Salt-deficient diet exacerbates cystogenesis in ARPKD via epithelial sodium channel (ENaC)

The Pathophysiology of Inherited Renal Cystic Diseases

Renal cystic diseases (RCDs) can arise from utero to early adulthood and present with a variety of symptoms including renal, hepatic, and cardiovascular manifestations. It is well known that common RCDs such as autosomal polycystic kidney disease and autosomal recessive kidney disease are linked to genes such as PKD1 and PKHD1, respectively. However, it is important to investigate the genetic pathophysiology of how these gene mutations lead to clinical symptoms and include some of the less-studied RCDs, such as autosomal dominant tubulointerstitial kidney disease, multicystic dysplastic kidney, Zellweger syndrome, calyceal diverticula, and more. We plan to take a thorough look into the genetic involvement and clinical sequalae of a number of RCDs with the goal of helping to guide diagnosis, counseling, and treatment.

TRPV4 functional status in cystic cells regulates cystogenesis in autosomal recessive polycystic kidney disease during variations in dietary potassium

Mechanosensitive TRPV4 channel plays a dominant role in maintaining [Ca2+ ]i homeostasis and flow-sensitive [Ca2+ ]i signaling in the renal tubule. Polycystic kidney disease (PKD) manifests as progressive cyst growth due to cAMP-dependent fluid secretion along with deficient mechanosensitivity and impaired TRPV4 activity. Here, we tested how regulation of renal TRPV4 function by dietary K+ intake modulates the rate of cystogenesis and mechanosensitive [Ca2+ ]i signaling in cystic cells of PCK453 rats, a homologous model of human autosomal recessive PKD (ARPKD). One month treatment with both high KCl (5% K+ ) and KB/C (5% K+ with bicarbonate/citrate) diets significantly increased TRPV4 levels when compared to control (0.9% K+ ). High KCl diet caused an increased TRPV4-dependent Ca2+ influx, and partial restoration of mechanosensitivity in freshly isolated monolayers of cystic cells. Unexpectedly, high KB/C diet induced an opposite effect by reducing TRPV4 activity and worsening [Ca2+ ]i homeostasis. Importantly, high KCl diet decreased cAMP, whereas high KB/C diet further increased cAMP levels in cystic cells (assessed as AQP2 distribution). At the systemic level, high KCl diet fed PCK453 rats had significantly lower kidney-to-bodyweight ratio and reduced cystic area. These beneficial effects were negated by a concomitant administration of an orally active TRPV4 antagonist, GSK2193874, resulting in greater kidney weight, accelerated cystogenesis, and augmented renal injury. High KB/C diet also exacerbated renal manifestations of ARPKD, consistent with deficient TRPV4 activity in cystic cells. Overall, we demonstrate that TRPV4 channel activity negatively regulates cAMP levels in cystic cells thus attenuating (high activity) or accelerating (low activity) ARPKD progression.

Modulation of P2X4 receptor activity by ivermectin and 5-BDBD has no effect on the development of ARPKD in PCK rats

Autosomal recessive polycystic kidney disease (ARPKD) is an inherited pathology caused mainly by mutations of the polycystic kidney and hepatic disease 1 (PKHD1) gene, which usually leads to end-stage renal disease. Previous studies suggested that the P2X purinoreceptor 4 (P2X₄ R) may play an important role in the progression of ARPKD. To test this hypothesis, we assessed the chronic effects of ivermectin (P2X₄ R allosteric modulator) and 5-BDBD (P2X₄ R antagonist) on the development of ARPKD in PCK/CrljCrl-Pkhd1pck/CRL (PCK) rats. Our data indicated that activation of ATP-mediated P2X₄ R signaling with ivermectin for 6 weeks in high dose (50 mg/L; water supplementation) decreased the total body weight of PCK rats while the heart and kidney weight remained unaffected. Smaller doses of ivermectin (0.5 or 5 mg/L, 6 weeks) or the inhibition of P2X₄ R signaling with 5-BDBD (18 mg/kg/day, food supplement for 8 weeks) showed no effect on electrolyte balance or the basic physiological parameters. Furthermore, cystic index analysis for kidneys and liver revealed no effect of smaller doses of ivermectin (0.5 or 5 mg/L) and 5-BDBD on the cyst development of PCK rats. We observed a slight increase in the cystic liver index on high ivermectin dose, possibly due to the cytotoxicity of the drug. In conclusion, this study revealed that pharmacological modulation of P2X₄ R by ivermectin or 5-BDBD does not affect the development of ARPKD in PCK rats, which may provide insights for future studies on investigating the therapeutic potential of adenosine triphosphate (ATP)-P2 signaling in PKD diseases.

Role of no table salt on hypertension and stroke based on large sample size from National Health and Nutrition Examination Survey database

Background

To assess the associations between no table salt and hypertension or stroke.

Methods

The data of 15,352 subjects were collected from National Health and Nutrition Examination Survey (NHANES) database. All subjects were divided into no hypertension or stroke group (n = 10,894), hypertension group (n = 5888), stroke group (n = 164) and hypertension and stroke group (n = 511). Univariate and multivariate logistic regression analysis was used to measure the associations of salt type used with hypertension and stroke and co-variables were respectively adjusted in different models.

Results

After adjusting age and gender, other salt intake was associated with 1.88-fold risk of hypertension (OR = 1.88, 95%CI: 1.44–2.46) and no table salt was associated with 1.30-fold risk of hypertension (OR = 1.30, 95%CI: 1.15–1.47). After adjusting age, gender, race, BMI, PIR, marital status, CVDs, whether doctors’ told them to reduce salt, and diabetes, the risk of hypertension was 1.23-fold increase in no table salt group (OR = 1.23, 95%CI: 1.04–1.46). After the adjustment of age and gender, the risk of hypertension and stroke was 3.33-fold increase (OR = 3.33, 95%CI: 2.12–5.32) in other salt intake group and 1.43-fold increase (OR = 1.43, 95%CI:1.17–1.74) in no table salt group.

Conclusion

Other salt intake or no table salt were associated with a higher risk of hypertension or hypertension and stroke.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12889-022-13722-8.

Effects of elevation of ANP and its deficiency on cardiorenal function

Atrial natriuretic peptide (ANP), encoded by Nppa, is a vasodilatory hormone that promotes salt excretion. Genome-wide association studies identified Nppa as a causative factor of blood pressure development, and in humans, ANP levels were suggested as an indicator of salt sensitivity. This study aimed to provide insights into the effects of ANP on cardiorenal function in salt-sensitive hypertension. To address this question, hypertension was induced in SSNPPA-/- (knockout of Nppa in the Dahl Salt-Sensitive (SS) rat background) or SSWT (wild type Dahl SS) rats by a high salt diet challenge (HS, 4% NaCl for 21 days). Chronic infusion of ANP in SSWT rats attenuated the increase in blood pressure and cardiorenal damage. Overall, SSNPPA-/- strain demonstrated higher blood pressure and intensified cardiac fibrosis (with no changes in ejection fraction) compared to SSWT rats. Furthermore, SSNPPA-/- rats exhibited kidney hypertrophy and higher glomerular injury scores, reduced diuresis, and lower sodium and chloride excretion than SSWT when fed a HS diet. Additionally, the activity of epithelial Na+ channel (ENaC) was found to be increased in the collecting ducts of the SSNPPA-/- rats. Taken together, these data show promise for the therapeutic benefits of ANP and ANP-increasing drugs for treating salt-sensitive hypertension.

Metformin improves relevant disease parameters in an autosomal dominant polycystic kidney disease mouse model

Autosomal dominant polycystic kidney disease (ADPKD), caused by mutations in the polycystin 1 (PKD1) or polycystin 2 genes, presents with progressive development of kidney cysts and eventual end-stage kidney disease with limited treatment options. Previous work has shown that metformin reduces cyst growth in rapid ADPKD mouse models via inhibition of cystic fibrosis transmembrane conductance regulator-mediated fluid secretion, mammalian target of rapamycin, and cAMP pathways. The present study importantly tested the effectiveness of metformin as a therapy for ADPKD in a more clinically relevant Pkd1RC/RC mouse model, homozygous for the R3277C knockin point mutation in the Pkd1 gene. This mutation causes ADPKD in humans. Pkd1RC/RC male and female mice, which have a slow progression to end-stage kidney disease, received metformin (300 mg/kg/day in drinking water vs. water alone) from 3 to 9 or 12 mo of age. As previously reported, Pkd1RC/RC females had a more severe disease phenotype as compared with males. Metformin treatment reduced the ratio of total kidney weight-to-body weight relative to age-matched and sex-matched untreated controls at both 9 and 12 mo and reduced the cystic index in females at 9 mo. Metformin also increased glomerular filtration rate, lowered systolic blood pressure, improved anemia, and lowered blood urea nitrogen levels relative to controls in both sexes. Moreover, metformin reduced gene expression of key inflammatory markers and both gene and protein expression of kidney injury marker-1 and cyclin-dependent kinase-1 versus untreated controls. Altogether, these findings suggest several beneficial effects of metformin in this highly relevant slowly progressive ADPKD mouse model, which may help inform new ADPKD therapies in patients.NEW & NOTEWORTHY Metformin treatment improved ADPKD disease severity in a relevant, slowly progressive ADPKD mouse model that recapitulates a PKD-associated PKD1 mutation. Relative to controls, metformin reduced kidney weight/body weight, cystic index and BUN levels, while improving GFR, blood pressure and anemia. Metformin also reduced key inflammatory and injury markers, along with cell proliferation markers. These findings suggest several beneficial effects of metformin in this ADPKD mouse model, which may help inform new ADPKD therapies in patients.

MicroRNA-210-5p alleviates cardiac fibrosis via targeting transforming growth factor-beta type I receptor in rats on high sodium chloride (NaCl)-based diet

The present study was designed to explore whether high sodium chloride (NaCl)-based diet (HSD) caused cardiac fibrosis regardless of blood pressure in Sprague-Dawley (SD) rats, and to further determine the effects and the underlying mechanisms of microRNA (miR)-210-5p on HSD-induced cardiac fibrosis in rats or NaCl-induced cardiac fibroblast activation in neonatal rat cardiac fibroblasts (NRCFs). The SD rats received 8% HSD, and NRCFs were treated with NaCl. The levels of collagen I, alpha-smooth muscle actin (α-SMA) and transforming growth factor-beta 1 (TGF-β1) were increased in the heart of hypertension (HTN), hypertension-prone (HP) and hypertension-resistant (HR) rats on HSD in vivo. NaCl increased the levels of collagen I, α-SMA and TGF-β1 in NRCFs in vitro. The level of miR-210-5p was reduced in both NBD-induced rats' hearts and NaCl-treated NRCFs, which was consistent with the results of miR high-throughput sequencing in NRCFs. The HSD or NaCl-induced increases of collagen I, α-SMA and TGF-β1 were inhibited by miR-210-5p agomiR in vitro and in vivo, respectively. miR-210-5p antagomiR could mimic the pathological effects of NaCl in NRCFS. Bioinformatics analysis and luciferase reporter assays demonstrated that TGF-β type I receptor (TGFBR1) was a direct target gene of miR-210-5p. These results indicated that HSD resulted in cardiac fibrosis regardless of blood pressure. The upregulation of miR-210-5p could attenuate cardiac fibroblast activation in NRCFS via targeting TGFBR1. Thus, upregulating miR-210-5p might be a strategy for the treatment of cardiac fibrosis.

Recent advances in understanding ion transport mechanisms in polycystic kidney disease

This review focuses on the most recent advances in the understanding of the electrolyte transport-related mechanisms important for the development of severe inherited renal disorders, autosomal dominant (AD) and recessive (AR) forms of polycystic kidney disease (PKD). We provide here a basic overview of the origins and clinical aspects of ARPKD and ADPKD and discuss the implications of electrolyte transport in cystogenesis. Special attention is devoted to intracellular calcium handling by the cystic cells, with a focus on polycystins and fibrocystin, as well as other calcium level regulators, such as transient receptor potential vanilloid type 4 (TRPV4) channels, ciliary machinery, and purinergic receptor remodeling. Sodium transport is reviewed with a focus on the epithelial sodium channel (ENaC), and the role of chloride-dependent fluid secretion in cystic fluid accumulation is discussed. In addition, we highlight the emerging promising concepts in the field, such as potassium transport, and suggest some new avenues for research related to electrolyte handling.

Insights Into the Molecular Mechanisms of Polycystic Kidney Diseases

Autosomal dominant (AD) and autosomal recessive (AR) polycystic kidney diseases (PKD) are severe multisystem genetic disorders characterized with formation and uncontrolled growth of fluid-filled cysts in the kidney, the spread of which eventually leads to the loss of renal function. Currently, there are no treatments for ARPKD, and tolvaptan is the only FDA-approved drug that alleviates the symptoms of ADPKD. However, tolvaptan has only a modest effect on disease progression, and its long-term use is associated with many side effects. Therefore, there is still a pressing need to better understand the fundamental mechanisms behind PKD development. This review highlights current knowledge about the fundamental aspects of PKD development (with a focus on ADPKD) including the PC1/PC2 pathways and cilia-associated mechanisms, major molecular cascades related to metabolism, mitochondrial bioenergetics, and systemic responses (hormonal status, levels of growth factors, immune system, and microbiome) that affect its progression. In addition, we discuss new information regarding non-pharmacological therapies, such as dietary restrictions, which can potentially alleviate PKD.

Predictors of progression in autosomal dominant and autosomal recessive polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) and autosomal recessive polycystic kidney disease (ARPKD) are characterized by bilateral cystic kidney disease leading to progressive kidney function decline. These diseases also have distinct liver manifestations. The range of clinical presentation and severity of both ADPKD and ARPKD is much wider than was once recognized. Pediatric and adult nephrologists are likely to care for individuals with both diseases in their lifetimes. This article will review genetic, clinical, and imaging predictors of kidney and liver disease progression in ADPKD and ARPKD and will briefly summarize pharmacologic therapies to prevent progression.

Non-Coding RNAs in Hereditary Kidney Disorders

Single-gene defects have been revealed to be the etiologies of many kidney diseases with the recent advances in molecular genetics. Autosomal dominant polycystic kidney disease (ADPKD), as one of the most common inherited kidney diseases, is caused by mutations of PKD1 or PKD2 gene. Due to the complexity of pathophysiology of cyst formation and progression, limited therapeutic options are available. The roles of noncoding RNAs in development and disease have gained widespread attention in recent years. In particular, microRNAs in promoting PKD progression have been highlighted. The dysregulated microRNAs modulate cyst growth through suppressing the expression of PKD genes and regulating cystic renal epithelial cell proliferation, mitochondrial metabolism, apoptosis and autophagy. The antagonists of microRNAs have emerged as potential therapeutic drugs for the treatment of ADPKD. In addition, studies have also focused on microRNAs as potential biomarkers for ADPKD and other common hereditary kidney diseases, including HNF1β-associated kidney disease, Alport syndrome, congenital abnormalities of the kidney and urinary tract (CAKUT), von Hippel-Lindau (VHL) disease, and Fabry disease. This review assembles the current understanding of the non-coding RNAs, including microRNAs and long noncoding RNAs, in polycystic kidney disease and these common monogenic kidney diseases.

The genetic background significantly impacts the severity of kidney cystic disease in the Pkd1RC/RC mouse model of autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD), primarily due to PKD1 or PKD2 mutations, causes progressive kidney cyst development and kidney failure. There is significant intrafamilial variability likely due to the genetic background and environmental/lifestyle factors; variability that can be modeled in PKD mice. Here, we characterized mice homozygous for the PKD1 hypomorphic allele, p.Arg3277Cys (Pkd1^RC/RC), inbred into the BALB/cJ (BC) or the 129S6/SvEvTac (129) strains, plus F1 progeny bred with the previously characterized C57BL/6J (B6) model; F1(BC/B6) or F1(129/B6). By one-month cystic disease in both the BC and 129 Pkd1^RC/RC mice was more severe than in B6 and continued with more rapid progression to six to nine months. Thereafter, the expansive disease stage plateaued/declined, coinciding with increased fibrosis and a clear decline in kidney function. Greater severity correlated with more inter-animal and inter-kidney disease variability, especially in the 129-line. Both F1 combinations had intermediate disease severity, more similar to B6 but progressive from one-month of age. Mild biliary dysgenesis, and an early switch from proximal tubule to collecting duct cysts, was seen in all backgrounds. Preclinical testing with a positive control, tolvaptan, employed the F1(129/B6)-Pkd1^RC/RC line, which has moderately progressive disease and limited isogenic variability. Magnetic resonance imaging was utilized to randomize animals and provide total kidney volume endpoints; complementing more traditional data. Thus, we show how genetic background can tailor the Pkd1^RC/RC model to address different aspects of pathogenesis and disease modification, and describe a possible standardized protocol for preclinical testing.

Non-coding RNAs and the mineralocorticoid receptor in the kidney

The final steps in the Renin-Angiotensin-Aldosterone signaling System (RAAS) involve binding of the corticosteroid hormone, aldosterone to its mineralocorticoid receptor (MR). The bound MR interacts with response elements to induce or repress the transcription of aldosterone-regulated genes. Along with the classic genomic targets of aldosterone that alter mRNA and protein expression, aldosterone also regulates the expression of non-coding RNAs (ncRNAs). Short ncRNAs termed microRNAs (miRs) have been shown to play a role in transducing aldosterone's actions via MR signaling. The role of miRs in homeostatic regulation of aldosterone signaling, and the potential for aldosterone-regulated miRs to act as feedback regulators of MR have been recently reported. In this review, the role of miRs in RAAS signaling and feedback regulation of MR in kidney epithelial cells will be discussed.

Inhibition of neprilysin with sacubitril without RAS blockage aggravates renal disease in Dahl SS rats

Salt-sensitive (SS) hypertension is accompanied with severe cardiorenal complications. In this condition, elevated blood pressure (BP) resulting from salt retention is associated with counterintuitively lower levels of atrial natriuretic peptide (ANP). In plasma, ANP is degraded by the neprilysin; therefore, pharmacological inhibition of this metalloprotease (i.e., with sacubitril) can be employed to increase ANP level. We have shown earlier that sacubitril in combination with valsartan (75 μg/day each) had beneficial effects on renal function in Dahl SS rats. The goal of this study was to evaluate the effects of a higher dose of sacubitril on renal damage in this model. To induce hypertension, male Dahl SS rats were fed a 4% NaCl diet (HS) for 21 days, and were administered sacubitril (125 μg/day) or vehicle via s.c. osmotic pumps. At the end of the HS challenge, both groups exhibited similar outcomes for GFR, heart weight, plasma electrolytes, BUN, and creatinine. Sacubitril exacerbated kidney hypertrophy, but did not affect levels of renal fibrosis. We also observed aggravated glomerular lesions and increased formation of protein casts in the sacubitril-treated animals compared to controls. Thus, in Dahl SS rats, administration of sacubitril without renin-angiotensin-system blockage had adverse effects on renal disease progression, particularly in regards to glomerular damage and protein cast formation. We can speculate that while ANP levels are increased because of neprilysin inhibition, there are off-target effects of sacubitril, which are detrimental to renal function in the SS hypertensive state.

MicroRNAs and Polycystic Kidney Disease

Important advances have been made regarding the diagnosis and management of polycystic kidney diseases. Care of patients with polycystic kidney diseases has moved beyond supportive care for complications and chronic kidney disease to new potentially disease-modifying therapies. Recently, the role of noncoding RNAs, in particular microRNAs, has been described in polycystic kidney diseases. microRNAs are involved in the regulation of gene expression, in which PKD1, PKD2, and other genes that contribute to the pathogenesis of polycystic kidney diseases are considerable participants. Seminal studies have highlighted the potential importance of microRNAs as new therapeutic targets and innovative diagnostic and/or prognostic biomarkers. Furthermore, an anti-miR-17 drug has advanced through preclinical autosomal dominant polycystic disease studies, and an anti-miR-21 drug has already cleared a phase 1 clinical trial. Most probably, new drugs in the microRNA research field will be yielded as a result of ongoing and planned therapeutic trials. To provide a foundation for understanding microRNA functions as a disease-modifying therapeutic drug in novel targeted therapies, in this narrative review we present an overview of the current knowledge of microRNAs in the pathogenesis of polycystic kidney diseases.

Aldosterone-induced microRNAs act as feedback regulators of mineralocorticoid receptor signaling in kidney epithelia

The final steps in the Renin-Angiotensin-Aldosterone signaling System (RAAS) involve binding of the corticosteroid hormone, aldosterone to its mineralocorticoid receptor (MR). The bound MR interacts with response elements to induce or repress the transcription of aldosterone-regulated genes. A well characterized aldosterone-induced gene is the serum and glucocorticoid-induced kinase (SGK1), which acts downstream to increase sodium transport in distal kidney nephron epithelial cells. The role of microRNAs (miRs) induced by extended aldosterone stimulation in regulating MR and SGK1 has not been reported. In these studies, miRs predicted to bind to the 3'-UTR of mouse MR were profiled by qRT-PCR after aldosterone stimulation. The miR-466a/b/c/e family was upregulated in mouse kidney cortical collecting duct epithelial cells. A luciferase reporter assay confirmed miR-466 binding to both MR and SGK1 3'-UTRs. Inhibition of miR-466 increased MR and SGK1 mRNA and protein levels. Inhibiting miR-466b and preventing its upregulation after aldosterone stimulation increased amiloride-sensitive sodium transport and sensitivity to aldosterone stimulation. In vivo upregulation of miR-466 was confirmed in distal nephrons of mice on low Na⁺ diets. Repression of MR and SGK1 by aldosterone-induced miRs may represent a negative feedback loop that contributes to a form of aldosterone escape in vivo.

Differential effects of low-dose sacubitril and/or valsartan on renal disease in salt-sensitive hypertension

Diuretics and renin-angiotensin system blockers are often insufficient to control the blood pressure (BP) in salt-sensitive (SS) subjects. Abundant data support the proposal that the level of atrial natriuretic peptide may correlate with the pathogenesis of SS hypertension. We hypothesized here that increasing atrial natriuretic peptide levels with sacubitril, combined with renin-angiotensin system blockage by valsartan, can be beneficial for alleviation of renal damage in a model of SS hypertension, the Dahl SS rat. To induce a BP increase, rats were challenged with a high-salt 4% NaCl diet for 21 days, and chronic administration of vehicle or low-dose sacubitril and/or valsartan (75 μg/day each) was performed. Urine flow, Na+ excretion, and water consumption were increased on the high-salt diet compared with the starting point (0.4% NaCl) in all groups but remained similar among the groups at the end of the protocol. Upon salt challenge, we observed a mild decrease in systolic BP and urinary neutrophil gelatinase-associated lipocalin levels (indicative of alleviated tubular damage) in the valsartan-treated groups. Sacubitril, as well as sacubitril/valsartan, attenuated the glomerular filtration rate decline induced by salt. Alleviation of protein cast formation and lower renal medullary fibrosis were observed in the sacubitril/valsartan- and valsartan-treated groups, but not when sacubitril alone was administered. Interestingly, proteinuria was mildly mitigated only in rats that received sacubitril/valsartan. Further studies of the effects of sacubitril/valsartan in the setting of SS hypertension, perhaps involving a higher dose of the drug, are warranted to determine if it can interfere with the progression of the disease.

Nephron-Specific Disruption of Polycystin-1 Induces Cyclooxygenase-2-Mediated Blood Pressure Reduction Independent of Cystogenesis

BACKGROUND

Hypertension often occurs before renal function deteriorates in autosomal dominant polycystic kidney disease (ADPKD). It is unknown whether the Pkd1 gene product polycystin-1-the predominant causal factor in ADPKD-itself contributes to ADPKD hypertension independent of cystogenesis.

METHODS

We induced nephron-specific disruption of the Pkd1 gene in 3-month-old mice and examined them at 4-5 months of age.

RESULTS

Kidneys from the Pkd1 knockout mice showed no apparent renal cysts, tubule dilation, or increased cell proliferation. Compared with control mice, Pkd1 knockout mice exhibited reduced arterial pressure during high salt intake; this associated with an increased natriuretic, diuretic, and kaliuretic response during the first 2-3 days of salt loading. The lower arterial pressure and enhanced natriuresis during high salt loading in Pkd1 knockout mice were associated with lower urinary nitrite/nitrate excretion and markedly increased urinary PGE₂ excretion, whereas GFR, plasma renin concentration, and urinary endothelin-1 excretion were similar between knockout and control mice. Kidney cyclooxygenase-2 protein levels were increased in Pkd1 knockout mice during high salt intake; administration of NS-398, a selective cyclooxygenase-2 inhibitor, abolished the arterial pressure difference between the knockout and control mice during high salt intake. Total kidney Na⁺/K⁺/2Cl^- cotransporter isoform 2 (NKCC2) levels were greatly reduced in Pkd1 knockout mice fed a high salt diet compared with controls.

CONCLUSIONS

These studies suggest that nephron polycystin-1 deficiency does not itself contribute to ADPKD hypertension and that it may, in fact, exert a relative salt-wasting effect. The work seems to comprise the first in vivo studies to describe a potential physiologic role for nephron polycystin-1 in the absence of cysts, tubule dilation, or enhanced cell proliferation.

Relationship between the renin-angiotensin-aldosterone system and renal Kir5.1 channels

Kir5.1 (encoded by the Kcnj16 gene) is an inwardly rectifying K+ (Kir) channel highly expressed in the aldosterone-sensitive distal nephron of the kidney, where it forms a functional channel with Kir4.1. Kir4.1/Kir5.1 channels are responsible for setting the transepithelial voltage in the distal nephron and collecting ducts and are thereby major determinants of fluid and electrolyte distribution. These channels contribute to renal blood pressure control and have been implicated in salt-sensitive hypertension. However, mechanisms pertaining to the impact of K ir4.1/Kir5.1-mediated K+ transport on the renin-angiotensin-aldosterone system (RAAS) remain unclear. Herein, we utilized a knockout of Kcnj16 in the Dahl salt-sensitive rat (SSKcnj16-/-) to investigate the relationship between Kir5.1 and RAAS balance and function in the sensitivity of blood pressure to the dietary Na+/K+ ratio. The knockout of Kcnj16 caused substantial elevations in plasma RAAS hormones (aldosterone and angiotensin peptides) and altered the RAAS response to changing the dietary Na+/K+ ratio. Blocking aldosterone with spironolactone caused rapid mortality in SSKcnj16-/- rats. Supplementation of the diet with high K+ was protective against mortality resulting from aldosterone-mediated mechanisms. Captopril and losartan treatment had no effect on the survival of SSKcnj16-/- rats. However, neither of these drugs prevented mortality of SSKcnj16-/- rats when switched to high Na+ diet. These studies revealed that the knockout of Kcnj16 markedly altered RAAS regulation and function, suggesting Kir5.1 as a key regulator of the RAAS, particularly when exposed to changes in dietary sodium and potassium content.

Knockout of P2rx7 purinergic receptor attenuates cyst growth in a rat model of ARPKD

The severity of polycystic kidney diseases (PKD) depends on the counterbalancing of genetic predisposition and environmental factors exerting permissive or protective influence on cyst development. One poorly characterized phenomenon in the cystic epithelium is abnormal purinergic signaling. Earlier experimental studies revealed the high importance of the ionotropic P2X receptors (particularly, P2X7) in the pathophysiology of the cyst wall. To study mechanisms of P2X7 involvement in cyst growth and aspects of targeting these receptors in PKD treatment we performed a CRISPR/SpCas9-mediated global knockout of the P2rx7 gene in PCK rats, a model of autosomal recessive PKD (ARPKD). A single base insertion in exon 2 of the P2rx7 gene in the renal tissues of homozygous mutant animals leads to lack of P2X7 protein that did not affect their viability or renal excretory function. However, PCK.P2rx7 rats demonstrated slower cyst growth (but not formation of new cysts) compared with heterozygous and PCK.P2rx7⁺ littermates. P2X7 receptors are known to activate pannexin-1, a plasma channel capable of releasing ATP, and we found here that pannexin-1 expression in the cystic epithelium is significantly higher than in nondilated tubules. P2X7 deficiency reduces renal pannexin-1 protein expression and daily urinary ATP excretion. Patch-clamp analysis revealed that lack of P2X7 increases epithelial sodium channel activity in renal tissues and restores impaired channel activity in cysts. Interpretation of our current data in the context of earlier studies strongly suggests that P2X7 contributes to cyst growth by increasing pannexin-1-dependent pathogenic ATP release into the lumen and reduction of sodium reabsorption across the cyst walls.

Progression of diabetic kidney disease in T2DN rats

Diabetic kidney disease (DKD) is one of the leading pathological causes of decreased renal function and progression to end-stage kidney failure. To explore and characterize age-related changes in DKD and associated glomerular damage, we used a rat model of type 2 diabetic nephropathy (T2DN) at 12 wk and older than 48 wk. We compared their disease progression with control nondiabetic Wistar and diabetic Goto-Kakizaki (GK) rats. During the early stages of DKD, T2DN and GK animals revealed significant increases in blood glucose and kidney-to-body weight ratio. Both diabetic groups had significantly altered renin-angiotensin-aldosterone system function. Thereafter, during the later stages of disease progression, T2DN rats demonstrated a remarkable increase in renal damage compared with GK and Wistar rats, as indicated by renal hypertrophy, polyuria accompanied by a decrease in urine osmolarity, high cholesterol, a significant prevalence of medullary protein casts, and severe forms of glomerular injury. Urinary nephrin shedding indicated loss of the glomerular slit diaphragm, which also correlates with the dramatic elevation in albuminuria and loss of podocin staining in aged T2DN rats. Furthermore, we used scanning ion microscopy topographical analyses to detect and quantify the pathological remodeling in podocyte foot projections of isolated glomeruli from T2DN animals. In summary, T2DN rats developed renal and physiological abnormalities similar to clinical observations in human patients with DKD, including progressive glomerular damage and a significant decrease in renin-angiotensin-aldosterone system plasma levels, indicating these rats are an excellent model for studying the progression of renal damage in type 2 DKD.

Pharmacological reversal of renal cysts from secretion to absorption suggests a potential therapeutic strategy for managing autosomal dominant polycystic kidney disease

Autosomal-dominant polycystic kidney disease (ADPKD) induces a secretory phenotype, resulting in multiple fluid-filled cysts. We have previously demonstrated that VX-809, a corrector of the cystic fibrosis transmembrane conductance regulator (CFTR), reduces cyst growth. Here, we show that in normal mice CFTR is located within the cells and also at the apical and basolateral membranes. However, in polycystic kidney disease (pkd1)-knockout mice, CFTR was located at the plasma membrane, consistent with its role in cAMP-dependent fluid secretion. In cystic mice, VX-809 treatment increased CFTR levels at the apical membrane and reduced its association with the endoplasmic reticulum. Surprisingly, VX-809 treatment significantly increased CFTR's co-localization with the basolateral membrane in cystic mice. Na⁺/H⁺ exchanger 3 (NHE3) is present in pkd1-knockout and normal mice and in proximal tubule-derived, cultured pkd1-knockout cells. VX-809 increased the expression, activity, and apical plasma membrane localization of NHE3. Co-localization of epithelial sodium channel (ENaC) with the plasma membrane was reduced in cysts in pkd1-knockout mice, consistent with an inability of the cysts to absorb fluid. Interestingly, in the cystic mice, VX-809 treatment increased ENaC levels at the apical plasma membrane consistent with fluid absorption. Thus, VX-809 treatment of pkd1-null mouse kidneys significantly affected CFTR, NHE3, and ENaC, altering the cyst phenotype from one poised toward fluid secretion toward one more favorable for absorption. VX-809 also altered the location of CFTR but not of NHE3 or ENaC in normal mice. Given that VX-809 administration is safe, it may have potential utility for treating patients with ADPKD.

ATP release into ADPKD cysts via pannexin-1/P2X7 channels decreases ENaC activity

Genetic predisposition is necessary for polycystic kidney disease (PKD) initiation, although there are other, incompletely identified downstream processes that are required for cyst growth. Their characterization may provide a unique opportunity for clinical interventions. One of the poorly studied phenomena in PKD is high ATP content in cysts. Unfortunately, neither origins of uncontrolled ATP release, nor consequences of abnormal purinergic signaling in relation to epithelial transport are well explored in the polycystic kidney. We tested the distribution of pannexin-1 (Panx1) and P2X7, two proteins potentially involved in ATP release, in the kidneys of the Pkd1RC/RC mice, a model of autosomal dominant PKD (ADPKD). Abundances of both proteins were abnormally increased in the cyst lining cells compared to non-dilated collecting ducts. To establish if pannexin-1 contributes to ATP release in the collecting ducts (CD), we measured luminal accumulation of ATP in M1 cell renal CD monolayers, and found that treatment with probenecid, a Panx1 blocker, prevents ATP release. Single channel patch clamp analysis of polarized M1 cells revealed that apical stimulation of P2X receptors with αβ-MeATP acutely reduces ENaC activity. We conclude that in ADPKD progression, an abnormal hyperexpression of both PANX1 and P2RX7 occurs in the cyst lining epithelial cells. High abundance of both proteins is not typical for non-dilated CDs but, when it happens in cysts, pannexin1/P2X7 cooperation elevates ATP release into the luminal space. High ATP level is a pathogenic factor facilitating cystogenesis by reducing ENaC-mediated reabsorption from the lumen.