Citrate therapy for polycystic kidney disease in rats

A combination of β-hydroxybutyrate and citrate ameliorates disease progression in a rat model of polycystic kidney disease

Our research has shown that interventions producing a state of ketosis are highly effective in rat, mouse, and cat models of polycystic kidney disease (PKD), preventing and partially reversing cyst growth and disease progression. The ketone β-hydroxybutyrate (BHB) appears to underlie this effect. In addition, we have demonstrated that naturally formed microcrystals within kidney tubules trigger a renoprotective response that facilitates tubular obstruction clearance in healthy animals but, alternatively, leads to cyst formation in PKD. The administration of citrate prevents microcrystal formation and slows PKD progression. Juvenile Cy/+ rats, a nonorthologous PKD model, were supplemented from 3 to 8 wk of age with water containing titrated BHB, citrate, or in combination to find minimal effective and optimal dosages, respectively. Adult rats were given a reduced BHB/citrate combination or equimolar control K/NaCl salts from 8 to 12 wk of age. In addition, adult rats were placed in metabolic cages following BHB, citrate, and BHB/citrate administration to determine the impact on mineral, creatinine, and citrate excretion. BHB or citrate alone effectively ameliorates disease progression in juvenile rats, decreasing markers of cystic disease and, in combination, producing a synergistic effect. BHB/citrate leads to partial disease regression in adult rats with established cystic disease, inhibiting cyst formation and kidney injury. BHB/citrate confers benefits via multiple mechanisms, increases creatinine and citrate excretion, and normalizes mineral excretion. BHB and citrate are widely available and generally recognized as safe compounds and, in combination, exhibit high promise for supporting kidney health in polycystic kidney disease.NEW & NOTEWORTHY Combining β-hydroxybutyrate (BHB) and citrate effectively slows and prevents cyst formation and expansion in young Cy/+ rats using less BHB and citrate than when used alone, demonstrating synergy. In adult rats, the combination causes a partial reversal of existing disease, reducing cyst number and cystic area, preserving glomerular health, and decreasing markers of kidney injury. Our results suggest a safe and feasible strategy for supporting kidney health in polycystic kidney disease (PKD) using a combination of BHB and citrate.

Metabolome analysis to investigate the effect of heavy metal exposure and chemoprevention agents on toxic injury caused by a multi-heavy metal mixture in rats

Heavy metal pollution is a significant threat to both the environment and living organisms. This is especially vital considering the persistent and cumulative nature of heavy metal exposure, which can lead to severe and chronic health consequences for individuals. Therefore, implementing effective treatments is critical to addressing the serious public health issues posed by heavy metal pollution. In this study, nontargeted metabolomics was carried out to investigate the metabolic changes associated with long-term low-dose intake of mixed heavy metal pollutants (MHMPs) in liver, kidney, and plasma samples of Sprague-Dawley (SD) rats with and without treatment to reveal the underlying toxic effects of MHMPs and the effects of chemoprevention agents, including epigallocatechin-3-gallate (EGCG), trisodium citrate dihydrate (TCD), and glutathione (GSH). In the liver, kidney, and plasma, we observed a total of 21, 69, and 16 metabolites, respectively, exhibiting significant differences (P < 0.05, fold change >1.2 or <0.83, and VIP ≥ 1) between the control group and the mixture group. The findings demonstrated that exposure to MHMPs leads to the dysregulation of numerous metabolic pathways, with a particular emphasis on purine metabolism and aminoacyl-tRNA biosynthesis with upregulated renal purine metabolites and downregulated hepatic purine metabolites as well as renal aminoacyl-tRNA biosynthesis-related metabolites. However, the application of chemical protectants was shown to partially restore the metabolic alterations induced by MHMPs, particularly purine metabolism-related metabolites, including hepatic adenine and renal adenine, guanine, guanosine, adenosine monophosphate (AMP), and hypoxanthine. In addition, liver adenosine, kidney inosine and L-phenylalanine were considered the main regulated sites based on their significant correlations with multiple heavy metals. Our study provides crucial insights into the toxicological mechanisms of heavy metal pollution and has the potential to guide the development of effective preventive strategies.

The citrate transporter SLC13A5 as a therapeutic target for kidney disease: evidence from Mendelian randomization to inform drug development

BACKGROUND

Solute carrier family 13 member 5 (SLC13A5) is a Na+-coupled citrate co-transporter that mediates entry of extracellular citrate into the cytosol. SLC13A5 inhibition has been proposed as a target for reducing progression of kidney disease. The aim of this study was to leverage the Mendelian randomization paradigm to gain insight into the effects of SLC13A5 inhibition in humans, towards prioritizing and informing clinical development efforts.

METHODS

The primary Mendelian randomization analyses investigated the effect of SLC13A5 inhibition on measures of kidney function, including creatinine and cystatin C-based measures of estimated glomerular filtration rate (creatinine-eGFR and cystatin C-eGFR), blood urea nitrogen (BUN), urine albumin-creatinine ratio (uACR), and risk of chronic kidney disease and microalbuminuria. Secondary analyses included a paired plasma and urine metabolome-wide association study, investigation of secondary traits related to SLC13A5 biology, a phenome-wide association study (PheWAS), and a proteome-wide association study. All analyses were compared to the effect of genetically predicted plasma citrate levels using variants selected from across the genome, and statistical sensitivity analyses robust to the inclusion of pleiotropic variants were also performed. Data were obtained from large-scale genetic consortia and biobanks, with sample sizes ranging from 5023 to 1,320,016 individuals.

RESULTS

We found evidence of associations between genetically proxied SLC13A5 inhibition and higher creatinine-eGFR (p = 0.002), cystatin C-eGFR (p = 0.005), and lower BUN (p = 3 × 10-4). Statistical sensitivity analyses robust to the inclusion of pleiotropic variants suggested that these effects may be a consequence of higher plasma citrate levels. There was no strong evidence of associations of genetically proxied SLC13A5 inhibition with uACR or risk of CKD or microalbuminuria. Secondary analyses identified evidence of associations with higher plasma calcium levels (p = 6 × 10-13) and lower fasting glucose (p = 0.02). PheWAS did not identify any safety concerns.

CONCLUSIONS

This Mendelian randomization analysis provides human-centric insight to guide clinical development of an SLC13A5 inhibitor. We identify plasma calcium and citrate as biologically plausible biomarkers of target engagement, and plasma citrate as a potential biomarker of mechanism of action. Our human genetic evidence corroborates evidence from various animal models to support effects of SLC13A5 inhibition on improving kidney function.

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.

Urinary Citrate Is Associated with Kidney Outcomes in Early Polycystic Kidney Disease

Low urinary citrate and crystal deposition accelerated cystogenesis in an experimental model of polycystic kidney disease (PKD).Hypocitraturia, frequently observed in patients with autosomal dominant PKD (ADPKD) could contribute to disease progression.Present findings suggest lower urinary citrate in early PKD was associated with faster eGFR decline and worse kidney survival.

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.

Gut-kidney axis in oxalate homeostasis

PURPOSE OF REVIEW

The gut-kidney axis plays a critical role in oxalate homeostasis, and better understanding of oxalate transport regulatory mechanisms is essential for developing novel therapies.

RECENT FINDINGS

Oxalate potentially contributes to chronic kidney disease (CKD) progression, CKD - and end stage renal disease (ESRD)-associated cardiovascular diseases, polycystic kidney disease (PKD) progression, and/or poor renal allograft survival, emphasizing the need for plasma and urinary oxalate lowering therapies. One promising strategy would be to enhance the bowel's ability to secrete oxalate, which might be facilitated by the following findings. Oxalobacter formigenes (O. formigenes)-derived factors recapitulate O. formigenes colonization effects by reducing urinary oxalate excretion in hyperoxaluric mice by inducing colonic oxalate secretion. Protein kinase A activation stimulates intestinal oxalate transport by enhancing the surface expression of the oxalate transporter SLC26A6 (A6). Glycosylation also stimulates A6-mediated oxalate transport. The colon adapts to chronic acidosis in rats through increased colonic oxalate secretion as previously reported in CKD rats, and A6-mediated enteric oxalate secretion is critical in reducing the body oxalate burden in CKD mice. Intestinal oxalate transport is negatively regulated by proinflammatory cytokines and cholinergic, purinergic, and adenosinergic signaling.

SUMMARY

These findings could facilitate the development of novel therapeutics for hyperoxalemia, hyperoxaluria, and related disorders if similar regulatory mechanisms are confirmed in humans.

Crystal deposition triggers tubule dilation that accelerates cystogenesis in polycystic kidney disease

The rate of disease progression in autosomal-dominant (AD) polycystic kidney disease (PKD) exhibits high intra-familial variability suggesting that environmental factors may play a role. We hypothesized that a prevalent form of renal insult may accelerate cystic progression and investigated tubular crystal deposition. We report that calcium oxalate (CaOx) crystal deposition led to rapid tubule dilation, activation of PKD-associated signaling pathways, and hypertrophy in tubule segments along the affected nephrons. Blocking mTOR signaling blunted this response and inhibited efficient excretion of lodged crystals. This mechanism of "flushing out" crystals by purposefully dilating renal tubules has not previously been recognized. Challenging PKD rat models with CaOx crystal deposition, or inducing calcium phosphate deposition by increasing dietary phosphorous intake, led to increased cystogenesis and disease progression. In a cohort of ADPKD patients, lower levels of urinary excretion of citrate, an endogenous inhibitor of calcium crystal formation, correlated with increased disease severity. These results suggest that PKD progression may be accelerated by commonly occurring renal crystal deposition which could be therapeutically controlled by relatively simple measures.

A cross-platform metabolomics workflow for volume-restricted tissue samples: application to an animal model for polycystic kidney disease

Metabolic profiling provides an unbiased view of the physiological status of an organism as a “function” of the metabolic composition of a measured sample.

Experiences and Perspectives of Polycystic Kidney Disease Patients following a Diet of Reduced Osmoles, Protein, and Acid Precursors Supplemented with Water: A Qualitative Study

BACKGROUND

Salt, protein, acid precursors, and fluid intake have been identified as factors that influence cyst growth in ADPKD. Unfortunately, the feasibility of following these dietary restrictions/enhancements from a patient's point-of-view has yet to be studied. The purpose of this study is to understand better the experiences of patients following a relatively complex dietary prescription targeting these factors.

METHODS

Twelve adults with ADPKD and kidney function >30ml/min/1.73m2 were recruited from the University of Kansas Medical Center Polycystic Kidney Disease clinic. In a qualitative design, semi-structured interviews of participants were conducted following a four week dietary intervention (experimental diet lower in sodium, protein, and acid precursors, and supplemented with water) either face-to-face or by telephone. All interviews were recorded, transcribed verbatim, and checked for accuracy. Transcripts were analyzed thematically for emerging themes.

RESULTS

Participants reported that eating less meat and more fruits and vegetables were the easiest components of the diet, whereas reaching the daily goal amount of fruits and vegetables and tracking the diet constantly were the most difficult components. Participants had little difficulty with fluid intake and reported the prescribed fluid goal as achievable. The tracking system for fruits and vegetables and protein was reported to be both helpful and intuitive, but tracking their intake on paper was tedious. Eating out was the most significant barrier to following the diet with some individuals avoiding restaurants in order to comply with the dietary prescription.

CONCLUSION

Participants on the experimental diet heightened their awareness of the consumption of dietary salt, protein, acid precursors, and fluid intake. Additionally, most participants believed adherence to the prescribed diet was feasible. However, participants wanted less cumbersome ways to track and monitor the diet, especially given that the prescribed diet is designed for lifelong adherence. Future studies should focus on targeting these specific dietary factors in larger groups of more ethnically and culturally diverse populations to help inform clinicians and how best to help diverse populations adhere to the dietary intervention.

TRIAL REGISTRATION

ClinicalTrials.gov NCT01810614.

Lack of Benefit of Early Intervention with Dietary Flax and Fish Oil and Soy Protein in Orthologous Rodent Models of Human Hereditary Polycystic Kidney Disease

Rationale for dietary advice in polycystic kidney disease (PKD) is based in part on animal studies that have examined non-orthologous models with progressive development of cystic disease. Since no model completely mimics human PKD, the purpose of the current studies was to examine the effects of dietary soy protein (compared to casein) or oils enriched in omega-3 fatty acids (fish or flax oil compared to soy oil) on early disease progression in two orthologous models of PKD. The models studied were Pkd2^WS25/- mice as a model of autosomal dominant PKD, and PCK rats as a model of autosomal recessive PKD. After 13 weeks of feeding, dietary fish (but not flax) oil resulted in larger kidneys and greater kidney water content in female Pkd2^WS25/- compared to control mice. After 12 weeks of feeding male PCK compared to control rats, both fish and flax compared to soy oil resulted in enlarged kidneys and livers, greater kidney water content and higher kidney cyst area in diseased rats. Dietary soy protein compared to casein had no effects in Pkd2^WS25/- compared to control mice. In PCK rats, kidney and liver histology were not improved, but lower proteinuria and higher urine pH suggest that soy protein could be beneficial in the long term. Therefore, in contrast to studies in non-orthologous models during the progressive development phase, these studies in orthologous PKD models do not support dietary advice to increase soy protein or oils enriched in omega-3 oils in early PKD.

Systematic biomarker discovery and coordinative validation for different primary nephrotic syndromes using gas chromatography-mass spectrometry

The goal of this study is to identify systematic biomarker panel for primary nephrotic syndromes from urine samples by applying a non-target metabolite profiling, and to validate their utility in independent sampling and analysis by multiplex statistical approaches. Nephrotic syndrome (NS) is a nonspecific kidney disorder, which is mostly represented by minimal change disease (MCD), focal segmental glomerulosclerosis (FSGS), and membranous glomerulonephritis (MGN). Since urine metabolites may mirror disease-specific functional perturbations in kidney injury, we examined urine samples for distinctive metabolic changes to identify biomarkers for clinical applications. We developed unbiased multi-component covarianced models from a discovery set with 48 samples (12 healthy controls, 12 MCD, 12 FSGS, and 12 MGN). To extensively validate their diagnostic potential, new batch from 54 patients with primary NS were independently examined a year after. In the independent validation set, the model including citric acid, pyruvic acid, fructose, ethanolamine, and cysteine effectively discriminated each NS using receiver operating characteristic (ROC) analysis except MCD-MGN comparison; nonetheless an additional metabolite multi-composite greatly improved the discrimination power between MCD and MGN. Finally, we proposed the re-constructed metabolic network distinctively dysregulated by the different NSs that may deepen comprehensive understanding of the disease mechanistic, and help the enhanced identification of NS and therapeutic plans for future.

Diet and polycystic kidney disease: A pilot intervention study

BACKGROUND & AIMS

Dietary sodium, protein, acid precursors, and water have been linked to cyst growth in polycystic kidney disease; yet, no studies in patients have examined the feasibility of using a dietary intervention that controls all of these factors. The aim of this study was to determine if a diet, appropriate for persons of most ages, reduces the excretion of sodium, urea, acid, and decreases mean urine osmolality while gaining acceptance by patients with autosomal dominant polycystic kidney disease (ADPKD).

METHODS

Twelve adults with ADPKD enrolled in a pre-post pilot feasibility study and served as their own controls. Individuals consumed their usual diet for one week then for four weeks followed an isocaloric diet lower in sodium and protein and higher in fruits, vegetables, and water. Three-day diet records and two 24-h urine samples were collected at baseline, week 2, and week 4 visits; blood pressure, weight, and serum were obtained at all three visits. A modified nutrition hassles questionnaire was completed on the last visit.

RESULTS

During the dietary intervention, subjects (n = 11) consumed less sodium, protein, and dietary acid precursors 36%, 28%, and 99%, respectively, and increased fluid intake by 42%. Urinary sodium, urea, net acid excretion, osmoles, and osmolality decreased 20%, 28%, 20%, 37%, and 15%, respectively; volume increased 35%. Urine changes were in accord with the diet record. Ninety-one percent of participants reported that none of the hassles were worse than "somewhat severe", and most participants felt "somewhat confident" or "very confident" that they could manage the new diet.

CONCLUSIONS

A majority of adult patients with ADPKD successfully prepared and followed a composite diet prescription with decreased sodium, protein, acid precursors, and increased fluid intake. This trail was registered at ClinicalTrials.gov (NCT01810614).

Acidosis: progression of chronic kidney disease and quality of life

Metabolic acidosis (MA) is relatively common in patients with chronic kidney disease (CKD) particularly in stages 4 and 5. It is assumed to play a contributory role in the development of several complications including bone disease, skeletal muscle wasting, altered protein synthesis, and degradation. Recent evidence also suggests that even mild acidosis might play a role in progressive glomerular filtration rate loss. Experimental and clinical studies suggest that correction of acidosis by alkali therapy attenuates these complications and improves quality of life. Despite several recent small and single-center studies supporting this notion, more robust evidence is required with regard to the long-term benefits of alkali therapy, type of alkali supplements, and the optimal level of serum bicarbonate.

Application of GC-MS coupled with chemometrics for scanning serum metabolic biomarkers from renal fibrosis rat

Renal interstitial fibrosis closely relates to chronic kidney disease and is regarded as the final common pathway in most cases of end-stage renal disease. Metabolomic biomarkers can facilitate early diagnosis and allow better understanding of the pathogenesis underlying renal fibrosis. Gas chromatography-mass spectrometry (GC/MS) is one of the most promising techniques for identification of metabolites. However, the existence of the background, baseline offset, and overlapping peaks makes accurate identification of the metabolites unachievable. In this study, GC/MS coupled with chemometric methods was successfully developed to accurately identify and seek metabolic biomarkers for rats with renal fibrosis. By using these methods, seventy-six metabolites from rat serum were accurately identified and five metabolites (i.e., urea, ornithine, citric acid, galactose, and cholesterol) may be useful as potential biomarkers for renal fibrosis.

Determinants of renal tissue hypoxia in a rat model of polycystic kidney disease

Renal tissue oxygen tension (Po2) and its determinants have not been quantified in polycystic kidney disease (PKD). Therefore, we measured kidney tissue Po2 in the Lewis rat model of PKD (LPK) and in Lewis control rats. We also determined the relative contributions of altered renal oxygen delivery and consumption to renal tissue hypoxia in LPK rats. Po2 of the superficial cortex of 11- to 13-wk-old LPK rats, measured by Clark electrode with the rat under anesthesia, was higher within the cysts (32.8 ± 4.0 mmHg) than the superficial cortical parenchyma (18.3 ± 3.5 mmHg). Po2 in the superficial cortical parenchyma of Lewis rats was 2.5-fold greater (46.0 ± 3.1 mmHg) than in LPK rats. At each depth below the cortical surface, tissue Po2 in LPK rats was approximately half that in Lewis rats. Renal blood flow was 60% less in LPK than in Lewis rats, and arterial hemoglobin concentration was 57% less, so renal oxygen delivery was 78% less. Renal venous Po2 was 38% less in LPK than Lewis rats. Sodium reabsorption was 98% less in LPK than Lewis rats, but renal oxygen consumption did not significantly differ between the two groups. Thus, in this model of PKD, kidney tissue is severely hypoxic, at least partly because of deficient renal oxygen delivery. Nevertheless, the observation of similar renal oxygen consumption, despite markedly less sodium reabsorption, in the kidneys of LPK compared with Lewis rats, indicates the presence of inappropriately high oxygen consumption in the polycystic kidney.

1H NMR-based metabolite profiling of plasma in a rat model of chronic kidney disease

Chronic kidney disease (CKD) is characterized by the gradual loss of the kidney function to excrete wastes and fluids from the blood. ¹H NMR-based metabolomics was exploited to investigate the altered metabolic pattern in rats with CKD induced by surgical reduction of the renal mass (i.e., 5/6 nephrectomy (5/6 Nx)), particularly for identifying specific metabolic biomarkers associated with early of CKD. Plasma metabolite profiling was performed in CKD rats (at 4- or 8-weeks after 5/6 Nx) compared to sham-operated rats. Principle components analysis (PCA), partial least squares-discriminant analysis (PLS-DA) and orthogonal partial least squares-discriminant analysis (OPLS-DA) score plots showed a significant separation between the groups. The resulting metabolic profiles demonstrated significantly increased plasma levels of organic anions, including citrate, β-hydroxybutyrate, lactate, acetate, acetoacetate, and formate in CKD. Moreover, levels of alanine, glutamine, and glutamate were significantly higher. These changes were likely to be associated with complicated metabolic acidosis in CKD for counteracting systemic metabolic acidosis or increased protein catabolism from muscle. In contrast, levels of VLDL/LDL (CH₂)_n and N-acetylglycoproteins were decreased. Taken together, the observed changes of plasma metabolite profiles in CKD rats provide insights into the disturbed metabolism in early phase of CKD, in particular for the altered metabolism of acid-base and/or amino acids.

Kidney injury accelerates cystogenesis via pathways modulated by heme oxygenase and complement

AKI accelerates cystogenesis. Because cystogenic mutations induce strong transcriptional responses similar to those seen after AKI, these responses may accelerate the progression of cystic renal disease. Here, we modulated the severity of the AKI-like response in Cys1(cpk/cpk) mice, a model that mimics autosomal recessive polycystic kidney disease. Specifically, we induced or inhibited activity of the renoprotective enzyme heme oxygenase (HO) and determined the effects on renal cystogenesis. We found that induction of HO attenuated both renal injury and the rate of cystogenesis, whereas inhibition of HO promoted cystogenesis. HO activity mediated the response of NFκB, which is a hallmark transcriptional feature common to both cystogenesis and AKI. Among the HO-modulated effects we measured, expression of complement component 3 (C3) strongly correlated with cystogenesis, a functionally relevant association as suggested by Cys1(cpk/cpk) mice with genetically induced C3 deficiency. Because both C3 deficiency and HO induction reduce cyst number and cyst areas, these two factors define an injury-stimulated cystogenic pathway that may provide therapeutic targets to slow the formation of new renal cysts and the growth of existing cysts.

Acidosis and progression of chronic kidney disease

PURPOSE OF REVIEW

Chronic kidney disease progressively impairs the ability of kidneys to excrete hydrogen ions owing to the reduced capacity of the kidney to synthesize ammonia resulting in metabolic acidosis. There is good experimental evidence that metabolic acidosis contributes to protein energy wasting disorder and progression of chronic kidney disease (CKD). However, there was a lack of robust clinical evidence to support these experimental observations.

RECENT FINDINGS

Three recent publications have confirmed the experimental evidence and the only randomized controlled study of its kind has suggested that the correction of acidosis by sodium bicarbonate in patients with advanced CKD is associated with attenuation of the rate of decline of renal function, reduction in the incidence of end stage renal disease and improvement of nutritional parameters.

SUMMARY

In light of these recent studies, it appears that this cheap and simple strategy, which is in line with current renal recommendations, has the potential of translating into significant economic, quality of life and clinical outcome benefits in an expanding pool of patients with CKD.

The Influence of G‐Protein β3‐Subunit Gene and Endothelial Nitric Oxide Synthase Gene in Exon 7 Polymorphisms on Progression of Autosomal Dominant Polycystic Kidney Disease

BACKGROUND

A significant phenotypical variability is observed in autosomal dominant polycystic kidney disease (ADPKD). The variability cannot be fully explained by the genetic heterogeneity of the disease. We examined the influence of G-protein beta3-subunit C825T polymorphism and endothelial nitric oxide synthase Glu298Asp polymorphism on the progression of ADPKD towards end stage renal failure (ESRF).

METHODS

306 ADPKD patients (pts) were analyzed; 261 pts (136 males, 125 females) with ESRF, with subgroup of 73 pts (44 males, 29 females) with ESRF before 45 years (rapid progressors), 46 pts (20 males, 26 females) with ESRF later than in 63 years (slow progressors) and 45 ADPKD pts (17 males, 28 females) in mean age 51 years with serum creatinine under 110 micromol/L (slow progressors) and 100 genetically unrelated healthy Czech subjects. DNA samples from collected blood were genotyped for G-protein beta3-subunit C825T genotype in exon 10 and for endothelial nitric oxide synthase Glu298Asp genotype in exon 7.

RESULTS

The G-protein beta3-subunit C825T genotype exhibited no significant differences among the groups of slow progressors (6.6% (6/91) TT, 54.9% (50/91) CT, 38.8% (35/91) CC), rapid progressors (9.6% (7/73) TT, 46.6% (34/73) CT, 43.8% (32/73) CC), ADPKD group with ESRF between 40-63 years (9.2% (13/142) TT, 50% (71/142) CT, 40.8% (58/142) CC) and control group (12% TT, 44% CT, 44% CC). When comparing the ages of ESRF of all patients with ESRF, we did not find significant differences in the ages: males TT--51.7+/-8.8 years, CT--51.9+/-10.3 years, CC--49.7+/-10.2 years and females TT--56+/-9.9 years, CT--53.2+/-8.5 years, CC--53.9+/-8.7 years. The endothelial nitric oxide synthase Glu298Asp and Asp29Asp genotypes were significantly more frequent in rapid progressors (9.6% (7/73) Asp/Asp, 39.7% (29/73) Asp/Glu, 50.7% (37/73) Glu/Glu) and in ADPKD group with ESRF between 40-63 years (11.3% (16/142) Asp/Asp, 41.5% (59/142) Asp/Glu, 47.2% (67/142) Glu/Glu) in comparison with slow progressors (8.8% (8/91) Asp/Asp, 24.2% (22/91) Asp/Glu, 67.0% (61/91) Glu/Glu) and with control group (8% Asp/Asp, 32% Asp/Glu, 60% Glu/Glu) (Chi-square test, p<0.05). Comparing the ages of ESRF of all patients with ESRF, we did not find significant differences in the ages in males with Asp/Asp--54.9+/-10.4 years, Asp/Glu--50.2+/-9.4 years, Glu/Glu--51.0+/-10.4 years. We found out in homozygous Asp/Asp females significantly earlier onset of ESRF (49.2+/-5.6 years) in comparison with heterozygous females (53.3+/-7.2 years) and with Glu/Glu homozygous females (54.8+/-9.7 years) (t-test, p<0.05).

CONCLUSION

We excluded the significance of G-protein beta3-subunit C825T polymorphism on the progression of ADPKD. We established the negative prognostic value of the carriers of Asp variant of eNOS polymorphism. Finding of new modifiers could have in future clinical consequences for ADPKD patients.

Volume Progression in Autosomal Dominant Polycystic Kidney Disease: The Major Factor Determining Clinical Outcomes

Autosomal dominant polycystic kidney disease (PKD) is a hereditary condition characterized by the progressive enlargement of innumerable renal cysts that contribute to life-altering morbidity early in the course of the disease. Evidence indicates that the rate of increase in kidney volume can be reliably measured by magnetic resonance or computed tomography imaging, thus providing objective means to judge the effectiveness of therapies that are targeted to the aberrant growth of renal tubules. It is now possible, therefore, to monitor the effectiveness of potential therapies on the signature abnormality in autosomal dominant PKD before irreversible damage has been done by the cysts. Evidence accumulated from human cross-sectional and longitudinal studies and longitudinal studies of PKD models in animals provide strong support for the view that reducing the rate of kidney volume enlargement will ameliorate the late-stage development of renal insufficiency.

Hypocitraturia as a risk factor for nephrocalcinosis after kidney transplantation

Calcium-oxalate crystal deposition in kidney transplant biopsy specimen led us to investigate the impact of calcineurin inhibitor treatment on urinary excretion of lithogenic and stone inhibitory substances in 53 children after successful kidney transplantation (KTx) receiving cyclosporine A (CsA) or tacrolimus. We compared the values obtained with those of 12 patients with recurrent nephrotic syndrome under CsA and of 6 patients with Rasmussen encephalitis (RE) under tacrolimus therapy. Renal ultrasound examinations were repeatedly performed. Hypocitraturia was found in 69% of patients, with KTx patients having a significantly lower urinary citrate excretion than those receiving calcineurin inhibitors for other reasons. Secondly, we found hyperoxaluria in 35% of patients, again especially in those after KTx. No significant difference in urinary substances was seen comparing CsA with tacrolimus treatment. Urolithiasis was found in one and calcium-oxalate crystal deposition in biopsy specimen of three KTx patients. Calcineurin inhibitor treatment can lead to significant hypocitraturia, especially in patients after KTx receiving the highest dose of medication. Hyperoxaluria is primarily the result of a removal of significant body oxalate stores, deposited during dialysis, but may not be suspected as a specific side effect of calcineurin inhibitor therapy. Both findings can increase the risk for urolithiasis or nephrocalcinosis.

Dietary potassium citrate does not harm the pcy mouse

Formation of multiple cysts in the kidneys occurs in several inherited diseases and often leads to terminal kidney failure. Because there is no definitive therapy to halt or slow the progression of renal cystic disease in people, numerous studies have examined possible therapies in animal models. Autosomal-dominant polycystic kidney disease (ADPKD) in the Han:SPRD rat is ameliorated when alkalinizing citrate salts are provided in drinking solutions. By contrast, pcy mice with cystic disease fare worse with the same treatment. We tested the hypothesis that pcy mice ingesting citrate salts in the feed would not be adversely affected by this treatment. Male homozygous pcy mice were given regular feed or 6% potassium citrate-supplemented feed and ad libitum access to water starting at 3 weeks of age. The survival curves of the treated and untreated mice were not significantly different. We conclude that treatment with potassium citrate in the feed does not affect the progression of renal cystic disease in the pcy mouse. This model closely resembles human adolescent nephronophthisis (NPHP3). Based on these findings, citrate treatment cannot be recommended for NPHP3. The fact that it did no harm, however, removes a significant barrier to its consideration as a therapy for ADPKD.

Micro-MRI methods to detect renal cysts in mice

BACKGROUND

Mouse models of disease, especially using transgenic and knockout technologies, are powerful tools to analyze the molecular basis of disease. We recently reported that a new dynamic micro-MRI method with dendrimer-based contrast agents can visualize renal structure and function in normal living mice and mice with acute renal failure. While MRI contrast enhancement is useful for detecting functional impairment of the kidneys, this technology has limitations in assessing morphologic changes, particularly cystic disease, because contrast-enhanced micro-MRI depicts cysts as low-intensity areas that cannot be distinguished from fibrotic foci.

METHODS

In the current study, we evaluated if micro-MRI employing a new three-dimensional MR hydrography signal sequence [three-dimensional fast imaging employing steady-state acquisition (3D-FIESTA)] can visualize chronic cystic changes without any contrast agents.

RESULTS

We were able to positively depict multiple renal cortical cysts of approximately 0.2 mm diameter in a mouse model of sickle cell disease and observe serial changes of renal cysts (>0.2 mm diameter) in cyclooxygenase-2 (COX-2) knockout mice during a 21/2-month period. Some cysts decreased in size over time.

CONCLUSIONS

Micro-MRI with 3D-FIESTA can depict cyst formation in the diseased kidneys of living mice without injection of contrast agents.

Calcium citrate ameliorates the progression of chronic renal injury

BACKGROUND

Metabolic acidosis is a consequence of chronic renal failure and it may produce bone demineralization, muscle proteolysis, and progression of chronic renal failure. The aim of this study was to evaluate the effects of correction of metabolic acidosis with calcium citrate in an experimental model of renal mass ablation.

METHODS

Wistar rats were subjected to 5/6 nephrectomy and were randomly assigned to one of 4 groups: nontreated (NFX); treated with calcium citrate (1.45 g/100 g feed) (NFX-CIT); treated with captopril (500 mg/L water) (NFX-CAP); or treated with both (NFX-CAP-CIT) during 1, 10, or 20 weeks. Body weight, systolic blood pressure, proteinuria, arterial bicarbonate concentration, urine citrate excretion, plasma calcium, and inulin clearance were measured. Histologic glomerular and tubulointerstitial damage scores were measured at 1, 10, and 20 weeks, and glomerular and tubular proliferating cell nuclear antigen (PCNA)-positive cells, alpha-smooth muscle actin, and desmin staining were studied by immunohistochemistry at 1 and 10 weeks.

RESULTS

The treated groups showed significantly less glomerular and tubulointerstitial cellular proliferation in the first week (P < 0.05), less glomerular cell transdifferentiation and higher plasma bicarbonate at 10 weeks (P < 0.05), as well as diminished histologic glomerular and tubulointerstitial damage scores at 20 weeks (P < 0.05). Inulin clearances were higher (P < 0.05), and urine protein excretion rates were lower (P < 0.05) than in the NFX non-treated group, but arterial blood pressure was not significantly different in the NFX-CIT group.

CONCLUSION

Calcium citrate slows the progression of chronic renal injury in the 5/6 NFX model. It improves metabolic acidosis and diminishes cell proliferation and transdifferentiation without changes in systolic blood pressure.

Recent advances in understanding the pathogenesis of polycystic kidney disease: therapeutic implications

Hereditary polycystic kidney disease (PKD) is a common cause of renal failure. Increasing knowledge is available regarding mechanisms of cyst development and progression, and renal functional deterioration in PKD. On the basis of this information and theories regarding the pathophysiology of these processes, studies to alter progression and potentially treat PKD have been reported. Cyst development and progression requires epithelial cell proliferation, transepithelial fluid secretion and extracellular matrix remodelling. Several interventions designed to inhibit cell proliferation or alter fluid secretion modify the progression of PKD in selected animal models. Renal functional deterioration appears to involve interstitial inflammation and fibrosis, and tubular apoptosis. Glucocorticoids with anti-inflammatory and antifibrotic properties slow the progression of cystic disease and renal functional deterioration in animal models of PKD. Other interventions, such as dietary modification and angiotensin antagonism, shown to be of benefit in non-PKD models of slowly progressive renal disease, are also of benefit in animal models of PKD. Caution should be used in extrapolating interventional studies in one animal model to another model and certainly to human disease, since examples exist in which treatments in one model of PKD have different effects in another model. Nonetheless, early attempts to determine whether potential treatments are tolerated and of potential benefit in patients with PKD are beginning to appear. Ultimately, treatment of PKD may involve efforts to identify patients at greatest risk for disease progression, thus allowing targeted therapy, use of surrogate markers for disease progression to assist assessment of therapeutic efficacy, and combination therapy to retard disease progression and renal functional deterioration in this common hereditary cause of chronic renal failure.

Oral supplement of six selective amino acids arrest progression renal failure in uremic patients

Certain amino acids such as glycine, L-aspartic acid, L-glutamic acid, L-glutamine, L-histidine and L-arginine taken orally by normal adults or patients with renal failure increase glomerular filtration rate (GFR). Twelve nondiabetic patients suffering from glomerulonephritis confirmed by renal biopsy previously, with creatinine clearances ranging from 15 to 24 ml minute/1.73, and on low protein diet 0.6 g/ kg/day, received an amino acid supplement daily in 2 or 3 doses for 1 year. At 4, 8 and 12 months creatinine clearance increased slightly (NS, NS, NS), 24 hour urine volume increased (P < or = 0.001, 001, 0.001), 24 hour albuminuria decreased (P < 0.001, 0.001, 0.001), serum urea increased (NS, NS, NS) serum albumin increased (NS, 0.05, 0.05), total cholesterol decreased slightly (NS, NS, 0.01), HDL increased slightly (0.05, 0.05, 0.05), LDL decreased (NS, 0.001, 0.001) triglycerides decreased (0.001, 0.001, 0.001), Apo B remained unchanged (NS, NS, NS), ROS/H2O2 decreased (0.001, 0,001, 0.001), Hct increased (NS, 0.01, 0.01) Hb increased (0.05, 0.05, 0.05), and serum phosphate decreased (0.01, 0.01, 0.01). After removal of supplements at the end of the year all parameters remained unchanged. We believe that a large controlled study should be undertaken to confirm these most encouraging findings.

Murine models of polycystic kidney disease: molecular and therapeutic insights

Numerous murine (mouse and rat) models of polycystic kidney disease (PKD) have been described in which the mutant phenotype results from a spontaneous mutation or engineering via chemical mutagenesis, transgenic technologies, or gene-specific targeting in mouse orthologs of human PKD genes. These murine phenotypes closely resemble human PKD, with common abnormalities observed in tubular epithelia, the interstitial compartment, and the extracellular matrix of cystic kidneys. In both human and murine PKD, genetic background appears to modulate the renal cystic phenotype. In murine models, these putative modifying effects have been dissected into discrete factors called quantitative trait loci and genetically mapped. Several lines of experimental evidence support the hypothesis that PKD genes and their modifiers may define pathways involved in cystogenesis and PKD progression. Among the various pathway abnormalities described in murine PKD, recent provocative data indicate that structural and/or functional defects in the primary apical cilia of tubular epithelia may play a key role in PKD pathogenesis. This review describes the most widely studied murine models; highlights the data regarding specific gene defects and genetic modifiers; summarizes the data from these models that have advanced our understanding of PKD pathogenesis; and examines the effect of various therapeutic interventions in murine PKD.

Recent advances in the cell biology of polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is a significant familial disorder, crossing multiple ethnicities as well as organ systems. The goal of understanding and, ultimately, curing ADPKD has fostered collaborative efforts among many laboratories, mustered on by the opportunity to probe fundamental cellular biology. Here we review what is known about ADPKD including well-accepted data such as the identification of the causative genes and the fact that PKD1 and PKD2 act in the same pathway, fairly well-accepted concepts such as the "two-hit hypothesis," and somewhat confusing information regarding polycystin-1 and -2 localization and protein interactions. Special attention is paid to the recently discovered role of the cilium in polycystic kidney disease and the model it suggests. Studying ADPKD is important, not only as an evaluation of a multisystem disorder that spans a lifetime, but as a testament to the achievements of modern biology and medicine.

Atubular glomeruli in a rat model of polycystic kidney disease

BACKGROUND

Autosomal-dominant polycystic kidney disease (ADPKD) is associated with a progressive decline in glomerular filtration rate (GFR) that often leads to end-stage renal disease. The basis for this decline in GFR is poorly understood.

METHODS

Glomeruli in heterozygous Han:SPRD rats with ADPKD and their normal litter mates were studied by light microscopy, using serial sectioning techniques. The connections of the renal corpuscles to proximal tubules were classified as normal, atrophied, or absent (atubular glomerulus). Renal corpuscles also were examined by scanning electron microscopy. Single nephron glomerular blood flows were determined using microspheres.

RESULTS

In the kidneys of six-month-old rats with ADPKD, 50% of the glomeruli were atubular and another 26% were associated with atrophied neck segments; these glomeruli were most often smaller in size than normal. About 16% of the glomeruli were hypertrophied and had normal connections to proximal tubules. Sclerotic changes in cystic kidney glomeruli were usually mild or moderate, and belied the failure of glomerular function. Glomerular blood flow in the cystic kidneys averaged half of normal and was markedly heterogeneous; the majority of small glomeruli displayed very low blood flows and a few showed relatively high blood flows. Fewer glomerular abnormalities were found in rats treated for five months with potassium citrate in their drinking water.

CONCLUSIONS

The diminished GFR in the rat with ADPKD can be accounted for largely by the formation of atubular glomeruli. Compensatory glomerular hypertrophy also is present and may contribute to the progression of the renal disease.

Chronic caffeine consumption exacerbates hypertension in rats with polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is a common inherited disorder frequently associated with renal failure, hypertension, and other abnormalities. The present study determined whether chronic caffeine intake in an animal model of this disease would affect renal structure and function and blood pressure. Heterozygous male Han:Sprague-Dawley rats with ADPKD and normal littermates were provided with either tap water or solutions of caffeine to drink, starting at 1 month of age. When rats were aged 6 months, glomerular filtration rate (GFR) and mean arterial blood pressure (MAP) were measured under Inactin (Byk Gulden, Konstanz, Germany) anesthesia. Caffeine intake had no effect on GFR or cyst development in rats with PKD. MAP was greater in rats with PKD than normal rats and was increased more by caffeine. The hypertensive effect of chronic caffeine intake could not be ascribed to direct pressor effects of angiotensin II. Based on our finding that caffeine exacerbates hypertension in rats with PKD, it may be prudent for patients with ADPKD to limit coffee consumption to four or fewer cups of caffeinated coffee per day, pending studies of humans.

Autosomal dominant polycystic kidney disease: modification of disease progression

Autosomal dominant polycystic kidney disease is a common inherited disorder, which is characterised by the formation of fluid-filled cysts in both kidneys that leads to progressive renal failure. Mutations in two genes, PKD1 and PKD2, are associated with the disorder. We describe the various factors that cause variation in disease progression between patients. These include whether the patient has a germline mutation in the PKD1 or in the PKD2 gene, and the nature of the mutation. Detection of mutations in PKD1 is complicated, but the total number identified is rising and will enable genotype-to-phenotype studies. Another factor affecting disease progression is the occurrence of somatic mutations in PKD genes. Furthermore, modifying genes might directly affect the function of polycystins by affecting the rate of somatic mutations or the rate of protein interactions, or they might affect cystogenesis itself or clinical factors associated with disease progression. Finally, environmental factors that speed up or slow down progress towards chronic renal failure have been identified in rodents.