach to probing tubular transport in health and disease. Here, we review how kidney tubular physiology is reflected in uEVs and, conversely, how uEVs may modify tubular transport. Clinically, uEV tubular transporter profiling has been applied to rare diseases, such as inherited tubulopathies, but also to more common conditions, such as hypertension and kidney disease. Although uEVs hold the promise to advance the diagnosis of kidney disease to the molecular level, several biological and technical complexities must still be addressed. The future will tell whether uEV analysis will mainly be a powerful tool to study tubular physiology in humans or whether it will move forward to become a diagnostic bedside test.

Kidney phosphate wasting predicts poor outcome in polycystic kidney disease.

BACKGROUND AND HYPOTHESIS: Patients with ADPKD have disproportionately high levels of fibroblast growth factor-23 (FGF-23) for their CKD-stage with only a subgroup that develops kidney phosphate wasting. We assessed factors associated with phosphate wasting and hypothesize that it identifies patients with more severe disease and predicts disease progression. METHODS: We included 604 patients with ADPKD from a multi-center prospective observational (DIPAK) cohort in 4 university medical centers in the Netherlands. We measured parathyroid hormone (PTH), total plasma FGF-23 levels and calculated the ratio of tubular maximum reabsorption rate of phosphate to glomerular filtration rate (TmP/GFR) with < 0.8 mmol/L defined as kidney phosphate wasting. We analysed the association of TmP/GFR with eGFR decline over time and the risk for a composite kidney outcome (≥ 30% eGFR decline, kidney failure or kidney replacement therapy). RESULTS: In our cohort (age 48 ± 12 years, 39% male, eGFR 63 ± 28 mL/min/1.73m2), 59% of patients had phosphate wasting. Male sex (coefficient -0.2, 95% confidence interval [CI] -0.2; -0.1), eGFR (0.002, 0.001-0.004), FGF-23 (0.1, 0.03-0.2), PTH(-0.2, -0.3; -0.06) and Copeptin(-0.08, -0.1; -0.08) were associated with TmP/GFR. Corrected for PTH, FGF-23 and eGFR, every 0.1 mmol/L decrease in TmP/GFR was associated with a greater eGFR decline of 0.2 ml/min/1.73m2/year (95% CI 0.01-0.3) and an increased hazard ratio of 1.09 (95% CI 1.01-1.18) of the composite kidney outcome. CONCLUSION: Our study shows that in patients with ADPKD phosphate wasting is prevalent and associated with more rapid disease progression. Phosphate wasting may be a consequence of early proximal tubular dysfunction and insufficient suppression of PTH.

Chronic kidney disease increases the susceptibility to negative effects of low and high potassium intake.

BACKGROUND AND HYPOTHESIS: Dietary potassium (K+) has emerged as a modifiable factor for cardiovascular and kidney health in the general population, but its role in people with chronic kidney disease (CKD) is unclear. Here, we hypothesize that CKD increases the susceptibility to negative effects of low and high K+ diets. METHODS: We compared the effects of low, normal, or high KChloride (KCl) diets and a high KCitrate diet for four weeks in male rats with normal kidney function and in male rats with CKD using the 5/6th nephrectomy model (5/6Nx). RESULTS: Compared to rats with normal kidney function, 5/6Nx rats on the low KCl diet developed more severe extracellular and intracellular hypokalemia and more severe kidney injury, characterized by nephromegaly, infiltration of T-cells and macrophages, decreased eGFR and increased albuminuria. The high KCl diet caused hyperkalemia, hyperaldosteronism, hyperchloremic metabolic acidosis and severe hypertension in 5/6Nx but not in sham rats. The high KCitrate diet caused hypochloremic metabolic alkalosis but attenuated hypertension despite higher abundance of the phosphorylated sodium chloride cotransporter (pNCC) and similar levels of plasma aldosterone and epithelial sodium channel (ENaC) abundance. All 5/6Nx groups had more collagen deposition than the sham groups and this effect was most pronounced in the high KCitrate group. Plasma aldosterone correlated strongly with kidney collagen deposition. CONCLUSIONS: CKD increases the susceptibility to negative effects of low and high K+ diets in male rats, although the injury patterns are different. The low K+ diet caused inflammation, nephromegaly and kidney function decline, whereas the high K+ diet caused hypertension, hyperaldosteronism and kidney fibrosis. High KCitrate attenuated the hypertensive but not the pro-fibrotic effect of high KCl, which may be attributable to K+-induced aldosterone secretion. Our data suggest that especially in people with CKD it is important to identify the optimal threshold of dietary K+ intake.

Serum sodium, cognition and incident dementia in the general population.

BACKGROUND: Low serum sodium may be associated with cognitive impairment and dementia in the general population, but the data remain inconclusive. Therefore, we aimed to determine the association of low serum sodium with cognitive function and incident dementia in the general population. METHODS: Participants from a prospective population-based cohort were eligible if data on serum sodium (collected between 1997 and 2008), dementia prevalence and dementia incidence were available (follow-up until 2018). Global cognitive function was assessed with the Mini-Mental State Examination (MMSE) and the general cognitive factor (G-factor, derived from principal component analysis of individual tests). Linear regression and Cox proportional-hazards models were used to assess associations of standardised continuous and categorised low serum sodium (mean - 1.96*SD: cut-off of 137 mmol/L) with overall cognitive function and incident dementia, respectively. RESULTS: In all, 8,028 participants free of dementia at baseline (mean age 63.6 years, 57% female, serum sodium 142 ± 2 mmol/L), including 217 participants with low serum sodium, were included. Cross-sectionally, continuous serum sodium and/or low serum sodium were not associated with the MMSE or G-factor. However, participants with low serum sodium performed worse on the Stroop and Purdue Pegboard tests. During a median follow-up of 10.7 years, 758 subjects developed dementia. Continuous serum sodium (hazard ratio (HR) 0.98, 95% confidence interval (CI) 0.92;1.05) and low serum sodium (HR 1.27, 95% CI 0.90;1.79) were not associated with a higher risk of incident dementia. CONCLUSION: We identified no significant associations of low serum sodium with overall cognitive functioning and risk of dementia. However, low serum sodium-including levels above the clinical cut-off for hyponatremia-was associated with impairments in selected cognitive domains including attention and psychomotor function.